Compared to the size of the US, there aren't a lot of offsets. Assuming we just wanted to stabilise output at 1990 levels, the US at that time was responsible for about 1/4 of greenhouse output. So on top of any other measures required to get back to 1990 levels, the US would have to pay for a 1/3 reduction everywhere other than the US. When you consider how much of that output is related to stuff exported to the US this becomes somewhat tricky. Fake offsets created by planting trees and so on just don't work at all on that scale - you'd need to plant an area the size of Texas... every year. And never allow a tree to die. I can't remember the exact details, but within 50 years the entire land area of the planet would be covered in forest.

On top of this, there is no prospect whatsoever of China and India agreeing to 1990 levels across the board, even if it's Sheriff George doing the 'negotiating'. So more likely the US would need to halve its 1990 levels in order to allow other countries to come up to the level the US is coming down to. Or at least some approximation to that.

To get that sort of global readjustment it would probably make more sense to have a carbon allowance like the UK proposal - each person gets X emissions per year and can use or sell them as they see fit. It could make indenture and slavery popular again, as well as promoting population growth if done badly. Whether it could be done well is an open question - it will be interesting to watch whoever does it first. The global system of course raises the problem of US exceptionalism (again), but we're going to have to address that eventually anyway, or you'll kill us all.