July 20, 2016

expert reaction to new research on Antarctic temperatures

Publishing in the journal Nature researchers from British Antarctic Survey (BAS) have reported how a regional cooling phase in the Antarctic is a result of natural forces temporarily masking the longer warming trend. These comments accompanied a briefing.

Prof. Andrew Shepherd, Director of the Centre for Polar Observation and Modelling at the University of Leeds, said:

“It’s completely unsurprising that in any long term temperature record there will be a decade of measurements that buck the trend, and there are few scientists left who believe that atmospheric warming will be the main cause of Antarctic instability over the next century. The real threat is ocean warming, which has triggered widespread loss of ice just around the corner in West Antarctica, and we should not lose sight of that because there are early signs in the satellite record of similar effects at the Peninsula too.”

Prof. Martin Siegert, Co-Director of the Grantham Institute, Imperial College London, said:

“This paper uncovers an important interplay between atmospheric processes and the weather, such as wind, in of the world’s most challenging and extreme environments. It begs a question as to the climate variability in other regions of Antarctica – where there is far more ice with the potential to melt and cause sea-level rise – as well as in the Arctic and other locations.

“Scientists have long understood that global warming reflects an average change in climate across the planet i.e. it won’t lead to temperatures rising at the same rate continuously, everywhere and at once – and some areas will cool. It is important that these new findings confirm this, and the details need to be unpicked and understood in order that we can predict and adapt to future change.

“The Antarctic Peninsula was one of the regions of the world where warming was greatest over the last 100 years, and now it has levelled off whilst other areas continue warming. The study certainly does not suggest that global warming has been halted, however, and it must not be misconstrued as such.”

Dr Ed Hawkins, climate research scientist at the National Centre for Atmospheric Science, University of Reading, said:

“Each of the Antarctic Peninsula measurement stations shows an overall warming trend since their measurements began, but those on the very tip of the Peninsula show an apparent cooling over the last couple of decades. That a very small part of the planet shows a short-term cooling is not in any way a surprise – it is what we expect from temporary natural variations in the atmospheric circulation interacting with a long-term warming trend. Understanding the causes of such variations is important but the context is that of warming temperatures in a region with very large stores of ice.”

Prof. Chris Rapley, Professor of Climate Science at UCL, said:

“We know from rising global mean sea level and global and surface temperature data that the energy balance of the planet has been upset and that the world is warming. We expect that in the polar regions the warming will be amplified as a result of the ice albedo feedback, in which the loss of white ice and snow cover results in increased absorption of solar heat. We expect that this process will amplify natural fluctuations in the polar regions as much as it does the secular warming.

“In the Arctic, where the surface of ocean and land is primarily covered by a thin and relatively fragile layer of snow or ice, we see the amplification occurring, driving substantial loss of snow and ice cover – indeed more rapidly than anticipated by climate scientists.

“However, in the Antarctic the situation is different. Apart from on the Antarctic Peninsula and certain coastal areas of the continent we do not see similar rapid changes. There are numerous reasons for this, but a key factor is the massive and extensive block of ice covering the continent, which acts a massive reservoir of cold, slowing down or ‘buffering’ large scale changes. Note though that the changes in coastal ice discharge that ARE taking place are globally significant through their impact on global mean sea level.

“The paper by Turner et al reports on temperature and shallow ice core data from the Antarctic Peninsula. These show that, following a rapid (“highly unusual”) rise in temperatures since the 1950s, there has been a small but statistically significant cooling (although, overall, the region has warmed). The authors provide a causal explanation related to changes in regional and mid latitude atmospheric circulation, but leave the cause of those changes unexplained. They note that the recent temperature variability observed lies (just) within the bounds of natural variability over the last 1000y revealed by the ice core.

“So far so good.

“However, in a field of science which in which the public discourse is polarised and contested, despite the community of experts having arrived at a firm view, it behoves scientists to explain very clearly the significance of their results and to place them in context.

“In this respect it is a pity that the BAS authors do not refer to a paper published on the 15th July by their own colleagues ¹ and others that addresses the retreat of 90% of the Peninsula’s glaciers since 1940 and shows that this is dominated by ocean warming. Nor do they place their result in the broader context, of the highly unusual peninsular ice shelf collapses. Whilst some of the ice shelves that disintegrated over recent decades at the northern tip of the Peninsula had collapsed and reformed previously in the Holocene (four thousand years ago), the more southerly collapses have been shown to be unusual over at least the last ten thousand years.

“Whilst the temperature fluctuations may be consistent with past natural variability, the Turner analysis does not rule out that they are in part or wholly driven by something else – or indeed that other changes to the Peninsula cryosphere are not significant from the perspective of anthropogenic climate change. Cook et al note, for example: ‘Scientists know that ocean warming is affecting large glaciers elsewhere on the continent, but thought that the atmospheric temperatures were the primary cause of all glacier changes on the Peninsula. We now know that’s not the case’”.

¹ A. J. Cook, P. R. Holland, M. P. Meredith, T. Murray, A. Luckman, D. G. Vaughan. Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science, 2016 DOI: 10.1126/science.aae0017

Prof. Jonathan Gregory, Climate Scientist at the University of Reading, said:

“The temperature records that the authors use are mostly in a small part of the Antarctic Peninsula and represent only 1% of Antarctica, which itself is only 3% of the world. There is always a lot of chaotically generated variability in local climate, and a particular period of 15 years may not be typical of longer-term trends. For these reasons, I do not expect this paper to change our expectation of future climate change or the assessment that human influence has been the dominant cause of global warming since the mid-20th century, although it is useful in improving scientific understanding of the complexity of regional climate variability.”

Dr Colin Summerhayes, Emeritus Associate of the Scott Polar Research Institute, said:

“On my recent annual lecture cruises to the Antarctic Peninsula the sea ice has increased to the point that it prevents my cruise ship from visiting Port Lockroy. The growth in sea ice coverage is consistent with what is being recorded at other places around Antarctica, though at odds with the rise in temperature recorded since the 1950s at the Ukraine’s Vernadsky research station (formerly the UK’s Faraday station) at the tip of the Peninsula. Turner et al now show that the increasing sea ice reflects a local cooling trend, and that the prior warming was not global but local. Does this mean that global warming has stopped? No, the behaviour of the Antarctic climate system cannot be taken as representative of the global climate system – not least because the strong wall of winds around the continent, strengthened by the growth in the ozone hole, helps to keep warm air away from the continent. At times that wall is breached, allowing warm winds down the peninsula between 1950 and 2000. The breach is now closed. Nevertheless, those same strong winds are driving the welling up of warm deep water at the coast, which is melting ice shelves from beneath and speeding up the supply of ice from the land to the sea, contributing to the global rise in sea level that is driven by global warming. Turner’s paper concludes with the observation that global climate models show the local warming and cooling of the air will give way to persistent warming before the end of the century.”

Prof. Martyn Tranter, Professor of Polar Biogeochemistry at the University of Bristol, said:

“The region looks anomalous, since it is cooling down after a period of warming, and this in a warming global climate.

“A combination of natural factors and processes are cooling this small area of Antarctica. The authors are very careful to explain that the regional meteorology has changed, leading to more cold air and less warm air circulating over the region. This also has the effect of piling cold sea ice onto the east coast of the Peninsula, which also cools the regional climate. The authors are careful to state that the most likely climate predictions for the region are that warming will resume, certainly by the latter part of this century.

“It is important not to interpret the cooling of this small area of Antarctica as evidence that the climate is not warming. The cooling here has very little influence on global climate change, and the overwhelming evidence is that the global climate is warming.”

‘Absence of 21st century warming on Antarctic Peninsula consistent with natural variability’ by John Turner et al. published in Nature on Wednesday 20 July 2016. 

Declared interests

None declared