(CNN) -- The United States has its first black president, but it may have to wait to get a green one. Right now, Barack Obama has his hands full.
"All of us will face doubts and difficulties in our own capitals," he told a United Nations summit this week, "as we try to reach a lasting solution to the climate challenge."
The "climate challenge" could offer Obama a chance to make his biggest impact on the planet and posterity, if he can find a way to cut U.S. greenhouse gas emissions.
As a presidential candidate, he promised a remarkable reduction of 80 percent by 2050. Democratic lawmakers in Congress even have legislation starting towards that goal. What matters more -- the economy or the environment? Tell us.
But this week's U.N. speech gave Obama his first opportunity as president to tell the world what he can actually achieve.
On the same day that China promised to reforest an area of land the size of Norway and Japan promised to cut emissions dramatically in little more than a decade, the U.S. president did his best to portray his country as newly committed to the cause.
But he stayed away from specific targets. Why?
"The politics in the United States are difficult," said former vice president Al Gore, who won the Nobel Peace Prize as an environmental crusader.
Obama faces challenges familiar world-wide; many voters insist there is an economic cost to environmentalism that they aren't willing to pay.
But Obama has an additional hurdle of his own: his enormous overhaul of the U.S. healthcare system is stuck in Congress and he is struggling to get it approved.
It's such a tough sell that it's eclipsed and maybe even endangered the rest of his agenda. The Democrats' emissions-cutting legislation is unlikely to move until healthcare is out of the way.
But the rest of the world isn't waiting.
The landmark 1997 Kyoto Protocol on climate change is set to expire. Negotiations on a successor treaty begin in Copenhagen in December.
The Obama administration will be there but, without congressional support or an approved emissions bill, it will have trouble making any real commitments.
Obama and the U.S. may get to global warming eventually, but it's not going to be right away.