So the president is now tackling emissions via the Environmental Protection Agency (EPA). They have proposed a rule whereby there will be a 30% cut from 2005 levels by 2030 (See the proposal here).
These include tough limits on coal-fired plants, so the ultimate outcome may be a huge switch to natural gas fueled stations which produce around half the carbon of coal stations for the same amount of energy.
First, though, there will be a year long consultation period, and then the various States have until June 30, 2016 to come up with their plans to implement the rule - A few months before Obama leaves office.
Individual States can achieve the reduction any way they choose, whether it's by more renewables, shutting down coal stations, cap-and-trade and so on. This flexibility will give the rule a much better chance of working, especially if there are legal challenges (Which, of course, there will be).
Interestingly, by using 2005 as the baseline year, the country's emissions are already down 15%, thanks to things like the shale gas revolution. There is so much more they can do to achieve the remainder.
Aside from cutting carbon emissions, the proposed rule will also have considerable health benefits as it will also reduce the air pollution produced by the coal-fired stations - That could save around $50 billion every year.
The new rule will have much wider implications though. It makes a global climate agreement far more likely come late 2015 in Paris, and it will up the ambition of countries like China and India who have been dragging their feet, citing the american lack of effort as a reason. Already various countries and blocs have welcomed the news. Suddenly the U.S. are being hailed as heroes, with Obama at their head.
Of course, there's a long way to go before the rule is in place. The fossil fuel industry will fight it all the way, and the Republicans will no doubt try to axe it or the EPA the first chance they get, out of sheer spite, but Obama has a very real chance here of leaving a lasting legacy: Future generations (our kids amongst them) may look back on this move as the game-changer that gave them hope.