That's my thought, not the researchers. An academic says the longer growing season for corn over the past 30 years accounts for part of the yield increases.
More generally, there seems to be some controversy over the causes of corn yield increases--better farming, better seeds, more inputs, better weather. The yield curves should be a source of discomfort to some in the organic farming community who believe that industrial ag exhausts the soil. May do so, but it's been a while.
And back on global warming--some in the environmental community preach that it's disastrous. I'd disagree, in part. It's change that will be disastrous to many and helpful to some and disconcerting to all. If we were just worried about humans, I'd like to see increased efforts on mitigation of effects--planning now to relocate sea-level communities, etc. But since we have to worry about the whole world, we also need to worry about things like a carbon tax.