It is deep winter in Antarctica, the time of year that the continent is shrouded in darkness and surrounded by millions of square miles of frozen ocean.

But this year there is a lot less sea ice than ever recorded before. And that is bad news for people around the world, because sea ice helps control how quickly the rest of Antarctica's ice melts, which contributes to global sea level rise.

Each year, the amount of sea ice around Antarctica fluctuates enormously. In the Antarctic summer, starting in December, the sea ice melts rapidly. Some seas around Antarctica are virtually ice-free by March. Then, as Antarctic winter approaches, the ice begins to grow again. At its most expansive, the sea ice covers an area the size of Antarctica itself – essentially, the frozen continent doubles in size.

But the amount of sea ice has been slowly decreasing in recent years. And, this year, the ice is significantly smaller than it ever has been. As of late June, almost a million square miles of ice was missing from the ocean around Antarctica. It's the smallest amount of ice ever measured around the continent since 1979, when satellites allowed scientists to track such events annually.

"It's a big enough deal to be alarming to climate scientists," says Ted Scambos, who studies Antarctica at the University of Colorado, Boulder. "We've seen a decline in sea ice cover since about 2016, but 2023 took a huge jump downward."

Less sea ice means more sea level rise

The missing ice is a problem for people far from Antarctica. While sea ice loss doesn't contribute directly to sea level rise – melting sea ice doesn't add any extra water to the ocean – Antarctic sea ice acts like a protective collar around the continent. The sea ice shields Antarctica's glaciers and massive ice shelves from ocean waves and above-freezing ocean water that hasten melting.

"Those ice shelves contain a lot of frozen water, and if they break and become a part of the ocean, that's going to raise global sea levels," says Marilyn Raphael, who studies Antarctic sea ice at the University of California, Los Angeles.

The West Antarctic ice shelf, which is the region of the continent that is melting most rapidly in response to climate change, contains enough water to raise global sea levels by about 10 feet.

The United States is in the crosshairs of that melting. Disappearing ice in West Antarctica will be the main driver of potentially catastrophic sea level rise in coastal Texas and other places along the Gulf of Mexico later this century.

Scientists are racing to figure out how much of that ice is likely to melt in the next hundred years, and to understand why the sea ice shrank so precipitously this year.

They are challenging questions to answer, because Antarctica and the ocean surrounding it are enormous, complex and remote. Unlike the Arctic, where humans have lived for millennia, the Antarctic requires specialized ships and planes to visit, and is basically inaccessible for half the year.

"This is a very surprising, difficult system to forecast and predict," says Scambos, who has been studying Antarctica for more than 30 years. One theory about why the sea ice is so small this year is that warm ocean water from other parts of the planet has started to mix with the layer of water at the surface, where sea ice forms.

"That's led to a little bit of heat in that water," Scambos explains, "and you have to get all the heat out to get it to the freezing point and then actually form ice. So, if you have a few tenths of a degree of heat in the water, [the freezing] goes much, much slower."

He adds: "That's likely to continue because there's just not much cold water left in the ocean pretty much anywhere."

Sea ice can't recover because of climate change

This year is likely not a one-off bad year for Antarctica's sea ice. One reason is that global temperatures are only rising, which means that the world's oceans are getting warmer and warmer.

Another reason is that missing ice begets more missing ice. Ice reflects much of the sun's energy, but ocean water is darker in color and absorbs more of that heat. Less sea ice means more exposed ocean water, which in turn absorbs more heat and makes it more difficult for ice to re-form.

"We're probably in for several years of low sea ice and Antarctica going forward," Scambos says.

What happens in the next month or so will influence what happens in the coming years, says Raphael. The sea ice is still growing, and Antarctic sea ice hits its maximum size in mid-September each year.

"We only have six more weeks, approximately," Raphael says. "If the ice doesn't reach a normal maximum, or close, that means we have less ice to melt," during the Antarctic summer. "So there's potential for even more ice loss next year."