AGW Observer

Observations of anthropogenic global warming

New research from last week 11/2012

Posted by Ari Jokimäki on March 19, 2012

One aspect of Arctic sea ice decrease is that ice-free season gets longer, as shown in one of this week’s studies. Another study suggests that when sea ice decreases, cloud cover increases. To continue with the Arctic theme, yet another study in this week’s batch discusses links between arctic amplification and extreme weather events.

In non-Arctic studies this week we have biases & tree rings, GHG’s & mass extinctions, hurricanes & volcanos, monsoon & global warming, United States & sea level, cities & weather, CRUTEM & update, Lake Ontario & temperature, soil & acidification, South Africa & temperature extremes, deforestation & droughts, and atmospheric nitrous oxide & fertilizers. In other words, lots of fun & science.

Slow growth and big tree biases in tree ring studies

Detecting evidence for CO2 fertilization from tree ring studies: The potential role of sampling biases – Brienen et al. (2012) [FULL TEXT]

Abstract: “Tree ring analysis allows reconstructing historical growth rates over long periods. Several studies have reported an increasing trend in ring widths, often attributed to growth stimulation by increasing atmospheric CO2 concentration. However, these trends may also have been caused by sampling biases. Here we describe two biases and evaluate their magnitude. (1) The slow-grower survivorship bias is caused by differences in tree longevity of fast- and slow-growing trees within a population. If fast-growing trees live shorter, they are underrepresented in the ancient portion of the tree ring data set. As a result, reconstructed growth rates in the distant past are biased toward slower growth. (2) The big-tree selection bias is caused by sampling only the biggest trees in a population. As a result, slow-growing small trees are underrepresented in recent times as they did not reach the minimum sample diameter. We constructed stochastic models to simulate growth trajectories based on a hypothetical species with lifetime constant growth rates and on observed tree ring data from the tropical tree Cedrela odorata. Tree growth rates used as input in our models were kept constant over time. By mimicking a standard tree ring sampling approach and selecting only big living trees, we show that both biases lead to apparent increases in historical growth rates. Increases for the slow-grower survivorship bias were relatively small and depended strongly on assumptions about tree mortality. The big-tree selection bias resulted in strong historical increases, with a doubling in growth rates over recent decades. A literature review suggests that historical growth increases reported in many tree ring studies may have been partially due to the big-tree sampling bias. We call for great caution in the interpretation of historical growth trends from tree ring analyses and recommend that such studies include individuals of all sizes.”

Citation: Brienen, R. J. W., E. Gloor, and P. A. Zuidema (2012), Detecting evidence for CO2 fertilization from tree ring studies: The potential role of sampling biases, Global Biogeochem. Cycles, 26, GB1025, doi:10.1029/2011GB004143.

Arctic amplification may be linked to extreme weather events

Evidence linking Arctic amplification to extreme weather in mid-latitudes – Francis & Vavrus (2012)

Abstract: “Arctic amplification (AA) – the observed enhanced warming in high northern latitudes relative to the northern hemisphere – is evident in lower-tropospheric temperatures and in 1000-to-500 hPa thicknesses. Daily fields of 500 hPa heights from the National Centers for Environmental Prediction Reanalysis are analyzed over N. America and the N. Atlantic to assess changes in north-south (Rossby) wave characteristics associated with AA and the relaxation of poleward thickness gradients. Two effects are identified that each contribute to a slower eastward progression of Rossby waves in the upper-level flow: 1) weakened zonal winds, and 2) increased wave amplitude. These effects are particularly evident in autumn and winter consistent with sea-ice loss, but are also apparent in summer, possibly related to earlier snow melt on high-latitude land. Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.”

Citation: Francis, J. A. and S. J. Vavrus (2012), Evidence linking Arctic amplification to extreme weather in mid-latitudes, Geophys. Res. Lett., 39, L06801, doi:10.1029/2012GL051000.

Decreased greenhouse effect might have contributed to Frasnian-Famennian mass extinction

Carbon isotopic evidence for the associations of decreasing atmospheric CO2 level with the Frasnian-Famennian mass extinction – Xu et al. (2012)

Abstract: “A perturbation of the global carbon cycle has often been used for interpreting the Frasnian-Famennian (F-F) mass extinction. However, the changes of atmospheric CO2 level (pCO2) during this interval are much debatable. To illustrate the carbon cycle during F-F transition, paired inorganic (δ13Ccarb) and organic (δ13Corg) carbon isotope analyses were carried out on two late Devonian carbonate sequences (Dongcun and Yangdi) from south China. The larger amplitude shift of δ13Corg compared to δ13Ccarb and its resultant Δ13C (Δ13C = δ13Ccarbδ13Corg) decrease indicate decreased atmospheric CO2 level around the F-F boundary. The onset of pCO2 level decrease predates that of marine regressions, which coincide with the beginning of conodont extinctions, suggesting that temperature decrease induced by decreased greenhouse effect of atmospheric CO2 might have contributed to the F-F mass extinction.”

Citation: Xu, B., Z. Gu, C. Wang, Q. Hao, J. Han, Q. Liu, L. Wang, and Y. Lu (2012), Carbon isotopic evidence for the associations of decreasing atmospheric CO2 level with the Frasnian-Famennian mass extinction, J. Geophys. Res., 117, G01032, doi:10.1029/2011JG001847.

Hurricane activity might decrease in the years following major volcanic eruptions

Atlantic hurricane activity following two major volcanic eruptions – Evan (2012)

Abstract: “In 1982 and 1991, two major volcanic eruptions loaded the stratosphere with long-lived sulfate aerosols, altering the global climate by redistributing longwave and shortwave radiation at the surface and throughout the atmosphere, cooling the surface and subsurface waters of the tropical oceans. Theory and observations demonstrate, through direct and indirect mechanisms, a causal relationship between tropical North Atlantic sea surface temperatures and seasonal Atlantic hurricane frequency, duration, and intensity. Therefore, it is plausible that hurricane activity in the seasons immediately following these eruptions is diminished. However, to date, such a theory remains untested. Here I use observations, reanalysis data, and output from a numerical model to suggest that the number, duration, and intensity of hurricanes in the years following the eruptions of El Chichón (1982) and Mount Pinatubo (1991) decreased via the aerosol direct effect. Determining the effects of each eruption on seasonal cyclone activity is complicated by simultaneous positive ENSO events; thus further study of the relationship between Atlantic tropical cyclones and major volcanic eruptions is warranted.”

Citation: Evan, A. T. (2012), Atlantic hurricane activity following two major volcanic eruptions, J. Geophys. Res., 117, D06101, doi:10.1029/2011JD016716.

Global monsoon area increases with global warming

Increase of global monsoon area and precipitation under global warming: A robust signal? – Hsu et al. (2012)

Abstract: “Monsoons, the most energetic tropical climate system, exert a great social and economic impact upon billions of people around the world. The global monsoon precipitation had an increasing trend over the past three decades. Whether or not this increasing trend will continue in the 21st century is investigated, based on simulations of three high-resolution atmospheric general circulation models that were forced by different future sea surface temperature (SST) warming patterns. The results show that the global monsoon area, precipitation and intensity all increase consistently among the model projections. This indicates that the strengthened global monsoon is a robust signal across the models and SST patterns explored here. The increase of the global monsoon precipitation is attributed to the increases of moisture convergence and surface evaporation. The former is caused by the increase of atmospheric water vapor and the latter is due to the increase of SST. The effect of the moisture and evaporation increase is offset to a certain extent by the weakening of the monsoon circulation.”

Citation: Hsu, P., T. Li, J.-J. Luo, H. Murakami, A. Kitoh, and M. Zhao (2012), Increase of global monsoon area and precipitation under global warming: A robust signal?, Geophys. Res. Lett., 39, L06701, doi:10.1029/2012GL051037.

In fastest Arctic sea ice decrease regions ice-free season is now 3 months longer

Regions of rapid sea ice change: An inter-hemispheric seasonal comparison – Stammerjohn et al. (2012)

Abstract: “This bi-polar analysis resolves ice edge changes on space/time scales relevant for investigating seasonal ice-ocean feedbacks and focuses on spatio-temporal changes in the timing of annual sea ice retreat and advance over 1979/80 to 2010/11. Where Arctic sea ice decrease is fastest, the sea ice retreat is now nearly 2 months earlier and subsequent advance more than 1 month later (compared to 1979/80), resulting in a 3-month longer summer ice-free season. In the Antarctic Peninsula and Bellingshausen Sea region, sea ice retreat is more than 1 month earlier and advance 2 months later, resulting in a more than 3-month longer summer ice-free season. In contrast, in the western Ross Sea (Antarctica) region, sea ice retreat and advance are more than 1 month later and earlier respectively, resulting in a more than 2 month shorter summer ice-free season. Regardless of trend magnitude or direction, and at latitudes mostly poleward of 70° (N/S), there is strong correspondence between anomalies in the timings of sea ice retreat and subsequent advance, but little correspondence between advance and subsequent retreat. These results support a strong ocean thermal feedback in autumn in response to changes in spring sea ice retreat. Further, model calculations suggest different net ocean heat changes in the Arctic versus Antarctic where autumn sea ice advance is 1 versus 2 months later. Ocean-atmosphere changes, particularly in boreal spring and austral autumn (i.e., during ∼March-May), are discussed and compared, as well as possible inter-hemispheric climate connections.”

Citation: Stammerjohn, S., R. Massom, D. Rind, and D. Martinson (2012), Regions of rapid sea ice change: An inter-hemispheric seasonal comparison, Geophys. Res. Lett., 39, L06501, doi:10.1029/2012GL050874.

In United States 10% of population lives below future sea level risk line

Tidally adjusted estimates of topographic vulnerability to sea level rise and flooding for the contiguous United States – Strauss et al. (2012) [FULL TEXT]

Abstract: “Because sea level could rise 1 m or more during the next century, it is important to understand what land, communities and assets may be most at risk from increased flooding and eventual submersion. Employing a recent high-resolution edition of the National Elevation Dataset and using VDatum, a newly available tidal model covering the contiguous US, together with data from the 2010 Census, we quantify low-lying coastal land, housing and population relative to local mean high tide levels, which range from ~0 to 3 m in elevation (North American Vertical Datum of 1988). Previous work at regional to national scales has sometimes equated elevation with the amount of sea level rise, leading to underestimated risk anywhere where the mean high tide elevation exceeds 0 m, and compromising comparisons across regions with different tidal levels. Using our tidally adjusted approach, we estimate the contiguous US population living on land within 1 m of high tide to be 3.7 million. In 544 municipalities and 38 counties, we find that over 10% of the population lives below this line; all told, some 2150 towns and cities have some degree of exposure. At the state level, Florida, Louisiana, California, New York and New Jersey have the largest sub-meter populations. We assess topographic susceptibility of land, housing and population to sea level rise for all coastal states, counties and municipalities, from 0 to 6 m above mean high tide, and find important threat levels for widely distributed communities of every size. We estimate that over 22.9 million Americans live on land within 6 m of local mean high tide.”

Citation: Benjamin H Strauss et al 2012 Environ. Res. Lett. 7 014033 doi:10.1088/1748-9326/7/1/014033.

Review of urban meteorology

Hidden climate change – urban meteorology and the scales of real weather – Janković & Hebbert (2012)

Abstract: “This paper discusses the scale at which the weather is experienced and modified by human activities in urban environment. The climates of built-up areas differ from their non-urban counterparts in many aspect: wind-flows, radiation, humidity, precipitation and air quality all change in the presence of human settlement, transforming each city into a singularity within its regional weather system. Yet this pervasive category of anthropogenic climate change has always tended to be hidden and difficult to discern. The paper first describes the sequence of discovery of the urban heat island since the early nineteenth century, and the emergence and consolidation of a scientific field devoted to the climatology of cities. This is followed by a discussion of various attempts to apply knowledge of climatic factors to the design and management of settlement. We find that real-world application of urban climatology has met with limited success. However, the conclusion suggests that global climate change gives a new visibility and practical relevance to urban-scale climate science.”

Citation: Vladimir Janković and Michael Hebbert, Climatic Change, DOI: 10.1007/s10584-012-0429-1.

Official paper for the new CRUTEM version

Hemispheric and large-scale land-surface air temperature variations: An extensive revision and an update to 2010 – Jones et al. (2012)

Abstract: “This study is an extensive revision of the Climatic Research Unit (CRU) land station temperature database that has been used to produce a grid-box data set of 5° latitude × 5° longitude temperature anomalies. The new database (CRUTEM4) comprises 5583 station records of which 4842 have enough data for the 1961–1990 period to calculate or estimate the average temperatures for this period. Many station records have had their data replaced by newly homogenized series that have been produced by a number of studies, particularly from National Meteorological Services (NMSs). Hemispheric temperature averages for land areas developed with the new CRUTEM4 data set differ slightly from their CRUTEM3 equivalent. The inclusion of much additional data from the Arctic (particularly the Russian Arctic) has led to estimates for the Northern Hemisphere (NH) being warmer by about 0.1°C for the years since 2001. The NH/Southern Hemisphere (SH) warms by 1.12°C/0.84°C over the period 1901–2010. The robustness of the hemispheric averages is assessed by producing five different analyses, each including a different subset of 20% of the station time series and by omitting some large countries. CRUTEM4 is also compared with hemispheric averages produced by reanalyses undertaken by the European Centre for Medium-Range Weather Forecasts (ECMWF): ERA-40 (1958–2001) and ERA-Interim (1979–2010) data sets. For the NH, agreement is good back to 1958 and excellent from 1979 at monthly, annual, and decadal time scales. For the SH, agreement is poorer, but if the area is restricted to the SH north of 60°S, the agreement is dramatically improved from the mid-1970s.”

Citation: Jones, P. D., D. H. Lister, T. J. Osborn, C. Harpham, M. Salmon, and C. P. Morice (2012), Hemispheric and large-scale land-surface air temperature variations: An extensive revision and an update to 2010, J. Geophys. Res., 117, D05127, doi:10.1029/2011JD017139.

Lake Ontario has warmed during last 40 years

On Recent Trends in Atmospheric and Limnological Variables in Lake Ontario – Huang et al. (2012)

Abstract: “The surface air and water temperatures increased at all seasonal and annual time scales during the last 40 years in Lake Ontario. The annual mean air and surface water temperatures have increased by 1.43±0.39°C and 1.26±0.32°C over 1970-2009, respectively. The air temperature increased at a faster rate than surface water temperature in winter and autumn, whereas in spring and summer the surface water temperature warmed faster than air temperature. The length of summer stratified season has increased by 12±2 days since early 1970s due to the increase in water temperature. The decline of surface wind speed over Lake Ontario resulted in shallower surface mixed layer and enhanced the summer thermal stratification, which increased summer surface water temperature more rapidly than air temperature.”

Citation: Anning Huang, Yerubandi R. Rao, and Weitao Zhang, Journal of Climate 2012, doi:

Decrease in Arctic sea ice concentration leads to increase in cloud cover

A cloudier Arctic expected with diminishing sea ice – Liu et al. (2012)

Abstract: “Arctic sea ice cover has decreased dramatically over the last three decades. Global climate models under-predicted this decline, most likely a result of the misrepresentation of one or more processes that influence sea ice. The cloud feedback is the primary source of uncertainty in model simulations, especially in the polar regions. A better understanding of the interaction between sea ice and clouds, and specifically the impact of decreased sea ice on cloud cover, will provide valuable insight into the Arctic climate system and may ultimately help in improving climate model parameterizations. In this study, an equilibrium feedback assessment is employed to quantify the relationship between changes in sea ice and clouds, using satellite-derived sea ice concentration and cloud cover over the period 2000–2010. Results show that a 1% decrease in sea ice concentration leads to a 0.36–0.47% increase in cloud cover, suggesting that a further decline in sea ice cover will result in an even cloudier Arctic.”

Citation: Liu, Y., J. R. Key, Z. Liu, X. Wang, and S. J. Vavrus (2012), A cloudier Arctic expected with diminishing sea ice, Geophys. Res. Lett., 39, L05705, doi:10.1029/2012GL051251.

Soils have been acidifying in northern China

Trends Significant soil acidification across northern China’s grasslands during 1980s-2000s – Yang et al. (2012)

Abstract: “Anthropogenic acid deposition may lead to soil acidification, with soil buffering capacity regulating the magnitude of any soil pH change. However, little evidence is available from large-scale or long-term observations. Here we evaluated changes in soil pH across northern China’s grasslands over a 20 year period using soil profiles obtained from China’s Second National Soil Inventory during the 1980s and a more recent regional soil survey in 2001-2005. A transect from the central-southern Tibetan Plateau to the eastern Inner Mongolian Plateau, where Kriging interpolation provided robust predictions of the spatial distribution of soil pH, was then selected to examine pH changes during the survey period. Our results showed that soil pH in the surface layer had declined significantly over the last two decades, with an overall decrease of 0.63 units (95% confidence interval = 0.54-0.73 units). The decline of soil pH was observed in both alpine grasslands on the Tibetan Plateau and temperate grasslands on the Inner Mongolian Plateau. Soil pH decreased more intensively in low soil carbonate regions, while changes of soil pH showed no significant associations with soil cation exchange capacity. These results suggest that grassland soils across northern China have experienced significant acidification from the 1980s to the 2000s, with soil carbonates buffering the increase of soil acidity. The buffering process may induce a large loss of carbon from soil carbonates and thus alter the carbon balance in these globally important ecosystems.”

Citation: Yuanhe Yang, Chengjun Ji, Wenhong Ma, Shifeng Wang, Shaopeng Wang, Wenxuan Han, Anwar Mohammat, David Robinson, Pete Smith, Global Change Biology, DOI: 10.1111/j.1365-2486.2012.02694.x.

Warm extremes have increased and cold extremes have decreased in South Africa

Trends in extreme temperature indices in South Africa: 1962–2009 – Kruger & Sekele (2012)

Abstract: “Trends in daily maximum and minimum extreme temperature indices were investigated for 28 weather stations in South Africa, not only for the common period of 1962–2009, but also for longer periods which the individual record lengths of the stations would allow. The utilized weather stations had limited gaps in their time series, did not undergo major moves, or had their exposure compromised during the study period, as to influence the homogeneity of their time series. The indices calculated were forthcoming from those developed by the WMO/CLIVAR Expert Team on Climate Change Detection and Indices (ETCCDI), but only those applicable to the South African climate were selected. The general result is that warm extremes increased and cold extremes decreased for all of the weather stations. The trends however vary on a regional basis, both in magnitude and statistical significance, broadly indicating that the western half, as well as parts of the northeast and east of South Africa, show relatively stronger increases in warm extremes and decreases in cold extremes than elsewhere in the country. These regions coincide to a large degree with the thermal regimes in South Africa which are susceptible to extreme temperatures. The annual absolute maximum and minimum temperatures do not reflect the general trends displayed by the other indices, showing that individual extreme events cannot always be associated with observed long-term climatic trends. The analyses of longer time series than the common period indicate that it is highly likely that warming accelerated since the mid-1960s in South Africa.”

Citation: A. C. Kruger, S. S. Sekele, International Journal of Climatology, DOI: 10.1002/joc.3455.

In eastern Australia they cut down forests and got droughts in return

A review and modelling results of the simulated response of deforestation on climate extremes in eastern Australia – Deo (2012)

Abstract: “The native vegetation cover in Australia has been modified extensively since the advent of European population. This was paralleled by increases in mean surface temperatures, decreases in mean rainfall and persistence of long-lasting and severe droughts, especially in eastern Australia. The purpose of this article is twofold: (1) to review the simulated response of deforestation on Australian droughts in light of the physics of land-surface processes, (2) to provide further analysis of the modelling results from the CSIRO Mark 3 Atmospheric Global Climate Model (AGCM) to quantify the changes in Australian droughts from the pre-European to modern-day land cover conditions. The simulated response for the austral summer for the modern-day period showed (1) a shift in the tails of the probability distribution functions of rainfall and temperature towards drier and warmer conditions, (2) a decrease in average rainfall between ~ 4–12%, (3) a reduction in average soil moisture by ~ 40%, (4) an increase dry spells by ~ 3–4 days, (5) a decrease in cumulative wet day rainfall between 10 and 25 mm day−1, (6) an increases in drought duration by ~ 6–12 consecutive days and an increase in drought severity by ~ 4–8%, (7) an average warming of ~ 0.4–3.6 °C, and an increase in dry spells by ~ 6–9 days for the 1982/83 El Niño event. These changes were statistically significant at the 95% confidence level using the non-parametric bootstrapping procedure. The simulated changes in atmospheric variables indicate that deforestation has been a contributing factor to the observed increases in drought severity and duration in eastern Australia.”

Citation: Ravinesh C. Deo, Atmospheric Research, Volume 108, May 2012, Pages 19–38,

Atmospheric nitrous oxide increase since 1940 is largely from nitrogen-based fertilizers

Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940 – Park et al. (2012)

Abstract: “The atmospheric nitrous oxide mixing ratio has increased by 20% since 1750 (ref. 1). Given that nitrous oxide is both a long-lived greenhouse gas and a stratospheric ozone-depleting substance, this increase is of global concern. However, the magnitude and geographic distribution of nitrous oxide sources, and how they have changed over time, is uncertain. A key unknown is the influence of the stratospheric circulation, which brings air depleted in nitrous oxide to the surface. Here, we report the oxygen and intramolecular nitrogen isotopic compositions of nitrous oxide in firn air samples from Antarctica and archived air samples from Cape Grim, Tasmania, spanning 1940–2005. We detect seasonal cycles in the isotopic composition of nitrous oxide at Cape Grim. The phases and amplitudes of these seasonal cycles allow us to distinguish between the influence of the stratospheric sink and the oceanic source at this site, demonstrating that isotope measurements can help in the attribution and quantification of surface sources in general. Large interannual variations and long-term decreasing trends in isotope composition are also apparent. These long-term trends allow us to distinguish between natural and anthropogenic sources of nitrous oxide, and confirm that the rise in atmospheric nitrous oxide levels is largely the result of an increased reliance on nitrogen-based fertilizers.”

Citation: S. Park, P. Croteau, K. A. Boering, D. M. Etheridge, D. Ferretti, P. J. Fraser, K-R. Kim, P. B. Krummel, R. L. Langenfelds, T. D. van Ommen, L. P. Steele & C. M. Trudinger, Nature Geoscience, 2012, doi:10.1038/ngeo1421.

CLASSIC OF THE WEEK: Unknown author (1895)

The Carbonic Acid Gas in the Atmosphere (1895) [FULL TEXT]

Abstract: No abstract.

Citation: Mon. Wea. Rev., 23, 300–301. doi:;2.

When each paper is published, it is notified in AGW Observer Facebook page and Twitter page. Here’s the archive for the research papers of previous weeks. If this sort of thing interests you, be sure to check out A Few Things Illconsidered. They also have a weekly posting containing lots of links to new research and other climate related news.

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