The Greenland ice sheet is losing ice. How much? Currently, it’s losing over 300 billion tonnes of ice every year. That’s more than the entire weight of Mount Everest. How can an ice sheet lose that much ice? You have to understand that ice sheets are big. They’re among the largest physical features on Earth, rising kilometres up into the air. Our planet has three major ice sheets – in Greenland, East Antarctica and West Antarctica. If all three ice sheets were to melt, they’d raise global sea levels by nearly 80 metres. The Greenland Ice Sheet is the smallest of the three ice sheets. Even so, its melting would still cause sea level rise of over 6 metres. When we look into the Earth’s past, we can see that the Greenland Ice Sheet is very sensitive to climate change. Around 400,000 years ago, when global temperatures were about 3 degrees Celsius warmer than now, the Greenland Ice sheet melted enough to raise global sea levels by four and a half meters.
Greenland loses ice in a few ways. First, Icebergs break off the end of glaciers. Ice also melts at the surface then runs off into the ocean. Some of this melt water drains into deep channels in the ice called “moulins”. If the water reaches the bottom of the ice sheet, this can serve as a lubricant at the base. This speeds up the flow of glaciers into the ocean. Also, the floating ice found along the edges of ice sheets can act as a cork, holding back the ice sheet and preventing glaciers from falling or melting into the ocean. Warming air and ocean temperatures are melting these ice shelves and sea ice along the coast, this is effectively popping the cork and letting the outlet glaciers flow faster into the ocean. However, the Greenland Ice Sheet is also gaining ice in its interior. This happens in winter, when snow falls or when summer meltwater refreezes.
Whether Greenland is causing sea levels to rise or fall depends on whether the total mass of Greenland’s ice is increasing or decreasing. Air temperatures in Greenland have increased by nearly 2 degrees Celsius over the past century and a half. Satellites have found that the surface area of ice melt on the Greenland Ice Sheet has doubled over the past decade. Satellite data also show that most of Greenland’s largest outlet glaciers are speeding up and losing more ice to the ocean. As a result, Greenland has been losing ice at an accelerating rate. So when you look at the whole picture, the Greenland Ice Sheet is now the largest individual contributor to global sea level rise. However, there is a misunderstanding about what’s happening on Greenland, because of a cherry pick looking only at the ice build up in its interior. Not considering the rest of Greenland created the myth that Greenland is gaining ice.
The Greenland Ice Sheet was relatively stable throughout the 1990s. It was losing more ice along its coastlines. But at the same time, it was gaining more ice in its interior due to more winter snowfall. The increase in snowfall was caused by warming temperatures, which led to more moisture in the atmosphere. More moisture in the air results in more precipitation, which in Greenland means more snowfall. So in the 1990s, losses around the coast were balanced by gains in the interior. But since the early 2000s, the amount of ice being lost in coastal areas began to exceed ice gains in the interior. And now this process is accelerating. Over the last few decades, the Greenland Ice Sheet has gone from being in near balance to losing about 300 cubic kilometres of ice each year.
Again, this is more than the entire mass of Mount Everest! While this is a startling statistic, even more startling is that ice loss from Greenland is on the increase. This means Greenland is contributing more to sea level rise over time. And if the lessons from the past are any indication it's that Greenland is highly sensitive to warmer temperatures and can contribute significantly to sea level rise..