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 evidence suggests that droughts will become more intense in many parts of the world if the planet keeps heating up — a trend that could disrupt the world's food supply.

1) Droughts are complex phenomena, 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 main ways this can happen. Less rain could fall on the region. The evaporation of moisture from soil could speed up, either because of hotter air temperatures or wind shifts. Or there could be less water to begin with — say, because there was less snowfall the previous winter. Quite often, it's some 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 endured 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) 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 steadily 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 could get dicey ...

4) Global warming 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 places like Colorado, the warmer winter meant less snowpack to provide water in the spring. In other regions, high temperatures are parching the soil and affecting the rate of evaporation.

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 as to exact regions. Here’s one effort by the National Center for Atmospheric Research to model what the world could look like in 2030 to 2039 under a “moderate” emissions scenario:

That’s a look at predicted Palmer Drought Severity Index around the world. Take a look at the United States, where the PDSI ranges as high as -6 to -7 in the Great Plains. For context, 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, there's a possibility of persisting drought conditions more severe as the Dust Bowl within the next two decades.

This 2011 review paper (pdf) by Aiguo Dai of the National Center on Atmospheric Research offers more detail about what's expected to happen in North America if the world keeps heating up. 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 told me, via e-mail, that expected natural changes in Pacific Ocean cycles could very well intensify drought 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 and heat stress can hurt crop yields. This year, the fall U.S. corn harvest is already expected to take a hit. So what about the future? A recent overview paper by John Antle of Montana State University concluded that, if the world keeps warming, U.S. farm production could drop by a modest 4 to 13 percent by 2030 (the Corn Belt and Southwest would suffer most, while farmers further north would get a boost). Yet some researchers have begun studying worst-case scenarios, too: A 2011 paper by Michael Roberts and Wolfram Schenkler estimated that average crop yields in existing U.S. farm regions could decline as much as 63 to 82 percent by the end of the century under especially rapid warming.

If that were to happen, the world would have to shift where it gets its food. Better farming techniques could 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.

For a more detailed look at how the world could boost its agricultural yields under stress, see Jonathan Foley's recent essay (pdf) in Scientific American. There are plenty of steps that can be taken, from limiting crop biofuels to reducing food waste. Still, 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-filled world.