Extended drought and Dust-Bowlification over large swaths of the habited Earth may be the most dangerous impact of unrestricted greenhouse gas emissions, as I’ve discussed many times (see Intro to global warming impacts: Hell and High Water).
That’s especially true since such impacts could well last centuries, whereas the actual Dust Bowl itself only lasted seven to ten years — see NOAA stunner: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe.
A must-read new study from the National Center for Atmospheric Research, “Drought under global warming: a review,” is the best review and analysis on the subject I’ve seen. It spells out for the lukewarmers and the delayers just what we risk if we continue to listen to the Siren song of “more energy R&D plus adapatation.”
The NCAR study is the source of the top figure (click to enlarge), which shows that in a half century, much of the United States (and large parts of the rest of the world) could experience devastating levels of drought — far worse than the 1930s Dust Bowl, especially since the conditions would only get worse and worse and worse and worse, while potentially affecting 10 to 100 times as many people. And this study merely models the IPCC’s “moderate” A1B scenario — atmospheric concentrations of CO2 around 520 ppm in 2050 and 700 in 2100. We’re currently on the A1F1 pathway, which would takes us to 1000 ppm by century’s end, but I’m sure with an aggressive program of energy R&D we could keep that to, say 900 ppm.
Indeed, the study itself notes that it has ignored well understood climate impacts that could worsen the situation:
As alarming as Figure 11 shows, there may still be other processes that could cause additional drying over land under global warming that are not included in the PDSI calculation. For example, both thermodynamic arguments124 and climate model simulations125 suggest that precipitation may become more intense but less frequent (i.e., longer dry spells) under GHG-induced global warming. This may increase flash floods and runoff, but diminish soil moisture and increase the risk of agricultural drought.
That is, even when it does rain in dry areas, it may come down so intensely as to be counterproductive.
The study notes that “Recent studies revealed that persistent dry periods lasting for multiple years to several decades have occurred many times during the last 500–1000 years over North America, West Africa, and East Asia.” Of course, those periods inevitably caused havoc on local inhabitants. Further, this study warns that by century’s end, even in this moderate scenario, many parts of the world could see extended drought beyond the range of human experience:
The large-scale pattern shown in Figure 11 appears to be a robust response to increased GHGs. This is very alarming because if the drying is anything resembling Figure 11, 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.
Most important, unlike most earlier droughts over the past millennium, these super-droughts will hit a planet with a population approaching 10 billion — and they will hit multiple areas simultaneously, making it exceedingly difficult for countries to receive significant outside aid or for large populations to migrate.
And as I have discussed, future droughts will be fundamentally different from all previous droughts that humanity has experienced because they will be very hot weather droughts (see Must-have PPT: The “global-change-type drought” and the future of extreme weather).
One can talk about adapting to this cataclysm, but fundamentally we are really talking more about misery and triage than what most people think of adaptation (see Real adaptation is as politically tough as real mitigation, but much more expensive and not as effective in reducing future misery: Rhetorical adaptation, however, is a political winner. Too bad it means preventable suffering for billions).
For media coverage of the NCAR report, see
- MSNBC: “Future droughts will be shockers, study says 1970s. Sahel disaster will seem mild compared to areas by 2030s, models project.”
- AFP: “Much of planet could see extreme drought in 30 years: study”
- Reuters: “Some of the world’s most populous areas — southern Europe, northern Africa, the western [2/3rds of the] U.S. and much of Latin America — could face severe, even unprecedented drought by 2100, researchers said Tuesday.”
While this study has received deserved attention, the UK Met Office came to a similar view four years ago in their analysis, projecting severe drought over 40% of the Earth landmass by century’s end (see “The Century of Drought”).
Also, “the unexpectedly rapid expansion of the tropical belt constitutes yet another signal that climate change is occurring sooner than expected,” noted one climate researcher in December 2007. A 2008 study led by NOAA noted, “A poleward expansion of the tropics is likely to bring even drier conditions to” the U.S. Southwest, Mexico, Australia and parts of Africa and South America.”
In 2007, Science (subs. req’d) published research that “predicted a permanent drought by 2050 throughout the Southwest” “” levels of aridity comparable to the 1930s Dust Bowl would stretch from Kansas to California. And they were also only looking at a 720 ppm case. The Dust Bowl was a sustained decrease in soil moisture of about 15% (“which is calculated by subtracting evaporation from precipitation”).
Because this subject is so important, I am going to repost NCAR’s release, “Climate change: Drought may threaten much of globe within decades:
The United States and many other heavily populated countries face a growing threat of severe and prolonged drought in coming decades, according to a new study by National Center for Atmospheric Research (NCAR) scientist Aiguo Dai. The detailed analysis concludes that warming temperatures associated with climate change will likely create increasingly dry conditions across much of the globe in the next 30 years, possibly reaching a scale in some regions by the end of the century that has rarely, if ever, been observed in modern times. 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.
In contrast, higher-latitude regions from Alaska to Scandinavia are likely to become more moist.
Dai cautioned that the findings are based on the best current projections of greenhouse gas emissions. What actually happens in coming decades will depend on many factors, including actual future emissions of greenhouse gases as well as natural climate cycles such as El Ni±o.
The new findings appear this week as part of a longer review article in Wiley Interdisciplinary Reviews: Climate Change. The study was supported by the National Science Foundation, NCAR’s sponsor.
“We are facing the possibility of widespread drought in the coming decades, but this has yet to be fully recognized by both the public and the climate change research community,” Dai says. “If the projections in this study come even close to being realized, the consequences for society worldwide will be enormous.”
While regional climate projections are less certain than those for the globe as a whole, Dai’s study indicates that most of the western two-thirds of the United States will be significantly drier by the 2030s. Large parts of the nation may face an increasing risk of extreme drought during the century.
Other countries and continents that could face significant drying include:
- Much of Latin America, including large sections of Mexico and Brazil
- Regions bordering the Mediterranean Sea, which could become especially dry
- Large parts of Southwest Asia
- Most of Africa and Australia, with particularly dry conditions in regions of Africa
- Southeast Asia, including parts of China and neighboring countries
The study also finds that drought risk can be expected to decrease this century across much of Northern Europe, Russia, Canada, and Alaska, as well as some areas in the Southern Hemisphere. However, the globe’s land areas should be drier overall.
“The increased wetness over the northern, sparsely populated high latitudes can’t match the drying over the more densely populated temperate and tropical areas,” Dai says.
A climate change expert not associated with the study, Richard Seager of Columbia University’s Lamont Doherty Earth Observatory, adds:
“As Dai emphasizes here, vast swaths of the subtropics and the midlatitude continents face a future with drier soils and less surface water as a result of reducing rainfall and increasing evaporation driven by a warming atmosphere. The term ‘global warming’ does not do justice to the climatic changes the world will experience in coming decades. Some of the worst disruptions we face will involve water, not just temperature.”
Future drought. These four maps illustrate the potential for future drought worldwide over the decades indicated, based on current projections of future greenhouse gas emissions. These maps are not intended as forecasts, since the actual course of projected greenhouse gas emissions as well as natural climate variations could alter the drought patterns.The maps use a common measure, the Palmer Drought Severity Index, which assigns positive numbers when conditions are unusually wet for a particular region, and negative numbers when conditions are unusually dry. 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 Wiley Interdisciplinary Reviews, redrawn by UCAR. This image is freely available for media use. Please credit the University Corporation for Atmospheric Research. For more information on how individuals and organizations may use UCAR images, see Media & nonprofit use*)
As an aside, this series of maps misses the drop in the PDSI we’ve already seen in the past few decades. The top two maps below from the study itself show how it used to be:
The NCAR release continues:
A portrait of worsening drought
Previous climate studies have indicated that global warming will probably alter precipitation patterns as the subtropics expand. The 2007 assessment by the Intergovernmental Panel on Climate Change (IPCC) concluded that subtropical areas will likely have precipitation declines, with high-latitude areas getting more precipitation.
In addition, previous studies by Dai have indicated that climate change may already be having a drying effect on parts of the world. In a much-cited 2004 study, he and colleagues found that the percentage of Earth’s land area stricken by serious drought more than doubled from the 1970s to the early 2000s. Last year, he headed up a research team that found that some of the world’s major rivers are losing water.
In his new study, Dai turned from rain and snow amounts to drought itself, and posed a basic question: how will climate change affect future droughts? If rainfall runs short by a given amount, it may or may not produce drought conditions, depending on how warm it is, how quickly the moisture evaporates, and other factors.
Droughts are complex events that can be associated with significantly reduced precipitation, dry soils that fail to sustain crops, and reduced levels in reservoirs and other bodies of water that can imperil drinking supplies. A common measure called the Palmer Drought Severity Index 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.
Dai turned to results from the 22 computer models used by the IPCC in its 2007 report to gather projections about temperature, precipitation, humidity, wind speed, and Earth’s radiative balance, based on current projections of greenhouse gas emissions. He then fed the information into the Palmer model to calculate the PDSI index. A reading of +0.5 to -0.5 on the index indicates normal conditions, while a reading at or below -4 indicates extreme drought. The most index ranges from +10 to -10 for current climate conditions, although readings below -6 are exceedingly rare, even during short periods of time in small areas.
By the 2030s, the results indicated that some regions in the United States and overseas could experience particularly severe conditions, with average decadal readings potentially dropping to -4 to -6 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.
The PDSI in the Great Plains during the Dust Bowl apparently spiked very briefly to -6, but otherwise rarely exceeded -3 for the decade (see here). So the numbers projected by Dai are beyond catastrophic, the damage would be incalculable.
Dai cautions that global climate models remain inconsistent in capturing precipitation changes and other atmospheric factors, especially at the regional scale. However, the 2007 IPCC models were in stronger agreement on high- and low-latitude precipitation than those used in previous reports, says Dai.
There are also uncertainties in how well the Palmer index captures the range of conditions that future climate may produce. The index could be overestimating drought intensity in the more extreme cases, says Dai. On the other hand, the index may be underestimating the loss of soil moisture should rain and snow fall in shorter, heavier bursts and run off more quickly. Such precipitation trends have already been diagnosed in the United States and several other areas over recent years, says Dai.
“The fact that the current drought index may not work for the 21st century climate is itself a troubling sign,” Dai says.
It may seem politically implausible that we are going to take serious action anytime soon, but that doesn’t mean our inaction is any less immoral. People need to understand what we risk if we stay anywhere near our current emissions path. The time for rapid deployment of every available low-carbon technology is still right now.
For a glimpse of what’s coming to the United States, see:
- Absolute must read: Australia today offers horrific glimpse of U.S. Southwest, much of planet, post-2040, if we don’t slash emissions soon”
- “Australia faces collapse as climate change kicks in”: Are the Southwest and California next?
- Dust Bowl-ification hits Eastern Australia “” next stop the U.S. Southwest.
Also, Bryan Walsh has a good piece on EcoCentric, “Water: Lake Mead Is at Record Low Levels. Is the Southwest Drying Up?”