All summer long, the Midwest has been roiled by the worst drought in half a century. And it's taking a surprisingly large toll on economic growth this year.

Not good for the economy. (Associated Press)

The U.S. economy grew at a 2 percent annual pace from July to September — but it would have grown significantly faster without the dry spell. All told, a drop in farm inventories shaved 0.42 points off growth last quarter, the Bureau of Economic analysis says. That's after the drought knocked 0.17 points off growth in April through June.

If anything, that's understating the drought's effects. BEA mainly looked at how the dry summer shriveled farm inventories — the crops, grain and cattle that are stored on farms — which dropped by $29 billion last quarter. But of course, agriculture can affect GDP in other ways, too. The United States is a major exporter of crops such as corn and soybeans. And drop in exports shaved a further 0.2 points off growth last quarter. Some of that was likely drought-related.

The government also notes that it paid $15 billion in crop insurance to farmers between July and September. That doesn't affect GDP, but it does give a sense for just how devastating and costly the drought was.

Analysts are hoping that the dry spell will only temporarily hurt growth. "Eventually, when crop yields return to normal, the rebound in farm inventories will boost GDP growth," notes Paul Ashworth of Capital Economics. But it's worth noting that some climate modelers expect droughts to become a more frequent feature of the U.S. landscape in the coming years. 

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 global warming continues apace. Rainfall won’t go away. In parts of the Midwest, it will even increase. But warmer air temperatures and increased evaporation are expected to dry out soils and make persistent droughts more likely in the next 20 to 50 years. Here’s one effort by NCAR to model what the world could look like in 2030 to 2039 under a “moderate” emissions scenario:

If those predictions pan out—and if this year's data is anything to go by—severe droughts could potentially put a troubling dent in the U.S. economy in the decades ahead.