AMNA NAWAZ: As temperature records are broken all over the planet this summer, scientists are also increasingly concerned about what's happening to the sea ice around Antarctica.

William Brangham, who's reported from that southernmost continent and has been talking with some of those researchers, is here now to explain.

Good to see you.

WILLIAM BRANGHAM: Hi.

AMNA NAWAZ: So, what is happening down there?

It's wintertime.

You would assume there is more ice, not less.

WILLIAM BRANGHAM: Right, you would assume that.

And, remember, Antarctica is a -- this colossal continent covered completely in ice.

It is the size of the United States and Mexico combined, covered with huge ice sheets and glaciers.

That's not what we're talking about.

We're talking about sea ice, which is frozen seawater, that -- and that ice grows every year from the periphery of Antarctica outwards.

It's like this incredible halo that grows out across the ocean.

We can see this NASA animation of what it looks like.

That growth of that sea ice is so massive, it doubles the size of the continent every single year.

But the problem is, is that, this year and last year, to a similar extent, that ice has not been growing nearly as quickly.

I want to put up this other chart here.

This shows the traditional, that gray -- that line at the top is what it normal growth of the sea ice is.

That red line below is where we are now.

It is a marked difference.

That is roughly the size of Alaska that is missing ice now in the Southern Ocean.

AMNA NAWAZ: It's remarkable to see in that graphic there.

So, you have been talking to researchers.

Do they know why this is happening?

WILLIAM BRANGHAM: It is a complicated picture.

And there are some leading suspects.

And these are all suspects that we have our fingerprints on.

The first one is, is that the ocean is warmer.

We have seen that all over the world.

As we burn coal and oil and gas and warm the atmosphere, that warms this planet, and the oceans absorb the bulk of that warming.

Warmer ocean water makes it hard to grow ice.

You can -- that's pretty standard physics.

The air is warmer as well.

That makes it harder to grow ice.

Wind is also complicit in this, changing wind patterns.

Wind is really important in how ice grows off of Antarctica and where it grows.

Those things are changing.

Again, scientists believe that we are one of the main drivers of this, but the exact mechanism is not totally clear.

AMNA NAWAZ: So what's the bigger concern in all of this?

If that sea ice is not growing at the rate it's expected to, what are the consequences of that?

WILLIAM BRANGHAM: This is the biggest issue here.

And the reason we care about this is that that ice provides all sorts of incredibly valuable things for us.

First off, that ice keeps the Southern Hemisphere cooler.

And you think of ice as this flat, white surface that reflects the sun's radiant energy off into space.

If that ice is not there, that sunlight hits the ocean, warms the oceans.

This is part of a very complicated engine that drives ocean currents globally and weather patterns globally.

So, disrupting that could cause huge problems.

This sea ice also protects the glaciers that are sitting up on land at in Antarctica.

We want those glaciers to stay there.

If they slip into the ocean, that raises sea level rise globally.

So that sea ice is like a buffer.

It's like a big inner tube protector around that ice.

We don't want that to go away.

And, thirdly, there are animals that live all over Antarctica.

And they in one way or another benefit from that ice.

Penguins, seals, whales, the tiny krill that live in the water there all spend part of their life on that sea ice.

And it's important.

One of the researchers I talked with recently is a woman named Marilyn Raphael.

She's a geologist at UCLA.

In addition to explaining a lot of this mechanism to me, I asked her how she sees this data.

Here's what she had to say.

MARILYN RAPHAEL, UCLA: There is a part of me that's scientifically interested in what's happening.

Like, what is at work here?

That is completely separate from the other part.

That's the citizen of the world apart that says, this is really shocking, and this -- it's not good.

It's not good news for our system, and not just the Antarctic system, but from a global climate system.

I look at the curve daily, but I look at that curve early, and I'm almost willing it to inch upwards.

But it's not listening to me.

WILLIAM BRANGHAM: Researchers all over the world are willing that graph to go up, to grow more of the sea ice.

But there is no evidence that it is.

We are about to hit the peak of the Antarctic winter, meaning, so if its chances of growing to its fullest extent are about to run out -- soon, the sun will return to the Southern Hemisphere, and that already shrunk and ice will continue to melt and break up and go away.

AMNA NAWAZ: Incredibly sobering information.

William Brangham, thank you for your expertise on this and your reporting.

WILLIAM BRANGHAM: My pleasure.