If we don’t pump the brakes on climate change, all of Greenland could eventually melt, adding about 24 feet to global sea levels. Add in the West Antarctic ice sheet, and we’re looking at a total of about 40 feet of potential sea level rise from these two ice sheets alone. The question is, when is this going to happen? In a few hundred years, or a few thousand?

NASA’s on a mission to find out, and that includes an effort to study melting ice in Greenland that began in the summer of 2015. Appropriately, it’s called OMG—Oceans Melting Greenland. Over a 5 year period, OMG is gathering temperature data, mapping the seafloor, and modeling the ocean currents that accelerate the melt.

OMG gathers much of its data from 240 probes dropped from a military plane around Greenland. The mission also uses ships to collect measurements of the depth and shape of the sea floor, and it’s even teaming up scientists at the University of Washington that use narwhals in their research. Turns out, narwhals like to hang out in the deep waters near glaciers, so they’ve been equipped with temperature and salinity monitors that send data back to NASA.

According to Josh Willis, the lead scientist at OMG, ”Every time we make a big discovery in Greenland, we find out that the probability of a really fast collapse of the Greenland ice sheet is higher than we previously expected.”

Not only is the ice melting from rising air temperatures, it’s also melting from below. Glaciers are basically rivers of ice—and like rivers, they flow downstream into the ocean. Near Greenland, they often flow into deep ocean water—sometimes as deep as 800 meters—giving the glaciers a nice warm Atlantic bath to accelerate their melt. Over the past 15 years, Greenland has lost about 4 trillion tons of ice, contributing about a centimeter to global sea level rise. Not much so far, but the truth is that Greenland is just getting started.

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And the ice sheet won’t have to melt completely before rising sea levels cause serious problems. As sea levels rise, storm surges from floods can reach further and further inland. With about 40 percent of the world’s population living near the coast, this could have dramatic consequences for humanity in just a few generations.

So, how long until the ice sheet disappears? OMG is still collecting data to figure this out. The good news is we still have a chance to prevent warming from getting completely out of hand—and a total meltdown of the Greenland ice sheet is likely still a few centuries away.