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Carbon Brief Staff

Carbon Brief Staff

28.08.2012 | 11:10am
ScienceMedia outlets put their own spin on Antarctic ice core records showing recent rapid warming
SCIENCE | August 28. 2012. 11:10
Media outlets put their own spin on Antarctic ice core records showing recent rapid warming

A study published in Nature last week documents temperature changes in the Antarctic Peninsula over the last 15,000 years – and predicts that if recent rapid warming continues, previously stable ice shelves could be threatened. Although the research makes no attempt to attribute the warming effect to any one cause, it would be hard to work that out from some of the coverage of the report online and in the media, which has absorbed the findings into familiar arguments about whether climate change is happening or not.

The ice sheet covering Antarctica is simply enormous, concealing an entire continent and extending into the surrounding ocean as great ice shelves. The research concentrated on documenting temperature changes in just one part of the continent – the Antarctic Peninsula – which has experienced rapid warming over the last few decades.

The study’s authors were, in fact, very careful to put the modern changes, observed in weather station records and satellite images, into the context of longer-term climate trends without any attempt to attribute their causes.

That hasn’t stopped several outlets from imposing their own narrative on the study. Climate skeptic IT blog The Register claims, for example, that this latest research proves “warming is nothing unusual”, despite the paper itself describing warming in recent decades as specifically ‘unusual’. Another skeptic blog, Watts Up With That, headlines its article “Antarctic peninsula was 1.3°C warmer than today 11,000 years ago”.

In contrast to the ‘nothing to see here’ stories, others have inferred that the warming is human-caused. The Australian headlines its article ‘Humans partly to blame for Antarctic ice shelf collapse: study’. NPR, meanwhile, says ‘Humans’ Role In Antarctic Ice Melt Is Unclear’. But the study itself doesn’t discuss the causes of warming.

What the study found

The research uses an ice core drilled from James Ross Island, just off the Antarctic Peninsula. Scientists were able to analyse the chemical composition of tiny air bubbles trapped in the ice to reconstruct temperatures over the last 15,000 years in the region.

The records show that as the earth emerged from glaciation about 11,000 years ago, temperatures on the Antarctic Peninsula reached roughly 1.3 degrees Celsius warmer than today. In these conditions some of the surrounding ice shelves retreated.

In time, the region began cooling and ice shelves regrew. The Antarctic Peninsula experienced stable conditions for a period of about 7000 years before cooling again – with temperatures reaching their lowest about 600 years ago.

The ice core shows that from around 1400, warming began again. While the warming was comparatively slow to begin with, temperatures over the last century rose at a rate of about 1.56 degrees Celsius per century, and the last 50 years saw warming happening even faster.

Over recent decades, a number of ice shelves on the peninsula have collapsed, which the  study links to warming in the region. Dr Nerilie Abram, co-author of the paper, suggests the gradual warming over many centuries left ice shelves “poised for the succession of collapses that we have witnessed over the last two decades”, during the recent rapid warming.

Unusual but not unprecedented warming

Putting recent temperature rise into context, the authors explain that the speed at which the Antarctic Peninsula has warmed over the last few decades “is unusual but not unprecedented”.

Weirdly, the original press release had contradicted this, stating:

“The scientists reveal that the rapid warming of this region over the last 100 years has been unprecedented…”

The press release was quickly changed – although not before The Register pointed it out. Lead author of the study, Dr Mulvaney, told us that this was an error made by an editor that was hastily corrected.

It should be noted though that the term ‘unusual’ might not really encapsulate how rapid the recent warming is. The ice core shows that over the last century, mean temperatures on the peninsula increased by about 1.56 degrees Celsius. The authors say that that this puts the last century in the top 0.3 per cent of hundred year periods from the last 2000, ranked by speed of warming. Or to put it another way, as Realclimate say, the paper shows the most recent warming is faster than 99.7% of any other given 100-year period in the last 2000 years.

So recent warming in the region isn’t unprecedented, but has happened this fast only very infrequently over the past two millennia.

Caused by climate change?

On its own, this one temperature record proves very little about changes happening to the climate on a global scale. It doesn’t tell us about the cause of temperature change in the region, or more broadly.

Factor this study into what’s being observed worldwide, however, and it contributes to the broader picture. Ice sheets in both the northern and southern hemisphere are experiencing loss at the same time – and that is out of the ordinary.

We’ve previously highlighted research comparing Arctic and Antarctic ice shelf retreat over the last twelve thousand years, which shows that the last few decades is the only point in that period when ice shelves have retreated simultaneously at both poles. Adding to this, other research shows that the current warming is the first instance over the last twenty thousand years in which both hemispheres have warmed simultaneously.

So while the Nature study’s ice core record of temperature change doesn’t speculate about causes, it documents recent warming, seen elsewhere in both the north and south hemispheres. Dr Mulvaney and his team predict that if the peninsula continues to warm, previously stable ice shelves will be threatened. And that could change the Antarctic Peninsula beyond anything that’s been witnessed in the last 15,000 years.

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