[bbctol]

I'm curious as to what supports the idea that deserts will re-green and agricultural output will increase? I am not at all an expert in the field, but I've always heard desertification and disappearing arable land as chief concerns of global warming, so I'd like to learn more.

[akiselev]

The increasing temperatures speed up evaporation in the water cycle which has a lot of impact on rainfall. Since wind speeds aren't increasing overall while temperature is, this means that rain clouds store more water and travel farther across land. This also means that the radius of the strip around the equator that can be considered tropical grows and pushes other biomes outward. The further north you get the more land becomes available for farming and if rains can reach them, a lot of fertile land will be unlocked for agriculture.

That's the theory anyway. The current impact of that evaporation seems to be increasing frequency and strength of hurricanes and monsoons. It is really hard to get fine grained reliable data on temperature and humidity across the entire planet, let alone account for unknown factors, so specific medium to long term impacts on agriculture are hard to pin down.