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Scientist says Earth's in 'new phase' of polar melting

• 21 Nov 2011, 16:13
• Verity Payne

According to British Antarctic Survey scientist Dr Dominic Hodgson we are currently in "a new phase of polar deglaciation".

Dr Hodgson's research compared Arctic and Antarctic ice shelf retreat over the last twelve thousand years and found that the last few decades is the only point in that period when ice shelves have retreated simultaneously at both poles. Ice shelves are floating bodies of ice, ranging from fifty to several hundreds of metres in thickness, that stick out to sea from continental ice sheets, like the ones covering Greenland and the Antarctic.

There have been major ice shelf break-ups in both the Arctic and Antarctic recently. The largest remaining Arctic ice shelf, the Ward Hunt Ice Shelf on Ellesmere Island in Canada, has spent the last decade disintegrating, releasing billions of cubic metres of fresh water and massive icebergs into the Arctic Ocean. Ice shelves have also been observed in retreat on the Antarctic Peninsula, with the Wilkins Ice Shelf observed to be breaking up, and the Larsen Ice Shelf retreating over the last decade.

Hodgson's commentary paper, published ahead of print on the website of US journal Proceeding of the National Academy of Sciences (PNAS), discusses a selection of recent research into ice shelf retreat. He raises the interesting point that ice shelf retreat has not happened in both polar regions at the same time, according to the 12,000 year records for both Arctic and Antarctic ice shelves which have recently retreated. In contrast, the past few decades have seen ice shelf retreat at both poles at the same time.

It is this pattern, along with the enhanced regional warming observed at the Arctic and Antarctic Peninsula, increased glacial melting and decrease in Arctic sea ice cover that leads Hodgson to suggest that:

"This first synchronous retreat of ice shelves is significant, not because of ice shelf contributions to global sea level, which are relatively minor, but because it is linked to wider changes in the cryosphere, the configuration of ocean currents, and both atmospheric and ocean temperatures."

Hodgson's thoughts echo another research paper, written by a Swedish geologist and published earlier this year, which reported that the current warming trend is the first instance over the last twenty thousand years in which both hemispheres have warmed simultaneously.

It is studies like these, studying records of geological climate change, which are part of the way scientists establish that the climate change we are currently experiencing is singular.

Unfortunately this conclusion is often lost when this sort of science is reported. A recent example of this is the Mail Online article which covered research looking at the twelve thousand year history of the Ward Ice Sheet. The Mail wrote:

"Images of the Arctic ice shelf cracking up are an icon of the damage wrought by global warming.

"But a team of researchers from the Universite Laval in Canada have found evidence that one ice shelf might have broken up before, 1,400 years ago - long before industrialisation had any impact on the planet."

The research found that the ice shelf grew and retreated naturally over the 12,000 year period, and for much of the time was absent altogether. Many Antarctic ice shelves show similar natural variability over the same time period.

The suggestion that because the climate has varied in the past the current warming trend in global average temperature must be natural is often made. But noting that something happened before without human influence does not demonstrate that humans are not causing it today.  

The world's climate has varied in the past, for many different reasons, some better understood than others. Over geological timescales, factors such as sun brightness, changes in the Earth's orbit, and greenhouse gas levels have all altered our climate. But even taking these factors into account, the climate change we are currently experiencing is singular and cannot be explained from natural variation alone - a human influence must be involved.