Why Are Glaciers Melting from the Bottom? It’s Complicated

Glaciers in Greenland and Antarctica are losing ice at alarming rates, and warmer air isn’t the only cause.

Scientists increasingly agree that warm ocean water is seeping beneath the ice and melting it from the bottom up.

It’s a growing problem in Greenland, scientists say, and it may already be the dominant driver of melting glaciers in Antarctica. But while scientists know that warm water is interacting with the ice front in both places, exactly what’s driving it there—and how climate change may be involved—is still an open question.

Scientists agree it’s not just the warming of the oceans, overall, that’s driving the process. Rather, complex systems involving winds and ocean circulation patterns are helping to drive naturally occurring warm water closer to the ice edge. Now, scientists are working to figure out exactly why that’s happening, whether climate change is playing a role, and what might be in store for the future of the world’s biggest ice sheets.

Trouble at the South Pole

Striking new research published last week in Nature Geoscience has once again raised the alarm about Antarctic melting and the ocean’s growing influence. An ambitious mapping project demonstrates that glaciers all around the Antarctic coastline, and particularly in West Antarctica, are retreating inland (Climatewire, April 4).

Scientists already generally agree that glacier retreat in Antarctica is largely being driven by warm water seeping underneath the ice—the process has been demonstrated by multiple studies in the last few years. As the ice melts, the point where it attaches to the bedrock at the bottom of the ocean (commonly known as the “grounding line”) recedes inland, which can cause the glaciers to become less stable and lose more ice over time.

The study only shows the extent of the ice losses and doesn’t delve into the exact mechanisms driving it. But by highlighting the amount of damage that’s actually occurring—the maps suggest that Antarctica is losing about 80 square miles of grounded ice area each year—it once again raises the question of what’s driving all that warm water. Determining exactly where it’s coming from, and how its influence could change in the coming decades, is one critical way scientists can improve their predictions about ice loss and sea-level rise for the future.

In an interview with E&E News about the research last week, lead study author Hannes Konrad of the University of Leeds noted that the process likely has less to do with the gradual climate-driven warming of the ocean than with specific physical processes that drive naturally occurring warm water to the ice front.

“It’s rather that in events, or in certain episodes, warm water is somehow pumped toward these glaciers and drives their retreat,” he said. “But the question is, why does this warm ocean water from time to time reach under the ice shelf?”

Increasingly, scientists believe that the winds around Antarctica are a big part of the answer. These winds can help drive the flow of naturally occurring warm water around the continent and, in the right conditions, push it closer to the ice sheet.

Much of the warm water affecting the Antarctic ice sheet is believed to belong to a large, naturally occurring warm mass known as “circumpolar deep water.” Originally formed from the mixing of waters originating in other, warmer parts of the globe, circumpolar deep water is now a fixture in the Southern Ocean.

This water mass is very salty and dense, causing it to sink beneath the colder, less dense water closer to the surface of the Southern Ocean. It can be found all around the perimeter of Antarctica, typically at a depth of around 1,600 feet below the surface, according to University of Washington glacier expert Eric Steig. It’s this circumpolar deep water that’s believed to be driving glacier melt around the continent.

“Although circumpolar deep water has probably warmed about a tenth of a degree in the last few decades, this warming is probably not the cause of the observed changes in the glaciers,” Steig said in an email to E&E News. “What is far more important is whether the already-warm water gets to the glacier front, under the floating ice shelves.”

Multiple studies in the last few years have suggested that changes in wind patterns around Antarctica can alter the ocean currents driving the movement of this warm bottom water, sometimes causing more of it to well up around the ice sheet than is usual.

Some of these studies indicate that temporary changes in these wind patterns may be strongly linked to the shift between El Niño and La Niña conditions. El Niño and La Niña events cause warming and cooling effects in different parts of the Pacific Ocean, and these heat transfers can in turn temporarily alter certain atmospheric circulation patterns in the tropics, which have rippling effects on winds as far south as Antarctica.

paper published earlier this year in Nature Geoscience, for instance, suggested that El Niño years are associated with greater rates of snowfall, but also higher rates of bottom melt in West Antarctica, likely linked to related changes in wind patterns. Other studies in the last few years have come to similar conclusions about El Niño-related increases in Antarctic melt rates.

A 2014 paper in Science, on the other hand, found that a strong La Niña event in 2012 may have had the opposite effect, allowing cooler waters greater access to the ice sheet and helping to reduce melt rates.

And it’s not just West Antarctica that’s vulnerable to changes in wind patterns. Recent research suggests that winds around Antarctica help drive bottom melting at Totten Glacier in East Antarctica, as well, a massive glacier sometimes referred to as Antarctica’s “sleeping giant.”

A November study published in Science Advances suggests that Totten’s ice loss tends to be greatest when nearby Antarctic winds are behaving in particular ways, helping to sweep colder surface waters aside and allow warmer bottom water to well up and seep beneath the ice shelf.

Links to climate change

Winds do naturally fluctuate from one year to the next to a certain extent. And the El Niño and La Niña events that may drive temporary changes in melt rates in West Antarctica are also natural phenomena. But Chad Greene, a University of Texas, Austin, glacier expert who led the recent Totten study, notes that climate change is also thought to have an influence on Antarctic wind patterns.

Around Totten Glacier, he notes, some models suggest that the winds driving certain major ocean currents circulating around Antarctica will become more intense as the climate warms, pushing these currents farther south. If this happens, these currents may help to drive cold surface water away from the pole and warm bottom water closer to the ice sheet.

In fast-melting West Antarctica, he noted, wind patterns may be more strongly linked to changes in the tropics. The strong link to El Niño suggests that ocean temperatures in the Pacific may generally be a significant driver of winds and melt rates around this part of the ice sheet. This means that long-term climate-driven warming in the Pacific could also have a gradual effect on the conditions affecting glacier melt.

According to Eric Rignot, a glacier expert at the University of California, Irvine, a major factor in this process is that the tropics are warming at a faster rate than the South Pole. (The opposite is true for the Arctic, which is warming faster than any other part of the planet.) This difference in warming rates causes a change in the temperature gradient between the equator and Antarctica, which alters the way air flows around the globe.

As a result, he said, some research predicts that eastward flowing wind currents may strengthen and push south toward Antarctica, resulting in “more transport of subsurface ocean heat toward the continent.”

According to Steig, the Pacific is already experiencing some changes that may be driving recent increases in warm water upwelling around Antarctica. For instance, he noted, scientists have observed a long-term warming trend in the western Pacific and a recent cooling trend in the eastern Pacific. Additionally, the regions of the Pacific strongly affected by El Niño have recently begun to shift.

But for the time being, he added, the direct connection to climate change is still “complex” and hinges on the extent to which the observed changes in the Pacific—which are in turn affecting the Antarctic—are being driven by human-caused global warming.

“This is an active area of study, but so far, the jury is out on this,” he said.

If these processes aren’t complex enough already, Rignot noted that the factors affecting warm water intrusion in Greenland are completely different from those in Antarctica. Because the Arctic is warming faster than other parts of the world, some research suggests that certain polar wind patterns are also changing. The results can be varied—sometimes, they may send cold polar air streaming south to Europe or the east coast of North America.

But “in other places, like Greenland, it brings tropical waters and warm air masses farther north than it used to,” Rignot noted.

That said, climate-driven changes in polar wind patterns are still something of a hot topic among climate scientists, who are still working to understand exactly how the rapidly warming Arctic might affect air and ocean circulation patterns elsewhere around the globe. And there are other challenges, as well, when it comes to understanding both the Greenland and the Antarctic ice sheets.

Collecting field data on ocean temperatures at the edge of the ice sheets is a challenge in and of itself, Rignot noted, particularly in remote Antarctica. Scientists are also still working to improve the ocean circulation models they use to predict how climate change will affect these processes.

“But I’m confident it’s going to change,” he added. “These models are coming along; there’s a lot more concerted effort to tackle these issues.”

That’s in large part because ice-ocean interactions have become such a critical component of climate scientists’ predictions about ice loss and sea-level rise. In Antarctica especially, ocean-driven melting is thought to be the dominant driver of ice loss. Because Antarctica isn’t currently warming as fast as some other parts of the world—and certainly not as fast as the Arctic—surface melt on the top of the ice sheet is less of an immediate problem (although with continued warming, it could certainly become a bigger factor).

In rapidly warming Greenland, the majority of ice loss still comes from surface melting, likely driven by rising air temperatures. But ocean-driven glacier melt is thought to be a growing driver and may compound the losses caused by the warming atmosphere.

In both places, the influence of warm water remains a convoluted research area, far more complicated than can be explained away by the overall warming of the oceans alone. Understanding where the water comes from and why, and even the natural processes that drive it to the ice edge, is complex enough. And scientists are still working to determine the extent to which human-caused climate change has already affected those processes and how they could change in the future.

But researchers have already made substantial progress in recent years, as the issue has come to the forefront of scientists’ attention, Rignot said. And he believes the growing interest will continue to advance the science quickly.

“I think we’re going to make a lot of progress in the coming years,” he said.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.

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