Unique measurements confirm vulnerability of Thwaites Glacier

The Thwaites Glacier (image 1) loses 60 billion tons of ice every year, enough to cover the entire Netherlands with a layer of ice over 1.5 meters thick. This meltwater currently accounts for 4 percent of the total global sea level rise. A recent study indicated that in the current warmer climate, the Thwaites Glacier is shrinking, and together with surrounding glaciers will cause the global sea level to rise by one meter. The ice loss is mainly caused by warm seawater melting the glacier from underneath.

Glaciers in Antarctica flow into the sea. Here they form floating ice shelves that hold the ice behind them in place (image 2). The thicker and more robust these ice shelves are, the slower glaciers can slide into the sea and contribute to sea level rise. In certain areas, especially in West Antarctica, these ice shelves float in seawater that is a few degrees warmer than freezing point. The rate at which seawater melts these ice shelves is one of the biggest sources of uncertainty in future projections of ice melt in Antarctica. For this reason, one of the main questions of sea level research at KNMI is: where and how fast do ice shelves melt due to warm seawater?

Worldwide, there is only one institute with the technical and scientific expertise for this: the British Antarctic Survey in the United Kingdom. With decades of experience in drilling with hot water, they, together with a team from South Korea, traveled to the grounding line of the Thwaites Glacier. An earlier attempt four years ago had to be aborted because the glacier was inaccessible due to too much sea ice.

For the first time, the temperature of the seawater beneath the kilometer-thick floating ice shelf near the grounding line has been measured

This year they succeeded in reaching the inhospitable area and conducting the crucial seawater measurements at the Thwaites grounding line. It took 48 hours to form a borehole 30 centimeters in diameter and 1000 meters deep (image 3). Through this borehole, they lowered measuring equipment into the underlying seawater and made the very first measurements of water temperatures and melt rates at this most critical point around Antarctica. Although not all measurement activities succeeded - other sensors got stuck in the freezing borehole - this campaign was an unprecedented success.

Although an official publication of the measurements will take some time, initial reports have already been released. These confirm that the seawater at the grounding line is about as warm as the deep, warm seawater in front of the ice shelf. This is consistent with calculations from our computer model and those of other groups worldwide.

The warm water penetrates beneath the ice shelf up to the grounding line

The measurements show how vulnerable Antarctica is to warming seawater, as it penetrates to the deepest parts under the ice shelf. At the same time, they show that we are on the right track with model development and that our models realistically represent the complex dynamics in Antarctica. This measurement campaign is therefore a turning point - in a positive sense - in our increasing understanding of the Antarctic ice sheet. Furthermore, this success once again demonstrates the importance of international cooperation in gathering collective knowledge.

KNMI climate report by Erwin Lambert