Biden, in a Burst of Climate Orders, Rejoins the Paris Agreement.

The president also canceled the Keystone XL pipeline and ordered federal agencies to begin the process of reinstating environmental regulations reversed under the Trump administration.

WASHINGTON — President Joseph R. Biden Jr. on Wednesday recommitted the United States to the Paris climate agreement, the international accord designed to avert catastrophic global warming, and ordered federal agencies to start reviewing and reinstating more than 100 environmental regulations that were weakened or rolled back by former President Donald J. Trump.

The moves represent a first step in healing one of the deepest rifts between the United States and the rest of the world after Mr. Trump defiantly rejected the Paris pact and seemed to relish his administration’s push to weaken or undo major domestic climate policies.

Mr. Biden has elevated tackling the climate crisis among his highest priorities. In addition to curbing global warming, he has vowed that ending the coronavirus pandemic, restoring the economy and addressing racial injustice will be the central causes of his administration.

“We’re going to combat climate change in a way we have not before,” Mr. Biden said in the Oval Office on Wednesday evening, just before signing the executive orders. Even so, he cautioned: “They are just executive actions. They are important but we’re going to need legislation for a lot of the things we’re going to do.”

WASHINGTON — President Joseph R. Biden Jr. on Wednesday recommitted the United States to the Paris climate agreement, the international accord designed to avert catastrophic global warming, and ordered federal agencies to start reviewing and reinstating more than 100 environmental regulations that were weakened or rolled back by former President Donald J. Trump.

The moves represent a first step in healing one of the deepest rifts between the United States and the rest of the world after Mr. Trump defiantly rejected the Paris pact and seemed to relish his administration’s push to weaken or undo major domestic climate policies.

Mr. Biden has elevated tackling the climate crisis among his highest priorities. In addition to curbing global warming, he has vowed that ending the coronavirus pandemic, restoring the economy and addressing racial injustice will be the central causes of his administration.

“We’re going to combat climate change in a way we have not before,” Mr. Biden said in the Oval Office on Wednesday evening, just before signing the executive orders. Even so, he cautioned: “They are just executive actions. They are important but we’re going to need legislation for a lot of the things we’re going to do.”

Under the Paris Agreement, nearly 200 nations have vowed to reduce planet warming emissions to avert the most disastrous consequences of climate change. A letter to the United Nations signed by Mr. Biden on Wednesday formally starts the 30-day process of bringing the United States back into the accord.

But analysts cautioned that Mr. Biden’s actions on day one must be quickly followed by a series of aggressive domestic climate policies to drastically lower the country’s emissions of planet-warming pollution from tailpipes, smokestacks and oil and gas wells.

Also on Wednesday, Mr. Biden rescinded the construction permit for the Keystone XL oil pipeline, which would have transported carbon-heavy oil from the Canadian oil sands to the Gulf Coast. Earlier in the day, TC Energy, a Canadian company, said that it was suspending work on the line.