Thursday, July 19, 2012

How Droughts Will Reshape the United States

More than half of the continental United States is currently suffering through the worst drought in 50 years, with heat and a lack of rain rippling through the middle of the country. Crops are wilting, soils are cracked, and some dried-out forests are catching fire. U.S. corn production in particular is dwindling.

So is this a glimpse at our hotter, drier future? It appears so. While severe dry spells can (and do) occur naturally, some recent U.S. droughts have been linked to the broad-scale warming of the planet. And if greenhouse gas emissions keep rising and temperatures keep ticking upward, scientists say, we can likely expect more serious and persistent droughts in the years ahead. Here’s what we know about our potentially parched future:

1) The current drought isn’t at Dust Bowl levels, though it’s very large from a historical standpoint. The worst drought month in recorded U.S. history came in July of 1934, the dessicated peak of the Dust Bowl, when 79.9 percent of the country experienced drought conditions (and 63 percent of the country was suffering from extreme drought). Last month’s drought isn’t nearly at that level—and it hasn’t persisted for years the way the multi-year Dust Bowl droughts did. But it’s still in the top 10 for the past century, according to a Weather Channel analysis of the Palmer Drought Severity Index:

As it happens, the current drought is also more widespread than the infamous 1936 heat wavewhen more than 5,000 Americans died and farmers endured widespread crop failures. So why the difference in impact? Some credit goes to modern agricultural practices. Back in the 1930s, overgrazing led to erosion and dust storms that caked the Plains and worsened the drought. Modern erosion control and drought-resistant crops have averted some of that damage. Though not all…
2) Current droughts may be hurting U.S. corn yields, but they’re not yet causing a global food crisis. The United States is the world’s largest exporter of corn and a key supplier of soybeans. And right now, U.S. corn and soy production appear to be wilting under the heat—the Department of Agriculture has cut its corn-production estimate by 12 percent. If output ends up falling, that would raise the price for corn, for biofuels, as well as for beef (since corn is used to feed livestock).
More broadly, however, analysts still don’t think we’re facing a situation like 2007 and 2008, when skyrocketing food prices triggered riots in dozens of countries from Haiti to Egypt. That’s because global wheat and rice supplies are holding fairly steady, at least for now.
3) Climate change may already be making some U.S. droughts more likely. Given that the United States experienced even more severe droughts in the 1930s and 1950s, when carbon emissions were lower than they are today, one might assume that modern U.S. droughts have little to do with global warming. And, indeed, tree-ring data suggest that droughts in North America have been part of a natural cycle in the past, often tied to strong La Niña events. Yet scientists are now trying to figure out if rising global temperatures are making present-day droughts more common than those natural conditions would suggest.
One recent attribution study (pdf) from researchers at NOAA and the Met Office concluded that global warming may have made last year’s severe drought in Texas more likely to occur. That is, if it weren’t for human-driven climate change, seasonal conditions in Texas should have looked more like they did in the 1960s, when rainfall was higher. That’s one early, albeit tentative piece of evidence that the impacts of global warming are already being felt. (The NOAA study concedes, however, that “attribution of single extreme events to anthropogenic climate change remains challenging.”)
4) And future global warming could seriously dry out the United States. Keep in mind that human activity has already warmed the planet about 0.8°C above pre-industrial levels. According to the International Energy Agency, we’re on pace to warm the planet by 6°C by the end of the century. That would be a big difference. And a growing body of scientific evidence suggests that warming could drastically increase droughts around the world, including in North America.
This 2011 review paper (pdf) by Aiguo Dai of the National Center for Atmospheric Research summarizes much of what’s known about climate change and drought. Computer models suggest that in North America, the rain won’t go away. But warmer temperatures and increased evaporation will dry out soils and make persistent droughts more likely in the next 20 to 50 years. (Again, these droughts aren’t unprecedented, but they’re expected to become more frequent.) Here’s one projection of what the world could look like mid-century under a “moderate” emissions scenario:

The Palmer Drought Severity Index mid-century under a moderate emissions scenario. Under the PDSI, "a reading of -4 or below is considered extreme drought." (Source: National Center for Atmospheric Research.)

That’s a model of predicted Palmer Drought Severity Index around the world by mid-century. Take a look at the United States, where the PDSI ranges from -4 to -8 in the Plains. As Joe Romm notes, the PDSI briefly spiked to -6 in the Plains 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.
5) Farmers can take steps to adapt, though a drier world will be tough to navigate. Given that climate change might already be making drought more likely, U.S. farmers will likely need to adapt no matter what else is done. A 2009 paper by John Antle of Montana State University went through the research on this. By 2030, farmers in the U.S. corn belt and Southwest are expected to see significant losses, which could be partly offset by gains up North. (Antle estimated the overall drop in U.S. agricultural production to be quite modest, between 4 percent to 13 percent.) Keep in mind, however, that drought projections seem to be growing more severe in recent years. So the economic impacts could be higher.
Better farming techniques can soften some of the damage, the way erosion control has prevented Dust Bowl-type storms. Antle’s paper suggests that the U.S. government may need to change some policies—for instance, subsidies for corn and soy can prevent adaptation by locking in current farming patterns. Meanwhile, 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. And droughts are looking increasingly tough to avoid.

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