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 , ocean temperature measurements  and borehole measurements  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 , melting glaciers , rising sea level , vanishing sea ice , etc.
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 . 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 . 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 .
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 . From the spectrum we can identify the effects of different molecules (such as carbon dioxide or other chemicals mankind is emitting to the atmosphere) . 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 . The expected cooling of the stratosphere has also been observed in measurements .
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 , but even before that there had been plenty of measurements 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 . 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 . We are able to measure the carbon dioxide concentration from the sunlight at the surface, from reflected sunlight by satellites  and from the outgoing thermal radiation of the Earth, that too by satellites . 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 .
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 . 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 . 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 . 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 .
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 . The amount of that decrease also agrees well of what is expected from the carbon dioxide concentration changes . We have also measured the thermal radiation to increase on the surface of the Earth . 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 . 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 . 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 . 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 . The situation is currently getting better, and latest research indicates that the cloud feedback is positive . 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.
4. UAH (University of Alabama in Huntsville) (link is directly to their data, they don’t seem to have a decent website)
16. Uherek (2006), [full text], “Stratospheric cooling”
18. Keeling (1960), [full text], “The concentration and isotopic abundances of carbon dioxide in the atmosphere”
23. Buchwitz (2008), [full text], “Visualization of the global distribution of greenhouse gases using satellite measurements”
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”
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].”