Honestly, I doubt that it will have *any* impact on global co2 emissions. Whether we are in or out. I predict that global co2 emissions will, for the next 5 years, approximately continue this trend:
http://www.energytrendsinsider.com/…/2012/07/Global-CO2.png… Whether the US rejoins, or not, whether the accords fall apart, or not, Whether the world "bands together" to get revenge on the US for withdrawing, or not.
Here's why. China and other Asian nations *according to the accord* are building coal plants literally as fast as possible. According to the accords, they will continue doing so for another (approximately) 15 years. That fact so completely dwarfs any gains that could realistically be made anywhere else as to make them irrelevant.
As for those "gains", they will STILL HAPPEN. Solar photovoltaic is below grid parity in many regions. Those regions will still, "accord" or no, purchase and install all the PV that can be produced. Wind power is competitive with coal, and cheaper than most other fueled energy sources. It will still continue being installed as quickly as turbines can be manufactured. Electrical and hybrid vehicles are now technologically viable, and quite a lot cheaper per mile to drive than traditional internal combustion vehicles. They will continue to be purchased as quickly as they can be manufactured.
What *will* change is what economists refer to as "malinvestment". Government programming has caused market distortions that have caused huge waste of precious resources. It makes *no sense* to install PV in upstate ny, a temperate region with a long winter at a northern lattitude. It makes FAR more to install them in Arizona or California, hot climates in more southern lattitudes. So, WHY do you see all those solar roofs wasting a third of their productive capacity in NY instead of Arizona? Because NY allows the tax credit for leased systems. Seriously. That's why.
In fact, it's likely that the withdrawal of the US from the accords will result in *less* co2 emissions due to the marketplace making better use of those resources, reducing malinvestment.