WASHINGTON — World leaders welcomed the United States’ official return to the Paris climate accord Friday, but mathematically trickier measures lie just ahead for President Joe Biden, such as setting a challenging national target in coming months for cutting harmful fossil fuel emissions.

And even as Biden noted the nation’s first day back in the climate pact, the globe’s dangerous warming was merely one of a long list of urgent problems he raised into a movie address to European leaders on Friday, a month in his administration. Before bringing weather difficulties, he touched on the global pandemic, sputtering national economies and tense relations with China, among other things that threaten to impede and delay handling the country’s standing as the world’s top carbon polluter after China.

Despite all the other challenges, Biden said, speaking to the Munich security conference,”we can’t wait or do the bare minimum to address climate change. This is a worldwide existential crisis, and all of us will suffer if we fail.”

Trump said shortly after he took office he would initiate the process of pulling the U.S. in the Paris accord, but it didn’t take effect until Nov. 4, 2020, because of provisions in the arrangement.

Officially, the United States was only out of the worldwide global climate pact for 107 days. It had been a part of Trump’s withdrawal from global allegiances in general along with his oft-stated but fictitious view that global warming was a laughably confused take by the world’s scientists.

More broadly, Trump reversed Obama-era initiatives to rein in oil, gas and coal emissions and opened new national lands and oceans to exploration and drilling. Biden is employed to overturn those steps and additionally has pledged a $2 trillion remake of U.S. power grids, transportation systems and other infrastructure to sharply cut fossil fuel pollution.

While Friday’s return is heavily symbolic, world leaders say that they expect America to establish its seriousness to the origin. They are especially excited for the United States to announce its new national 2030 target for cutting fossil fuel emissions, which scientists agree are altering the Earth’s climate and worsening the extremes of drought, hurricanetherefore, flooding and other all-natural disasters.

United Nations Secretary-General Antonio Guterres said Thursday the official American reentry”is very important,” as is Biden’s statement that the U.S. will go back to supplying climate aid to poorer nations, as promised in 2009.

“It is not about how many days. It’s the political symbolism the biggest economy refuses to see the chance of addressing climate change” She was one of the primary forces in beating out the largely voluntary 2015 agreement where nations set their own aims to decrease greenhouse gases.

1 fear was that other nations would follow America in abandoning the climate struggle, but none did, Figueres said. She said the real issue has been four decades of climate inaction by the Trump administration. American cities, states and companies still worked to decrease heat-trapping carbon dioxide but without the involvement of the federal government.

“We have lost too long,” Figueres explained.

Inger Andersen, the environment program manager at the United Nations, said America has to establish its leadership to the rest of the world, but she said she has no doubt that it will when it submits its required emissions cutting goals.

“We hope they’ll translate into a very significant reduction of emissions, and they’ll be a good example for other states to follow,” Guterres said.

The Biden administration is working now on a goal that balances significant cuts in emissions with political and fiscal realities. Deciding a U.S. emissions goal by April, when Biden intends to sponsor world leaders to get an Earth Day summit, could assist the administration prod other nations for demanding emissions cuts too. That spring assembly should observe nations start”to place the down payments on the table,” John Kerry, Biden’s climate envoy, said Friday.

Republican leaders are fighting with it.

Wyoming Sen. John Barrasso, the top Republican on the Senate energy panel, has criticized Biden for rejoining the Paris accord, tweeting:”Returning to the Paris climate agreement will increase Americans’ energy expenses and won’t solve climate change. The Biden administration will set unworkable aims for the United States while China and Russia can continue with business as usual.”

University of Maryland environment professor Nate Hultman, who worked on the Obama administration’s official Paris goal, said he anticipates a 2030 target of cutting carbon dioxide emissions between 40 percent and 50% by the 2005 baseline levels.

A longtime international goal, included in the Paris accord with an even more rigorous goal, is to keep warming below 2 degrees Celsius (3.6 degrees Fahrenheit) because pre-industrial times. The planet has warmed 1.2 degrees Celsius (2.2 degrees Fahrenheit) since that time.

The United States’ return to the Paris accord and an ambitious target for emissions cuts would make limiting warming”to well under two degrees — not only to 2 degrees but below 2 degrees — more likely,” said researcher Zeke Hausfather, energy and climate manager for the Breakthrough Institute.