> Although I've read in several places that once global warming is engaged, you cannot reverse it because of stuff like melting methane ice spots in the tundra.

I'm fairly confident that point of no return has already been passed. Permafrost has stared to melt a while ago, causing bacteria to take hold of things they couldn't for x amount of years, catching up on releasing co2 and such that would otherwise have been in the atmosphere a long time ago already.