AGW Observer

Observations of anthropogenic global warming

Posted by Ari Jokimäki on February 8, 2010

We have lot of observations of the Earth surface warming during the last few decades. Surface temperature measurements over land and ocean [1,2], satellite measurements [3,4], weather balloon measurements [5], ocean temperature measurements [6] and borehole measurements [7] all show clear increase in the average temperature of the Earth. Supporting these are the indicators of warming, such as changes in the behavior and ranges of species [8], melting glaciers [9], rising sea level [10], vanishing sea ice [11], etc.

Figure 1. Earth’s surface temperature presented with 11-year floating means. Data is from references [1] and [2].

Carbon dioxide has been shown to be able to intercept part of the thermal radiation in numerous laboratory experiments, starting with the results John Tyndall published in 1859 [12]. Already he was able to prove the simple fact that carbon dioxide intercepts some of the thermal radiation. Since then the carbon dioxide’s ability to intercept thermal radiation has been studied in various conditions, for example in different pressures, different carbon dioxide concentrations, different gas mixtures, different frequency bands, and so on [13]. All these laboratory experiments have one thing in common; they all have made our knowledge of carbon dioxide properties more precise. Today we are even more certain that carbon dioxide intercepts part of the thermal radiation. In addition to that we have a good understanding of how it does it and how large is the effect it causes [13].

In addition to having measured the ability of carbon dioxide to intercept thermal radiation in laboratories, we have also measured the same properties directly from the atmosphere. For example, with satellites we are able to measure the spectrum (which is basically the intensity of the radiation at different frequencies) of the outgoing thermal radiation (also known as outgoing longwave radiation, OLR), and it shows same features as the spectra measured in laboratories [14]. From the spectrum we can identify the effects of different molecules (such as carbon dioxide or other chemicals mankind is emitting to the atmosphere) [15]. All the greenhouse gases are seen in the thermal radiation spectrum just because they have the ability to intercept the thermal radiation. Each gas molecule have their characteristic frequencies at which they intercept the thermal radiation. Therefore the spectrum has holes at each characteristic frequency of each molecule (if there is enough of those molecules in the atmosphere). From the spectrum it also can be determined how much there are the gases in question in the atmosphere. These direct measurements of the ability of carbon dioxide to intercept heat in laboratories and in atmosphere don’t reveal any properties that would disagree with the theory of the anthropogenic (man-made) global warming.

According to the theory, the sunlight first heats up the ground and ocean surface and the warmed up surface then emits heat as thermal radiation. The thermal radiation emitted by the surface then meets the greenhouse gases in the atmosphere. The greenhouse gases intercept part of the radiation and then emit it again to a random direction which causes about half of the radiation to return back towards the surface. When the amount of greenhouse gases increases in the atmosphere, they intercept more of the thermal radiation and more of the radiation will then return back to the surface. So there is more thermal radiation flying around in the lower parts of atmosphere and that means warming. Correspondingly, there is less thermal radiation in the upper atmosphere, especially in the stratosphere (at about 15-50 km height), and that means cooling [16]. The expected cooling of the stratosphere has also been observed in measurements [17].

So we know that carbon dioxide can intercept thermal radiation. In addition to that, we know that the carbon dioxide concentration of the atmosphere has been rising steadily for decades. We know that from the measurements of the atmospheric carbon dioxide concentration, which have been taken directly from the atmosphere with several different methods. We have measured the carbon dioxide concentration from air samples. Accurate measurements were started by Charles Keeling in 1950’s [18], but even before that there had been plenty of measurements[19] but with not enough accuracy to determine the long time changes in the carbon dioxide concentration of the atmosphere. These samples are still taken from many places all over the Earth [20]. Samples are taken for example from air planes or the from the surrounding air of the measurement stations. Many measurement stations now use automatic sampling. We have also measured the carbon dioxide concentration in many different ways from the atmosphere by utilizing spectral measurements [19]. We are able to measure the carbon dioxide concentration from the sunlight at the surface, from reflected sunlight by satellites [21] and from the outgoing thermal radiation of the Earth, that too by satellites [22]. It is noteworthy that when we measure the carbon dioxide concentration from the outgoing thermal radiation of the Earth, we are in fact measuring directly the amount of greenhouse effect caused by the carbon dioxide. Today it is beginning to be a routine to measure the carbon dioxide concentration from satellites in different places of the Earth with short time intervals, so we have knowledge of how carbon dioxide concentration varies in different regions [23].

Figure 2. Annual means of carbon dioxide concentration as measured from three different locations: South pole, Hawaii (Mauna Loa) and Alaska. The data is from reference [20].

So we know that carbon dioxide can intercept thermal radiation and that the atmospheric carbon dioxide concentration is increasing. In addition to those, we know that the increase in carbon dioxide concentration is mainly from the fossilized carbon burned by mankind. We know that because the carbon from the fossilized carbon is slightly different than the average carbon in the atmosphere. The difference is in the mass of the carbon atom in the carbon dioxide molecule. Atoms that are alike otherwise, but their masses are different, are called isotopes. In practice, the difference is caused by the amount of neutrons in the nucleus of the carbon atom. The natural isotopes of carbon are ¹²C, ¹³C, and ¹⁴C, where ¹²C is the lightest and ¹⁴C is heaviest. When trying to determine the source of the atmospheric carbon dioxide increase, the isotope ¹⁴C is the most relevant. Isotope ¹⁴C is also called as radiocarbon. Unlike isotopes ¹²C and ¹³C, the radiocarbon is not stable but decays by itself with a half-life of 5730 years [24]. New radiocarbon is being produced in the atmosphere when cosmic rays are reacting with nitrogen atoms, so there always is little radiocarbon in the atmosphere, even if it has limited lifetime. Because of that, living plants also use some radiocarbon during photosynthesis which means that when living plants are destroyed (by burning for example), they release also some radiocarbon among other isotopes. But fossil fuels (oil, coal) don’t have radiocarbon at all. Fossil fuels are from plants fossilized millions of years ago and because radiocarbon slowly decays by itself, there’s practically no radiocarbon left in fossil fuels. Therefore, when burning fossil fuels, no radiocarbon is released to the atmosphere. We are able to measure the carbon isotopes in the atmosphere and such measurements have shown that the radiocarbon content of the atmosphere has decreased while the carbon dioxide concentration has increased [25]. This can be explained only if the increase in the atmospheric carbon dioxide concentration is from fossil fuels (and it is called Suess effect).

We also have another isotopic method that gives a result that supports the radiocarbon method. Plants have a preference to lighter isotopes, so they have the isotope ¹²C the most. The most of the carbon from fossils is from ancient plants and isotopes ¹²C and ¹³C are stable isotopes (meaning that they don’t decay by themselves), so with those isotopes the carbon in fossils is almost the same as carbon in modern plants. Atmospheric measurements have shown that the carbon dioxide increase is from carbon dioxide that has lot of lightest carbon isotope (¹²C). It has been observed that in atmosphere the isotope ¹²C is getting more common while isotope ¹³C is getting more rare. This is exactly the expected result, if the carbon dioxide increase is from fossilized carbon or from modern plants. Furthermore, the isotope ¹³C has been getting rare at the same time as fossil fuel emissions have increased, so based on that it is more likely that the carbon dioxide increase is from fossil fuels [26]. When we also note the evidence cited above relating to the isotope ¹⁴C, it is practically certain that the increase is from fossil fuels. The increase in atmospheric carbon dioxide calls for so large amounts of carbon anyway, that it is difficult to get so much from anywhere else than from the wood mankind is burning (and from the forests that get destroyed for that) and from fossil fuels (oil, coal). For example volcanoes, which are thought to be significant sources for carbon dioxide, release much less carbon dioxide to the atmosphere than from the emissions of the mankind. The volcanic carbon dioxide emissions are only about 1 % of the emissions of the mankind [27].

So we know that carbon dioxide can intercept thermal radiation and that the atmospheric carbon dioxide concentration is increasing because of mankind. In addition to those, we know that the outgoing thermal radiation from the Earth is decreasing at the spectral bands of greenhouse gases and that the atmosphere is emitting more thermal radiation back to the surface. Those too we know from direct measurements. We have measured the thermal radiation decreasing precisely in those parts of the spectrum which are known to be intercepted by carbon dioxide [28]. The amount of that decrease also agrees well of what is expected from the carbon dioxide concentration changes [29]. We have also measured the thermal radiation to increase on the surface of the Earth [30]. Also in this case the amount of increase fits well to the expected effects of greenhouse gas concentrations and we have even measured the change from the spectral bands of carbon dioxide [31]. So it seems that the outgoing thermal radiation from the Earth is decreasing at carbon dioxide spectral bands and that the decrease would show on the Earth’s surface as an increase in carbon dioxide spectral bands.

In addition to the things mentioned above, we also have lots of research results which indicate that carbon dioxide had significant role also in past climate changes. In the past mankind wasn’t pushing carbon dioxide to the sky, but back then the changes in sunlight arriving to the surface of the Earth first caused a little bit of warming, which then caused carbon dioxide concentration to increase in the atmosphere. This was mainly because of changes in the warming oceans. A warming ocean can cause atmospheric carbon dioxide concentration to increase by at least three ways; the warming affects the solubility of carbon dioxide to seawater, increases the ocean mixing (which flushes out more carbon dioxide from the depths of the ocean), and affects the biological activity in the ocean (and biological activity uses carbon dioxide). So increased atmospheric carbon dioxide concentration and the effects from it then strongly amplify the original warming [32]. In past ages the carbon dioxide has sometimes been very high, many times higher than today, and usually those times have been much warmer than today and for those times that weren’t warmer we have a good reason why they weren’t [32, 33].

Most of the information above is from direct measurements without having to use theories or climate models. So, just by using measurements we have been able to determine that carbon dioxide is causing the warming of the Earth (increased thermal radiation on the Earth’s surface causes the surface to warm), and during the last decades there hasn’t been other known factors that could have caused the observed warming. However, carbon dioxide by itself can only cause little warming to the Earth’s surface temperature, perhaps about one degrees of Celsius on average when carbon dioxide concentration is doubled. When carbon dioxide warms the Earth, the warming causes some things. The warming for example causes snow and ice to melt. When snow and ice melt from certain region revealing the bare ground or ocean surface, the ability to reflect light changes in that region. Snow and ice reflect sunlight much better than bare ground or the ocean surface. When less sunlight is reflected back to the space, there’s more sunlight that stays and heats up the ground and ocean. This is an example of the warming effect from the carbon dioxide causing a consequence that has the effect of causing more warming. The consequences like that are called feedbacks. If a consequence has an effect of causing more warming, it is called a positive feedback, and if a consequence restrains the warming, it is called a negative feedback.

In the studies of feedbacks it has turned out that there are lot of positive feedbacks but only little negative feedbacks. The most important feedbacks are the changes in water vapour and in cloudiness. The water vapour feedback has been determined in many measurements to be clearly positive [34]. When atmosphere warms, it can hold bigger water vapour concentration and because water vapour is a strong greenhouse gas, it causes much more warming. It is generally known that the biggest uncertainty, when predicting the amount of the future warming, has to do with the changes in cloudiness. Problem is mostly due to us not having enough stable and precise long time measurements, so that we would have been able to measure the changes in cloudiness due to warming with enough accuracy [35]. The situation is currently getting better, and latest research indicates that the cloud feedback is positive [36]. If this turns out to be true, it means that the total heating effect from carbon dioxide is large, because the changes in cloudiness has pretty much been the only factor that could have been a strong negative feedback. If clouds are a positive feedback too, as it seems in the light of latest research, there aren’t much things anymore that could restrain the strong warming in the future.

I thank Esko, Kaitsu, Jari, Matti and Timo for their good comments on my text to make it better.

References

1. NASA GISS (Goddard Institute for Space Studies) surface temperature analysis

2. HadCRUT3 (Hadley Centre ja Climate Research Unit, East Anglia) surface temperature analysis

3. RSS (Remote Sensing Systems)

4. UAH (University of Alabama in Huntsville) (link is directly to their data, they don’t seem to have a decent website)

5. HadAT: Upper-air temperatures from weather balloons

6. Pierce et al. (2006), [abstract, full text], “Anthropogenic Warming of the Oceans: Observations and Model Results”

7. Huang et al. (2000), [abstract, full text], “Temperature trends over the past five centuries reconstructed from borehole temperatures”

8. Parmesan (2006), [abstract, full text], “Ecological and Evolutionary Responses to Recent Climate Change”

9. Zemp et al. (2009), [abstract, full text], “Six decades of glacier mass-balance observations: a review of the worldwide monitoring network”

10. Church & White (2006), [abstract, full text], “A 20th century acceleration in global sea-level rise”

11. Comiso et al. (2008), [abstract, full text], “Accelerated decline in the Arctic sea ice cover”

12. Wikipedia: Tyndall’s Setup For Measuring Radiant Heat Absorption By Gases

13. AGW Observer: Papers on laboratory measurements of CO2 absorption properties

14. Niro et al. (2005), [abstract, full text], “Spectra calculations in central and wing regions of CO₂ IR bands between 10 and 20 μm. III: atmospheric emission spectra”

15. Clerbaux et al. (2003), [abstract, full text], “Trace gas measurements from infrared satellite for chemistry and climate applications”

16. Uherek (2006), [full text], “Stratospheric cooling”

17. AGW Observer: Papers on temperature trends in stratosphere

18. Keeling (1960), [full text], “The concentration and isotopic abundances of carbon dioxide in the atmosphere”

19. AGW Observer: Papers on atmospheric carbon dioxide concentration measurements

20. CDIAC Trends: Atmospheric Carbon Dioxide and Carbon Isotope Records

21. Buchwitz et al. (2007), [abstract, full text], “First direct observation of the atmospheric CO₂ year-to-year increase from space”

22. Chédin et al. (2003), [abstract, full text], “First global measurement of midtropospheric CO2 from NOAA polar satellites: Tropical zone”

23. Buchwitz (2008), [full text], “Visualization of the global distribution of greenhouse gases using satellite measurements”

24. Wikipedia: Suess effect

25. Levin & Hessheimer (2000), [abstract, full text], “Radiocarbon – a unique tracer of global carbon cycle dynamics”

26. RealClimate, Steig (2004), “How do we know that recent CO₂ increases are due to human activities?”

27. Hards (2005), [full text], “Volcanic Contributions to Global Carbon Cycle”

28. AGW Observer: Papers on changes in OLR due to GHG’s

29. Griggs & Harries (2004), [abstract, full text], “Comparison of spectrally resolved outgoing longwave data between 1970 and present”

30. AGW Observer: Papers on changes in DLR

31. Evans & Puckrin (2006), [abstract, full text], “Measurements of the Radiative Surface Forcing of Climate”

32. AGW Observer: Papers on GHG role in historical climate changes

33. Royer (2008), [abstract, full text], “Linkages between CO₂, climate, and evolution in deep time”

34. AGW Observer: Papers on water vapor feedback observations

35. Loeb et al. (2007), [abstract], “Variability in global top-of-atmosphere shortwave radiation between 2000 and 2005″. Relevant quote: “As a minimum, radiation budget instruments should be stable enough to detect a change in net cloud forcing corresponding to a 25% cloud feedback. A 25% cloud feedback would reduce or amplify the influence of the anthropogenic radiative forcing by the same amount. Estimates of anthropogenic total radiative forcing in the next few decades are 0.6 Wm⁻² per decade [IPCC, 2001, Figure 9.13]. A 25% cloud feedback would change cloud net radiative forcing by 25% of the anthropogenic radiative forcing, or 0.15 Wm⁻² per decade. The global average shortwave (SW) or solar reflected cloud radiative forcing by clouds is ~50 Wm⁻², so that the observation requirements for global broadband radiation budget to directly observe such a cloud feedback is approximately 0.15/50 = 0.3% per decade in SW broadband calibration stability [Ohring et al., 2005]. Achieving this stability per decade in calibration is extremely difficult and has only recently been demonstrated for the first time by the ERBS and CERES broadband radiation budget instruments [Wong et al., 2006; Loeb et al., 2007].”

36. Clement et al. (2009), [abstract, full text], “Observational and Model Evidence for Positive Low-Level Cloud Feedback”

Posted in AGW evidence | Leave a Comment »

Yet another new climate science blog

Posted by Ari Jokimäki on February 7, 2010

With a few friends we have started a new blog called “Ilmastotieto” (Finnish word for “climate information”). Here’s the link. As you can guess from the title, it’s in Finnish. So from now on I’ll be writing there as well. I don’t think it will make my posting here much less as I probably will be publishing the articles I write there in Finnish here in English. There’s one already that I will publish here as soon as I have translated it to English. It will probably make the paperlist postings here little slower, though.

Posted in General | Leave a Comment »

Papers on black carbon

Posted by Ari Jokimäki on February 5, 2010

This is a list of papers on the climatic effects of black carbon. This subject was suggested by Kaj Luukko in this thread. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Evaluation of black carbon estimations in global aerosol models – Koch et al. (2009) “We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. … In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low.” [Full text]

Global and regional climate changes due to black carbon – Ramanathan & Carmichael (2008) A review paper. “Anthropogenic sources of black carbon, although distributed globally, are most concentrated in the tropics where solar irradiance is highest. … Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions.” [Full text]

Present-day climate forcing and response from black carbon in snow – Flanner et al. (2007) “We apply our Snow, Ice, and Aerosol Radiative (SNICAR) model, coupled to a general circulation model with prognostic carbon aerosol transport, to improve understanding of climate forcing and response from black carbon (BC) in snow. … Applying biomass burning BC emission inventories for a strong (1998) and weak (2001) boreal fire year, we estimate global annual mean BC/snow surface radiative forcing from all sources (fossil fuel, biofuel, and biomass burning) of +0.054 (0.007–0.13) and +0.049 (0.007–0.12) W m⁻², respectively.” [Full text]

20th-Century Industrial Black Carbon Emissions Altered Arctic Climate Forcing – McConnell et al. (2007) “Black carbon (BC) from biomass and fossil fuel combustion alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about its emission or deposition histories. Measurements of BC, vanillic acid, and non–sea-salt sulfur in ice cores indicate that sources and concentrations of BC in Greenland precipitation varied greatly since 1788 as a result of boreal forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a sevenfold increase in ice-core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 watts per square meter, which is eight times the typical preindustrial forcing value.” [Supporting information]

Aerosol organic carbon to black carbon ratios: Analysis of published data and implications for climate forcing – Novakov et al. (2005) “Measurements of organic carbon (OC) and black carbon (BC) concentrations over a variety of locations worldwide have been analyzed to infer the spatial distributions of the ratios of OC to BC. Since these ratios determine the relative amounts of scattering and absorption, they are often used to estimate the radiative forcing due to aerosols. … The OC to BC ratios, ranging from 1.3 to 2.4, appear relatively constant and are generally unaffected by seasonality, sources, or technology changes, at the locations considered here. The ratios compare well with emission inventories over Europe and China but are a factor of 2 lower in other regions. The reduced estimate for OC/BC in aerosols strengthens the argument that reduction of soot emissions maybe a useful approach to slow global warming.” [Full text]

Climate response of direct radiative forcing of anthropogenic black carbon – Chung & Seinfeld (2005) “The equilibrium climate effect of direct radiative forcing of anthropogenic black carbon (BC) is examined by 100-year simulations in the Goddard Institute for Space Studies General Circulation Model II-prime coupled to a mixed-layer ocean model. … The climate sensitivity of BC direct radiative forcing is calculated to be 0.6 K W⁻¹ m², which is about 70% of that of CO₂, independent of the assumption of BC mixing state. The largest surface temperature response occurs over the northern high latitudes during winter and early spring.” [Full text]

Climate sensitivity to black carbon aerosol from fossil fuel combustion – Roberts & Jones (2004) “However, it is unclear how the climate sensitivity to black carbon aerosol forcing compares with the sensitivity to greenhouse gas forcing. Here we investigate this question using the HadSM4 configuration of the Hadley Centre climate model, extended by the addition of interactive black carbon and sulphate aerosol schemes. The results confirm earlier suggestions that the climate sensitivities are not necessarily similar and indicate that the black carbon sensitivity may be weaker.”

A modeling study on the climate impacts of black carbon aerosols – Wang (2004) “A three-dimensional interactive aerosol-climate model has been developed and used to study the climatic impact of black carbon (BC) aerosols. When compared with the model’s natural variability, significant global-scale changes caused by BC aerosols occurred in surface latent and sensible heat flux, surface net long-wave radiative flux, planetary boundary layer height, convective precipitation (all negative), and low-cloud coverage (positive), all closely related to the hydrological cycle. … The result of this study suggests that with a constant annual emission of 14 TgC, BC aerosols do not cause a significant change in global-mean surface temperature. The calculated surface temperature change is determined by a subtle balance among changes in surface energy budget as well as in the hydrological cycle, all caused by BC forcing and often compensate each other. The result of this study shows that the influences of BC aerosols on climate and environment are more significant in regional scale than in global scale.” [Full text]

Global atmospheric black carbon inferred from AERONET – Sato et al. (2003) “The spectral range of AERONET allows discrimination between constituents that absorb most strongly in the UV region, such as soil dust and organic carbon, and the more ubiquitously absorbing black carbon (BC). … We find that the amount of BC in current climatologies must be increased by a factor of 2–4 to yield best agreement with AERONET, in the approximation in which BC is externally mixed with other aerosols. The inferred climate forcing by BC, regardless of whether it is internally or externally mixed, is ≈1 W/m², most of which is probably anthropogenic.” [Full text]

Large historical changes of fossil-fuel black carbon aerosols – Novakov et al. (2003) “We estimate historical trends of fossil-fuel BC emissions in six regions that represent about two-thirds of present day emissions and extrapolate these to global emissions from 1875 onward. Qualitative features in these trends show rapid increase in the latter part of the 1800s, the leveling off in the first half of the 1900s, and the re-acceleration in the past 50 years as China and India developed. We find that historical changes of fuel utilization have caused large temporal change in aerosol absorption, and thus substantial change of aerosol single scatter albedo in some regions, which suggests that BC may have contributed to global temperature changes in the past century.” [Full text]

Climate Effects of Black Carbon Aerosols in China and India – Menon et al. (2002) “In recent decades, there has been a tendency toward increased summer floods in south China, increased drought in north China, and moderate cooling in China and India while most of the world has been warming. We used a global climate model to investigate possible aerosol contributions to these trends. We found precipitation and temperature changes in the model that were comparable to those observed if the aerosols included a large proportion of absorbing black carbon (“soot”), similar to observed amounts. Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the large-scale circulation and hydrologic cycle with significant regional climate effects.” [Full text]

Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols – Jacobson (2001) “Here I simulate the evolution of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an internal mixture, largely due to coagulation and growth of aerosol particles. This finding implies a higher positive forcing from black carbon than previously thought, suggesting that the warming effect from black carbon may nearly balance the net cooling effect of other anthropogenic aerosol constituents. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH₄, suggesting that black carbon may be the second most important component of global warming after CO₂ in terms of direct forcing.”

A global black carbon aerosol model – Cooke & Wilson (1996) “A global inventory has been constructed for emissions of black carbon from fossil fuel combustion and biomass burning. … The modeled values of black carbon mass concentration compare within a factor of 2 in continental regions and some remote regions but are higher than measured values in other remote marine regions and in the upper troposphere.” [Full text]

Effect of black carbon on the optical properties and climate forcing of sulfate aerosols – Chýlek et al. (1995) “We study the optical properties of anthropogenic sulfate aerosols containing black carbon using a recently developed exact solution of the scattering problem for a spherical particle (sulfate aerosol) containing an eccentrically located spherical inclusion (black carbon). … The black carbon within the sulfate aerosol reduces the expected sulfate direct cooling effect by about 0.034 W/m² for each 1% of the black carbon to sulfate mass mixing ratio. Thus the presence of black carbon within sulfate in the background aerosol does not significantly change the previous estimates of the global aerosol direct cooling effect. However, in regions where the black carbon in sulfate concentrations are of the order of 5% or more, the local and regional effects are significant.”

Posted in AGW evidence | Leave a Comment »

Papers on CO2 records from ice cores

Posted by Ari Jokimäki on February 2, 2010

This is a list of papers on the past CO₂ records measured from the ice cores. Specifically this list contains papers that present new or improved carbon dioxide records from ice cores. The list is based to the Ice Core Data Sets at NOAA. The subject for this list was suggested by Magnus W here. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

High-resolution carbon dioxide concentration record 650,000–800,000 years before present – Lüthi et al. (2008) “So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years. Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present.” [Data]

Atmospheric CO₂ and Climate on Millennial Time Scales During the Last Glacial Period – Ahn & Brook (2008) “We compared CO₂ variations on millennial time scales between 20,000 and 90,000 years ago with an Antarctic temperature proxy and records of abrupt climate change in the Northern Hemisphere. CO₂ concentration and Antarctic temperature were positively correlated over millennial-scale climate cycles, implying a strong connection to Southern Ocean processes.” [Full text] [Data]

Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years – Kawamura et al. (2007) “Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. … Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation.” [Supplement] [Data]
Data description: “CO2 record from the Dome Fuji ice core (Antarctica) covering the last 260 kyr with relatively low resolution.”

Stable Carbon Cycle–Climate Relationship During the Late Pleistocene – Siegenthaler et al. (2005) “A record of atmospheric carbon dioxide (CO₂) concentrations measured on the EPICA (European Project for Ice Coring in Antarctica) Dome Concordia ice core extends the Vostok CO₂ record back to 650,000 years before the present (yr B.P.). Before 430,000 yr B.P., partial pressure of atmospheric CO₂ lies within the range of 260 and 180 parts per million by volume. This range is almost 30% smaller than that of the last four glacial cycles; however, the apparent sensitivity between deuterium and CO₂ remains stable throughout the six glacial cycles, suggesting that the relationship between CO₂ and Antarctic climate remained rather constant over this interval.” [Full text] [Data]

Supporting evidence from the EPICA Dronning Maud Land ice core for atmospheric CO₂ changes during the past millennium – Siegenthaler et al. (2005) “Here we present a new detailed CO₂ record from the Dronning Maud Land (DML) ice core, drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA) and some new measurements on a previously drilled ice core from the South Pole. The DML CO₂ record shows an increase from about 278 to 282 parts per million by volume (ppmv) between ad 1000 and ad 1200 and a fairly continuous decrease to a mean value of about 277 ppmv around ad 1700.” [Full text] [Data]

Evidence for substantial accumulation rate variability in Antarctica during the Holocene, through synchronization of CO₂ in the Taylor Dome, Dome C and DML ice cores – Monnin et al. (2004) “High resolution records of atmospheric CO₂ concentration during the Holocene are obtained from the Dome Concordia and Dronning Maud Land (Antarctica) ice cores. These records confirm that the CO₂ concentration varied between 260 and 280 ppmv in the Holocene as measured in the Taylor Dome ice core. However, there are differences in the CO₂ records most likely caused by mismatches in timescales. Matching the Taylor Dome timescale to the Dome C timescale by synchronization of CO₂ indicates that the accumulation rate at Taylor Dome increased through the Holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature. This result shows that different locations experienced substantially different accumulation changes, and casts doubt on the often-used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature, at least for coastal locations.” [Full text] [Data]

High-resolution Holocene N₂O ice core record and its relationship with CH₄ and CO₂ – Flückiger et al. (2002) “Here we fill this gap with a high-resolution N2O record measured along the European Project for Ice Coring in Antarctica (EPICA) Dome C Antarctic ice core. On the same ice we obtained high-resolution methane and carbon dioxide records. This provides the unique opportunity to compare variations of the three most important greenhouse gases (after water vapor) without any uncertainty in their relative timing. The CO₂ and CH₄ records are in good agreement with previous measurements on other ice cores.” [Full text] [Data]

Atmospheric CO₂ Concentrations over the Last Glacial Termination – Monnin et al. (2001) “A record of atmospheric carbon dioxide (CO₂) concentration during the transition from the Last Glacial Maximum to the Holocene, obtained from the Dome Concordia, Antarctica, ice core, reveals that an increase of 76 parts per million by volume occurred over a period of 6000 years in four clearly distinguishable intervals. The close correlation between CO₂ concentration and Antarctic temperature indicates that the Southern Ocean played an important role in causing the CO₂ increase. However, the similarity of changes in CO₂ concentration and variations of atmospheric methane concentration suggests that processes in the tropics and in the Northern Hemisphere, where the main sources for methane are located, also had substantial effects on atmospheric CO₂ concentrations.” [Data]

Atmospheric CO₂ concentration from 60 to 20 kyr BP from the Taylor Dome Ice Core, Antarctica – Indermühle et al. (2000) “A high‐resolution record of the atmospheric CO₂ concentration from 60 to 20 thousand years before present (kyr BP) based on measurements on the ice core of Taylor Dome, Antarctica is presented. This record shows four distinct peaks of 20 parts per million by volume (ppmv) on a millennial time scale. Good correlation of the CO₂ record with temperature reconstructions based on stable isotope measurements on the Vostok ice core (Antarctica) is found.” [Full text] [Data]

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica – Petit et al. (1999) “Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.” [Full text] [Data]
Data description: “They include the deuterium content of the ice (δD_ice, a proxy of local temperature change), the dust content (desert aerosols), the concentration of sodium (marine aerosol), and from the entrapped air the greenhouse gases CO₂ and CH₄, and the δ¹⁸O of O₂ (hereafter δ¹⁸O_atm) which reflects changes in global ice volume and in the hydrological cycle.”

Ice Core Records of Atmospheric CO₂ Around the Last Three Glacial Terminations – Fischer et al. (1999) “High-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 ± 400 years after the warming of the last three deglaciations. Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere.” [Full text] [Data]

Dual modes of the carbon cycle since the Last Glacial Maximum – Smith et al. (1999) “We have measured the CO₂ concentration and δ¹³CO₂ of air trapped in ice from Taylor Dome, Antarctica, across the last glacial termination in order to develop a time series for the carbonisotope composition of atmospheric CO₂ and to constrain the mechanisms of carbon cycling between the main sources and sinks of atmospheric CO₂. The atmospheric CO₂ concentration data (Fig. 1) form a record comparable to that from Byrd and Vostok.” [Full text] [Data]

Holocene carbon-cycle dynamics based on CO₂ trapped in ice at Taylor Dome, Antarctica – Indermühle et al. (1999) “A high-resolution ice-core record of atmospheric CO₂ concentration over the Holocene epoch shows that the global carbon cycle has not been in steady state during the past 11,000 years. Analysis of the CO₂ concentration and carbon stable-isotope records, using a one-dimensional carbon-cycle model,uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial-scale changes of atmospheric CO₂ concentrations.” [Full text] [Data]

Natural and anthropogenic changes in atmospheric CO₂ over the last 1000 years from air in Antarctic ice and firn – Etheridge et al. (1996) “A record of atmospheric CO2 mixing ratios from 1006 A.D. to 1978 A.D. has been produced by analysing the air enclosed in three ice cores from Law Dome, Antarctica. The enclosed air has unparalleled age resolution and extends into recent decades, because of the high rate of snow accumulation at the ice core sites. The CO2 data overlap with the record from direct atmospheric measurements for up to 20 years.” [Data]

CO₂ measurements from polar ice cores: more data from different sites – Staffelbach et al. (1991) “In this paper, we will discuss possible small deviations of the CO₂ concentration in air bubbles from that of the atmosphere at the time of enclosure. To do this, new results from Crête (Central Greenland) ice cores, covering the period since the beginning of industrialisation are presented, showing a good agreement with the data from Antarctic ice cores. In addition, the record of the atmospheric CO₂ concentration during the transition from the last glaciation to the Holocene and the fast variations in the concentration of atmospheric CO₂ during parts of the last glaciation, as suggested by Greenland ice core data, will be discussed.”

CO₂ record in the Byrd ice core 50,000–5,000 years bp – Neftel et al. (1988) “To achieve this, we have studied a great number of samples from the deep ice core from Byrd station, Westantarctica, drilled in 1968. These measurements allow us to reconstruct the atmospheric CO₂ concentration in the time period 50,000–15,000 yr bp in great detail.” [Full text] [Data]

Vostok ice core provides 160,000-year record of atmospheric CO₂ – Barnola et al. (1987) “Direct evidence of past atmospheric CO₂ changes has been extended to the past 160,000 years from the Vostok ice core. These changes are most notably an inherent phenomenon of change between glacial and interglacial periods. Besides this major 100,000-year cycle, the CO₂ record seems to exhibit a cyclic change with a period of some 21,000 years.” [Data]

Evidence from polar ice cores for the increase in atmospheric CO₂ in the past two centuries – Neftel et al. (1985) “An ice core from Siple Station (West Antartica) that allows determination of the enclosed gas concentration with very good time resolution has recently become available. We report here measurements of this core which now allow us to trace the development of the atmospheric CO₂ from a period overlapping the Mauna Loa record back over the past two centuries.”

Posted in AGW evidence | 5 Comments »

Papers on Amazon and global warming

Posted by Ari Jokimäki on February 1, 2010

This is a list of papers on the effect of global warming to the Amazon rainforest. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (February 3, 2010): 2 x Phillips et al. (2009) added, thanks to Skeptical Science for pointing them out (see the comment section below).

Drought Sensitivity of the Amazon Rainforest – Phillips et al. (2009) “We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. … Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 x 10¹⁵ to 1.6 x 10¹⁵ grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.” [Full text] [Conference abstract]

Changes in Amazonian Forest Biomass, Dynamics, and Composition, 1980–2002 – Phillips et al. (2009) “Long-term, on-the-ground monitoring of forest plots distributed across Amazonia provides a powerful means to quantify stocks and fluxes of biomass and biodiversity. Here we examine the evidence for concerted changes in the structure, dynamics, and functional composition of old-growth Amazonian forests over recent decades. … The most likely driver(s) of changes are recent increases in the supply of resources such as atmospheric carbon dioxide, which would increase net primary productivity, increasing tree growth and recruitment, and, in turn, mortality. In the future the growth response of remaining undisturbed Amazonian forests is likely to saturate, and there is a risk of these ecosystems transitioning from sink to source driven by higher respiration (temperature), higher mortality (drought), or compositional change (functional shifts toward lighterwooded plants).” [Full text]

Exploring the likelihood and mechanism of a climate-change-induced dieback of the Amazon rainforest – Malhi et al. (2009) “We examine the evidence for the possibility that 21st-century climate change may cause a large-scale “dieback” or degradation of Amazonian rainforest. … We then examine climate simulations by 19 global climate models (GCMs) in this context and find that most tend to underestimate current rainfall. … Our analysis suggests that dry-season water stress is likely to increase in E. Amazonia over the 21st century, but the region tends toward a climate more appropriate to seasonal forest than to savanna.” [Full text]

Interactions among Amazon land use, forests and climate: prospects for a near-term forest tipping point – Nepstad et al. (2008) “Rising worldwide demands for biofuel and meat are creating powerful new incentives for agro-industrial expansion into Amazon forest regions. Forest fires, drought and logging increase susceptibility to further burning while deforestation and smoke can inhibit rainfall, exacerbating fire risk. If sea surface temperature anomalies (such as El Niño episodes) and associated Amazon droughts of the last decade continue into the future, approximately 55% of the forests of the Amazon will be cleared, logged, damaged by drought or burned over the next 20 years, emitting 15–26 Pg of carbon to the atmosphere.” [Full text]

Towards quantifying uncertainty in predictions of Amazon ‘dieback’ – Huntingford et al. (2008) “We analyse how the modelled vegetation cover in Amazonia responds to (i) uncertainty in the parameters specified in the atmosphere component of HadCM3 and their associated influence on predicted surface climate. … The potential for human-induced climate change to trigger the loss of Amazon rainforest appears robust within the context of the uncertainties explored in this paper.” [Full text]

Climate Change, Deforestation, and the Fate of the Amazon – Malhi et al. (2008) “The forest biome of Amazonia is one of Earth’s greatest biological treasures and a major component of the Earth system. This century, it faces the dual threats of deforestation and stress from climate change. Here, we summarize some of the latest findings and thinking on these threats, explore the consequences for the forest ecosystem and its human residents, and outline options for the future of Amazonia.” [Full text]

New views on an old forest: assessing the longevity, resilience and future of the Amazon rainforest – Maslin et al. (2005) “The aim of this paper is to investigate the longevity and diversity of the Amazonian rainforest and to assess its likely future. Palaeoclimate and palaeoecological records suggest that the Amazon rainforest originated in the late Cretaceous and has been a permanent feature of South America for at least the last 55 million years. The Amazon rainforest has survived the high temperatures of the Early Eocene climate optimum, the gradual Cenozoic cooling, and the drier and lower carbon dioxide levels of the Quaternary glacial periods. Two new theories for the great diversity of the Amazon rainforest are discussed – the canopy density hypothesis and the precessional-forced seasonality hypothesis. We suggest the Amazon rainforest should not be viewed as a geologically ephemeral feature of South America, but rather as a constant feature of the global Cenozoic biosphere. The forest is now, however, entering a set of climatic conditions with no past analogue. The predicted future hotter and more arid tropical climates may have a disastrous effect on the Amazon rainforest.” [Full text]

The role of ecosystem-atmosphere interactions in simulated Amazonian precipitation decrease and forest dieback under global climate warming – Betts et al. (2004) “A suite of simulations with the HadCM3LC coupled climate-carbon cycle model is used to examine the various forcings and feedbacks involved in the simulated precipitation decrease and forest dieback. Rising atmospheric CO₂ is found to contribute 20% to the precipitation reduction through the physiological forcing of stomatal closure, with 80% of the reduction being seen when stomatal closure was excluded and only radiative forcing by CO₂ was included. … The precipitation reduction is enhanced by 20% by the biogeophysical feedback, and 5% by the carbon cycle feedback from the forest dieback.” [Full text]

Contrasting simulated past and future responses of the Amazonian forest to atmospheric change – Cowling et al. (2004) “We contrasted HadCM3LC simulations of Amazonian forest at the last glacial maximum (LGM; 21 kyr ago) and a Younger Dryas–like period (13–12 kyr ago) with predicted responses of future warming to provide estimates of the climatic limits under which the Amazon forest remains relatively stable. Our simulations indicate that despite lower atmospheric CO₂ concentrations and increased aridity during the LGM, Amazonia remains mostly forested, and that the cooling climate of the Younger Dryas–like period in fact causes a trend toward increased above–ground carbon balance relative to today. … Although elevated atmospheric CO₂ contributes to a positive enhancement of plant carbon and water balance, decreased stomatal conductance and increased plant and soil respiration cause a positive feedback that amplifies localized drying and climate warming. We speculate that the Amazonian forest is currently near its critical resiliency threshold, and that even minor climate warming may be sufficient to promote deleterious feedbacks on forest integrity.” [Full text]

Using a GCM analogue model to investigate the potential for Amazonian forest dieback – Huntingford et al. (2004) “A combined GCM analogue model and GCM land surface representation is used to investigate the influences of climatology and land surface parameterisation on modelled Amazonian vegetation change. … The timing of forest dieback is found to be sensitive to the initial pre-industrial climate, as well as uncertainties in the representation of land-atmosphere CO₂ exchange. … Further advances are required in both GCM rainfall simulation and land-surface process representation before a clearer picture will emerge on the timing of possible Amazonian forest dieback.”

A crisis in the making: responses of Amazonian forests to land use and climate change – Laurance (1998) “At least three global-change phenomena are having major impacts on Amazonian forests: (1) accelerating deforestation and logging; (2) rapidly changing patterns of forest loss; and (3) interactions between human land-use and climatic variability. Additional alterations caused by climatic change, rising concentrations of atmospheric carbon dioxide, mining, overhunting and other large-scale phenomena could also have important effects on the Amazon ecosystem.”

Posted in Climate science | 2 Comments »

Papers on natural variability

Posted by Ari Jokimäki on January 29, 2010

This is a list of papers on natural variability of Earth’s climate. This subject was suggested by PeterPan here. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Long-term natural variability and 20th century climate change – Swanson et al. (2009) “Here we present a technique that objectively identifies the component of inter-decadal global mean surface temperature attributable to natural long-term climate variability. Removal of that hidden variability from the actual observed global mean surface temperature record delineates the externally forced climate signal, which is monotonic, accelerating warming during the 20th century.” [Full text]

Changes in the phase of the annual cycle of surface temperature – Stine et al. (2009) “Here we show that the phase of the annual cycle of surface temperature over extratropical land shifted towards earlier seasons by 1.7 days between 1954 and 2007; this change is highly anomalous with respect to earlier variations, which we interpret as being indicative of the natural range. Significant changes in the amplitude of the annual cycle are also observed between 1954 and 2007. These shifts in the annual cycles appear to be related, in part, to changes in the northern annular mode of climate variability, although the land phase shift is significantly larger than that predicted by trends in the northern annular mode alone.” [Full text]

What is causing the variability in global mean land temperature? – Hoerling et al. (2008) “Diagnosis of climate models reveals that most of the observed variability of global mean land temperature during 1880–2007 is caused by variations in global sea surface temperatures (SSTs). Further, most of the variability in global SSTs have themselves resulted from external radiative forcing due to greenhouse gas, aerosol, solar and volcanic variations, especially on multidecadal time scales. Our results indicate that natural variations internal to the Earth’s climate system have had a relatively small impact on the low frequency variations in global mean land temperature. It is therefore extremely unlikely that the recent trajectory of terrestrial warming can be overwhelmed (and become colder than normal) as a consequence of natural variability.” [Full text]

Multidecadal Climate Variability in Observed and Modeled Surface Temperatures – Kravtsov & Spannagle (2008) “This study identifies interdecadal natural climate variability in global surface temperatures by subtracting, from the observed temperature evolution, multimodel ensemble mean based on the World Climate Research Programme’s (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset. The resulting signal resembles the so-called Atlantic multidecadal oscillation (AMO) and is presumably associated with intrinsic dynamics of the oceanic thermohaline circulation (THC). … Evidence suggests that the AMO influence on secular trends in the global-mean surface temperature can arise via direct, regional contribution to the surface temperature evolution, as well as via an indirect route linked to variability of the oceanic uptake of CO₂ associated with AMO-related THC changes.” [Full text]

How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006 – Lean & Rind (2008) “To distinguish between simultaneous natural and anthropogenic impacts on surface temperature, regionally as well as globally, we perform a robust multivariate analysis using the best available estimates of each together with the observed surface temperature record from 1889 to 2006. The results enable us to compare, for the first time from observations, the geographical distributions of responses to individual influences consistent with their global impacts. We find a response to solar forcing quite different from that reported in several papers published recently in this journal, and zonally averaged responses to both natural and anthropogenic forcings that differ distinctly from those indicated by the Intergovernmental Panel on Climate Change, whose conclusions depended on model simulations.” [Full text]

A signature of persistent natural thermohaline circulation cycles in observed climate – Knight et al. (2005) “Using a 1400 year climate model calculation, we are able to simulate the observed pattern and amplitude of the AMO. The results imply the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). This relationship suggests we can attempt to reconstruct past THC changes, and we infer an increase in THC strength over the last 25 years. Potential predictability associated with the mode implies natural THC and AMO decreases over the next few decades independent of anthropogenic climate change.” [Full text]

A Comparison of Surface Air Temperature Variability in Three 1000-Yr Coupled Ocean–Atmosphere Model Integrations – Stouffer et al. (2000) “This study compares the variability of surface air temperature in three long coupled ocean–atmosphere general circulation model integrations. It is shown that the annual mean climatology of the surface air temperatures (SAT) in all three models is realistic and the linear trends over the 1000-yr integrations are small over most areas of the globe. … Assuming that the simulation of variability of the global mean SAT is as realistic on longer timescales as it is for the shorter timescales, then the observed warming of more than 0.5 K of the SAT in the last 110 yr is not likely to be due to internally generated variability of the coupled atmosphere–ocean–sea ice system. Instead, the warming is likely to be due to changes in the radiative forcing of the climate system, such as the forcing associated with increases in greenhouse gases.” [Full text]

ENSO-like Interdecadal Variability: 1900–93 – Zhang et al. (1997) “A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976–77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. … By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the “ENSO cycle-related” variability on the interannual timescale, and the other a linearly independent “residual” comprising all the interdecadal variability in the record.” [Full text]

Global and regional variability in a coupled AOGCM – Tett et al. (1997) “The variability of near surface temperature on global and regional spatial scales and interannual time scales from a 1000 year control integration of the Hadley Centre coupled model (HADCM2-CTL) are compared with the observational record of surface temperature. The model succeeds in reproducing the observed patterns of natural variability, with high variability over the northern continents and low variability over much of the tropics. The model global mean variability has similar strength to observed global mean variability on time scales less than 20 years. The warming seen in the historical record is outside the range of natural variability as simulated in HADCM2-CTL.”

Robust estimation of background noise and signal detection in climatic time series – Mann & Lees (1996) “We present a new technique for isolating climate signals in time series with a characteristic red noise background which arises from temporal persistence. … We apply our methodology to historical climate and paleoclimate time series examples. Analysis of a ~ 3 million year sediment core reveals significant periodic components at known astronomical forcing periodicities and a significant quasiperiodic 100 year peak.”

Model assessment of the role of natural variability in recent global warming – Stouffer et al. (1994) “Here we evaluate the observed warming trend using a 1,000-year time series of global temperature obtained from a mathematical model of the coupled ocean–atmosphere–land system. We find that the model approximately reproduces the magnitude of the annual to interdecadal variation in global mean surface air temperature. But throughout the simulated time series no temperature change as large as 0.5 °C per century is sustained for more than a few decades. Assuming that the model is realistic, these results suggest that the observed trend is not a natural feature of the interaction between the atmosphere and oceans.”

An oscillation in the global climate system of period 65–70 years – Schlesinger & Ramankutty (1994) “Here we apply singular spectrum analysis to four global-mean temperature records, and identify a temperature oscillation with a period of 65–70 years. Singular spectrum analysis of the surface temperature records for 11 geographical regions shows that the 65–70-year oscillation is the statistical result of 50–88-year oscillations for the North Atlantic Ocean and its bounding Northern Hemisphere continents. These oscillations have obscured the greenhouse warming signal in the North Atlantic and North America. Comparison with previous observations and model simulations suggests that the oscillation arises from predictable internal variability of the ocean–atmosphere system.”

Natural variability of the climate system and detection of the greenhouse effect – Wigley & Raper (1990) “Here we show how the ocean may produce low-frequency climate variability by passive modulation of natural forcing, to produce substantial trends in global mean temperature on the century timescale. Simulations with a simple climate model are used to determine the main controls on internally generated low-frequency variability, and show that natural trends of up to 0.3 °C may occur over intervals of up to 100 years. Although the magnitude of such trends is unexpectedly large, it is insufficient to explain the observed global warming during the twentieth century.”

Internally and Externally Caused Climate Change – Robock (1978) “A numerical climate model is used to simulate climate change forced only by random fluctuations of the atmospheric heat transport. This short-term natural variability of the atmosphere is shown to be a possible “cause” not only of the variability of the annual world average temperature about its mean, but also long-term excursions from the mean.” [Full text]

Posted in AGW evidence | Leave a Comment »

Comments on Schwartz et al. (2010), version 2

Posted by Ari Jokimäki on January 27, 2010

I first wrote about Schwartz et al. (2010) here but as some of it was based on my misunderstandings I wrote it again, resulting text below. I mistakenly thought that ocean thermal lag would cause 0.5 K cooling to the observed temperature at all climate sensitivity values and then interpreted their results based on that mistake, which lead me to believe that they had forgot to include the ocean thermal lag to their final analysis. Also, my calculated value of 0.5 K was wrong (0.3 K would have been correct), see the comment section of the first version. So here’s the second, and hopefully more correct version:

Newly published paper by Schwartz et al. (2010) (abstract, full text) has been claimed to show that theory of AGW is false or that “global warming has been cancelled”, etc. The claims are based on this statement in the paper’s introduction:

However, the observed increase of GMST over the industrial period is less than 40% of what would be expected from present best estimates of Earth’s climate sensitivity and the forcing (imposed change in energy balance, W m^-2) by the observed increases in GHGs.

(GMST = global mean surface temperature, GHG = greenhouse gases).

In other words, they determine expected temperature rise from greenhouse gas forcing and the climate sensitivity, then look at the observed temperature rise and compare the two. Not surprisingly, they found that the two are different. I said “not surprisingly” because they only looked at greenhouse gas forcing and I know that there are other forcings at play and I’m also quite sure that some of those forcings work in different direction than greenhouse gases, aerosols for example. Just a simple example of the situation would be that if GHG’s would cause a warming of 2K and aerosols would cause cooling of 1K, then the resulting warming from these two would be 1K meaning that the observed warming would be only 50 % of the expected warming from GHG’s.

So, the 40 % number they give doesn’t represent the total overall situation, but it only represents the situation if only greenhouse gases are considered and the rest forcings are ignored. Now, Schwartz et al. know this because it is the subject of their paper to study what causes the difference, so Schwartz et al. are not claiming that observed temperature is less than 40 % of the expected all-forcing-temperature. Yet, it is the 40 % number that is the one they are now parroting all over the Internet as if it would represent the total overall situation.

It would be the same as if I would calculate that aerosols in the air would cause cooling of certain amount and then I would note that global temperature has been rising instead of expected cooling from aerosols. I would then say that I will now consider why there is such a difference but somebody else would just quote me on the observed temperatures not showing the expected cooling and would then spread that word as a proof that the theory of aerosols has been now shown wrong.

Well, at this point we are only in the introduction section of the Schwartz et al. and we already have handled most of the false claims circulating in the Internet about this. But Schwartz et al. do have things to say even beyond the introduction.

Rest of the paper

Schwartz et al. are studying if the difference between the observed and expected greenhouse gas warming is due four main things:

- Natural variation in global temperature.
- Lack of attainment of equilibrium.
- Overestimate of climate sensitivity.
- Countervailing forcings over the industrial period.

They calculated that the expected warming from GHG’s would have been 2.1 K. They said that the observed temperature increase had been 0.8 K.

Natural variation in global temperature can cause up to 0.2 K of cooling according to them. This is how they found it out:

We use variation in preindustrial global temperature as inferred from proxy records, mainly tree rings, ice cores, corals, and varved sediments to estimate the likely magnitude of any natural cooling over the 150-year interval of the instrumental record.

Proxies? Tree-rings??? Surely any self-respecting climate denier at last now will dump this paper as a heretic production. Well, seriously, I think that’s reasonable approach to get a rough idea. However, it’s also bad news for those who think that the global warming is from natural variability. According to Schwartz et al. observed = 0.8 K and natural variability = 0.2 K. That means the observed warming is 400 % of the expected maximum warming from natural variability – a worse result than the observed versus expected from GHG’s.

Note that natural variability can work for both directions, it can cause cooling or warming.

Lack of attainment of equilibrium is a fancy way of saying that there might be delays in the climate system so that not all the warming from GHG’s has yet been realised in surface temperature but is instead hiding somewhere. Ocean is the most obvious and important place to hide the warming from GHG’s. They determine that 0.37 W/(m²) of the forcing could be hiding in the ocean, and they say that it corresponds to 22 % of the warming discrepancy.

Note that this effect works only to one direction, it has a cooling effect on global surface temperature.

Overestimate of climate sensitivity suggests that the climate sensitivity would be lower than the expected range. That would explain the discrepancy. They note that IPCC limit for very “unlikely” climate sensitivity is 1.5 K and they say that the observed warming would require the climate sensitivity to be even lower than that. The situation is presented in their Figure 2. There they present the observed warming as a horizontal line and they have added the natural variability as a horizontal band around the observed line. The expected warming from GHG’s is presented as an increasing line. One can see that when accounting for natural variability, the expected warming goes out of the band at climate sensitivity of about 1.7 K. That already is within IPCC very unlikely limits, and approaching the “likely” limit of 2.0 K.

Countervailing forcings over the industrial period also have an effect to the global temperature. They are discussing aerosol forcing here. In their Figure 2 they present also how aerosol forcing would effect the situation (however, see below for a minor error in the Figure 2 relating to this). The red lines in Figure 2 are with aerosol forcing; three lines for different amounts of assumed aerosol forcing. They say that with the IPCC best estimate aerosol forcing (1.2 W/m²) the warming “would be compatible with the lower end of the IPCC “likely” range of climate sensitivity”, but actually if we would consider the possible natural variability, we can see from their Figure 2 that resulting climate sensitivity would approach the nominal 3 K value, the climate sensitivity might in that case be about 2.8 K (but of course, as natural variation might work to other direction too, the resulting sensitivity might also be only about 1.6 K).

They then enter to a discussion about the methods of determining the climate sensitivity and possible actions for improving the aerosol forcing uncertainty.

Some notes

Their Figure 2 is not very clear, so it might be good to go explain some of the things in it. The yellow line there represents the GHG-forcing only. The black line represents the greenhouse forcing minus the ocean thermal lag of 0.37 W/m². This is the expected forcing from the GHG’s without taking other forcings to consideration. The effect of aerosol forcing (and tropospheric ozone forcing) is then presented in three red lines. The highest line has aerosol forcing of 0.6 W/m², the middle red line has aerosol forcing of 1.2 W/m², and the lowest red line has aerosol forcing of 2.4 W/m². The values of 0.6 and 2.4 are from the IPCC best estimate 5 % and 95 % range.

However, here is apparently a minor mistake (thanks to AJ for catching that, see the comment section of the first version), the highest and lowest red lines are drawn too high so that at the equilibrium climate sensitivity of 1 K/(W/m²) they are 0.15 K too high. The reason for this is unknown.

The equilibrium climate sensitivity of 1 K/(W/m²) (corresponding to CO₂ doubling temperature of 3.7 K) is a good point to see how the numbers add because at that equilibrium sensitivity the forcing and temperature have numerically the same value (i.e. the ocean thermal lag forcing of 0.37 W/m² is 0.37 K at that point). Let us check if the numbers add up. Here are the numbers as I have “measured” (based on the pixel amounts in the image) them from the Figure 2 (the difference from the yellow line is given in parentheses):

Yellow line:  2.63 K (0.00 K)
Black line:   2.26 K (0.38 K)
Red 0.6 line: 2.13 K (0.50 K)
Red 1.2 line: 1.36 K (1.28 K)
Red 2.4 line: 0.33 K (2.31 K)

As described above, the yellow line presents the GHG forcing which is said to be 2.6 W/m², so at this point it would be expected to be 2.6 K. My measured value agrees very well with that.

The black line is expected to be 0.37 K below the yellow line at that point, and the measured difference from the figure is 0.38, so that agrees well.

The highest red line has the forcings of GHG’s, tropospheric ozone, ocean thermal lag, and aerosol forcing of 0.6 W/m², so they would add up to (2.6 + 0.35 – 0.37 – 0.6) W/m² = 1.98 W/m². Here my measured value is 2.13 K, which is 0.15 K higher than it should be (this was discussed briefly above).

The middle red line has the same forcings as the highest red line, but now the aerosol forcing is 1.2 W/m², so now all the forcings would add up to (2.6 + 0.35 – 0.37 – 1.2) W/m² = 1.38 W/m². Here my measured value is 1.36 K, a good agreement.

The lowest red line has the same forcings as the highest red line, but now the aerosol forcing is 2.4 W/m², so now all the forcings would add up to (2.6 + 0.35 – 0.37 – 2.4) W/m² = 0.18 W/m². Here my measured value is 0.33 K, so here again is the 0.15 K discrepancy discussed above.

Note that solar and volcanic forcing have not been mentioned. They should be included in the natural variability, so there is no need to handle them separately here.

I’m little disappointed of the lack of references to the preceeding studies on the subject. For example, Lean & Rind (2008) determined the relative sizes on forcings, finding no such problems as Schwartz et al. are suggesting.

As I have been making my paperlists, I have read a lot of introduction sections of papers because there the existing research on the subject in question is given and also the references to the key papers on the subject. I was quite amazed when I had read the introduction section of this Schwartz et al. paper. There isn’t a single reference to peer-reviewed papers, but they only reference IPCC 4th assessment report once. I don’t recall seeing any other papers with so poor introduction section.

Note that James Annan has also made some comments on this (thanks to Paul Middents for pointing it out in the comment section of the first version).

Conclusion

Claims in the Internet about Schwartz et al. are largely based on misunderstanding and not reading the paper beyond the abstract and/or introduction chapter. However, there is an apparent actual point in Schwartz et al. that other factors contributing to the difference of observed and expected warming are not enough suggesting that we have some forcings wrong or that climate sensitivity is somewhat smaller than we have thought.

Schwartz et al. do make an important point about aerosol forcing, the fact that it has large uncertainty. But I’m not quite sure that’s exactly a new finding.

Posted in Climate science, Denialist claims | Leave a Comment »

Comments on S c h w a r t z et al. (2010)

Posted by Ari Jokimäki on January 24, 2010

UPDATE: I misunderstood some points about this paper and made some false comments about it here. Therefore I wrote a new version. Use the new version because all the information in this version is not correct.

Newly published paper by Schwartz et al. (2010) (abstract) has been claimed to show that theory of AGW is false or that “global warming has been cancelled”, etc. The claims are based on this statement in the paper’s introduction:

However, the observed increase of GMST over the industrial period is less than 40% of what would be expected from present best estimates of Earth’s climate sensitivity and the forcing (imposed change in energy balance, W m^-2) by the observed increases in GHGs.

(GMST = global mean surface temperature, GHG = greenhouse gases).

Rest of the paper

Schwartz et al. are studying if the difference between the observed and expected greenhouse gas warming is due four main things:

- Natural variation in global temperature.
- Lack of attainment of equilibrium.
- Overestimate of climate sensitivity.
- Countervailing forcings over the industrial period.

They calculated that the expected warming from GHG’s would have been 2.1 K. They said that the observed temperature increase had been 0.8 K. That means that they are looking to find 2.1 K – 0.8 K = 1.3 K of cooling from the above mentioned four things.

Natural variation in global temperature can cause up to 0.2 K of cooling according to them. This is how they found it out:

We use variation in preindustrial global temperature as inferred from proxy records, mainly tree rings, ice cores, corals, and varved sediments to estimate the likely magnitude of any natural cooling over the 150-year interval of the instrumental record.

Note that natural variability can work for both directions, it can cause cooling or warming.

Note that this effect works only to one direction, it has a cooling effect on global surface temperature.

Overestimate of climate sensitivity suggests that the climate sensitivity would be lower than the expected range. That would explain the discrepancy. They note that IPCC limit for very “unlikely” climate sensitivity is 1.5 K and they say that the observed warming would require the climate sensitivity to be even lower than that. That, however ignores the other factors causing the cooling mentioned above. The situation is presented in their Figure 2. There they present the observed warming as a horizontal line and they have added the natural variability as a horizontal band around the observed line. The expected warming from GHG’s is presented as an increasing line. One can see that when accounting for natural variability, the expected warming goes out of the band at climate sensitivity of about 1.7 K. That already is within IPCC very unlikely limits, and approaching the “likely” limit of 2.0 K.

However, they haven’t included the “Lack of attainment of equilibrium” value of 0.5 K discussed above. If we would include that, we would get a possible climate sensitivity of 2.2 K, well within the IPCC “likely” limits. This wouldn’t even include the aerosol forcing, which is likely to be substantially negative. With aerosol forcing of the size IPCC has determined to be the best estimate we would get even higher possible climate sensitivity (one that would agree quite well with IPCC limits), approaching 4 K. I have reconstructed some relevant parts of their Figure 2 and I have added the 0.5 K lines there as well. See the Figure 1 below.

Figure 1. Reconstruction of the relevant parts of Schwartz et al. Figure 2 – the warming of Earth’s surface (X-axis) as a function of climate sensitivity (Y-axis). Expected increase of global mean surface temperature for GHG’s only (black), expected increase of global mean surface temperature for GHG’s and aerosol’s based on IPCC’s best estimate (green), observed increase of global mean surface temperature (blue thick line) and the possible effect of natural variability to that (blue thin lines), and observed increase of global mean surface temperature when ocean thermal sink has been accounted for (red thick line) and the possible effect of natural variability to that (red thin lines).

Countervailing forcings over the industrial period also have an effect to the global temperature. Aerosol forcing we already discussed briefly above and it is the only forcing they are discussing here. Here too they discuss Figure 2 in a manner that is ignoring other factors. They say that with the IPCC best estimate aerosol forcing the warming “would be compatible with the lower end of the IPCC “likely” range of climate sensitivity”, but actually if we consider the natural variability and the warming wasted to the ocean, we can see from their Figure 2 that resulting climate sensitivity could easily be 4 K. Here are my estimates for the climate sensitivity (values in Kelvins) based on their reconstructed Figure 2 presented above as Figure 1:

                      GHG      GHG + aero
Observed              1.3         2.2
Obs & natural      1.0 - 1.6   1.7 - 2.7
Obs + ocean           2.1         3.5
Obs + ocean & nat  1.8 - 2.4   3.0 - 4.0

Schwartz et al. do make an important point about aerosol forcing, the fact that it has large uncertainty. But I’m not quite sure that’s exactly a new finding.

So, at this point it seems I’m disagreeing with them a little. In my opinion they are stressing the low end of their results and not considering the high end much. In fact the warming that goes to the ocean is quite certain component, so they definitely should have considered that in their Figure 2.

They then enter to a discussion about the methods of determining the climate sensitivity and possible actions for improving the aerosol forcing uncertainty.

Conclusion

To me it seems that Schwartz et al. are mistaken and their point seems to rise from the fact that they didn’t consider ocean thermal lag when they determined the whole situation. They considered the ocean thermal lag separately but did not include it to their discussion of Figure 2 describing the overall situation. When the ocean thermal lag is included, the results seem to agree well with the IPCC values and the best estimate of the climate sensitivity would be 3-4 K.

I’m also little disappointed of the lack of references to the preceeding studies on the matter. For example, Lean & Rind (2008) determined the relative sizes on forcings, finding no such problems as Schwartz et al. are suggesting.

UPDATE (January 25, 2010):
I’ll add one note. As I have been making my paperlists, I have read a lot of introduction sections of papers because there the existing research on the subject in question is given and also the references to the key papers on the subject. I was quite amazed when I had read the introduction section of this Schwartz et al. paper. There isn’t a single reference to peer-reviewed papers, but they only reference IPCC 4th assessment report once. I don’t recall seeing any other papers with so poor introduction section. Also note that Schwartz had problems before with the ocean’s role in his 2007 paper, see the comment section below (thanks to Paul Middents for pointing that out).

Posted in Climate science, Denialist claims | 13 Comments »

Papers on methane emissions

Posted by Ari Jokimäki on January 23, 2010

This is a list of papers on methane emissions from a variety of different sources. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Escape of methane gas from the seabed along the West Spitsbergen continental margin – Westbrook et al. (2009) “More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150–400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1°C over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process becomes widespread along Arctic continental margins, tens of Teragrams of methane per year could be released into the ocean.” [Full text]

Tropical methane emissions: A revised view from SCIAMACHY onboard ENVISAT – Frankenberg et al. (2008) “Methane retrievals from near-infrared spectra recorded by the SCIAMACHY instrument onboard ENVISAT hitherto suggested unexpectedly large tropical emissions. Even though recent studies confirm substantial tropical emissions, there were indications for an unresolved error in the satellite retrievals. Here we identify a retrieval error related to inaccuracies in water vapor spectroscopic parameters, causing a substantial overestimation of methane correlated with high water vapor abundances. We report on the overall implications of an update in water spectroscopy on methane retrievals with special focus on the tropics where the impact is largest. The new retrievals are applied in a four-dimensional variational (4D-VAR) data assimilation system to derive a first estimate of the impact on tropical CH₄ sources. Compared to inversions based on previous SCIAMACHY retrievals, annual tropical emission estimates are reduced from 260 to about 201 Tg CH4 but still remain higher than previously anticipated.” [Full text]

Anomalies of methane in the atmosphere over the East Siberian shelf: Is there any sign of methane leakage from shallow shelf hydrates? – Shakhova et al. (2008) “Extremely high concentrations of methane (up to 8 ppm) in the atmospheric layer above the sea surface along with anomalously high concentrations of dissolved methane in the water column (up to 560 nM, or 12000% of super saturation), registered during a summertime cruise over the ESS in September 2005, were analyzed together with available data obtained during previous and subsequent expeditions to distinguish between possible methane sources of different origin, potential, and mobility. Using indirect evidence it was shown that one such source may be highly potential and extremely mobile shallow methane hydrates, whose stability zone is seabed permafrost-related and could be disturbed upon permafrost development, degradation, and thawing. … …we consider release of up to 50 Gt of predicted amount of hydrate storage as highly possible for abrupt release at any time. That may cause ~12-times increase of modern atmospheric methane burden with consequent catastrophic greenhouse warming.”

Early anthropogenic CH₄ emissions and the variation of CH₄ and ¹³CH₄ over the last millennium – Houweling et al. (2008) “The main idea is that emissions of isotopically depleted CH₄, from, for example, rice cultivation, domestic ruminants, and waste treatment started increasing earlier than the isotopically enriched emissions from fossil fuel, which started with the start of industrialization. However, because the observed increase of atmospheric methane only started around 1750 A.D., these preindustrial anthropogenic emissions must have been accompanied by a net reduction of natural CH₄ sources during the Little Ice Age (LIA) compensating for the increase of anthropogenic emissions during that period. Results of transient box model simulations for the last millennium show that under the new hypothesis a close agreement can be obtained between model and measurements.” [Full text]

Methane hydrate stability and anthropogenic climate change – Archer (2007) “The hydrate reservoir is so large that if 10% of the methane were released to the atmosphere within a few years, it would have an impact on the Earth’s radiation budget equivalent to a factor of 10 increase in atmospheric CO₂. … Hydrates are releasing methane to the atmosphere today in response to anthropogenic warming, for example along the Arctic coastline of Siberia. However most of the hydrates are located at depths in soils and ocean sediments where anthropogenic warming and any possible methane release will take place over time scales of millennia. Individual catastrophic releases like landslides and pockmark explosions are too small to reach a sizable fraction of the hydrates. The carbon isotopic excursion at the end of the Paleocene has been interpreted as the release of thousands of Gton C, possibly from hydrates, but the time scale of the release appears to have been thousands of years, chronic rather than catastrophic.” [Full text]

Methane emissions from terrestrial plants under aerobic conditions – Keppler et al. (2006) “Here we demonstrate using stable carbon isotopes that methane is readily formed in situ in terrestrial plants under oxic conditions by a hitherto unrecognized process. Significant methane emissions from both intact plants and detached leaves were observed during incubation experiments in the laboratory and in the field. If our measurements are typical for short-lived biomass and scaled on a global basis, we estimate a methane source strength of 62–236 Tg yr^-1 for living plants and 1–7 Tg yr^-1 for plant litter (1 Tg = 10¹² g).” [Full text]

Late Quaternary Atmospheric CH₄ Isotope Record Suggests Marine Clathrates Are Stable – Sowers (2006) “One explanation for the abrupt increases in atmospheric CH₄, that occurred repeatedly during the last glacial cycle involves clathrate destabalization events. Because marine clathrates have a distinct deuterium/hydrogen (D/H) isotope ratio, any such destabilization event should cause the D/H ratio of atmospheric CH₄ (δD_CH4) to increase. Analyses of air trapped in the ice from the second Greenland ice sheet project show stable and/or decreasing δD_CH4 values during the end of the Younger and Older Dryas periods and one stadial period, suggesting that marine clathrates were stable during these abrupt warming episodes. Elevated glacial δD_CH4 values may be the result of a lower ratio of net to gross wetland CH₄ emissions and an increase in petroleum-based emissions.”

Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming – Walter et al. (2006) “Thaw lakes in North Siberia are known to emit methane, but the magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable. Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. … We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent. Furthermore, the Pleistocene age (35,260–42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost.” [Full text]

Assessing Methane Emissions from Global Space-Borne Observations – Frankenberg et al. (2005) “In the past two centuries, atmospheric methane has more than doubled and now constitutes 20% of the anthropogenic climate forcing by greenhouse gases. Yet its sources are not well quantified, introducing uncertainties in its global budget. We retrieved the global methane distribution by using spaceborne near-infrared absorption spectroscopy. In addition to the expected latitudinal gradient, we detected large-scale patterns of anthropogenic and natural methane emissions. Furthermore, we observed unexpectedly high methane concentrations over tropical rainforests, revealing that emission inventories considerably underestimated methane sources in these regions during the time period of investigation (August through November 2003).” [Full text]

The distribution of methane on the Siberian Arctic shelves: Implications for the marine methane cycle – Shakhova et al. (2005) “We present two years of data obtained during the late summer period (September 2003 and September 2004) for both the ESS and LS shelves. According to our data, the surface layer of shelf water was supersaturated up to 2500% relative to the present average atmospheric methane content of 1.85 ppm. Anomalously high concentrations (up to 154 nM or 4400% supersaturation) of dissolved methane in the bottom layer of shelf water suggest that the bottom layer is somehow affected by near-bottom sources. Considering the possible formation mechanisms of such plumes, we favor thermo-abrasion and the effects of shallow gas or gas hydrates release.” [Full text]

Global inventory of methane clathrate: sensitivity to changes in the deep ocean – Buffett & Archer (2004) “We present a mechanistic model for the distribution of methane clathrate in marine sediments, and use it to predict the sensitivity of the steady-state methane inventory to changes in the deep ocean. … Preferred values for these parameters are taken from previous studies of both passive and active margins, yielding a global estimate of 3×10¹⁸ g of carbon (3000 Gton C) in clathrate and 2×10¹⁸ g (2000 Gton C) in methane bubbles. The predicted methane inventory decreases by 85% in response to 3 °C of warming.” [Full text]

Measurements of an anomalous global methane increase during 1998 – Dlugokencky et al. (2001) “The increased growth rate during 1998 corresponds to an increase in the imbalance between CH₄ sources and sinks equal to ∼24 Tg CH₄, the largest perturbation observed in 16 years of measurements. We suggest that wetland and boreal biomass burning sources may have contributed to the anomaly.”

Continuing decline in the growth rate of the atmospheric methane burden – Dlugokencky et al. (1998) “Measurements have revealed that although the global atmospheric methane burden continues to increase with significant interannual variability, the overall rate of increase has slowed. Here we present an analysis of methane measurements from a global air sampling network that suggests that, assuming constant OH concentration, global annual methane emissions have remained nearly constant during the period 1984–96, and that the decreasing growth rate in atmospheric methane reflects the approach to a steady state on a timescale comparable to methane’s atmospheric lifetime.”

Changing concentration, lifetime and climate forcing of atmospheric methane – Lelieveld et al. (1998) “Here, we review sources and sink estimates and we present global 3D model calculations, showing that the main features of the global CH₄ distribution are well represented. The model has been used to derive the total CH₄ emission source, being about 600 Tg yr^-1. Based on published results of isotope measurements the total contribution of fossil fuel related CH₄ emissions has been estimated to be about 110 Tg yr^-1.”

Methane emissions from cattle – Johnson & Johnson (1995) “Ruminant livestock can produce 250 to 500 L of methane per day. This level of production results in estimates of the contribution by cattle to global warming that may occur in the next 50 to 100 yr to be a little less than 2%.” [Full text]

A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992 – Dlugokencky et al. (1994) “Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983–1991 was (11.6±0.2) ppbv yr⁻¹, but the increase in 1992 was only (1.8±1.6) ppbv. In the Southern Hemisphere, the average increase (1983–1991) was (11.1±0.2) ppbv yr⁻¹, and the 1992 increase was (7.7±1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man’s activities.”

Role of methane clathrates in past and future climates – MacDonald (1990) “Methane occurrences and the organic carbon content of sediments are the bases used to estimate the amount of carbon currently stored as clathrates. The estimate of about 11,000 Gt of carbon for ocean sediments, and about 400 Gt for sediments under permafrost regions is in rough accord with an independent estimate by Kvenvolden of 10,000 Gt. … The sensitivity of clathrates to surface change, the time scales involved, and the large quantities of carbon stored as clathrate indicate that clathrates may have played a significant role in modifying the composition of the atmosphere during the ice ages. The release of methane and its subsequent oxidation to carbon dioxide may be responsible for the observed swings in atmospheric methane and carbon dioxide concentrations during glacial times.”

Methane Emission From Natural Wetlands: Global Distribution, Area, and Environmental Characteristics of Sources – Matthews & Fung (1987) “A global data base of wetlands at 1º resolution has been developed from the integration of three independent global, digital sources: (1) vegetation, (2) soil properties and (3) fractional inundation in each 1º cell. … The annual methane emission from wetlands is ∽110 Tg, well within the range of previous estimates (11-300 Tg).”

Posted in AGW evidence | Leave a Comment »

Viewing Angle on ISCCP Problems

Posted by Ari Jokimäki on January 20, 2010

International Satellite Cloud Climatology Project (ISCCP) provides measurements of global cloud cover. See the project website for the introduction (see also Rossow & Schiffer, 1991, for basic description of ISCCP cloud data products). They offer the longest satellite dataseries for global cloud cover. ISCCP data has been widely used to study trends in global cloud cover. For example, Pallé et al. (2004) have used ISCCP data to determine a decrease in Earth’s reflectance. Figure 1 shows the global cloud cover trends in ISCCP data, and there we can see that the global cloud cover has apparently decreased rather strongly between about 1987 and 2000.

Figure 1. The global cloud cover from ISCCP data. (Image is loaded from from ISCCP website.)

Couple of other examples of papers using ISCCP data in cloud cover research:

Pinker et al. (2005):

Long-term variations in solar radiation at Earth’s surface (S) can affect our climate, the hydrological cycle, plant photosynthesis, and solar power. … Here we present an estimate of global temporal variations in S by using the longest available satellite record. We observed an overall increase in S from 1983 to 2001 at a rate of 0.16 watts per square meter (0.10%) per year; this change is a combination of a decrease until about 1990, followed by a sustained increase. The global-scale findings are consistent with recent independent satellite observations but differ in sign and magnitude from previously reported ground observations. Unlike ground stations, satellites can uniformly sample the entire globe.

Hatzianastassiou et al. (2005):

The monthly mean shortwave (SW) radiation budget at the Earth’s surface (SRB) was computed on 2.5-degree longitude-latitude resolution for the 17-year period from 1984 to 2000, using a radiative transfer model accounting for the key physical parameters that determine the surface SRB, and long-term climatological data from the International Satellite Cloud Climatology Project (ISCCP-D2). … Significant increasing trends in DSR and net DSR fluxes were found, equal to 4.1 and 3.7 Wm⁻², respectively, over the 1984–2000 period (equivalent to 2.4 and 2.2 Wm⁻² per decade), indicating an increasing surface solar radiative heating. … The surface solar heating occurs mainly in the period starting from the early 1990s, in contrast to the commonly reported decreasing trend in DSR through the late 1980s, found also in our study.

The apparent decrease in global cloud cover has also been used as an argument against anthropogenic warming (but mainly outside of scientific literature). But when the sources of these arguments are consulted, almost without exception we run into ISCCP cloud cover data being used as the source. In the following, I will show you why it is false to claim global cloud cover trends based on ISCCP data.

Problematic satellite viewing angle

Norris (2000) searched for suspicious spatial patterns from ISCCP cloud data, and found some:

The most striking feature of Figure 2 is the circular patch of positive correlation centered on 0°N, 0°E. It seems unlikely that a natural process would produce such a regular pattern, but it is interesting to note that the pattern almost exactly resembles the footprint of Meteosat data incorporated into ISCCP (Meteosat is the European geostationary satellite).

Norris concluded:

Accordingly, the ISCCP time series presented in Figure 1 is probably spurious, and any resemblance to time series of other parameters (e.g., Marsh and Svensmark, 2000) is merely coincidental.

Campbell (2004) showed images of the same problem. Campbell showed simply the cloudiness on the map of the world in certain time period and pointed out that there were clear “seams” showing in the maps. Campbell noted:

Qualitatively one suspects that different view angles are affecting the products.

Campbell then showed a graph that had cloudiness plotted for one latitude band, and the satellite positions were marked to the graph. The cloudiness clearly increased further from the satellite positions, so the cloudiness seemed to be increasing with satellite viewing angle. Campbell also made an effort of quantifying the apparent effect; the cloudiness data was divided to different bins by viewing angle, and average cloudiness was calculated for the bins. The resulting graph shows almost linear increase in cloudiness with the increase in viewing angle. Campbell presented the finding also in the form of an equation that describes the viewing angle dependence of cloudiness in ISCCP data.

So Campbell had established that the amount of measured cloudiness in ISCCP data is larger when the satellite viewing angle is larger. Next Campbell entered into a discussion of trends. Campbell noted an important thing:

As the ISCCP project progressed, different numbers of geo satellites were available. This had the unforeseen effect of changing the average view angle as the data set has been accumulated.

And it turns out that the number of satellites increased, and that then decreased the average viewing angle. As higher viewing angle was associated with higher cloudiness, decreasing average viewing angle decreases the amount of measured clouds. Campbell showed that it has indeed happened in the ISCCP data and the result is that the long decreasing trend in ISCCP cloud data seems to be an artifact of satellite viewing angle. Campbell then applied a correction based on the equation for the viewing angle dependency. The result still shows a decreasing trend but clearly smaller than without the correction. However, the corrected map still showed some “seams” so the correction was only rough one, and there were some calibration issues Campbell suspected might still be at play. Campbell concluded:

A substantial part of the ISCCP trend is due to systematic changes in the view angle over the 18 years of the data analysis. From an analysis of the AVHRR data alone, increasing cloudiness at steeper view angles is obvious.

Pallé (2005) also noted that there possibly is viewing angle problems with ISCCP data. Campbell (2006) continued to define a correction to the ISCCP viewing angle problem among some other problems (some diurnal corrections). After corrections, not much cloud trends were left. However, Campbell noted that there still were a discontinuity in the data:

But there is a very distinct discontinuity in all the cloud amount time series in October 2001 when NOAA 16 AVHRR data replaced NOAA 14 data.

Evan et al. (2007) also studied the viewing angle issue. They first noted how the ISCCP data had been used, including most of the above mentioned studies, and they then said:

However, these trends in total cloudiness have not been observed in surface [Norris, 2005] and other satellite [Jacobowitz et al., 2003; Wylie et al., 2005] cloud records.

In their figure 2 they showed similar map as Norris and Campbell did previously of the cloud variability, showing the circular features suggestive of the viewing angle problem. They said:

These circles correspond to so-called geostationary ‘‘footprints’’ that describe the area observed by each satellite. At the center of these footprints a satellite sensor’s viewing angle is perpendicular to the surface, corresponding to a satellite zenith angle of 0 degrees. At the footprint edges the satellite zenith angle is much higher, corresponding to a longer path length through the atmosphere that light must travel before it is detected by the sensor.

They then performed a test where they created two dataseries from ISCCP data; one from large satellite viewing angles and one from small viewing angles. It turned out that the dataseries with large viewing angles (black solid line in their figure 3) showed far stronger decrease in clouds than the one with small viewing angle (black dashed line in their figure 3). They also showed how changes in viewing angle over time from large viewing angle areas (grey solid line in figure 3) matched very well the step changes in the change in cloudiness (compare grey and black solid lines in figure 3). They also listed some events in the satellite network (launches of new satellites, repositioning of satellites, etc.) that might have affected the data, and the listed events matched the step changes in tha data rather well. One such event was a shift that occurred in 2001. That shift seems to be related to a change in reference satellite (Knapp, 2008).

Evan et al. then showed an example of one location which had large satellite viewing angle where there was a known satellite network event, and showed how the cloud data clearly jumped at the event (see their figure 4), while a nearby region that had small viewing angle showed trendless line at the same time. To finish the paper, Evan et al. suggested some actions that could be done to correct the problem.

Berthier et al. (2008) also found some potential problems with ISCCP data:

Comparisons of CTH developed from LITE, for 2 weeks of data in 1994, with ISCCP (International Satellite Cloud Climatology Project) cloud products show that the cloud fraction observed from spaceborne lidar is much higher than that from ISCCP. Another key result is that ISCCP products tend to underestimate the CTH of optically thin cirrus clouds.

They considered the viewing angle problem to be one possible source for the differences they found. The viewing angle problem was also briefly touched in Norris (2008).

Conclusion

It seems that the finding of Norris, Campbell, and Evan et al. about ISCCP satellite viewing angle (by changes in satellite network that affect the viewing angles) creating a spurious trend is a real problem. I think they have shown it beyond reasonable doubt. ISCCP website lists known and fixed errors in ISCCP data but there doesn’t seem to be any mention of this problem.

Many studies have made conclusions relating to clouds based on the apparent long term decrease in ISCCP data, but all that research is in doubt because the trend at least partly is not real. Furthermore, ISCCP cloud data is the longest satellite record we have on the subject, so it would be important to issue an official correction to this problem. When the trend is corrected, interesting things might be found, as shown by Clement et al. (2009) who corrected for the problem and found that cloud feedback is positive. As a long record, ISCCP data would be suitable to that kind of studies.

References

Berthier, S., Chazette, P., Pelon, J., Baum, B., 2008, “Comparison of cloud statistics from spaceborne lidar systems”, Atmos. Chem. Phys., 8, 6965-6977, 2008, [Full text]

Campbell, G. Garrett, 2004, “View angle dependence of cloudiness and the trend in ISCCP cloudiness”, 13th Conference on Satellite Meteorology and Oceanography, [Full text]

Campbell, G. Garrett, 2006, “Diurnal and Angular Variability of Cloud Detection: Consistency Between Polar and Geosynchronous ISCCP Products”, 14th Conference on Satellite Meteorology and Oceanography, [Full text]

Clement, Amy C., Burgman, Robert, Norris, Joel R., 2009, “Observational and Model Evidence for Positive Low-Level Cloud Feedback”, Science 24 July 2009, Vol. 325. no. 5939, pp. 460 – 464, DOI: 10.1126/science.1171255, [Full text]

Evan, Amato T., Heidinger, Andrew K., Vimont, Daniel J., 2007, “Arguments against a physical long-term trend in global ISCCP cloud amounts”, Geophys. Res. Lett., 34, L04701, doi:10.1029/2006GL028083, [Full text]

Hatzianastassiou, N., Matsoukas, C., Fotiadi, A. Pavlakis, K. G., Drakakis, E., Hatzidimitriou, D., Vardavas, I., 2005, “Global distribution of Earth’s surface shortwave radiation budget”, Atmos. Chem. Phys. Discuss., 5, 4545-4597, [Full text]

Knapp, Kenneth R., 2008, “Calibration Assessment of ISCCP Geostationary Infrared Observations Using HIRS”, Journal of Atmospheric and Oceanic Technology, Volume 25, Issue 2 (February 2008), DOI: 10.1175/2007JTECHA910.1, [Full text]

Norris, Joel R., 2000, “What Can Cloud Observations Tell Us About Climate Variability?”, Space Science Reviews, Volume 94, Numbers 1-2 / November, 2000, DOI 10.1023/A:1026704314326, [Full text]

Norris, J. R., 2008, “Observed Interdecadal Changes in Cloudiness: Real or Spurious?”, Advances in Global Change Research, 33, DOI 10.1007/978-1-4020-6766-2, [Full text]

Pallé, E., Goode, P. R., Montañés-Rodríguez, P., Koonin, S. E., 2004, “Changes in Earth’s Reflectance over the Past Two Decades”, Science 28 May 2004, Vol. 304. no. 5675, pp. 1299 – 1301, DOI: 10.1126/science.1094070, [Full text]

Pallé, E., 2005, “Possible satellite perspective effects on the reported correlations between solar activity and clouds”, Geophys. Res. Lett., 32, L03802, doi:10.1029/2004GL021167, [Full text]

Pinker, R. T., Zhang, B., Dutton, E. G., 2005, “Do Satellites Detect Trends in Surface Solar Radiation?”, Science 6 May 2005, Vol. 308. no. 5723, pp. 850 – 854, DOI: 10.1126/science.1103159

Rossow, William B., Schiffer, Robert A., 1991, “ISCCP Cloud Data Products”, Bulletin of the American Meteorological Society, Volume 72, Issue 1 (January 1991), DOI: 10.1175/1520-0477(1991)0722.0.CO;2

Posted in Climate science, Denialist claims | Leave a Comment »

« Previous Entries

AGW Observer

Observations of anthropogenic global warming

Recent Posts

Recent Comments

Pages

Categories

Blogroll

Observations of anthropogenic global warming

References

Yet another new climate science blog

Papers on black carbon

Papers on CO2 records from ice cores

Papers on Amazon and global warming

Papers on natural variability

Comments on Schwartz et al. (2010), version 2

Rest of the paper

Some notes

Conclusion

Comments on S c h w a r t z et al. (2010)

Rest of the paper

Conclusion

Papers on methane emissions

Viewing Angle on ISCCP Problems

Problematic satellite viewing angle

Conclusion

References