In a joint announcement, Presidents Barack Obama and Xi Jinping laid out plans that will form their basic positions as the world heads into an intense year of negotiations that many hope will lead to a binding new global deal on cutting emissions at a summit in Paris a year from now.

The U.S. intends to cut emissions by up to 28% from 2005 levels by 2025, while China said its emissions would peak by 2030, adding that it would try for an earlier date. China, which has relied overwhelmingly on coal to fuel its development for the last three decades, also said it would try to make sure at least 20% of its energy supplies come from non-fossil fuels by 2030.

Although the commitments don’t go meaningfully beyond what the two countries have already outlined in their domestic political debates, the sight of the world’s two largest polluters in apparent harmony in recognizing the need for action was hailed by outside observers.

“This is a very welcome joint statement and a very timely one,” said Fatih Birol, chief economist at the International Energy Agency, at a press conference in London.

John Sauven,Greenpeace’s executive director in the U.K., called it “a major political breakthrough that many thought impossible. The world’s biggest economies–China, the U.S. and the E.U.–are now firmly set on a path towards a low-carbon economy and there’s no looking back.”

The E.U. last month agreed in principle to cut its carbon emissions by 40% from a 1990 baseline by 2030.

All the same, Sauven reckoned that “the targets announced are not yet as ambitious as scientists say they should be if we are to stop the worst ravages of climate change.”

The I.E.A. still expects carbon dioxide emissions to grow uninterruptedly through 2040, however, as India and other parts of the emerging world offset reductions in the U.S., China and Europe.

[fortune-brightcove videoid=3666787681001]
The Obama-Xi announcement followed the first formal talks between the two in over a year, and capped a visit that has delivered far more than had been expected. On Tuesday, the two had already agreed to a reduction in trade tariffs on a wide range of high-tech goods, which is expected to pave the way for a global deal at the World Trade Organization by the end of the year.

By contrast, there has been little public sign of tension over issues such as cyber-espionage and regional security, which have dominated U.S.-Chinese relations in the last 12 months.

“If the United States is going to continue to lead the world in addressing global challenges, then we have to have the second largest economy and the most populous nation on earth as our partner,” Reuters reported Obama as saying.

Xi meanwhile said “the Pacific Ocean is broad enough to accommodate the development of both China and the United States and our two countries should work together to contribute to security in Asia.”