Hours after he was sworn in, President Joe Biden announced the US plans to reenter the Paris climate accord, the landmark international agreement signed in 2015 to limit global warming, in a sign of Biden's urgency to address the climate crisis.

The US abandoned the agreement late last year on former President Donald Trump's orders. Trump spent much of his time in office weakening many of the country's bedrock climate and environmental guardrails.

Experts say that rejoining the agreement s is a significant step by the Biden administration to reverse the climate policies of the last four years. But now comes the hard work.

As he takes the reins of the executive branch, the challenges that Biden faces rival any confronted by his 45 predecessors -- an out-of-control pandemic, a sputtering economy and the threat of right-wing extremist violence stoked by viral misinformation.

Biden's action sends a strong message that the US is prepared to cooperate in the fight against climate change and seek to reclaim the leadership role it once held, experts say. Under the agreement, countries are expected to enhance their commitments to curb greenhouse gas emissions every five years.

"It sends a very important signal to the rest of the world on one of the biggest problems we face," said John Holdren, a professor of environmental science and policy at Harvard University who served as director of the White House Office of Science and Technology Policy under President Barack Obama. "I think it is very important that the US demonstrates once again that it will take the global climate change challenge seriously."

"Welcome back to the Paris Agreement," French President Emmanuel Macron tweeted Wednesday.

Biden is also signing a raft of executive orders aimed at gutting Trump's climate and environmental policies. The orders direct federal agencies to review dozens of rules from the Trump era and aim to reverse any that are "harmful to public health, damaging to the environment, unsupported by the best available science, or otherwise not in the national interest."

He also directed all federal agencies to review the threats posed by greenhouse gas emissions, especially to the poor, communities of color and young people, who are most vulnerable to the effects of climate change.