Something very unusual — and unnerving — is happening on this planet of ours. The chart below shows the total extent of floating sea ice in the Earth’s oceans at any given point in time. Normally it waxes and wanes with the seasons.

But ever since September, as the red line shows, global sea ice has utterly collapsed, following a pattern never seen before. On January 14, total sea ice extent was at its lowest level since satellite records began in 1978 — and likely the lowest it’s been for thousands of years. And yes, global warming is an important part of the story here.

There are two major sources of sea ice in the world. There’s the ice covering the open water in the North Pole that tends to melt during the Northern Hemisphere summer and then refreezes in the winter. And then there’s the sea ice that surrounds the coast of Antarctica that usually recedes in the Southern Hemisphere summer (which is now) and refreezes later in the year.

What’s so odd is that both are near record lows for this time of year. Hence the ominous plunge seen in the chart above. Let’s break these two down:

1) Arctic sea ice is at record winter lows. Because of global warming, Arctic sea ice in the North has been steadily declining for decades. In September 2012, Arctic sea ice extent reached an all-time record low. Then in 2016, sea ice extent fell to its lowest October, November, and December levels ever — months when the ice normally begins to grow again as colder weather sets in:

So what happened? Well, 2016 was a record hot year globally, and the past few months have been exceptionally warm in the Arctic, with warmer-than-average ocean temperatures and persistent winds bringing in warm air from lower latitudes. That has prevented the Arctic sea ice from refreezing as quickly as it usually does in the winter. (According to NASA’s Walt Meier, persistent winds may have also inhibited sea ice from pushing southward.)

The map below from NASA shows how Arctic sea ice extent in November 2016 compared with previous years. It’s a very noticeable change:

And no, this is not a fluke: Researchers have estimated that up to 95 percent of the long-term decline in Arctic sea ice has been driven by human activity. Not only has global warming heated the region drastically (the Arctic is warming even faster than the rest of the planet, for reasons explained here), but dark soot particles from factories and cars in Europe and Asia are traveling north, settling on the ice, and absorbing extra sunlight.

That said, natural variability still plays a significant role. In 2012, a large storm in August helped break up the slushy sea ice and cause it to melt even more rapidly. That was one reason why we saw a record low minimum in 2012 but then a slight rebound the following years. That's also why scientists expect Arctic sea ice extent to bounce around erratically in the years ahead — even though the overall trend will be down.

2) Antarctic sea ice is at mysterious lows. Now, the disappearance of Arctic sea ice is technically old news. What’s new is that the sea ice around Antarctica has also been at record lows in November, December, and parts of January. That’s what’s driving the breathtaking lows in global ice right now. Here’s a chart from the National Snow and Ice Data Center showing Antarctic sea ice extent:

And here, via NASA, is what Antarctic sea ice extent looked like in November 2016 compared with the historical median:

Scientists aren’t sure whether this year’s retreat of Antarctic ice is related to global warming at all — there’s so much more natural variability in Antarctica because the ice is thinner than in the Arctic (it disappears almost completely every summer) and the ice along the Antarctic coasts is more vulnerable to rough weather.

In recent years, Antarctic sea ice has actually been expanding slightly even as the planet gets warmer, and scientists still aren’t exactly sure why (weather patterns and even the ozone hole have been mentioned as possible culprits). But in 2016, a sudden shift in wind patterns caused southern sea ice to collapse. What drove that shift in wind patterns is tougher to say.

Because of that volatility in Antarctica, it’s too soon to say whether this year’s collapse in global sea ice is an aberration. But we do know that total sea ice has been trending downward over time — driven by steep losses in the Arctic — even before this year. Blip or no, the broader trend is plenty disturbing.

Why should anyone care about sea ice?

One important thing to note here is that we’re talking about sea ice that’s already floating in the ocean — when it melts and disappears, it doesn’t directly affect global sea levels. (The ice was already displacing its own weight; much like when an ice cube melts in a drink, it doesn’t raise the water level.) So the disappearance of sea ice won’t, on its own, flood our coastal cities.

But melting sea ice can have important indirect effects. For instance: As more and more sea ice in the Arctic vanishes, more of the ocean underneath is exposed to sunlight. Because the ocean is darker than the bright ice, it absorbs more heat — and the broader region heats up more quickly. (This “feedback” effect appears to be quite significant in the Arctic.)

That’s important because right next door to all that Arctic sea ice is the massive ice sheet sitting atop land in Greenland. When that ice heats up and melts, it flows off of the land into the ocean, which really does raise sea levels. Greenland's ice sheet is currently 1.9 miles thick and contains enough ice to raise global sea levels by about 25 feet in all. And that ice sheet is indeed shrinking:

Greenland’s ice sheet is losing mass

The vanishing sea ice can matter for other reasons too. In the Arctic, we could see oil companies or shipping lines move into the newly open waters. And there’s some debate over whether the disappearing Arctic sea ice could muck up winter weather patterns in North America (though this is still being hotly debated).

But disappearing sea ice is also an important bellwether. By piling greenhouse gases into the atmosphere, we’ve set in motion all sorts of drastic planetary changes we can no longer control — changes that keep shocking and surprising us with each passing year.

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