Now, you could generally assume that all that air eventually has to condense somewhere, but the problem is that we've now raised the potential threshold of what precipitation entails. So we've got drier soil (which is worse at holding moisture when it rains), more potential for precipitation, and a longer buildup time between precipitation threshold events. We've just created feedback loop that encourages flash flooding, making it more difficult to farm all areas, both old farmland and new potentially arable land.
Its fun to learn about the complex system dynamics of climate physics, it'd just be more fun if it didn't come with a side serving of impending catastrophe.
And probably it will have local climate effects if not global ones.
Many areas are in fact predicted to have increased precipitation.
> What borderline arable lands will become arable as a result of climate change, if any?
Anywhere with water will experience forestation. With higher CO2 levels, the earth will definitively become greener. But don't hold your breath for humans (pun intended).