Glaciers Are Once Again Retreating on a Worldwide Basis
© Global Warming Images / WWF
Fast-forwards to the past
13 June 2019
Nosotros often hear near how our apace changing climate is already accelerating the melting of glaciers and ice sheets across the globe. Only what nigh the opposite: could the melting polar ice sheets themselves disrupt climate, creating a feedback loop that leads to runaway environmental change? To explore that idea, NICK GOLLEDGE brought together an international squad of climate scientists—and the results were alarming.
Inside A FROZEN TENT at the end of July 2010, Dorthe Dahl-Jensen, a Danish paleoclimatologist, announced the completion of the N Greenland Eemian Water ice Drilling (NEEM) project. The team had finally hit bedrock, 2,537 metres below the surface of the Greenland Ice Canvass. Dahl-Jensen and her team had spent 3 summers camped high on the meridian plateau, painstakingly drilling to recover cores that would tell them how the ice canvas had changed during the Eemian period 125,000 years ago.
This interval—the final time World was warmer than information technology is today—is of import because it poses a worrying question. Boilerplate air temperatures then were simply a degree or so higher up current temperatures, yet the global sea level was more than 6 metres higher than information technology is now. How could this be possible? As we caput toward an artificially warmer world, can the NEEM ice cadre records give united states of america a window into our future?
1 fashion to address this question is to utilise computer models to simulate these periods of the past, and then use the same models to project into the futurity. For this we demand three things: bear witness of how the earth (especially the polar water ice sheets) looked during warmer periods; predictions of how the climate may change in the future; and a numerical ice-sheet model that incorporates the necessary physical equations to accurately simulate the catamenia of an entire ice sheet.
In our recent work, we used a model developed by a team at the University of Alaska, Fairbanks. Over the last 10 years, nosotros have used this model to predict how both the Greenland and Antarctic ice sheets have evolved in the past, besides as how they might change in the time to come. Past checking our by simulations confronting geological records equally well as ice cadre records from sites such as NEEM, we accept developed conviction that the model can be used reliably.
DYNAMIC THINNING
Climate models suggest that our current governmental commitments to mitigating greenhouse gas emissions will atomic number 82 to average global warming of 3°C to
4°C past the end of the century. When nosotros used our model to simulate how the ice sheets would respond to this warming, nosotros found they lost ice, not only through melting at their surface, but likewise through dynamic thinning, a process through which ice-sheet outlet glaciers are melted by rut from seawater, thinning their lowest regions. Considering this thinning happens most swiftly at the terminus, or end of the glacier, the overall gradient of the ice surface gets steeper, which in turn encourages the ice to flow more than quickly and discharge even more of its mass into the ocean.
In Greenland, our model suggested that this would happen to the greatest extent in the northwest sector of the water ice sheet. Using a climate model, we so calculated that this meltwater would ultimately detect its way into the North Atlantic. And this is where the problems begin: even though the amount of meltwater released is tiny compared with the volume of the ocean, it is buoyant because it contains no table salt, then information technology floats on the sea surface. Normal sea circulation relies on a process known as convective overturning, in which relatively warm water rises from depth at loftier latitudes, releases heat to the temper and, once cooler, begins to sink.
But this sinking relies on the water being salty, and therefore dense. Adding freshwater, even if information technology is common cold, prevents this sinking process. As the meltwater from Greenland flows due south, it meets warm water from the Gulf of Mexico existence carried north. When the Gulf Stream moves east, information technology releases so much rut into the temper that northwest Europe is kept far warmer than areas at comparable latitudes in eastern Canada. As this electric current slows down, however, this estrus transport reduces, and northwest Europe cools. Simply the heat in the ocean remains—and it has to go somewhere.
A DANGEROUS POSITIVE FEEDBACK LOOP
With the rate of sinking suppressed, there is less vertical mixing in the h2o cavalcade, so the warmer, deeper layers retain their heat. This heat is specially dangerous for ice sheets, considering many outlet glaciers flow through deep troughs that tin can exist hundreds of metres beneath sea level and below the cold sea surface. Instead, these glaciers are flowing into the deeper, warmer parts of the h2o column, where the extra oestrus—trapped by the reduced overturning—accelerates the melting of the submarine water ice fronts. The glaciers once again go steeper, menses more than chop-chop and discharge even more ice.
Nosotros found that meltwater from the Greenland Ice Canvass can accelerate its own retreat past up to 40 per cent.
These processes course a cocky-reinforcing loop, also known every bit positive feedback. Even so the complexity of combining water ice sail models with global climate models has meant that this feedback was ignored in previous predictions. Past including this loop, nosotros found that meltwater from the Greenland Ice Canvas can accelerate its ain retreat by upwards to 40 per cent.
Furthermore, the disruption to sea apportionment acquired by water ice sheet meltwater extends beyond the Arctic, reducing air temperatures in parts of Siberia and the Aleutian Islands, but producing con- siderably warmer temperatures across Svalbard and equally far north equally the North Pole. Worse withal, the belch of ice sheet meltwater into the oceans seems to upset circulation patterns in a way that can dilate yr-to-year climate variations, producing unreliable weather that can exist far warmer or colder than average from i year to the side by side.
Contempo research has shown that the levels of warming expected over the coming centuries have no analogue in the recent past. In fact, conditions by the terminate of this century will be similar to those last seen three 1000000 years ago, when the sea level was effectually twenty metres higher than information technology is now. More than
alarmingly, some studies have suggested that positive feedbacks could trigger an unstoppable chain of events that would lead to a "hothouse" globe entirely different from the one we inhabit today.
Whether nosotros cull to fast-forward into that kind of future is down to united states. But evidence from the by suggests that unless we rapidly reduce our emissions, the changes we are setting in move now may play out for hundreds—or more likely, thousands—of years.
NICK GOLLEDGE is an associate professor in the Antarctic Research Centre at Victoria University of Wellington in New Zealand. He uses computer simulations of the Greenland and Antarctic ice sheets to understand how they behave nether unlike climates and what this might hateful for global ocean levels.
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Source: https://arcticwwf.org/newsroom/the-circle/arctic-check-up/fast-forward-to-the-past/
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