Bidding to rally rich and surging nations alike, President Obama wants the world's top polluters to keep driving toward a deal to halt global warming.
Nearing six months on the job, Obama has some momentum: a new agreement among developed and emerging nations to cap rising global temperatures, plus good will from his peers for repositioning the U.S. as an aggressive player in the debate.
Yet when Obama helps lead a gathering of the world's major economies here Thursday, he will run smack into the same old problem: Neither the wealthy nor the developing countries think the other side is doing enough. And only when the pollution emitters work together on a binding plan will a climate strategy work, experts say.
Even victory came with a setback on Wednesday. The G-8 nations set a goal of cutting all greenhouse gas emissions in half by 2050, but developing nations refused to go along.
Confronting global warming — a trend scientists say could unleash devastating droughts, floods and disease if left unchecked — is a dominant theme again at this year's G-8 summit of Britain, Canada, France, Germany, Italy, Japan, Russia and the United States.
Obama will take part in discussions all day on climate and a host of economic issues, and the number of countries represented at the table will just keep growing.
First, the traditional industrialized powers will expand their forum to other strategic economies: Brazil, China, India, Mexico and South Africa, plus a special invitee, Egypt.
And Obama later will help lead a forum of major economies that also includes Australia, Indonesia and South Korea. Together, including the U.S., the represented countries account for about 80 percent of the emissions of the heat-trapping gases blamed for global warming.
The results this week will be a pivotal marker of what could happen in talks in December in Copenhagen, when the United Nations tries to conclude a new worldwide climate deal.
"This will also be an opportunity for the president and the other leaders to discuss what they can do collectively to add political momentum to the negotiations," Mike Froman, a national security aide leading the administration's G-8 efforts, said ahead of Thursday's events.
The two blocs — the richest countries and the fastest growing ones — did strike an important agreement Wednesday. Their unified position now is that global temperature should be kept from rising by more than 3.6 degrees (2 degrees Celsius).
That's the point at which the Earth's climate system would fall into perilous instability, according to the United Nations' chief panel on climate change.
The U.S. and the other G-8 nations set a new goal of reducing their greenhouse gas emissions by 80 percent or more by 2050, part of their global goal of a 50 percent cut.
More steps by developed and developing countries will be announced Thursday, Froman said.
But the emerging countries are refusing to commit to specific reduction targets.
They are upset that the industrialized G-8 has not been forthcoming on either midterm emissions reductions — well before 2050 — or pledges of financing and transferring technology to the developing world. And they worry that major reductions could hamper their economies.
"Support from the G-8 is only the first step in what is likely to be a long and difficult process," said Guy Caruso, a senior adviser for the energy and national security program at the Center for Strategic and International Studies, a think tank in Washington.
"The Major Economies Forum recognizes this reality," he said. "The bottom line is that the industrialized countries will need to provide the incentives to the emerging economies."
Obama began his agenda Thursday by meeting with Brazil's president, Luiz Inacio Lula da Silva, who has called on rich nations to bear more of the cost of fighting global warming. Silva greeted the president by presenting him with a gold Brazilian soccer jersey with the number 5 on it.
The Silva meeting is a late add. It comes during the slot when Obama was to have met with Chinese President Hu Jintao, who returned home to deal with an outbreak of ethnic violence.
Hu's departure is seen by analysts as weakening the chances that the U.S. and other G-8 countries can advance climate talks at this summit with China and a few of its close peers.