As farmers in the United States slog through the country’s largest drought in 50 years, a lot of people are asking about the connection between global warming and the arid landscape in the Midwest. Is climate change causing this drought? Didn’t the United States suffer worse droughts in the past? And what will happen if the planet keeps heating up?
Those aren’t simple questions. So here’s a guide to what we know about the link between climate change and drought. The best single reference on this topic is the Intergovernmental Panel on Climate Change’s (IPCC) 2012 report on extreme events, which synthesized the existing scientific research.
And the short version is this: Droughts have multiple causes. The United States has suffered worse droughts in the past. It’s not yet clear whether we’ve reached the point where global warming is making droughts worse again, at least in North America. But most evidence suggests that droughts will become more intense in many parts of the world if the planet keeps heating up, which could disrupt the world’s food supply.
1) Droughts are complex beasts, with many different causes. A drought occurs when a region stays abnormally dry for a long enough period to cause an imbalance in the water cycle. There are three ways this can happen. Less rain could fall on the region. The evaporation of moisture from soil could speed up, either because of hotter temperatures or wind shifts. Or there could be less water to begin with — say, because there was less snowfall the previous winter. Or a combination of these three things. That makes drought tougher to model than, say, heat waves.
2) Yes, North America has had worse droughts in the past. Scientists have looked at data from tree rings and found (pdf) that North America suffered brutal “megadroughts” during the medieval period. These droughts were similar in intensity to today’s dry spells, but lasted 20 to 40 years and were possibly linked to massive La Niña ocean events:
Fortunately, we haven’t seen anything that bad in recent times. The worst droughts in the modern era occurred in the 1930s (the infamous Dust Bowl) and the 1950s, though the drought for June 2012 was one of the 10 worst months in the past century.
3) So far, climate change has had a mixed effect on droughts around the world in recent times. All the carbon dioxide we’ve put into the air has warmed the world about 0.8°C above pre-industrial levels. According to the IPCC, scientists have “medium confidence” that climate change has altered drought patterns worldwide. But there’s no single pattern. Parts of Europe and Africa appear to be drying out. But in North America, droughts have actually become “shorter, less frequent, less severe, and cover a smaller portion of the country over the last century.” (The big exception is in the Southwest.)
How can this be? Again, go back to the fact that droughts have many different causes. As one 2007 study by David Easterling and three other NOAA researchers found, the United States has been getting hotter since 1950, which has dried out soils. But the country has also received more rainfall. And the latter factor has dominated recently. The NOAA authors concluded that severe droughts in the U.S. would have been about 50 percent bigger since 1980 had it not been for that extra rain. So far, we’ve been lucky. But what happens if North America keeps warming and the heat starts outweighing the boost in rainfall? Then things get dicey …
4) Climate change may now be exacerbating droughts in the United States, though it’s hard to say how much. As Climate Central’s Andrew Freedman reports, most experts agree that the biggest driver behind this year’s U.S. drought was La Niña, a periodic cooling of the ocean surface which mucks with weather patterns. As a result, the Midwest and Southwest regions are getting less rain. But the United States is also much hotter this year, part of an overall warming trend. And that extra heat has done two things. In Colorado, a warm winter meant less snowpack to provide water in the spring. In other areas, high temperatures are parching the soil.
But trying to figure out how much worse global warming made a single drought is difficult. One recent attribution study (pdf) from researchers at NOAA estimated that rising global temperatures may have made Texas’s severe drought in 2011 more likely to occur. But the study conceded that “attribution of single extreme events to anthropogenic climate change remains challenging.” And several scientists have criticized the NOAA attribution study as overly hasty — see Texas A&M’s John-Nielsen Gammon for a critique.
5) It’s likely that droughts will continue to get worse as the planet heats up. As the IPCC report notes, scientists have more confidence about what the future holds if we keep heating the planet. Climate models tend to agree that droughts will get more intense and frequent in the Mediterranean, in central North America, Mexico, northeast Brazil and southern Africa, though there are still uncertainties. Here’s one projection of what the world could look like mid-century under a “moderate” emissions scenario:
That’s a model of predicted Palmer Drought Severity Index around the world by mid-century, assuming the world keeps warming. Take a look at the United States, where the PDSI ranges from -4 to -8 in the Great Plains. The PDSI briefly spiked to -6 in that area during the Dust Bowl, but it rarely exceeded -3 for the rest of the 1930s. In other words, it’s a forecast of drought conditions more severe than they were during the Dust Bowl.
This 2011 review paper (pdf) by Aiguo Dai of the National Center for Atmospheric Research offers more detail about North America. Rainfall won’t go away. In parts of the Midwest, it will even increase. But modeling suggests that warmer air temperatures and increased evaporation will dry out soils and make persistent droughts more likely in the next 20 to 50 years. Dai also notes by e-mail that expected changes in Pacific Ocean cycles are likely to intensify these trends for the United States in the coming decades. (His study on this is forthcoming in the journal Nature Climate Change.)
6) Farmers can take steps to adapt, though a drier world will be tougher to navigate. Large droughts can hurt crop yields. This year, the fall corn harvest is expected to take a hit. And what about the future? A recent overview paper by John Antle of Montana State University concluded that U.S. farm production could drop by between 4 and 13 percent by 2030, with the Corn Belt and Southwest suffering most and farmers further north getting a boost. Other research has estimated that average crop yields in current U.S. farm regions could decline by as much as 63 to 82 percent by the end of the century under especially fast warming.
If that happens, the world will have to shift where it gets its food. Better farming techniques can soften some of the damage, the way erosion control has prevented Dust Bowl-type storms. And Tom Philpott argues that organic farming may need to play a larger role in the future. While the practice often produces lower yields than industrial farming, a recent Nature paper found that soils managed with organic techniques tend to hold more water and perform better in droughts.
But maintaining the world’s food supply — at a time, it should be noted, when the global population is expected to keep growing to 10 billion — isn’t going to be an easy task in a hotter, drought-plagued world.
Source: Wonkblog