Many parts of federal policy shift back and forth over time. Taxes rise and fall, as do spending on anti-poverty programs and the military. If a package of policies doesn’t pass one year, it might pass in a future year, and the long-term trajectory of the United States probably won’t be affected much.

Climate policy is different.

The world has already warmed to dangerous levels. Heat waves, wildfires, droughts and severe storms have become more common. The Arctic is melting, and seas are rising. If countries do not act quickly to slow their emissions of greenhouse gases — and, by extension, slow global warming — the damage could be catastrophic, scientists have warned.

The U.S. has a uniquely important role in fighting climate change. It has produced far more greenhouse gases over the course of history than any other nation and remains a leading emitter today. In recent years, the U.S. has done considerably less to reduce emissions than Europe. The U.S. also remains the world’s most powerful country, with the ability to influence climate policy in China, India and elsewhere.

Until yesterday, the Democratic Party seemed as if it were on the verge of squandering a major opportunity to combat climate change. Democrats control both Congress and the presidency, and yet they had been unable to agree on a package of climate policies to accelerate the use of clean energy and reduce emissions. Senator Joe Manchin had been blocking any deal, and the Senate is so closely divided that the Democrats cannot afford to lose a single vote.

Yesterday, however, Manchin appeared to change his mind. He announced that he had agreed to include hundreds of billions of dollars for climate and energy programs in a bill that would also reduce prescription drug prices, raise taxes on the affluent and shrink the federal deficit.

If Manchin and other Democrats remain united, it would be a very big deal. “This has the opportunity to be an enormous breakthrough for climate progress,” Sam Ricketts, co-founder of Evergreen Action, an environmental group, told The Times.

It’s especially significant because congressional Republicans have almost uniformly opposed policies to slow climate change (a contrast with conservatives in many other countries). And it remains unclear whether Democrats will again control both Congress and the White House anytime soon. If Congress fails to pass a climate bill this summer, it may not do so for years — while the ravages of climate change worsen.

After all the recent bickering among Democrats, I know that many people remain skeptical that they actually have a deal until Congress has passed a bill. That skepticism makes sense. Last night’s announced deal between Manchin and Chuck Schumer, the Democratic leader in the Senate, is different from a complete bill that can pass in both the Senate and the House.

But I would say this: If this tentative agreement leads to legislation, it will probably have more lasting importance than anything else President Biden signs in his first two years in office.

Lives Lived: Tony Dow found fame at a young age as Wally Cleaver on the 1950s sitcom “Leave It to Beaver.” He first resented the way the role had typecast him, but said that changed with age: “At 40, I realized how great the show was.” He died at 77.

SPORTS NEWS FROM THE ATHLETIC

A sweep and a trade: The Mets finished off a two-game sweep of the Yankees yesterday, but just minutes after the final out, the Bronx Bombers traded for Andrew Benintendi, one of the top bats on the market.

DK Metcalf “holds in”: The Seahawks wide receiver attended practice Wednesday, but refused to participate while the team worked on a new contract for him.

Mike Trout’s rare condition: The Angels’ superstar outfielder is dealing with a rare back condition, a team trainer said. There is no timetable for his return to the lineup, though Trout said he planned to play again this season.

ARTS AND IDEAS

Have you seen this boot?

The producers of the Broadway revival of “Into the Woods” are looking for a special prop: a huge, inflatable boot that hung over the theater’s facade in the 1980s. The boot returned for the show’s 2002 revival, but was stashed away when the weather got bad. Now, nobody knows where it is, James Barron writes.

“It was literally the beacon that called us all to the theater,” the producer Jordan Roth said. “I think why it captured our imagination was the way it really physicalized this impossible balance of the show between whimsy and weight.”

Some suspect it was cut into pieces. Others say producers just haven’t looked in the right spot. “It’s in storage,” said Michael David, the executive producer for the original run. “I just don’t know where in storage.”

PLAY, WATCH, EAT

What to Cook