JOHN YANG: This past week, the EPA said it is reconsidering the scientific finding that greenhouse gases are a danger to public health.

That finding has been at the center of this country's fight against climate change.

This comes as research shows average Global temperatures in 2024 likely rose above a 1.5 degrees Celsius threshold that for years has been an environmental red line.

Ali Rogin spoke with Michael Mann, the director of the center for Science, Sustainability and the Media at the University of Pennsylvania.

ALI ROGIN: Michael, thank you so much for being here.

Simply put, how big a deal is it for the planet to exceed this 1.5 degree threshold?

MICHAEL MANN, University of Pennsylvania: Well, it's a big deal, but it should be put in context.

What happened this year was that we had an El Nino event.

It's a natural warming event that adds to the global temperature, so it boosted it temporarily above that 1.5 Celsius mark.

What we're really concerned about is when the trend line itself crosses 1.5 Celsius, roughly 3 degrees Fahrenheit.

And that'll probably happen in little more than a decade if we continue on this current course.

And so I sort of liken it to, you know, here in Philadelphia, where I live, if you get a 50-degree or 60-degree day in late February, it doesn't mean winter's over, but it's an indication that the end of winter is probably coming.

This is an indication that we're getting perilously close to this very dangerous threshold.

ALI ROGIN: And what happens if we do continue to exceed this number?

That'll be a sign that things are continuing to move in that direction in a more permanent way, right?

MICHAEL MANN: Yeah.

And so, you know, it's not like a cliff that we go off at that level of warming.

It's more like a dangerous highway that we're going down.

Every bit of additional warming makes things worse, but conversely, every bit of warming that we can prevent makes things better.

And so it's important for us to, you know, get off the burning of fossil fuels as quickly as we can, limit warming as much as we can.

And this is just an indication that we are, you know, headed towards ever more dangerous levels of climate change.

ALI ROGIN: And what is it about that 1.5 degree change?

I've read that it's not so much that number itself, it's that it more about what it represents.

Is that correct?

MICHAEL MANN: It is, because it's a continuous scale.

And, you know, the devastating wildfires we've seen out west, the hurricane season, the tremendous flooding that we saw in parts of the southeastern U.S. the mortality that came with that, this is an indication that we are already seeing truly dangerous climate consequences.

And they will just get more and more dangerous.

We will see more frequent and more intense weather extremes of the sort that we've seen if we continue down this road.

And so we need to, you know, reduce our carbon emissions as quickly as possible to prevent things from getting even worse.

ALI ROGIN: President Trump has started the process of withdrawing from the Paris Climate Agreement, which set this 1.5 degree threshold, also easing restrictions on oil and gas production.

How does that impact efforts to avoid further exceeding this threshold?

And what about the general political climate elsewhere in the world?

MICHAEL MANN: The real problem here is that this sort of sends a signal to the rest of the world that the United States isn't willing to honor its commitments.

We're the world's largest legacy carbon polluter.

We've put more carbon pollution into the atmosphere than any other country, and that's all the climate cares about.

It cares about the cumulative carbon emissions.

Over time, we've contributed more to the warming of the planet than any other country.

Fortunately, we have seen some degree of resolve.

The EU remains committed.

China seems to be stepping up.

I think they see an opportunity for actually taking leadership in the vacuum left by U.S. disengagement under the new Trump administration.

So it's a global problem.

It requires global cooperation.

And so we will have to see what other countries do.

ALI ROGIN: And in terms of that global participation, there's long been a debate about the responsibility of developing countries and what they have to do versus developed first world countries like the United States.

How does the reaching and exceeding of this 1.5 degree threshold move the needle on that debate?

MICHAEL MANN: You know, the combination of the fact that we're getting perilously close to this, you know, new dangerous milestone, and the fact that this is happening in an era of U.S. disengagement.

And it does provide an excuse to developing countries who can rightly say, look, you had access to two centuries of cheap, dirty fossil fuel energy to grow your economy, and if you're not willing to do something to help the world deal with this problem, then why shouldn't we get our own opportunity to grow our economies?

Many of these developing countries have contributed the least to the problem, and they're already bearing the brunt of the problem.

They're seeing some of the worst consequences.

They have the least wealth and resilience to deal with the consequences that are already happening.

We have to provide some degree of assistance to developing countries to help them deal with the catastrophic consequences they're already dealing with, and to help them sort of leapfrog past the dirty fossil fuel stage of economic development.

We want to see them bypass that.

We can't afford them to make the same mistakes that we made, and we have to make it worth their while to move directly to clean, renewable energy sources.

ALI ROGIN: Michael Mann, director of the center for Science, Sustainability and the Media at the University of Pennsylvania, thank you so much for joining us.

MICHAEL MANN: Thank you.