'The Dust Bowl that never ends'

Climate Change Drying Out Southwest Now, With Worse To Come For A Third Of The Planet

Joe Romm

Think Progress,

11 April, 2014

Two new studies confirm that warming-driven climate change is already drying the U.S. Southwest and other parts of the globe. More worrisome, nearly a third of the world’s land faces drying from rising greenhouse gases — including two of the world’s greatest agricultural centers, “the U.S. Great Plains and a swath of southeastern China.”

These studies add fuel to the growing bonfire of concerns about climate change and food security. As I wrote in the article on Dust-Bowlification I did for the journal Nature in 2011, “Feeding some 9 billion people by mid-century in the face of a rapidly worsening climate may well be the greatest challenge the human race has ever faced.”

The fact that global warming is already drying out large parts of the planet — and that it is on track to get much, much worse — is well understood by climate scientists. Because this drying may be the single most consequential climate impact, confusionists try to blow smoke on it.

The first study is “Atmosphere and Ocean Origins of North American Droughts,” by Columbia’s Richard Seager and NOAA’s Martin Hoerling, in the Journal of Climate (subs. required, full text here). It concludes:

Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the American Southwest, thereby prolonging the duration and severity of naturally occurring droughts.

…. rising greenhouse gases will lead to a steady drying of southwest.”

So, yes, climate change is already worsening the length and strength of droughts in this country.

Of course, the more important question is: What’s going to happen in the future if we don’t slash CO2 emissions fast? It’s clear the U.S. Southwest will keep drying out. But the problem will be vastly more widespread according to the second study, “Global warming and 21st century drying” in Climate Dynamics by Cook et al.

The Columbia University news release explains the bleak conclusions:

Published this month in the journal Climate Dynamics, the study estimates that 12 percent of land will be subject to drought by 2100 through rainfall changes alone; but the drying will spread to 30 percent of land if higher evaporation rates from the added energy and humidity in the atmosphere are considered. An increase in evaporative drying means that even regions expected to get more rain, including important wheat, corn and rice belts in the western United States and southeastern China, will be at risk of drought.

Interestingly, this study has a similar finding to a study by the Met Office’s Hadley Centre for Climate Prediction and Research that was the subject of one of the earliest posts on Climate Progress, in October 2006: “One third of the planet will be desert by the year 2100, say climate experts in the most dire warning yet of the effects of global warming.”

This new 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.” It finds “increased evaporative drying will probably tip marginally wet regions at mid-latitudes like the U.S. Great Plains and a swath of southeastern China into aridity.”

This study vindicates leading climatologist James Hansen when he warned in 2012 that the Great Plains — one of America’s breadbaskets — was at risk of semipermanent drought. It’s not a big surprise he was correct given that Hansen himself co-authored one of the first journal articles ever written on the impact of global warming on increased evaporation. His 1990 Journal of Geophysical Research study, “Potential evapotranspiration and the likelihood of future drought,” projected that severe to extreme drought in the United States, then occurring every 20 years or so, could become an every-other-year phenomenon by mid-century.

The news release for the new study explains:

Much of the concern about future drought under global warming has focused on rainfall projections, but higher evaporation rates may also play an important role as warmer temperatures wring more moisture from the soil, even in some places where rainfall is forecasted to to increase, say the researchers.

This is a central point missed by many drought discussions that focus only on rainfall amounts. In February, the journal Science had an excellent article on this point, “Climate Change: A Drier Future?” with this useful figure:

Aridity increases in warmer climates, leading to expansion of dry climate zones. Evaporation and precipitation increase modestly, but on land, evaporative demand (broken wavy arrows) increases faster than precipitation, because the strong increases in air temperature and consequently saturated water vapor concentration over land (red bars at lower right) exceed growth in actual water vapor concentration (blue bars). Increases in sensible and latent heat (associated, respectively, with temperature and water vapor, and represented by the area of each bar) have the same sum over land and ocean, with sensible heat increasing more over land than oceans and latent heat increasing more over oceans. Relative humidity (ratio of blue to red bar length) decreases over land. (PET is “potential evapotranspiration.”)

The bottom line of the Science article is one that everyone in policymaking, agriculture, climate science, and the media who is concerned with the future of drought and food production should set to memory:

As the above considerations show, focusing on changes in precipitation, as typical in high-profile climate reports, does not tell the whole story — or perhaps even the main story — of hydrological change. In particular, it obscures the fact that in a warmer climate, more rain is needed. Many regions will get more rain, but it appears that few will get enough to keep pace with the growing evaporative demand.

We have been warned about this by leading climatologists for nearly a quarter of a century, now. The time to act was a long time ago, but now is infinitely better than later if you are at all concerned about how we are going to feed 9 billion people post-2050.