(Update: Today, China announced that it, too, will pledge limits on greenhouse gas emissions.)

It’s official — President Obama will lead the U.S. delegation at the Copenhagen climate change conference, and he will promise the world a 17 percent decrease (from 2005 levels)  in U.S. greenhouse gas emissions by 2020. His pledge mirrors the levels contained in climate change legislation being considered by Congress.

Although this will not improve chances of an actual treaty being reached at Copenhagen, the expectation is for a political agreement among the major nations. Such an agreement depends on what China brings to the conference, and that remains a mystery. China and the U.S. are the planet’s two biggest contributors to global warming.