For several years, China's role in international politics has been that of an overgrown teenager: large and powerful, but clumsy and developing. And no issue exemplified this role more than in the environment. No country in the world emits more carbon dioxide than China, which surpassed the United States in 2007. But China has long claimed that because it bore less responsibility for overall carbon emissions than did the West, it was unfair to ask China to accept any reductions in emissions.

China, apparently, has had a change of heart. In one of the most important deals since the two countries re-established diplomatic relations in 1979, China and the United States struck a historic, important deal on climate change at the Asia Pacific Economic Cooperation (APEC) summit in Beijing on Wednesday. The United States will reduce carbon emissions to 26 to 28 percent below 2005 levels by 2030, while in that same span, China's energy use will peak as non-fossil fuels comprise 20 percent of the country's energy profile.

For the United States, this represents double the commitment that Washington has previously proclaimed. But for China, the commitment is very attainable. A 2011 paper by climate scientists at Lawrence Berkeley National Laboratory projected carbon emissions to peak in China that year anyway. Investments in wind and solar energy, too, mean China is already projected to have 15 percent of its energy provided by non-fossil fuels.

But even if politics down the line keeps the two countries from meeting these commitments, the fact that Beijing and Washington came together to reach a deal at all is significant. In 2009, meetings at the United Nations Climate Change Conference in Copenhagen ended in failure when China refused to limit its carbon emissions in any meaningful way. Mark Lynas, a Guardian correspondent present at the meetings, characterized Chinese intransigence as rising out of fear.

Why did China, in the words of a UK-based analyst who also spent hours in heads of state meetings, "not only reject targets for itself, but also refuse to allow any other country to take on binding targets?" The analyst, who has attended climate conferences for more than 15 years, concludes that China wants to weaken the climate regulation regime now "in order to avoid the risk that it might be called on to be more ambitious in a few years' time."

So why did China change its mind now?

With each passing year, climate change continues to grow in importance. In the five years since Copenhagen, China has faced increased public concern over environmental issues. Last year, major protests erupted in Kunming, a provincial city in southwest China, over the proposed construction of a nearby paraxylene plant, which residents deemed potentially harmful. China also contends with the drying up of the Yellow River, one of the country's most important waterways, as well as rapid desertification in its north and west. Much of China's population and economic power live coastal cities threatened by rising sea levels. Crippling smog in its biggest cities has becoming a running joke.

But China's diplomatic offensive at the APEC summit in Beijing was about much more than just climate change. China and the U.S. also signed agreements on visa and trade policy and are nearing agreement on two military accords. Xi Jinping also met (albeit reluctantly) with Shinzo Abe, Japan's prime minister and a regional adversary. And in the weeks before the summit, China pledged to better manage maritime disputes with Vietnam, another historic rival.

This flurry of diplomatic activity, says Elizabeth Economy, a China expert at the Council on Foreign Relations, reflects the country's desire to burnish its international image.

"There was a strong desire not to have the APEC summit marred by conflict and disagreement," she said.

China's new spirit of cooperation does not mean the country will retreat from its core interests. Despite Xi Jinping's meeting with Shinzo Abe, China has not withdrawn its sovereignty claim over the Senkaku Islands, an archipelago administered by Japan. Beijing has also refused to relax its tight restrictions on foreign news reporters, who face an increasingly hostile environment in the country. And, just hours after the triumphant announcement of China's deal with the U.S., it was revealed that Chinese hackers had penetrated American weather and satellite systems.

In contrast to his predecessor, the dour Hu Jintao, China is now represented by a leader unafraid to assume his country's prominent role in major international issues.

"Xi Jinping has ambitions to be a global leader, and a global player, in ways that his predecessor didn't," said Economy.

In the weeks leading up to his meeting with President Obama, Xi Jinping called repeatedly for a "great power relationship." With China's actions in Beijing, he's got it.