Why the Slowdown in Agricultural Productivity?

From Greed, Green and Grains, via Mark Thoma:

Two words:

Climate Change  Global Warming.

Well, there may be more to it.  Like reduced public research and pathogens like wheat stem rust.

But new research by my colleagues David Lobell and Wolfram Schlenker, along with Justin Costa-Roberts shows that warming has hurt corn and wheat yields on all continents except North America:

Farms across the planet produced 3.8 percent less corn and 5.5 percent less wheat than they could have between 1980 and 2008 thanks to rising temperatures, a new analysis estimates. These wilting yields may have contributed to the current sky-high price of food, a team of U.S. researchers reports online May 5 in Science. Climate-induced losses could have driven up prices of corn by 6.4 percent and wheat by 18.9 percent since 1980.

The article was embargoed until 2pm today, but it's already circulating.

Why not North America, or why not yet in North America?  I think any climatological change in the Corn Belt could be disastrous. Don't fear, if you say nothing is happening, nothing is happening, right Republicans?