The US economy has only 7% of the world population, but it contributes to over 25% of the global gas emissions that is affecting the entire world. The current economic turmoil has already cost the US badly and it is running on 1.3 trillion dollar deficit this year which is nearly 8 times the deficit it suffered two years back.
US is therefore most unlikely to commit anything substantial on the emission reduction, as it will add to the already unmanageable deficit. That only means that other countries also might be able to avoid making any commitment on reduction of gas emission from their end. So do not expect anything tangible from the UN climate change summit happening this week.