President Barack Obama has stumbled on an unusual partner in his quest to combat climate change - China.
The world's two biggest emitters of heat-trapping greenhouse gases are finding common cause in efforts to reduce global warming, cooperation the US says could clear the way for other developing, heavily polluting nations like India and Brazil to get on board, too.
Sceptics question whether either nation will follow through on lofty aspirations. Still, the budding agreements are allowing the two rivals to present a positive front at a time when tensions are running high over espionage, alleged cybertheft and American fugitive Edward Snowden.
Last week, top American and Chinese officials announced new joint initiatives, including cutting emissions from heavy-duty vehicles and upping energy efficiency of buildings, transport and industry.
They also agreed to team up on large-scale experiments with ''carbon capture'' - technology to isolate carbon dioxide from power plant emissions so it can be safely stored. Lack of commercially viable technology has been a major barrier to making plants cleaner in the US and abroad.
A month earlier, Obama and Chinese President Xi Jinping met in the California desert for a summit aimed at forging closer ties.
The sole concrete achievement was a deal to phase out hydrofluorocarbons, a potent greenhouse gas used in refrigerators and air conditioners.
''This is a priority for the president and for me,'' Vice President Joe Biden said during a speech on Asian relations, specifically mentioning the accord with China.
''The impact of climate change also has an impact as growth as well as security.''
The world's most populous country, China, has long been perceived globally as an unabashed polluter, but has started to change its tone.
In 2007 China's notoriously pragmatic and economy-focused government called, in a national strategic document, for an ''ecological civilisation,'' reflecting a move toward balancing environmental protection with development.
China's environmental imperatives are clear: Suffocating smog in Beijing, rising sea levels and polluted water and soil that can stifle development.
Already vying with the US as the world's largest manufacturer, China looks at policies that constrain industry growth differently than other, largely agricultural developing nations in Africa or Asia.
Beijing may also see renewable and clean energy as a growing global fad and want to ensure they're not left out.
In 2010, China's government spent more than US$30 billion (NZ$36b) subsidising its solar panel industry, US energy officials said. And the US shale natural gas boom is attracting major Chinese investment, too.
The Chinese Embassy in Washington didn't respond to a request for comment.
For Obama, jumpstarting the global climate change effort is a key to his second-term agenda and his legacy.
Reducing US greenhouse gases will only go so far. Mounting emissions planetwide could blunt the impact of what Obama does at home.
That's where China could play a critical role.
''China is a huge weight in the global system,'' said Jonathan Pershing, the Energy Department's top climate official and a former US climate negotiator, in an interview.
''It has a developing country framework, so other developing countries say, 'Certainly if China can do it, we can do it, too.'''
Although China is unlikely to actively nudge other nations, the overriding US concern is that China not spoil global agreements that would otherwise proceed, Pershing said. That's because climate accords tend to be governed by consensus.
Such a phenomenon was on display in 2009 when China was accused of wrecking a stronger agreement in Copenhagen over its resistance to specific emissions limits.
The upbeat tone on climate comes as a series of disputes have complicated Obama's attempts to improve relations with China and strengthen US influence in Asia.
Washington has complained loudly to Beijing about cyberhacking and intellectual property. The White House also says bilateral relations were dealt a serious blow by China's refusal to extradite Snowden, the National Security Agency leaker, before he fled Hong Kong.
At the same time, Snowden's revelations about US surveillance emboldened Chinese officials to argue Obama's complaints about cybersecurity are hypocritical.
And China's growing investment and influence in sub-Saharan Africa, where it's surpassed the US as the largest trading partner, was on display last month as Obama traveled the continent.
''There are pressures on the US and China to do something about global warming, and it happens to fit in with the idea of expanding cooperation to try to contain and hopefully reverse the growing strategic rivalry,'' said a former US Ambassador to China, J. Stapleton Roy.
The White House hopes that political pressure from China's expanding middle class may spur China to further action.
It remains to be seen whether the cooperation on climate will extend much beyond diplomatic niceties. After all, the US and China have at times made similar pronouncements, said Elizabeth Economy, a senior fellow at the Council on Foreign Relations.
''There are many, many good ideas that simply don't come to fruition,'' Economy said.
''There are reasons why it hasn't worked very well in the past. As far as I can see, those same reasons exist today.''