BRUSSELS/WASHINGON—If he wins next Tuesday’s U.S. presidential election, Joe Biden has pledged to give the country a leading role in global efforts to curb climate change. Experts are wondering how ambitious his plans would be on slashing emissions this decade.

President Donald Trump, who has rejected mainstream science on climate change, does not have a plan to address global warming. The Republican president rejected the 2015 Paris agreement early in his first term, and the United States is set to formally exit the deal to rein in global emissions on Nov. 4, the day after the election.

His Democratic challenger, former Vice President Biden, has pledged that if he wins and takes office in January he would immediately rejoin the international accord, and commit the country to reach net zero emissions by 2050.

A push by Biden to re-assert U.S. leadership on the issue would likely also include a pledge to slash emissions by 2030.

That is because under the Paris agreement progress towards curbing climate change is measured by countries’ pledges to rein in emissions this decade. All countries have been asked to update those pledges, but so far, few have.

“Credibility begins at home,” said John Podesta, a counselor on climate and energy to former President Barack Obama, who is now an informal adviser to Biden.

Podesta’s think-tank, the Center for American Progress, has recommended to the Biden campaign that the United States commits to cut greenhouse gas emissions 43% by 2030, from 2005 levels.

“I think the campaign will take a hard look at that,” he told Reuters.

Researchers at The Rhodium Group agreed a 43% goal would put the country on track for net-zero emissions by 2050—a pathway which, if followed globally, would avoid the worst impacts of climate change.

Executive Action?

However, as the world’s biggest historical emitter of greenhouse gases and the biggest economy, “the U.S. should clearly do more than merely the global average,” said Joeri Rogelj, a climate scientist at Imperial College London.

A fair U.S. contribution would require more than a 70% cut in emissions this decade, from 2005 levels, he said.

The Biden campaign has not yet set an economy-wide emissions target for 2030, but his climate plan features a goal to eliminate carbon emissions from the power sector by 2035.

With lethal heatwaves, wildfires and floods intensifying around the world, scientists say the world’s ability to avert catastrophic climate change could slip out of reach, if emissions do not drop sharply this decade.

A Biden administration could get a boost from climate policies already in place in many states, cities and businesses. Together, these efforts could curb U.S. emissions 37% this decade, from 2005 levels, according to Bloomberg Philanthropies’ “America’s Pledge” initiative.

“Maximizing that potential will require strong, decisive action from federal leaders in the White House and in Congress,” said Antha Williams, head of Bloomberg Philanthropies climate programs.

But Biden’s ability to implement his $2 trillion plan to slash emissions would also depend on whether Democrats wrest control of the Senate from the Republicans next week. If they do not, he may need to take executive action.

Executive action is more vulnerable to lawsuits. Trump has faced over 100 environment-related lawsuits for his moves to unwind regulations meant to protect health and the environment in his bid to lower costs for the energy and auto industries.