The US generates 13% of global CO2. If we reduced emissions by 20%, that would cut us down to about 10% of current global emissions.
However, Obama agreed to let China increase their emissions for the next 16 years. At China’s current growth rates, there will be a huge increase in global CO2 emissions, which will place any US reductions in the noise.
The reason that China agreed to it, is because it puts American industry at a competitive disadvantage due to increased fuel costs. It was a win-win for China. They do nothing, while Obama shoots the US in the foot.