Probably the best idea is to start thinking about risky geoengineering remediation techniques.
If U.S. does nothing, then each individual EU country and Japan and Canada will all be individually too small to do it. EU won't take orders from non-NATO countries. And EU can't risk it's credibility on something that expensive that's not backed by USA. It would fuel exit-populist propaganda too much. Meanwhile USA is not breaking apart in similar fashion.
Looking from a Nordic country, this is the only reason why electing Trump would be a sad mistake. Otherwise Trump is just funny.
In general, most methods that try to manipulate the planet's radiation budget (eg giant sunscreen in space, white paint, SO2 in the stratosphere) are either prohibitively expensive and risky or require constant upkeep, allowing a full-blown catastrophe to happen should any short- to medium-term problem occur with the upkeep. At least those could be reversed if needed though.
The methods that try to remove carbon are a safer bet, but safely storing CO2 is hard and might not be safe (volcanic lakes have killed thousands by releasing CO2), especially longterm. And storing it in different form such as coal.. well, it's kind of pointless to first burn the coal just to remake it, especially from an energy and thus financial point of view. Storing it in the ocean's biomatter is again risky as we don't have a proper picture of what might happen.
IMHO, the risky geoengineering techniques should be tried on Venus for a few millenia+, before we need them to stabilize the climate against natural variations (an ice age would majorly suck as well, probably far more that global warming. Even if the timescale is different.)
( http://www.iup.uni-heidelberg.de/institut/studium/lehre/Clim... for material on this)
All humanity needs to do is spend about $200 billion per year on mining and crushing olivine rocks and spreading it in the ocean. https://eclipsenow.wordpress.com/olivine/