This past July, Brandon Overstreet inched toward the edge of a brilliant blue river winding across a vast ice sheet in Greenland. The rushing water at his feet was only a degree above freezing—a result of massive amounts of ice melting.

Just downstream from where he stood, the river plunged into a moulin, a roaring vertical waterfall within the ice sheet that Overstreet called “the most terrifying thing I can imagine.”

Overstreet carried a bag with 23 meters     (75 feet) of rope inside. His mission: to throw the bag across the river to scientists on the other side. One misstep and the cold water would sweep him instantly down the moulin and beneath the ice to his death. “Swimming was not an option,” says Overstreet.

For Overstreet and his team, the dangerous work is worth it: They’re trying to better understand how a warming world is melting Greenland’s ice—and how coastal communities could be affected by the additional water flowing into the world’s oceans as a result.

As the temperature rises, Earth’s glacier ice melts. Meltwater flows into the oceans, raising the level of the seas. The Intergovernmental Panel on Climate Change, a leading authority on climate change, estimates that ice melt will raise sea levels 1 meter (3 feet) by the year 2100.

That estimate comes from surveys that scientists have taken to reveal how much ice is melting and how quickly. The data also suggest that there’s enough ice in the Greenland ice sheet alone to raise the world’s sea levels by 7 meters (23 feet) if all of it were to melt.

Since Greenland’s temperatures hover around freezing, an increase of just a few degrees creates ideal melting conditions—a scenario that greatly concerns scientists.

Last summer, a helicopter dropped Overstreet on one side of the rushing river of meltwater they planned to study. The chopper brought the rest of his team to the other side. Overstreet’s job was to set up a cable that would bridge the river so the team could study it. He hurled bags of rope over and over again to his teammates, each time falling short—until finally, one made it across the river.

Once the cable was set up, the scientists attached a specially designed boogie board to it, which floated on the river. The board was equipped with instruments to measure the river’s depth, width, temperature, and rate of flow. That allowed them to precisely track how fast the ice was melting.

Scientists have watched Earth’s glaciers shrink for decades. But this was the first time anyone had directly measured a large river of meltwater every hour for 72 hours straight.