News in Science

Antarctic warming a tale of two ice cores

Ice melt Ice cores taken from two regions of the Antarctic give a mixed story on the impact of human-induced climate change on the icy continent.

Summer ice melting on the Antarctic Peninsula is at its highest in 1000 years, report scientists in today's Nature Geoscience journal.

But an analysis of the West Antarctic Ice Sheet stretching back 2000 years indicates "rapid ice loss may not be all that unusual" in that area of the continent, report another group of scientists in the same journal.

The studies add to the international effort to better understand the causes of environmental change in Antarctica and to make more accurate projections about the direct and indirect contribution of Antarctica's ice shelves and glaciers to global sea level rise.

Dr Nerilie Abram, of the Research School of Earth Sciences at the Australian National University, says her team studied a 364-metre ice core drilled from James Ross Island on the Antarctic Peninsula.

"This is an area right near the tip of the Antarctic Peninsula," she says. "The reason we went to this place is it is a region that is warming faster than anywhere else in the southern hemisphere over the past 50 years."

By examining the ice core, the team wanted to track how unique this warming was.

'Rare record'

However when they extracted the ice core, the researchers realised it also contained a "rare record" of melting events -- only the second of its kind from the Antarctic continent.

Abram says the core shows summer ice melting has increased 10-fold in the past 1000 years with the most rapid increase occurring in the past 50 years.

The coolest conditions and lowest melt at the ice core site occurred from about AD 1410 to 1460 when mean temperatures were about 1.6°C cooler than between 1981 to 2000.

"At that time (1410) around 0.5 per cent of snow that fell each year melted and refroze," says Abram.

While temperatures have gradually warmed since then "the melt record actually doesn't show large changes in melt until we get to the warming over the past 50 years", she adds.

Today about 5 per cent of annual snowfall melts each year, Abram says.

Critically, the record shows a "non-linear relationship" between ice melt and climate warming, she says.

"What that means is that the Antarctic Peninsula has warmed to a level where even small increases in temperature can now lead to a big increase in summer ice melt. This has important implications for ice instability and sea level rise in a warming climate."

However, Abram says the findings for the Antarctic Peninsula cannot be extrapolated to the whole southern continent.

She says while warming in this area can be attributed at least partly to human-induced climate change and the resultant strengthening of westerly winds, changes to the West Antarctic Ice Sheet cannot be so easily linked to this.

2000-year record

This view is supported by the second study published in Nature Geoscience led by Professor Eric Steig at the University of Washington.

His team, which included Dr Ailie Gallant, from Monash University, analysed a new ice core from the West Antarctic Ice Sheet Divide that goes back 2000 years, along with a number of ice cores dating back 200 years.

The divide is the highest point on the West Antarctic Ice Sheet and marks the division where ice "flows" toward either the Weddell or Ross seas.

Gallant says their work focused on the isotope, oxygen 18; higher levels of oxygen 18 indicate higher air temperatures.

She says while they discovered large increases in temperatures during the 1990s, there were several decades that exhibited similar climate patterns in the past 200 years.

"Then when we looked over 2000 years, there were a few other blips in about 1 per cent of the record," says Gallant.

"What this tells us is that what we saw in the 1990s is very unusual but not necessarily unprecedented."

Steig says while recent changes in climate and ice thinning are dramatic in the West Antarctic Ice Sheet, they cannot be attributed with confidence to human-induced global warming.

But the same is not true for the Antarctic Peninsula where rapid ice loss is even more dramatic, he adds.

The research shows the West Antarctic climate of the 1940s and 1830s would be similar to modern conditions. Along with the 1990s, these decades were also periods of unusual El Niño activity.

Gallant says their research suggests these decadal variations in temperature are linked to wind circulation patterns in the tropical Pacific Ocean and sea surface temperatures.

She says the study highlights the need to better understand how tropical Pacific climate will change in the future as it will influence what happens to the West Antarctic Ice Sheet.