A new climate study, “Global warming and 21st century drought,” is making waves through the blogosphere. It’s the latest in “worse than we thought” gloom and doom narratives. Authors Benjamin I. Cook and colleagues at the Lamont-Doherty Earth Observatory find that warming will not only decrease precipitation in dry regions but also increase evaporation from soils, causing more drought than previously predicted.
So although the IPCC Working Group I report concluded that, in the 21st century, Atlantic Ocean circulation collapse is “very unlikely,” ice sheet collapse is “exceptionally unlikely,” and catastrophic release of methane from melting permafrost is “very unlikely,” we’re still on eve of destruction.
As summarized in the study’s press release, by 2100 drought could afflict 30% of the Earth’s surface, “crops could wither in multiple regions simultaneously,” and “food price shocks could become far more common.” These results are “consistent with” the latest IPCC report, which “also predicts a strong chance of soil moisture drying in the Mediterranean, southwestern United States and southern African regions.”
The Cook team’s study is “one of the first to use the latest climate simulations to model the effects of both changing rainfall and evaporation rates on future drought.” Well, climate models haven’t exactly performed brilliantly in replicating global temperatures, have they? What do actual climate data and climate history tell us about the relationship between warming and drought?
There is a rip-roaring (and often funny) discussion of warming and drying on Watts Up With That? Particularly impressive are the research talents of commenter “Jimbo,” who keeps posting abstract after abstract of studies that point to conclusions opposite those of Cook et al. Here are some excerpts.
Warming and drying: recent decades (Earth is greening, some dry areas are getting wetter, little change in overall global soil moisture during the past 60 years)
Abstract – 28 June 2013 Randall J. Donohue et al Impact of CO2 fertilization on maximum foliage cover across the globe’s warm, arid environments Satellite observations reveal a greening of the globe over recent decades. . . .The direct CO2 effect on vegetation should be most clearly expressed in warm, arid environments where water is the dominant limit to vegetation growth. . . . Satellite observations, analyzed to remove the effect of variations in precipitation, show that cover across these environments has increased by 11%. Geophysical Research Letters – Volume 40, Issue 12, pages 3031–3035 http://onlinelibrary.wiley.com/doi/10.1002/grl.50563/abstract
Abstract – 16 October 2012 Changes in the variability of global land precipitation Fubao Sun et al On our warming climate there is a general expectation that the variability of precipitation (P) will increase at daily, monthly and inter-annual timescales. Here we analyse observations of monthlyP (1940–2009) over the global land surface using a new theoretical framework that can distinguish changes in global Pvariance between space and time. We report a near-zero temporal trend in global meanP. Unexpectedly we found a reduction in global land P variance over space and time that was due to a redistribution, where, on average, the dry became wetter while wet became drier. . . .Geophysical Research Letters – Volume 39, Issue 19 DOI: 10.1029/2012GL053369 http://onlinelibrary.wiley.com/doi/10.1029/2012GL053369/abstract
Letter To Nature – 11 September 2012 Justin Sheffield et al Little change in global drought over the past 60 years. . . .Here we show that the previously reported increase in global drought is overestimated because the PDSI [Palmer Drought Severity Index] uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. . . . http://www.nature.com/nature/journal/v491/n7424/full/nature11575.html
Africa, warming and drying: paleoclimate evidence (ice ages, not warm interglacials, promote African drying; Africa was wetter during the Holocene Optimum)
Abstract – History of Climate and Forests in Tropical Africa During the Last 8 Million Years There have been major changes in climate and in the composition and distribution of forest during the last 8 Myr. During the last world glacial maximum (peak 18,000 yr B.P.) the climate was dry and cold and forest much reduced and fragmented. The last glacial period as a whole (12,000–70,000 B.P.) was dry in tropical Africa and so too were most of the other 20 major ice ages which have occurred since 2.43 Myr B.P., in comparison with intervening interglacials . . . .http://link.springer.com/chapter/10.1007/978-94-017-3608-4_8
The Aquatic Civilization of Middle Africa J. E. G. Sutton Ahmadu Bello University, Zaria Between the ninth and third millennia B.C. [i.e. the warmer-than-present Holocene Optimum] wetter conditions prevailed over most of Africa. Lakes and rivers were fuller and some of the internal basins were temporarily linked, especially in the ‘Middle African’ belt. This comprises the southern Sahara and Sahel, stretching from the Upper Niger to the Middle Nile, with a south-easterly extension into the Upper Nile basin and the East African rift valleys. This situation was exploited by people who developed a decidedly aquatic economy and culture. From their waterside camps and settlements archaeologists have recovered bones of fish and aquatic animals which these people ate, as well as the distinctive harpoon-heads carved from bone with which they obtained them, and also pottery, bearing peculiar decoration executed with fish-bones and water-shells, made in imitation of (fishing-) baskets. Boating and other cultural developments are deducible. . . . .http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=3236492
Australia, warming and drying: paleoclimate evidence (Australia was wetter during the Holocene Optimum)
Abstract Holocene climate change in arid Australia from speleothem and alluvial records New high-resolution MC-ICPMS U/Th ages and C and O isotopic analyses from a Holocene speleothem in arid south-central Australia provide evidence for increased effective precipitation (EP) relative to present at c. 11.5 ka and c. 8—5 ka, peak moisture at 7—6 ka, and onset of an arid climate similar to present by c. 5 ka. . . . http://hol.sagepub.com/content/20/7/1093.short
Warming and rainforests: paleoclimate evidence (tropical rain forests survived the rapid warming of the Eocene Thermal Maximum; rainforest diversity increased during past warmings)
Abstract Carlos Jaramillo et. al – Science – 12 November 2010 Effects of Rapid Global Warming at the Paleocene-Eocene Boundary on Neotropical Vegetation Temperatures in tropical regions are estimated to have increased by 3° to 5°C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress. doi: 10.1126/science.1193833 http://www.sciencemag.org/content/330/6006/957
Abstract Carlos Jaramillo & Andrés Cárdenas – Annual Reviews – May 2013 Smithsonian Tropical Research Institute Global Warming and Neotropical Rainforests: A Historical Perspective There is concern over the future of the tropical rainforest (TRF) in the face of global warming. Will TRFs collapse? The fossil record can inform us about that. Our compilation of 5,998 empirical estimates of temperature over the past 120 Ma indicates that tropics have warmed as much as 7°C during both the mid-Cretaceous and the Paleogene. . . . The TRF did not collapse during past warmings; on the contrary, its diversity increased. The increase in temperature seems to be a major driver in promoting diversity. doi: 10.1146/annurev-earth-042711-105403 http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-042711-105403
Warming and biodiversity: paleoclimate evidence (previous warm periods benefited species from salamanders to birds to mammals)
Abstract PNAS – David R. Vieites – 2007 Rapid diversification and dispersal during periods of global warming by plethodontid salamanders . . . .Salamanders underwent rapid episodes of diversification and dispersal that coincided with major global warming events during the late Cretaceous and again during the Paleocene–Eocene thermal optimum. The major clades of plethodontids were established during these episodes, contemporaneously with similar phenomena in angiosperms, arthropods, birds, and mammals. Periods of global warming may have promoted diversification and both inter- and transcontinental dispersal in northern hemisphere salamanders . . . . http://www.pnas.org/content/104/50/19903.full
Abstract ZHAO Yu-long et al – Advances in Earth Science – 2007 The impacts of the Paleocene-Eocene thermal maximum (PETM) event on earth surface cycles and its trigger mechanism The Paleocene-Eocene Thermal Maximum (PETM) event is an abrupt climate change event that occurred at the Paleocene-Eocene boundary. The event led to a sudden reversal in ocean overturning along with an abrupt rise in sea surface salinity (SSSs) and atmospheric humidity. An unusual proliferation of biodiversity and productivity during the PETM is indicative of massive fertility increasing in both oceanic and terrestrial ecosystems. Global warming enabled the dispersal of low-latitude populations into mid-and high-latitude. Biological evolution also exhibited a dramatic pulse of change, including the first appearance of many important groups of “modern” mammals (such as primates, artiodactyls, and perissodactyls) and the mass extinction of benlhic foraminifera….. 22(4) 341-349 DOI: ISSN: 1001-8166 CN: 62-1091/P http://220.127.116.11/wxdata/En/Show.asp?id=8613
Abstract Systematics and Biodiversity – Volume 8, Issue 1, 2010 Kathy J. Willis et al 4 °C and beyond: what did this mean for biodiversity in the past? How do the predicted climatic changes (IPCC, 2007) for the next century compare in magnitude and rate to those that Earth has previously encountered? . . . This perspective article focuses on intervals in time in the fossil record when atmospheric CO2 concentrations increased up to 1200 ppmv, temperatures in mid- to high-latitudes increased by greater than 4 °C within 60 years, and sea levels rose by up to 3 m higher than present. For these intervals in time, case studies of past biotic responses are presented to demonstrate the scale and impact of the magnitude and rate of such climate changes on biodiversity. . . .What emerges from these past records is evidence for rapid community turnover, migrations, development of novel ecosystems and thresholds from one stable ecosystem state to another, but there is very little evidence for broad-scale extinctions due to a warming world. Based on this evidence from the fossil record, we make four recommendations for future climate-change integrated conservation strategies. DOI: 10.1080/14772000903495833 http://www.tandfonline.com/doi/abs/10.1080/14772000903495833#.UztDAp3D86Y
Warming and drying: field experiment (warming can increase soil moisture)
Abstract - Plants reverse warming effect on ecosystem water balance Models predict that global warming may increase aridity in water-limited ecosystems by accelerating evapotranspiration. We show that interactions between warming and the dominant biota in a grassland ecosystem produced the reverse effect. In a 2-year field experiment, simulated warming increased spring soil moisture by 5–10% under both ambient and elevated CO2. Warming also accelerated the decline of canopy greenness (normalized difference vegetation index) each spring by 11–17% by inducing earlier plant senescence. Lower transpirational water losses resulting from this earlier senescence provide a mechanism for the unexpected rise in soil moisture. Our findings illustrate the potential for organism–environment interactions to modify the direction as well as the magnitude of global change effects on ecosystem functioning.
“Jimbo” does not presume to forecast the future based on the studies above. His point rather is that “you can cherry pick what you want to back your claim. The IPCC is a political organization charged with cherry picking for the desired results.”