The US and China this week announced a landmark agreement to cut greenhouse gas emissions sharply by 2030, as part of a deal that President Barack Obama described as a "major milestone." As the New York Times reports, the deal was brokered between Obama and his Chinese counterpart, President Xi Jinping, over the course of nine months, before being announced Wednesday at the Asia-Pacific Economic Cooperation (APEC) summit in Beijing.

China and the US are the world's top two carbon emitters, accounting for more than a third of global greenhouse gas emissions, and their cooperation is seen as critical to spurring broader action across the globe ahead of a climate change summit in Paris next year. In an op-ed in today's New York Times, Secretary of State John Kerry described the deal as "historic."

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

The agreement calls for the US to reduce its greenhouse gas emissions by 26 to 28 percent below 2005 levels by 2025, and for China to reach peak carbon emission levels by 2030. Beijing also agreed to increase its share of non-fossil fuel energy use to 20 percent of primary energy consumption by 2030, compared to less than 10 percent in 2013. The country had previously set a target of 15 percent by 2020.

China is the world's largest largest carbon emitter, after overtaking the US in 2007, but has taken aggressive steps to curb emissions and increase investment in renewable energy amid growing public outrage over pollution levels. Today's announcement marks the first time that Beijing has agreed to peak its emissions, with the White House saying it expects the country to "succeed in peaking its emissions before 2030 based on its broad economic reform program, plans to address air pollution, and implementation of President Xi's call for an energy revolution."

The US will have to double the pace of its carbon reduction in order to reach its target, which the White House hopes to achieve through regulations on cars and power plants previously implemented by the Obama administration. But further emission cuts will likely face opposition from Republicans, who made strong gains in this month's mid-term elections, raising some doubts over the feasibility of today's pact.

"Our economy can’t take the president’s ideological war on coal that will increase the squeeze on middle-class families and struggling miners," Sen. Mitch McConnell (R - KY), presumed Senate majority leader, said in a statement following Wednesday's announcement. "This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs."

Massive fossil fuel subsidy programs may also hinder the development of cleaner energies across the globe. In its annual World Energy Outlook, released today, the International Energy Agency (IEA) reported that subsidies to fossil fuel industries reached $550 billion worldwide in 2013, more than four times those for renewable energy. The Paris-based IEA projects renewable power generation to increase dramatically by 2040 — particularly in emerging economies — though it stressed that continued fossil fuel subsidies have slowed investment in cleaner energy.

Some environmental experts have raised concerns over the pact announced Wednesday, saying it doesn't go far enough to reduce emissions, but others praised it as an important step ahead of next year's climate summit in Paris, where world leaders will look to broker a global agreement.

"This is the most important climate relationship in the world," Timothy Wirth, vice chairman of the United Nations Foundation, said in a statement. "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."