This will not happen.
A final emissions reduction target will be submitted, the U.S. said, once the U.S. Congress enacts domestic legislation requiring carbon pollution cuts. But such legislation has an uncertain fate in the Senate.Regardless of this, let's look at the numbers.
2005 US emissions = 7256.9 mt CO2e
First of all, let's note that its very unlikely that the US can determine its GHG emissions to one part in 100,000. There's no uncertainty on this number. It's clearly just what some GHG model spit out, and those who ran it have given on reason why we should believe its incredible accuracy.
In any case, to meet this target, 2020 US emissions must be less than 6023.2 mt CO2e. Now
2007 US emissions = 7282.4 mt CO2e
So we must achieve an average annual reduction of 1.5% to meet this target by 2020.
Can we do this in an arena were everyone believes economic growth must increase by 2-3%/yr? I don't see it happening.
And EVEN IF WE DID: This emission rate is far, far above what scientists say is needed -- which is essentially 0 mt/yr.