Drones capable of operating in extreme Arctic conditions have given scientists an unprecedented view of how Greenland's meltwater lakes drain, helping to explain why so much ice was lost this year. The results suggest most predictions for sea level rise may be too optimistic.

As the world heats up, it is inevitable the ice sheets of Greenland and Antarctica will lose volume, raising the height of the oceans. However, the speed with which this will occur remains a topic of much uncertainty; estimates range from disturbing to apocalyptic over the next few decades.

For greater precision, we need to understand better the behavior of the water that melts on the ice sheets' surface. We know lakes form in the summer, but the water that stays there until refrozen by winter doesn't affect sea levels. On the other hand, when a meltwater lake drains through cracks in the ice, the effects can be diabolical. Not only is the water likely to find its way to the oceans, but in the process, it makes the base of the glacier slippery, greatly speeding up the movement of the ice above to where the whole thing will turn to liquid.

To investigate further, Tom Chudley, a PhD student at the University of Cambridge, sent drones over Store Glacier, Greenland, to capture events researchers have not been able to witness in person. The 2018 formation of Lake 028 reactivated a fracture that had drained a lake at the same site the year before. It took less than five hours for 4.8 million cubic meters of water to cascade to bedrock, an average rate of more than seven Olympic swimming pools a second. The water raised the glacier by 55 centimeters (22 inches), lubricating its motion in the process.

"It's possible we've under-estimated the effects of these glaciers on the overall instability of the Greenland Ice Sheet," Chudley said in a statement. 

In Proceedings of the National Academy of Sciences, Chudley reports a third of the lake's volume didn't drain, meaning we can't rule out the possibility that the lakes we see haven't partially drained already. Failure to consider partial draining may have contributed to the underestimation of meltwater's effects.

Team leader Dr Poul Christoffersen added that the Store Glacier itself is not the core of the problem. "These glaciers are already moving quite fast, so the effect of the lakes may not appear to be as dramatic as it is on slower-moving glaciers elsewhere,” However, if previously slow-moving glaciers are having their path to the sea greased by similar processes, it could signal future acceleration.

Greenland is currently the largest contributor to global sea level rise. If similar effects start to be seen in Antarctica, we can kiss our coastal cities goodbye.