Ideally, global warming, but it would be fair to view that as pointless when dissipating the extra CO2 doesn't necessarily return the trees and the problem would degrade again in a couple hundred years. You'd have to introduce a new fuel source that is sought after, clean, and eternal. Which would be two wishes.

So you have to define it as both of your options, since the loss of either worsens the other. Turn the whole environment back to where it was in the 1200s, overrun the streets with bears, see if I care. It'll give 'em something to do. Especially the Amazonian avocado farmers.