There isn’t a reputable scientist in the world – even if they support the notion of climate change – who would predict a specific rise in temperature over any time period. Three degrees is a number picked out of a hat and has nothing to do with reality.
The ploy to piggyback Obama’s proposal on a 1992 law violates the whole idea of separation of powers by giving the president treaty ratifying authority that the Constitution clearly reserved for the Senate. It’s cynically transparent in its skirting the Constitution to make legal what ordinarily would be illegal.
For the president, democracy is just too hard.