Science & Technology

Warm waters are accelerating Greenland’s glacier retreat

To measure water depth and salt content, the OMG project flew a probe into a fjord along the coast of Greenland. Shown here is one such fjord whose glaciers are undercut by warm water. Credit: NASA / JPL-Caltech

Scientist NASAThe Oceans Melting Greenland mission explores the depths of the island’s warming coastal waters to better predict future sea level rise.

Greenland’s melting glaciers plunge into the Arctic Ocean via steep coves, or fjords. Major factors in rising world sea levels In response to climate change. A better understanding of how warming seawater affects these glaciers will help improve their fate predictions. Such predictions could be used in turn by communities around the world to better prepare for floods and mitigate damage to coastal ecosystems.

However, researchers have long lacked measurements of fjord depth along the rocky coast of Greenland. Without this information, it would be very difficult to accurately assess the amount of seawater allowed in the fjord and how it would affect glacier melting. By measuring their fjords one by one Science Advances For the first time quantified how warming coastal waters affect Greenland’s glaciers.

OMG mission

The OMG mission also measured the depth and salt content of the Greenland fjords, which are occupied by end-of-sea glaciers by boat. These measurements complement the measurements provided by probes dropped from the aircraft to create the most comprehensive survey of melting glaciers in Greenland to date. Credit: NASA

For the past five years, scientists on the Oceans Melting Greenland (OMG) mission have studied these marine termination glaciers in the air and by ship. Of the 226 glaciers surveyed, 74 in the deep fjord accounted for almost half of the total ice loss from Greenland (previously monitored by satellite) between 1992 and 2017. I found. These glaciers showed the most undercuts. The salty water at the bottom of the fjord melts the bottom of the glacier and crushes the ice above. In contrast, 51 glaciers extending into shallow fjords or shallow ridges had the least undercuts, contributing only 15% of total ice loss.

“I was surprised at how biased these findings were. The largest and deepest glaciers are undercut much faster than the smaller glaciers in shallow water,” said the study as a postdoctoral student at the University of California, Irvine. Said Michael Wood, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Southern California. Irvine, California. “In other words, the largest glaciers are most sensitive to warming water, and they are what actually drive Greenland’s ice loss.”

For Greenland glaciers, the larger the glacier, the faster it melts. And the culprit is the depth of the fjord they occupy. Deep fjords take in warmer seawater than shallow fjords, accelerating the undercut process.

Undercut and delivery

Greenland has one of only two ice sheets on the planet. The ice there is over 2 miles (3 km) thick in some places. At the edge of Greenland, vast glaciers extending from the ice sheet slowly move down the valley like ice conveyor belts, pouring into fjords, melting or breaking (or delivering) as icebergs. The ice is replenished by snowfall, which is compressed into ice packs over time.

Greenland iceberg

Warm sea water accelerates the rate at which Greenland glaciers melt and deliver, or break to form icebergs. This causes the glacier to recede toward land, accelerating the loss of ice from the Greenland ice sheet. Credit: NASA / JPL-Caltech

If the ice sheet is balanced, the amount of snow that accumulates on top will be about the same as the ice lost due to melting, evaporation, and labor. However, previous observations have shown that the ice sheet has been out of balance since the 1990s. Melting is accelerating and labor is increasing. In other words, the percentage of ice lost in the ocean exceeds the supply from the ice sheet. This causes the ice sheet to shrink and the glacier to recede towards land.

The main causes of such glacier retreat are the undercut process caused by two factors: the amount of snowmelt flowing from the glacier and the warm layer of saltwater at the bottom of the fjord. During the summer, when the temperature rises, the glacier surface heats up, creating a pool of snowmelt. These pools leak from the ice and drain from the glaciers of the river below the surface. As the snowmelt water flows into the sea, you will encounter warm salt water at the bottom of the fjord.

Since glacier melt water does not contain salt, it is less dense than salt water and rises as a plume. The plume drags warm sea water into contact with the bottom of the glacier. The amount of undercut depends on the depth of the fjord, the warmth of the seawater, and the amount of snowmelt flowing from under the glacier. As the climate warms, the amount of snowmelt water increases and the seawater temperature rises. There are two factors that drive the undercut process.

The presence of warm seawater and snowmelt from the glaciers that plunge into the Greenland fjords melt the glaciers from below. This is a process called undercut, which breaks debris as icebergs. OMG Principal Investigator Josh Willis explains how the process works in this animation. Credit: NASA /JPL-California Institute of Technology

These findings suggest that glacier ice loss may be underestimated at least twice if climate models do not take into account undercuts from warm seas.

The study also provides insight into why many of Greenland’s glaciers did not recover after the rapid warming of seawater, which rose nearly two degrees between 1998 and 2007. Celsius.. Ocean warming paused between 2008 and 2017, but glaciers have already experienced extreme undercuts in the last decade and have continued to accelerate and retreat.

“We have known for more than a decade that warm waters play a major role in the evolution of the Greenland ice sheet,” said Eric Lignott, OMG Deputy Senior Researcher at UCI and JPL. I will. “But for the first time, we were able to quantify the undercut effect and demonstrate its dominant impact on glacier retreat over the last two decades.”

To the depth of ice

In its sixth year, the OMG mission has performed a huge task of measuring seawater temperature and salt content across the Greenland coast. Every summer from 2016, the team drops 250-300 probes from the aircraft and measures how water temperature and salt change with depth, mapping fjord depths that are otherwise inaccessible. Spent a few weeks for.

This data complements other surveys in the region, including OMG measurements by boat (started in 2015) and observations collected from NASA and the US Geological Survey’s Landsat satellite. Glacier research growth About the interaction between ice and the sea. During this time, the OMG team was able to get a detailed view of how quickly the 226 glaciers surveyed melted and which retreated fastest.

OMG is planning a campaign in the summer of 2021. The team hopes that continuous measurements of ocean conditions will be invaluable for refining the prediction of ice loss and will ultimately help prepare the world for the rising ocean future. I will.

“When the ocean speaks, the Greenland ice sheet listens,” said Josh Willis, also JPL’s senior researcher at OMG. “This gang of 74 glaciers in the deep fjords really feels the influence of the ocean. It’s these discoveries that help us predict how fast the ice will eventually shrink. And It’s an important tool for both this generation and the next generation. “

Reference: Michael Wood, Eric Lignott, Ian Fenty, Lu’an, Anders Björk, Michelle van den Broke, Tiran Kai, Emily Kane, Dimitris Menemenris, Romane Milan, Matthew Morigem, “Marine coercion promotes glacier retreat in Greenland” by Jeremy Muginot Brice Noël, Bernd Scheuchl, Isabella Velicogna, Josh K. Willis, Hong Zhang, January 1, 2021 Science Advances..
DOI: 10.1126 / sciadv.aba7282

Warm waters are accelerating Greenland’s glacier retreat Warm waters are accelerating Greenland’s glacier retreat

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