If you look at the effects on carbon dioxide on plant life in isolation from everything else they're right. On the other hand higher temperatures will slow down growth leading to a small net positive or negative depending on which staple crop we're talking about. And on the gripping hand shifting patterns of rainfall are liable to totally screw everything up in a way that could be totally disastrous but which is very hard to predict.

But a reluctance to admit that you're wrong is a big part of human politics. Maybe we can find some way to reframe the issue so that it doesn't seem like a retreat to them?