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The U.S. and China, which together account for more than a third of all of the world's greenhouse gas emissions, have negotiated a sweeping agreement to cut emissions drastically by 2030, a deal that President Barack Obama called a "major milestone" Wednesday at the Asia-Pacific Economic Cooperation summit in Beijing.

The White House said the U.S. would seek by 2025 to reduce greenhouse gas emissions by 26 percent to 28 percent below a baseline level from 2005. At the same time, China said it intended to begin reversing the rise of its carbon emissions by 2030 and to increase the share of nuclear, wind, solar and other zero-emission power to 20 percent of all of its energy consumption by that year.

At a joint news conference with Chinese President Xi Jinping, Obama said he hoped the deal — the first time China has ever agreed to "peak" its carbon emissions — would jump-start negotiations with an eye toward reaching a worldwide climate agreement in Paris next year.

"We have a special responsibility to lead the world effort to combat global climate change," Obama said. "We hope to encourage all major economies to be ambitious."

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U.N. Secretary-General Ban Ki-moon congratulated Obama and Xi for "an important contribution to the new climate agreement to be reached in Paris next year."

"The joint announcement signals that the transition towards a low-carbon, climate-resilient future is accelerating," Ban said.

To an extent, the two world superpowers are playing catchup. The European Union earlier this year pledged to cut its emissions by at least 40 percent below 1990 levels by 2030.

On the U.S. side, the agreement would double the pace of carbon reduction from 1.2 percent a year through 2020 to 2.3 percent to 2.8 percent a year afterward. The White House said that ambitious target could be met under existing laws and that it would generate as much as $93 billion in "net benefits" from improved public health and reduced pollution.

As for China, Beijing's targets represent "a serious commitment to finally shift the Chinese economy away from coal," said Neera Tanden, president of the Center for American Progress, a nonprofit public policy institute.

"The United States and China have just turned the international climate change negotiation on its head," Tandeen said.

"This is the most important climate relationship in the world," said Timothy Wirth, vice chairman of the United Nations Foundation. "If the two biggest players on climate are able to get together, from two very different perspectives, the rest of the world can see that it's possible to make real progress."

The deal is the second agreement that the U.S. has cast as a breakthrough in relations between Washington and Beijing during the APEC meeting. U.S. Trade Representative Michael Froman said Tuesday that for the first time since 1996, the countries had agreed to cut trade tariffs on information technology products.