GEOFF BENNETT: Earth Day is tomorrow, one of the key events in the environmental movement.

It comes as a federal judge has temporarily blocked the EPA from canceling $14 billion in climate grants approved by the Biden administration, marking a setback to President Trump's agenda to freeze climate spending across the government.

As part of our Tipping Point series, William Brangham spoke recently with a leading environmentalist about the opposition to Mr. Trump's plan for the climate.

WILLIAM BRANGHAM: 2024 was the hottest year on record, capping a decade where almost every year broke the previous year's record high.

Carbon emissions, which help drive that warming, are also at record levels.

President Trump and his administration argue that this is not a problem and that trying to address it only hurts the economy and puts the U.S. at a competitive disadvantage.

On his first day in office, president trump withdrew the U.S. from the Paris climate accord.

He's tried to freeze billions in funding for clean energy projects, wants to boost coal production and declared a -- quote -- "national energy emergency" to increase domestic fossil fuels.

While Trump's actions face legal challenges, environmentalists are sounding the alarm.

And among them is Bill McKibben.

He's the author of several books on climate change, the founder of the grassroots climate campaign called 350.org, and Third Act, which organizes older Americans to work on climate justice.

Bill McKibben, great to have you on the program.

What are some of your greatest immediate concerns about the Trump administration?

BILL MCKIBBEN, Environmentalist: Well, William, we're seeing an incredible rollback pretty much of all environmental regulation dating back to 1970.

We're coming up on the 55th anniversary of Earth Day, and it was in the immediate aftermath of that we started basically regulating pollution, and now we're deregulating pollution of all kinds.

The most serious consequences are what's happening around climate and energy, and they're serious for two reasons.

One, as you say, the planet is getting hotter and hotter and hotter all the time.

March was the hottest March we have ever measured on this planet.

And, two, we're making a series of extremely foolish choices about energy.

We're the only place in the world that's decided that somehow coal is the future of the planet.

And we're going to have our lunch eaten by the rest of the world, which has quite rightly figured out that sun and wind and the batteries to store their power when the sun goes down or the wind drops are the cheapest, cleanest, easiest, fastest way forward.

So, on both counts, we're making just the most savage mistakes.

WILLIAM BRANGHAM: It also seems the administration is defunding a lot of the scientific investigation into climate change and how it might play out into the future.

BILL MCKIBBEN: Yes, we have -- look, the U.S. was the place where we first understood what was happening.

We were the first people to measure carbon in the atmosphere.

The people that built the computer models that helped us gave us the warnings about what was coming.

And those are precisely the programs that are now being chopped off.

Even the programs where we measure the amount of carbon in the atmosphere or the temperature of the Earth are under assault, as if, by not measuring it, it might go away.

But that's not how physics works.

There are a series of questions, a series of issues where people can differ in their opinions.

And I'm at odds with the Trump administration about many issues.

But this isn't a question of opinions.

This is a question of physics and of chemistry, well-understood physics and chemistry.

And willfully blinding ourselves to it is -- has to rank high on the list of dumbest things that governments have ever done.

WILLIAM BRANGHAM: What about the argument that this administration makes that the concerns about climate change and the impacts it will have on our world are, they argue, exaggerated or overblown or so far in the distance that to exert energy and money to address them is an enormous cost?

BILL MCKIBBEN: Tell that, first of all, to the good people of Los Angeles, say, who watched large parts of their city burn after the hottest, driest weather on record.

But then also go talk to the people in the insurance industry who are canceling policies up and down, East and West across this country because climate change has so scrambled their ability to predict what's going to happen that they can no longer figure out how to make a going business of writing insurance.

If you don't think that's an economic drag on us going forward, you haven't paid much attention to how capitalism works.

Insurance is an absolutely key part of where we are.

The real bottom line, though, is that we could be making huge positive steps right now.

We live on a planet where, as of three or four years ago, the cheapest way to make power is to point a sheet of glass at the sun.

The solar cell, an American invention at Bell Labs in 1954 is the most important piece of technology on the planet.

But right at the moment, we're ceding all that to the Chinese, who are building something like two-thirds of all the clean energy on planet Earth.

They're going to own the future and we're going to have some coal mines.

WILLIAM BRANGHAM: The administration's argues that all of the talk about the harms that climate change is causing and will cause into the future is exaggerated and that that is what is driving this climate anxiety documented among the young people of this world.

What do you say to that?

BILL MCKIBBEN: I spend a lot of time with young people.

And what I find makes them anxious is the fact that nobody in authority now is doing anything to address what's an obvious -- obviously the biggest danger facing their future.

They're not stupid.

You can't wish this away.

They have been studying physics and chemistry in school.

What they want is people in leadership to stand up and show them that they take seriously what scientists are saying.

We got a warning from science.

We got it in time to act.

So far, we have ignored that warning.

And that is truly, truly sad, and it should make us anxious.

WILLIAM BRANGHAM: All right, that is Bill McKibben of 350.org and Third Act.

Always great to talk to you.

Thank you so much.

BILL MCKIBBEN: William, a great pleasure.