melting | Eco Bio III Millennio

The Antarctic ice shelf is under threat from a silent, invisible agency – and the rate of melting of glaciers has trebled in the last two decades.

The ocean waters of the deep circumpolar current that swirl around the continent have been getting measurably warmer and nearer the ocean surface over the last 40 years, and now they could be accelerating glacier flow by melting the ice from underneath, according to new research.

And a separate study reports that the melting of the West Antarctic glaciers has accelerated threefold in the last 21 years.

West Antarctic ice sheet – a potential 4.8m of sea level rise

If the West Antarctic ice sheet were to melt altogether – something that is not likely to happen this century – the world’s sea levels would rise by 4.8 metres, with calamitous consequences for seaboard cities and communities everywhere.

Researchers from Germany, Britain, Japan and the US report in Science journal that they base their research on long-term studies of seawater temperature and salinity sampled from the Antarctic continental shelf.

This continued intrusion of warmer waters has accelerated the melting of glaciers in West Antarctica, and there is no indication that the trend is likely to reverse.

Other parts of the continent so far are stable – but they could start melting for the first time. “The Antarctic ice sheet is a giant water reservoir”, said Karen Heywood, professor of environmental sciences at the University of East Anglia, UK.

“The ice cap on the southern continent is on average 2,100 metres thick and contains 70% of the world’s fresh water. If this ice mass were to melt completely, it could raise global sea level by 60 metres. That is not going to happen, but it gives you an idea of how much water is stored there.”

Temperatures in the warmest waters in the Bellinghausen Sea in West Antarctica have risen from 0.8°C in the 1970s to about 1.2°C in the last few years.

“This might not sound much, but it is a large amount of extra heat available to melt the ice“, said Sunke Schmidtko, an oceanographer at the Geomar Helmholtz Centre for Ocean Researchin Kiel, Germany, who led the study. “These waters have warmed in West Antarctica over 50 years. And they are significantly shallower than 50 years ago.”

Unpredictable consequences on ice and ecology

The apparent rise of warm water, and the observed melting of the West Antarctic ice shelf, could be linked to long-term changes in wind patterns in the Southern Ocean. Although melting has not yet been observed in other parts of the continent, there could be serious consequences for other ice shelves.

The shelf areas are also important for Antarctic krill – the little shrimp that plays a vital role in the Antarctic ocean food chain – as they serve as protective ‘nurseries’ for the young crustaceans. Warming ice shelves may have unpredictable consequences for spawning cycles, krill abundance, and wider ocean biodiversity.

Meanwhile, according to US scientists writing in Geophysical Research Letters, the glaciers of the Amundsen Sea in West Antarctica are shedding ice faster than any other part of the region.

Tyler Sutterley, a climate researcher at the University of California Irvine, and NASA space agency colleagues used four sets of observations to confirm the threefold acceleration.

They took their data from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites, from a NASA airborne project called Operation IceBridge, from an earlier satellite called ICESat, and from readings by the European Space Agency’s Envisat satellite.

Glaciers losing 16 billion tonnes of ice a year

The observations spanned the period 1992 to 2013 and enabled the researchers to calculate the total loss of ice, and also the rate of change of that loss. In all, during that period the continent lost 83 billion tonnes of ice per year on average.

After 1992, the rate of loss accelerated by 6.1 billion tonnes a year, and between 2003 and 2009 the melt rate increased by 16.3 gigatonnes a year on average. So the increasing rate of loss is now nearly three times the original figure.

“The mass loss of these glaciers is increasing at an amazing rate”, said Isabella Velicogna, Earth system scientist at both UC Irvine and the NASA Jet Propulsion Laboratory.

Tim Radford writes for Climate News Network.

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Ice sheets respond slowly to changes in climate, because they are so massive that they themselves dominate the climate conditions over and around them.

But once they start flowing faster towards the shore and melting into the ocean the process takes centuries to reverse. Ice sheets are nature’s freight trains: tough to start moving, even harder to stop.

We know this process has been going back and forth throughout history – it’s why we’ve had ice ages and warm periods. But until now we haven’t known exactly how quickly ice sheets retreated and reformed.

New research published in the journal Nature Communications gives us an answer, and it isn’t great news.

It turns out sea levels often rose at scary rates in response to natural climate changes, long before mankind began pumping carbon into the atmosphere.

In the short-term sea level is affected by ocean warming and so-called ‘thermal expansion‘, or melting glaciers based on land. These changes can occur quickly – within a decade – but their impact on sea level is relatively small, in the tens of centimetres.

Collapsing ice sheets can cause big sea level rises

The drivers of longer-term sea level rise, over decades or centuries, are the continental ice sheets of Greenland and Antarctica.

On the fringes of these ice sheets are ‘ice shelves’ stretching far out into the ocean. Ice shelves can be hundreds of meters thick and, because 90% of ice in water floats below the surface, they remain ‘grounded’ on the sea floor as long as the sea is less deep than 90% of the ice shelf thickness.

Where the sea floor is deeper or the ice shelf gets thinner, there will be an area of floating land ice; here, warming ocean water can get underneath and melt the ice. Once sufficiently destabilised, an ice shelf can break up catastrophically.

Such an ice shelf collapse takes the brakes off the ice stream that feeds into the ice shelf, and land ice starts to flow much quicker towards the ocean.

Ice flow is a relatively slow process, and it takes some forcing to get a major ice sheet to systematically respond (like trying to set a fully loaded freight train into motion). Once moving, however, it will be equally hard to arrest that movement (like trying to stop a moving, fully loaded freight train).

Still, we cannot ignore it, because the sheer volume of land ice on Earth is enormous – equivalent to more than 65m of global sea level rise; Greenland alone accounts for 6 to 7m, West Antarctica for some 5-6m, and East Antarctica for the remainder. These melting ice sheets will dominate major sea level changes for centuries to come.

Diving into deep-sea data

We can learn something about what to expect by examining sea level changes during the past five ice-age cycles (past half million years), especially through comparing them with the total amount of ice on the planet at the time.

During a peak ice age, Earth held almost three times as much land ice as it holds today. For instance, during the most recent ice age the ice sheet over North America was 10-20% larger than the one we see today over all of Antarctica.

During warm periods in between ice ages the sea was often close to its present level but occasionally reached up to 8 or 9m above today’s shoreline – the equivalent of melting 1.3 Greenlands today.

To get a sense of how quickly the sea went up and down, we need highly detailed and well-dated records. Over the past decade I’ve led a team of scientists at the University of Southampton and the Australian National University who have developed such records using data from the Red Sea.

The Red Sea has a very shallow and narrow connection with the open Indian Ocean. It also evaporates quickly – the equivalent of 2m of water each year – so new water must constantly flow in to top up sea levels and to avoid it getting too salty.

But such inflow is restricted by the tiny gap between Djibouti and Yemen, and in the past that connection was even smaller. As a result, the Red Sea was much saltier during previous ice ages, when sea level stood more than 100m below the present.

Using microfossils from drill cores from the sea floor we can measure salinity through time and translate this to sea level changes in the Red Sea connection with the Indian Ocean. We were able to assess timings more accurately by comparing these sea level records to climate records from caves, which can be precisely dated by looking at radioactive decay in uranium.

Sea level rise by the metre

So now we had a detailed sea level record, with a well-defined timescale. Finally, we could work out rates of past sea level changes, and compare changing sea levels with well-dated reconstructions of temperature and CO₂ changes (from ice cores).

This allowed us to assess the speed of some 120 sea level rises in the past. Previously, this was possible only for one recent event. Now, for the first time, we had the information to look at how sea levels responded to natural climate change.

It appears the sea level could rise as quickly as 5.5m per century. However this only happened at the abrupt endings of ice ages, starting with about three times the modern ice volume. When starting with double the modern ice volume or less, sea levels did not rise faster than 2m per century. When global ice volume was similar to the present, the sea typically rose less than 1 to 1.5m per century.

So it seems the fastest losses of ice occur when there is more ice. Not much of a surprise, perhaps, but now at least we have some real numbers to say how fast, and how much ice.

And the speed the sea can rise during periods with modern ice volumes is still worrying – a 1m rise this century would hugely affect millions of people. Given that Earth has achieved these rates even when warming was much slower than today, such a rise is very possible.

How long will it take?

In the 120 different events we looked at, ice sheets went from initial change to maximum retreat within 400 years 68% of the time, and within 1,100 years for 95%. In other words: once triggered, ice sheet reduction (and therefore sea level rise) kept accelerating relentlessly for many centuries.

Research we carried out previously found that modern sea level rise seems to be conforming to what we would expect from (high end) natural responses to warming.

That is: after 150 years of increasing (man made) warming, the ice sheets would only recently be reaching the point where they start making a noticeable contribution to sea level rise.

But that time has come and, once ice sheets start to melt, the freight train is in motion. It will then keep moving for many centuries to come, no matter how hard we stamp on the brakes.

Eelco Rohling is Professor of Ocean and Climate Change at the University of Southampton. He receives funding from the UK Natural Environment Research Council, and from the Australian Research Council.

This article was originally published on The Conversation. Read the original article.

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