WASHINGTON (AP) — The world’s hopes for curbing climate change hinge on action by two giant nations whose relations are deteriorating: China and the United States. The two countries both say they are intent on retooling their economies to burn less climate-wrecking coal, oil and gas. But tensions between them threaten their ultimate success.

China and the United States are the world's No. 1 and No. 2 carbon polluters, respectively, pumping out nearly half of the fossil fuel fumes that are warming the planet's atmosphere.

The fast cuts in carbon needed to stave off the worst of climate change are all but impossible unless these countries work together and basically trust each other's pledges. During the Trump administration, the U.S. used China's emissions as an excuse not to act, and in the past China pointed to U.S. historical emissions as a reason to resist action.

New details of how quickly China plans to reduce carbon emissions will be revealed Friday when Beijing releases its next Five Year Plan. And in April, President Joe Biden is expected to announce the United States' own new targets for emissions cuts.

The U.S. and China both have appointed veteran envoys as their global climate negotiators, John Kerry and Xie Zhenhua. But while the two senior statesmen worked well together in laying groundwork for the 2015 Paris climate accord, now they face new challenges.

U.S.-China climate diplomacy threatens to be overshadowed by what the United States sees as Beijing's menacing policies toward Hong Kong, Taiwan and the South China Sea, conflict over human rights and trade, and U.S. claims of Chinese espionage.

Meanwhile, Chinese officials are upset about restrictions imposed by the Trump administration on trade, technology, Chinese media and students in the U.S., and the State Department's declaration this year that atrocities against China's Muslim minorities are a “genocide.”

Kerry, a secretary of state under President Barack Obama who was brought back to be Biden's climate envoy, recently told reporters: “Those issues" with China "will never be traded for anything that has to do with climate. That’s not going to happen.” But Kerry also called the climate “a standalone issue” with China, drawing criticism from China and from some human-rights advocates in the U.S.

Can climate talks between the two countries survive their other geopolitical battles?

“That's, I think, the huge question,” said John Podesta, who oversaw the Obama administration's climate efforts and is close to the Biden administration.

“Can you create a lane where you get cooperation on climate" while more contentious issues are dealt with separately? Podesta asked. “Or do they wind up interfering?"

Xie Zhenhua may help the odds. With his appointment as climate envoy last month, Xie is reprising the role he held during pivotal U.N. climate conferences that struck the world's first major commitments on reducing emissions from fossil fuels.

Prior to his appointment, Xie led a research effort at Tsinghua University in Beijing to map ways for China to stop contributing to global warming by midcentury. His research underpinned President Xi Jinping’s surprise pledge in September that China planned to go carbon neutral by 2060 — the first time the country announced a net-zero target.

Joanna Lewis, an expert in China energy and environment at Georgetown University, called Xie "a visionary, and very influential in setting China’s domestic policy targets,” as well as a skilled negotiator.

Xie's appointment "was a huge overture toward the United States, and particularly to John Kerry,” said Angel Hsu, an expert on China and climate change at the University of North Carolina, Chapel Hill.

Biden has pledged the U.S. will switch to an emissions-free power sector within 14 years, and have an entirely emissions-free economy by 2050. Kerry is also pushing other nations to commit to carbon neutrality by then.

Behind the dry numbers, massive spending on infrastructure and technology is needed to switch to a more energy-efficient economy, running on wind, solar and other cleaner-burning fuels. And Biden has a narrow majority in Congress to push his agenda, with Republicans, as well as some Democrats, opposing his plans.

Climate scientists say countries need to move fast to avert catastrophic temperature rises.

In 2019, coal accounted for 58% of China’s total primary energy consumption, according to the U.S. Energy Information Administration.

Last year, as China's government directed economic relief money to infrastructure projects during the pandemic, the country actually upped its net power capacity from coal — by about the equivalent of 15 Hoover Dams, or 30 gigawatts — according to the Global Energy Monitor and the Centre for Research on Energy and Clean Air. China also funds building of coal-fired power plants abroad, partly to build influence.

Many experts question whether the construction of coal-fired plants is driven by demand, or simply meant to stimulate the economy during a downturn. Either way, the brand-new coal plants have consequences.

“Every new coal plant that China builds is basically locking in carbon emissions for the next 50 years,” said Georgetown's Lewis.

The most important questions now, said Deborah Seligsohn, an expert in Chinese governance and air pollution at Villanova University, are: “How soon can China’s carbon emissions peak, and at what level?"

She is watching closely to see what targets are incorporated in the next Five Year Plan, and into China’s updated pledges for emission cuts under the Paris climate accord.

The key, climate negotiators say, will be making it worth China's while — financially and in terms of its international standing — to slow down its building and funding of new coal plants and speed up spending on clean energy.

Biden has reached out to European allies as a first step, trying to build consensus among China's trade partners about market and trade-based rewards and disincentives as a way of prodding China to reduce reliance on coal.

“None of these countries are wanting to save the planet and be completely selfless about this," Christiana Figueres, who helped broker the landmark climate deal in 2015, told The Associated Press. "Only if it also serves their interest.”

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Knickmeyer reported from Oklahoma City. AP Science Writer Seth Borenstein in Washington contributed to this report.