The United States and China unveiled what is being touted as ambitious targets Wednesday to reduce greenhouse gases, aiming to inject fresh momentum into the global fight against climate change ahead of a make-or-break treaty to be finalized next year.

President Barack Obama announced that the U.S. would move much faster in cutting pollution, with a goal to reduce it 26 percent to 28 percent by 2025 from 2005 levels. Earlier in his presidency, Obama set a goal to cut emissions by 17 percent by 2020.

Chinese President Xi Jinping, whose country's emissions are still growing as it builds new coal plants, didn't commit to cut emissions by a specific amount. Rather, he set a target for China's emissions to peak by 2030 or earlier if possible. He also pledged to increase the share of energy that China will derive from sources other than fossil fuels, such as solar and wind.

Both leaders were attending the Asia-Pacific Economic Cooperation (APEC) summit, hosted this year by Beijing. Their two countries have long been at loggerheads over global targets, with each saying the other should bear more responsibility for cutting emissions of gases blamed for heating up the atmosphere.

The breakthrough by the world's two largest polluters reflected their desire to display a united front on climate change, blunting arguments from developing countries that have balked at demands that they get serious about cutting emissions. The inequality of climate change is that rich nations developed using polluting resources while poor nations are often hard hit by environmental changes such as rising sea levels. Environmental advocates have long held that without agreement between the top carbon polluters, few other countries would cut emissions.

Scientists argue that drastic measures must be taken if the world is to limit global warming to the UN's target of 3.6 degrees Fahrenheit over preindustrial levels and that failing to do so could have disastrous results.

In a statement, U.N. Secretary General Ban Ki-moon commended China and the U.S. for demonstrating the leadership "that the world expects of them" as the world’s two largest economies. The agreement, he said, "will give the international community an unprecedented chance to succeed at reaching a meaningful, universal agreement in 2015."

Yet it was unclear how feasible it would be for either country to meet their goals, and Obama's pledge was sure to confront tough opposition from ascendant Republicans in Congress.

In an op-ed in The New York Times, Secretary of State John Kerry said the "milestone" 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."

David Sandalow, formerly a top environmental official at the White House and the Energy Department, called it "the most important bilateral climate announcement ever."

Obama's target, expected to serve as the U.S. contribution in a worldwide treaty to be finalized next year in Paris, came months before it had been expected. The U.S. has sought to show aggressive action on climate change in order to spur other nations to offer ambitious contributions too.

In June the Obama administration announced plans to cut harmful emissions at existing power plants by 30 percent from 2005 levels, a significant move against climate change. The new rule on carbon dioxide emissions, which is set to be finalized next year after an extended comment period, is intended to form the centerpiece of Obama's plans to reduce pollution linked to global warming.

For China, the commitment to cap emissions marked a turning point in China's evolution on global warming and its responsibility to deal with the problem. China accounts for about 30 percent of global emissions but has gotten serious only in recent years as the large-scale impact on health and quality of life in China has come into focus, exacerbated by smothering smog in Beijing's skies.

Earlier this year Chinese Premier Li Keqiang said he was "declaring war" on pollution as he addressed widespread and very public complaints about pollution, a topic that has drawn much negative international attention when air pollution has forced schools to close and grounded flights. He referred to the fouling of the country's air, water and soil as "nature's red-light warning against the model of inefficient and blind development."

The U.S., which never ratified the Kyoto Protocol on climate change, pledged to cut emissions before, with mixed results.

Blair Palese, CEO of 350.org, a movement that promotes awareness about climate change, welcomed the announcement, telling Al Jazeera that announcing such goals "sends a message, especially right before the G-20 summit, that this will not just be a change for and in the environment, but also in the business sector. It means that energy and fuel will now be looked at differently."

But she said it is "unrealistic to expect an end to the fossil fuel addiction."

Environmental advocates in Congress heralded the joint announcement as a game changer that would undermine opposition. "Now there is no longer an excuse for Congress to block action," said Sen. Barbara Boxer, D-Calif., who chairs the Senate's environmental panel.

But Republicans signaled that they would seek to thwart Obama's efforts when the GOP controls the Senate next year, pointing out that Obama was saddling future presidents with a tough-to-meet goal.

"This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs," said likely next Senate Majority Leader Mitch McConnell, R-Ky.