Most attempts to depict the sea ice use composite satellite imagery, like this. NASA

A new map reveals how much the ocean’s ice has shrunk since the 1970s.

The sea ice in the Arctic Ocean grows and shrinks with the seasons. From September to March, it expands in the darkness, locking water into island-sized solids. During the summer months, it recedes, as the water rejoins the atmosphere and sea.

On September 11 of this year, the sea ice in the Arctic Ocean reached its annual minimum. 2015’s minimum was the fourth-smallest ever recorded, and it nearly tied with the third-smallest on record. Which makes a certain amount of sense: In the satellite era, the ten worst years for Arctic sea ice have been the last ten.

In some maps, it can sometimes be hard to discern the northern sea ice. In satellite images—often the best representation available—the oceanic ice melds into the larger Arctic ice cap, which includes glaciers and ice sheets in Greenland, Canada, and Russia. That’s why I like this visualization, from developers at Mapbox, a mapping and data startup based here in D.C. It shows how the sea ice has changed—not the entire ice cap—through the satellite record. In particular, I think it reveals how bad things have gotten recently—and how small the oceanic ice was in 2012, the lowest ever recorded.

(If you’re on a phone, it might be best to try the full-screen version.)

Mapbox has also placed the sea-ice levels next to Earth’s mean atmospheric temperature. Year to year, that number doesn’t always obviously correlate with sea-ice levels: 2012 was a cooler (though above-average) year for the globe at large, though it was very warm in the Arctic Ocean.

But it shows the unmistakable trend for both: a warmer, wetter planet.

The chart’s data comes from the National Snow and Ice Data Center, a collaboration between NOAA and the University of Colorado. This is the oldest data set for sea ice available—it goes back to 1978. (That’s a pretty long time, at least for satellites: Landsat, the longest continuously operated satellite-data program, only started six years earlier.)

The age of the data helps explain why the edges of the ice cap look so jagged. “That’s a characteristic of the initial fleet of sensors they were flying to take these measurements back in the 1970s. They would measure in 25-by-25-kilometer grid squares,” said Matt Irwin, Mapbox’s government and humanitarian lead.

“There’s stuff today that you can put on a shoebox-sized nano-sat that will measure down to a one-by-one-kilometer grids. But what [scientists] want, and what’s super critical for getting a sense of the sea-ice change over time, is consistency in that data set, so they actually want to use the same resolution and are continuing to fly similar kinds of sensors as they were flying back in ’78.”

Irwin said the team considered smoothing edges of the ice cap, but chose to keep them for accuracy. “We thought it was a super cool way to show the pipeline from natural phenomenon to sensor to data to map,” he told me.

Though the lowest sea-ice-minimum in the satellite era last happened a few years ago, the smallest sea-ice maximum was recorded very recently: February 25, 2015.

This post originally appeared on The Atlantic.

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