The goal of Skeptical Science is to explain what peer reviewed science has to say about global warming. Our ability to put aside our preconceptions and beliefs and to evaluate things objectively - through the cold lens of rationality and reason - says a lot about ourselves.
Much ado has been made recently in the media and the blogosphere of recent extreme weather events around the world: the flooding in Tennessee and Pakistan, the Moscow heat waves, record drought in the Amazon, and yet more flooding in Queensland and Brazil.
So let's leave the hype and agendas behind and focus instead on one of the basic juxtapositions of the warming world we live in: the co-existence of droughts and floods.
As of the time the IPCC came out with the AR4, predicted extremes in precipitation and droughts were being observed:
"In a warmer future climate, most Atmosphere-Ocean General Circulation Models project increased summer dryness and winter wetness in most parts of the northern middle and high latitudes. Summer dryness indicates a greater risk of drought. Along with the risk of drying, there is an increased chance of intense precipitation and flooding due to the greater water-holding capacity of a warmer atmosphere. This has already been observed and is projected to continue because in a warmer world, precipitation tends to be concentrated into more intense events, with longer periods of little precipitation in between. Therefore, intense and heavy downpours would be interspersed with longer relatively dry periods. Another aspect of these projected changes is that wet extremes are projected to become more severe in many areas where mean precipitation is expected to increase, and dry extremes are projected to become more severe in areas where mean precipitation is projected to decrease."
Other studies demonstrated that the percentage of Earth’s land area stricken by serious drought more than doubled from the 1970s to the early 2000s.
This pattern continues today. 2010 witnessed flooding in Nashville, Pakistan, Queensland and Brazil. The Amazon experienced its 2nd once-in-a-century drought, a bare 5 years after the previous once-in-a-century drought.
It has been more than 3 years since the last assessment (IPCC AR4) and the next (IPCC AR5) is still years away. To help bridge this gap in our knowledge comes a new review study, Drought Under Global Warming, A Review, led by Aiguo Dai and his team. Together they take on the formidable task of assessing the field to date.
A common measure called the Palmer Drought Severity Index (PDSI) classifies the strength of a drought by tracking precipitation and evaporation over time and comparing them to the usual variability one would expect at a given location in the low (equatorial) and middle latitudes. The more negative the number, the more drier the conditions.
Using an ensemble of 22 computer climate models and a comprehensive index of drought conditions, as well as analyses of previously published studies, the paper finds most of the Western Hemisphere (along with large parts of Eurasia, Africa, and Australia) may be at threat of extreme drought this century. Note that in the paper they re-normalize the index to the local conditions; there would be no way to explain why the Sahara desert appears to be less arid than the USA. With the renormalization, the PDSI can thus be considered an anomaly.
Another finding of the paper is that precipitation was the dominant driver for changes in the terrestrial water budget before the early 1980's. Changes since then are not only attributed to anthropogenic greenhouse gas induced (human-induced GHG) surface warming, but that those same human-induced GHG increases have contributed to the observed drying trend since the 1980's.
It was also found that despite increased precipitation, due to increased surface temperatures and increased surface runoff, higher atmospheric demand for moisture results in drier soils.
Figure 1: Current Palmer Drought Severity Index [PDSI] 2000-2009. A reading of -4 or below is considered extreme drought. Regions that are blue or green will likely be at lower risk of drought, while those in the red and purple spectrum could face more unusually extreme drought conditions. (Courtesy University Corporation for Atmospheric Research [UCAR])
Tomorrow waits for no one. What can we expect, based on this work?
By the 2030s, the results indicated that some regions in the United States and overseas could experience particularly severe conditions, with average decadal PDSI readings potentially dropping to -4 to -6 (extreme drought) in much of the central and western United States as well as several regions overseas, and -8 or lower in parts of the Mediterranean. By the end of the century, many populated areas, including parts of the United States, could face readings in the range of -8 to -10, and much of the Mediterranean could fall to -15 to -20. Such readings would be almost unprecedented.
Figure2: PDSI 2030-2039. (Courtesy UCAR)
Extreme conditions experienced earlier are expected to continue, and even worsen. In fact, droughts may become so widespread and so severe that current drought indices may no longer be adequate to quantify them.
Figure 3: PDSI 2060-2069. (Courtesy UCAR)
If the drying is anything resembling that shown in these figures, a very large population will be severely affected in the coming decades over the whole United States, Southern Europe, Southeast Asia, Brazil, Chile, Australia and most of Africa.
Figure 4: PDSI 2090-2099. (Courtesy UCAR)
The paper closes on a somber note:The above figures are themselves based on IPCC AR4 SRES A1B (medium GHG) emission scenarios. Current Business-As-Usual (BAU) emissions are running higher than that (tracking the A1F1, or high emissions, scenario). Thus, potential exists for conditions to be worse than depicted above. And that is not a comforting thought.
Aiguo Dai is a researcher with the National Center for Atmospheric Research (NCAR). His past work includes (but is not limited to):
Posted by Daniel Bailey on Friday, 18 February, 2011
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