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Roz Pidcock

Roz Pidcock

15.04.2013 | 11:00am
ScienceAntarctic peninsula melt “unique in the past 1000 years”, and other lessons in ice sheet melt
SCIENCE | April 15. 2013. 11:00
Antarctic peninsula melt “unique in the past 1000 years”, and other lessons in ice sheet melt

Along with the rest of the world, Antarctica is warming up. Two new studies show the effect of warming on ice loss in two different regions of Antarctica – and they tell slightly different stories. The Antarctic Peninsula is showing clear signs of human activity but in West Antarctica it’s more difficult to see. So why the difference?

Antarctic Peninsula

Satellite data shows from 1992 to 2011, Antarctica lost more than 70 billion tonnes of ice, and the rate of loss is now twice as high as it was in the 1990s. But ice isn’t being lost at the same speed across the continent.

The first new paper examines changes in ice melt in the Antarctic Peninsula – the northernmost part of the Antarctic mainland. The research, led by Nerilie Abram from the British Antarctic Survey, is published this week in the journal Nature Geoscience.

As temperatures rise above zero degrees in summer on the Antarctic Peninsula, the surface of the ice melts and refreezes. To look for changes in the rate of ice melt over time, the scientists examined the thickness of individual melt layers in a 1,000 year old ice core. 

Scientists don’t have temperature records extending back that far but the chemical composition of the ice can also act as a proxy – or natural record – of past temperature. 

The data showed that 600 years ago, temperatures were about 1.6 degrees Celsius below what they are now. During this period – which was the coolest in the 1,000 year record – just 0.5 per cent of the ice that accumulated on the peninsula melted in summer.

That’s in sharp comparison to now. According to the research, summer melting rates are about five per cent – that’s a ten-fold increase in 600 years.

Importantly, most of this increase in the speed of surface melt happened in the second half of the 20th century in response to rapidly rising temperatures. Dr Robert Mulvaney, co-author on the paper told us the results show unequivocally:

that the melting we observe at this site in the past few decades has no similar analogue in the past 1000 years – here we can say the level of melting observed today is unique in the context of the past 1000 years.”

 Abram For Roz

Changes in temperature on the Antarctic Peninsula over the past 1,000 years  in green and surface ice melt as a percentage of total snow fall in red. Source: Abram et al., ( 2013)

So why does Antarctic ice loss matter? Mulvaney explains that what’s happening right across the globe is important but, “climate models suggest some regions are more susceptible to human-induced change, and point to the Antarctic Peninsula as particularly susceptible.”

Mulvaney explained how losing ice from the surface of the Antarctic Peninsula destabilises ice shelves – sheets of ice that extend from the land out into the Southern Ocean. These floating ice shelves act as buffers, keeping the ice sheet’s interior in place. If they disappear, the region’s glaciers can speed up, meaning more ice loss. Mulvaney told us:

[T]he loss of Peninsula ice shelves in the past few decades, while not directly adding to sea level (they are already floating), can cause an increase in flow of ice draining from the continent as glaciers lose the buttressing effect of the ice shelves, and flow more rapidly.”

Even though only a small fraction of the snow that falls each year melts, this means the Antarctic Peninsula is losing ice overall.

But how serious is this? Any increase in ice loss will have a relatively small effect on sea-level rise, Mulvaney told us, because there is not a huge mass of ice on the Peninsula. Much more critical for future sea level rise, he says, is West Antarctica, where a much larger mass of ice is available.

West Antarctica

As it turns out, another study in Nature Geoscience this week takes a closer look at the West Antarctic ice sheet. Even though it’s one of the fastest warming places on earth, the impact of human induced warming isn’t as easy to spot, the research suggests.

The scientists, led by Professor Eric Steig of the University of Washington, analysed the oxygen signature of ice core layers going back 2,000 years. This time, the chemical signature acted as a way of examining past changes in atmospheric circulations – which are the main cause of ice melt in the region.

The results showed ice melt since the 1950s increasing in line with rapidly warming atmospheric temperatures, as the graph below shows. However, this was in large part due to a period of particularly strong melting in the 1990s linked to a natural cycle in the tropical Pacific Ocean, known as El Niño.

Two other decades over the past two centuries have seen similar rates of ice melt, according to the research. So although recent melt rates “do not seem to be unprecedented”, they are “near the upper limit of the range of natural variability”.

West Antarctica _Steig

The oxygen signature of the ice core indicates changes in atmospheric circulation in West Antarctica over the past 2,000 years –  the main driver of ice loss.  The top right corner shows temperature change in the region since 1950. Source: Steig et al., ( 2013)

So why is human influence on the West Antarctic harder to see? Ice loss in West Antarctica is driven largely by the El Niño circulation patterns in the tropics. Unlike the Antarctic Peninsula, this produces such large natural fluctuations in ice loss that Stieg concludes it’s not yet possible to clearly detect the signal from human-induced climate change.

But this may change, according to the research, if climate change alters tropical circulation patterns and there’s a knock on effect on West Antarctica. As the researchers say in the paper:

“[How tropical climate will change in the future] represents a significant source of uncertainty in West Antarctic climate, and in the contribution of the West Antarctic ice sheet to sea level over the next century.”

Different regions, different influences

Satellite data show that, as a whole, Antarctica is losing ice rapidly. Earth’s other vast ice mass – the Greenland ice sheet – is also losing ice – far more rapidly than Antarctica. The earth is definitely losing ice overall, with no signs of slowing.

But these two papers are a useful reminder of the complexities of ice sheets and what’s causing them to melt. As with other parts of the world, once you get down to a regional scale, teasing out how the effects of climate change vary gets harder. What is pretty clear in one region may be less so in another.

 

Abram, N. et al., (2013) Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geoscience,  doi:10.1038/ngeo1787

Steig, E. J. et al., (2013) Recent climate and ice sheet changes in West Antarctica compared with the past 2,000 years. Nature Geoscience,  doi:10.1038/ngeo1778

 


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