Question: Did Chinese agree to global warming agreement because it doesn't change anything or because they know that with Republicans controlling the Senate no treaty will be approved?

The promise of 20% of energy from non fossil fuels actually requires a slower growth rate for those sources than has been true up until now.  Meanwhile, the US has agreed to a dramatically greater reduction in carbon emissions.

As part of its plan, the U.S. said Wednesday it would double the average pace of its carbon-dioxide reductions after 2020, eyeing an overall reduction of greenhouse-gas emissions of between 26% and 28% by 2025, compared with 2005 levels. . . .

But the agreement requires that the Senate pass the UN Treaty.  It seems extremely convenient for China to make this agreement immediately after Republicans take control of the Senate.  It is unlikely that an agreement would have been ratified with the Democrats in control, but with the Republicans there, it will allow Obama to attack them when the treaty is defeated.