Well, we had to read that headline twice. Awesome how much a day can change things: from denying climate change, the US government may be doing an about-face and now leading efforts to diminish emissions.
Senator John Kerry, speaking in Poznan, Poland, at the current climate conference, said the USA will take the lead, if (big if) other countries also pledge to curb emissions. His statement is available on the BBC website.
Meanwhile, Mexico has already made a bold statement, announcing it intends to cut emissions in half by 2050, should the president approve the plan in February. Note that the stated goals are "aspirational;" they are not binding commitments.
The Poznan meeting "marks the halfway point in a two-year process, agreed at the UN meeting last December in Bali, that aims to establish a new global deal on climate change by next year’s gathering in Copenhagen."