Kinda obvious, though: it's not like Obama could realistically pledge more. He entered into this agreement unilaterally as it is, with no vote of Congress (treaties do require 2/3rds majority), so pledging the "fair share" would be politically risky at the time. The strategy was to get HRC elected, and get democratic majority in the House/Senate, and then pony up the funds. None of those things quite worked out the way establishment was hoping it would.

So now the only avenue available to address the issue is re-negotiation of the agreement. Assuming the world leaders do actually want to save the climate, and not just bilk the US out of $100B/yr, they have no choice but to negotiate.