Jun 14 2018

New Review of Antarctic Ice

A new review of the past few decades of satellite data published in the most recent edition of Nature tells the story of what is happening to ice on Antarctica. The result is probably exactly what you think – the ice is melting, at an accelerating rate. But the details are interesting.

First, we should note that Antarctica is a continent. There is land under all that ice, unlike the Arctic which is floating ice without any land. This makes a huge difference. While Arctic ice melts, the water fits exactly into the space previously displaced by the ice, so does not result in any sea-level rise. There are other effects to be concerned about, such as the effect on ecosystems and the effects of all that fresh water melting into the North Atlantic.

When ice sitting on top of land melts, however, that is new water finding its way to the sea, resulting in direct sea-level rise. As water gets warmer, it also expands, which further causes sea level rise. Also, the weight of all that ice pushes down the land, and when the mass of that ice decreases the land actually lifts up a bit.

The Antarctic ice system is also more complex than the Arctic. There are different glacier systems, which terminate at the ocean, and then there is the surrounding sea ice. There are several ways to estimate the extent of ice also, not just land coverage. Scientists need to also measure the thickness of the ice, the relationship between land and sea ice, the weight of the ice (the gravity it produces), and look at the under side of the glaciers that can erode away from warm sea water.

With that background – what did the new review find?

Glaciologists usually talk of three distinct regions because they behave slightly differently from each other. In West Antarctica, which is dominated by those marine-terminating glaciers, the assessed losses have climbed from 53 billion to 159 billion tonnes per year over the full period from 1992 to 2017.

On the Antarctic Peninsula, the finger of land that points up to South America, the losses have risen from seven billion to 33 billion tonnes annually. This is largely, say scientists, because the floating ice platforms sitting in front of some glaciers have collapsed, allowing the ice behind to flow faster.

East Antarctica, the greater part of the continent, is the only region to have shown some growth. Much of this region essentially sits out of the ocean and collects its snows over time and is not subject to the same melting forces seen elsewhere. But the gains are likely quite small, running at about five billion tonnes per year.

That is a net loss of 187 billion tonnes of ice per year. This is the glacial ice sitting on top of land, the kind that contributes to sea level rise. The review further estimates that this ice loss is contributing to the total sea level rise of 3 mm per year, making it a major contributor, and eclipsing Greenland as a source. In total, since 1992 Antarctica has lost 2.7 trillion tonnes of ice contributing 7.5 mm to total sea level rise.

They further estimate, projecting trends into the future, that we will see 50-60 cm of sea level rise by the end of the century.

Antarctic sea ice also has a somewhat complex story to tell. The paper summarizes their findings:

The waxing and waning of Antarctic sea ice is one of Earth’s greatest seasonal habitat changes, and although the maximum extent of the sea ice has increased modestly since the 1970s, inter-annual variability is high, and there is evidence of longer-term decline in its extent.

The apparent increase in the sea ice surrounding Antarctica may be largely an short term artifact, concealing a longer term trend of decline, which is now apparent with longer periods of observation. But also, the sea ice alone does not tell the real story of Antarctic ice. As the continental glacial ice breaks off and crashes into the sea, that becomes sea ice. So increasing the rate of terminal glacial loss increases sea ice, so if you focus only on the sea ice you might think that ice is increasing.

Even with this factor, however, there is a long term trend of decreasing extent of antarctic sea ice.

We now have multiple satellite systems observing Antarctic ice in several different ways, and when all that data over the last three decades is put together, it tells a clear story. Total Antarctic ice is decreasing, and at an accelerating rate.

In the past climate change deniers have told a deliberately deceptive narrative by focusing on only a subset of the information. Ice in the Eastern glacier has increased modestly, and short term sea ice has increased. They throw out these cherry picked facts to create the impression that ice is increasing in some locations, and decreasing elsewhere, it’s all part of natural variation, and there is nothing to worry about. Sometimes they imply or even explicitly state the false premise that if global warming were real, ice should be decreasing everywhere all the time.

That false premise is also a straw man, because that is not what scientists believe or claim. There are still natural fluctuations, and a complex interaction of many facets of the climate. So you can get a distorted picture by focusing on short intervals of time, or only some locations.

The real picture of global climate change (emphasis on global) emerges when we take the broadest view possible. We need to look at the whole picture over a long time period – long enough that the long term signal rises above short term noise.

This paper does that for Antarctic ice, and the picture that emerges is clear. Antarctic ice is melting, contributing steadily to sea level rise.

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