In the year 1912, an iceberg, which likely calved off of the Jakobshavn Glacier in Greenland, floated into the path of the Titanic, leading to the "unsinkable" ship's tragic demise. Today, Earth’s glaciers are in retreat, shedding many such bergs into the ocean and triggering sea level rise. But new satellite data shows that oddly, the Jakobshavn Glacier has actually grown thicker in recent years, after decades of discharging tons of ice into the ocean.

During the 2000s, the Jakobshavn was the fastest flowing glacier on Greenland, moving at about 10.5 miles per year, reports the BBC’s Jonathan Amos. Satellite data showed that the front end of the massive ice river retreated by about 66 feet per year during that time. The glacier, which spills into Disko Bay, is one of the major pathways for ice from Greenland’s ice sheet—the largest in the world behind Antarctica—to enter the ocean and cause sea level rise. Harry Cockburn at The Independent reports that between 2000 and 2010 the Jakobshavn Glacier alone contributed 1 millimeter to sea level rise.

But a funny thing began occurring in 2013: the Jakobshavn’s ice front stopped thinning and instead began thickening and the trend continued through 2017, according to the European Space Agency, which monitors the glacier with various satellites as part of its Climate Change Initiative. That means the glacier is now flowing more slowly and is no longer retreating inland.

“It’s a complete reversal in behavior and it wasn’t predicted,” study leader Anna Hogg from Leeds University and the UK Centre for Polar Observation and Modelling, tells Amos. “The question now is: what’s next for Jakobshavn? Is this just a pause, or is it a switch-off of the dynamic thinning we’ve seen previously?”

Hogg and her colleagues, who recently presented the research at the Living Planet Symposium in Milan, think they know why the abrupt change in the glacier took place. In the late 2000s and into 2013, warmer ocean waters in Disko Bay infiltrated the fjord where the glacier resides, causing it to melt more quickly at its terminus, or lowest end, which is also sometimes called its “toe” or “snout.”

“In recent years, however, temperature measurements show that ocean water in Disko Bay has experienced a series of cooler years – more than one degree lower than mean temperature previously observed,” Hogg says in the press release. “This has reduced the rate of ice melt on Jakobshavn Isbrae [Glacier].”

But that doesn’t mean the Jakobshavn is necessarily growing overall. Snowfall is not keeping up the amount of ice lost by the glacier, meaning that over the long term, the glacier is still shrinking.

What this all means for sea-level rise, however, is complicated. The Jakobshavn Glacier drains just 7 percent of Greenland’s ice, reports Cockburn, and thinning and melting are still occurring all over the island. In fact, a study in PNAS released in January found that Greenland’s ice sheet is melting four times faster than it was in 2003.

But most of that ice loss isn’t coming from glaciers calving off into the sea. It turns out the warming of the atmosphere is also melting ice, which is discharged via rivers and streams that flow into the ocean. Interactions between the ice and warming atmosphere, researchers are beginning to understand, are important elements in the melting of the ice sheet as well as loss of glacial ice.

“The rate of sea-level contribution from Greenland has slowed in recent years and it’s because some of the biggest ice evacuators like Jakobshavn aren’t contributing as much as they used to.” Hogg tells Amos at the BBC. “All this is a reminder of how unpredictable glaciers can be. We didn't predict this change in behavior, and if Jakobshavn does start thinning and retreating again—we can’t predict when that will happen.”