BEIJING — The United States and China are the world’s two largest economies, two largest consumers of energy, and two largest emitters of greenhouse gases. Together we account for about 40 percent of the world’s emissions. 
 
 We need to solve this problem together because neither one of us can solve it alone. Even if the United States somehow eliminated all of our domestic greenhouse gas emissions, it still wouldn’t be enough to counteract the carbon pollution coming from China and the rest of the world. Likewise, even if China went down to zero emissions, it wouldn’t make enough of a difference if the United States and the rest of the world didn’t change direction. 
 
 That’s the reality of what we’re up against. That’s why it matters that the world’s most consequential relationship has just produced something of great consequence in the fight against climate change. 
 
 Today, President Obama and Chinese President Xi Jinping are jointly announcing targets to reduce carbon emissions in the post-2020 period. By doing this – together, and well before the deadline established by the international community – we are encouraging other countries to put forward their own ambitious emissions reduction targets soon and to overcome traditional divisions so we can conclude a strong global climate agreement in 2015. 
 
 Our announcement can inject momentum into the global climate negotiations, which resume in less than three weeks in Lima, Peru, and culminate next year in Paris. The commitment of both presidents to take ambitious action in our own countries, and work closely to remove obstacles on the road to Paris, sends an important signal that we must get this agreement done, that we can get it done, and that we will get it done. 
 
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 This is also a milestone in the United States-China relationship, the outcome of a concerted effort that began last year in Beijing, when State Councilor Yang Jiechi and I started the United States-China Climate Change Working Group. It was an effort inspired not just by our shared concern about the impact of climate change, but by our belief that the world’s largest economies, energy consumers and carbon emitters have a responsibility to lead. 
 
 The targets themselves are also important. Ambitious action by our countries together is the foundation to build the low-carbon global economy needed to combat climate change. The United States intends to reduce net greenhouse gas emissions by 26 to 28 percent below 2005 levels by 2025 – a target that is both ambitious and feasible. It roughly doubles the pace of carbon reductions in the period from 2020 to 2025 as compared to the period from 2005 to 2020. It puts us on a path to transform our economy, with emissions reductions on the order of 80 percent by 2050. It is grounded in an extensive analysis of the potential to reduce emissions in all sectors of our economy, with significant added benefits for health, clean air, and energy security. 
 
 Our target builds on the ambitious goal President Obama set in 2009 to cut emissions in the range of 17 percent below 2005 levels by 2020. We are on track to meet that goal, while creating jobs and growing the economy, with the help of a burgeoning clean energy sector. Since the president took office, wind energy production has tripled and solar energy has increased by a factor of ten. This summer, the Environmental Protection Agency proposed the first carbon pollution standards for existing power plants, which account for a third of United States carbon pollution. 
 
 The Chinese targets also represent a major advance. For the first time China is announcing a peak year for its carbon emissions – around 2030 – along with a commitment to try to reach the peak earlier. That matters because over the past 15 years, China has accounted for roughly 60 percent of the growth in carbon dioxide emissions world-wide. We are confident that China can and will reach peak emissions before 2030, in light of President Xi’s commitments to restructure the economy, dramatically reduce air pollution and stimulate an energy revolution. 
 
 China is also announcing today that it would expand the share of total energy consumption coming from zero-emission sources (renewable and nuclear energy) to around 20 percent by 2030, sending a powerful signal to investors and energy markets around the world and helping accelerate the global transition to clean-energy economies. To meet its goal, China will need to deploy an additional 800 to 1,000 gigawatts of nuclear, wind, solar and other renewable generation capacity by 2030 – an enormous amount, about the same as all the coal-fired power plants in China today, and nearly as much as the total electricity generation capacity of the United States. 
 
 There is no question that all of us will need to do more to push toward the de-carbonization of the global economy. But in climate diplomacy, as in life, you have to start at the beginning, and this breakthrough marks a fresh beginning. Two countries regarded for 20 years as the leaders of opposing camps in climate negotiations have come together to find common ground, determined to make lasting progress on an unprecedented global challenge. Let’s ensure that this is the first step toward a world that is more prosperous and more secure. ||||| “The stage is set for the possibility of getting this right, but on the other hand there are all these wrinkles and hurdles at the last minute,” Secretary of State John Kerry said in an interview last week. 
 
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 Earlier Attempts 
 
 If you are thinking you have seen this before, you are right. Since 1992, United Nations negotiators have held annual summit meetings aimed at drafting a global climate change treaty. They came close to a deal twice. 
 
 In 1997, world leaders signed the Kyoto Protocol, which assigned the largest economies legally binding targets for cutting their greenhouse gas emissions. There were two big problems: Kyoto assigned targets to the developed world, but made no requirements of developing economies like India and China, which today are two of the largest greenhouse gas polluters. President Bill Clinton never sent the deal to the Senate for ratification because it would not have passed, thus exempting the world’s largest economy from action. 
 
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 In 2009 in Copenhagen, Mr. Obama and other world leaders drafted a new pact to replace the Kyoto deal. It would have committed developing countries like China to action, but failed to achieve the unanimous consent required for legal enforcement, and has served as little more than a voluntary agreement. 
 
 What Is Different This Time? 
 
 The biggest is the breakthrough agreement between the United States and China. Last year the world’s two largest carbon polluters and historically the biggest obstacles to a deal announced plans to jointly enact emissions reduction policies. 
 
 Mr. Obama pledged that by 2025, the United States would cut emissions up to 28 percent from 2005 levels, largely through aggressive environmental rules on greenhouse gases from power plant smokestacks and vehicle tailpipes. President Xi Jinping announced that by 2017, China would begin a national cap-and-trade system placing limits on industrial emissions and requiring companies to pay for government-issued permits to pollute. Mr. Xi also has pledged that China’s emissions will begin to drop no later than 2030. 
 
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 Since then, more than 170 countries, representing more than 90 percent of global carbon emissions, have put forth proposals to reduce emissions. Those plans, expected to form the core of a new agreement, represent a major change from the usual brinkmanship of climate negotiations. 
 
 ‘It Is Not the Endpoint’ 
 
 Another difference between Paris and the Kyoto and Copenhagen deals: Negotiators have managed expectations. At the conferences in Japan and Denmark, world leaders promoted the expected pacts as treaties that would save the planet. No one is saying that about Paris. For one thing, a Paris deal would not take the legal form of a traditional treaty. And it would not save the planet, but only be a strong first step. 
 
 The objective of climate change policy is to curb greenhouse gas emissions enough to avoid a global average temperature increase of 3.6 degrees Fahrenheit. Beyond that threshold, scientists say, there will be devastating effects. Research shows the globe has already warmed about 1.7 degrees since the Industrial Revolution. 
 
 Together the more than 170 national plans for Paris would still allow the planet to warm by as much as 6 degrees, according to several independent and academic analyses. Scientists say that that level of warming is still likely to cause food shortages and widespread extinctions of plant and animal life. 
 
 Still, that would count as progress. Without the Paris climate policies, scientists say, the planet is headed toward a far more destructive temperature increase of more than 8 degrees Fahrenheit. 
 
 “The national plans will not put us on the path to lower temperatures enough, but they will bend the temperature curve,” said Janos Pasztor, the United Nations assistant secretary general for climate change. “While it is significant progress, it is not the endpoint. It is a turning point. It must mark the floor, not the ceiling.” 
 
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 Ratcheting Up Commitments 
 
 That means there is a lot more work to come. A key word in the Paris talks is “ratchet,” meaning that negotiators want to schedule countries to reconvene every five to 10 years with more stringent emissions plans to ratchet up their Paris commitments. 
 
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 It sounds simple, but the concept is controversial. In many countries, including the United States, pollution reduction plans are already causing domestic political upheaval. The idea that world leaders and lawmakers would have to go back to voters with even tougher policies is a hard sell. Many countries say that any “ratchet” meetings should not come until at least 10 years out, but American negotiators are pushing for tougher new targets every five years. 
 
 The Bill for Tougher Policies 
 
 Who would pay is another big obstacle. 
 
 In Copenhagen, Hillary Rodham Clinton, then the secretary of state, pledged that by 2020 the developed world would send developing countries $100 billion annually to help them make the transition to cleaner energy and pay for the effects of climate change, including floods and droughts. 
 
 The idea, American negotiators said, was that the money would come from public appropriations and private investments. A report this year by the Organization for Economic Cooperation and Development found that about $62 billion of so-called climate finance money, from public and private sources, went from developed to developing countries in 2014. But in Paris, many developing countries want to lock down an agreement binding wealthier countries to spend far more of their own money to help them. 
 
 “The cost of action is not $100 billion,” said Prakash Javadekar, the Indian environment minister. “It is trillions; $100 billion is just a reparation.” 
 
 That argument, and the word “reparations,” are likely to be heard until the talks end. 
 
 Agreeing on Verification 
 
 The Paris negotiators have to agree that governments meet their pledges in a way that does not violate nations’ sovereignty. Advocates of a climate pact cheered when Mr. Xi announced China’s plans, but this month newly published government data showed that China was burning 17 percent more coal a year than the government had previously disclosed. That raised questions about the credibility of the Chinese plan and is expected to emerge as a major sticking point in Paris. 
 
 Legally Binding vs. Peer Pressure 
 
 Traditionally, negotiators have sought to forge a legally binding treaty that needed ratification by the governments of the participating countries to have force. 
 
 There is no way to get that in this case, because of the United States. A treaty would be dead on arrival on Capitol Hill without the required two-thirds majority vote in the Republican-controlled Senate. 
 
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 So the voluntary plans are taking the place of mandatory, top-down targets — or what one negotiator called “legal jujitsu” that avoids the legal definition of a treaty. 
 
 Here’s another way to think about it: You are given a school assignment, but there won’t be any punishment if you don’t turn it in. However, you are required to attend an assembly with your peers and to show them your homework. The hope is that the “student leaders,” the largest economies, will createa dynamic of international peer pressure. 
 
 What Could Go Wrong 
 
 The United Nations is a unique institution. It passes new pacts only by unanimous consent. So in a system designed to ensure that even the smallest, poorest nations have equal voices, a single country can upend a deal. That is what happened when fewer than a half dozen countries opposed the Copenhagen accord. 
 
 After Paris, a single wild-card nation — eyes are on Malaysia, Saudi Arabia, Sudan or Venezuela — could bring the process down.
– The US and China rank as the world's two largest polluters, but a surprise deal is turning them into the world's two largest pollution fighters: The nations announced that they'll be working in tandem on long-term greenhouse-gas controls that would cut emissions by nearly a third over the next 15 years or so, CNN reports. "As the world's two largest economies, energy consumers, and emitters of greenhouse gases, we have a special responsibility to lead the global effort against climate change," President Obama said in a joint press conference with Chinese President Xi Jinping today at the end of the APEC summit. The move, which was months in the making, came as a surprise to experts expecting a much more limited scope, the Wall Street Journal reports. Calling it a "game changer," Mother Jones notes that it could heavily influence next year's climate talks in Paris. Key mandates include the US reducing emissions by 26% to 28% by the year 2025 (compared to the year 2005), as well as doubling the rate of emissions reduction each year between 2020 and 2025 to an average 2.3% to 2.8%, the AP reports. China is pledging to stop increasing emissions by 2030 (even earlier, if possible), as well as increase energy from zero-emission sources such as wind, solar, and nuclear to 20% by 2030. Although some worry about recent GOP gains in government as a possible roadblock, a senior administration official tells CNN that "we believe we can proceed with the authority we already have." Secretary of State John Kerry writes in a New York Times op-ed that "this is … a milestone in the United States-China relationship," a "fresh beginning," and "the first step toward a world that is more prosperous and more secure."