July 2, 2019

expert reaction to Antarctic sea ice trends

A new study reports the change in Antarctic sea ice over the last 40 years, showing increases followed by decreases at faster rates than the Arctic.

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

“Growth of Antarctic sea ice over the past few decades had been a relief, because it offset some of the rapid losses in the Arctic. But half of that growth has been wiped out in just a few years, and now that sea ice is retreating in both hemispheres we should be concerned about our planet’s temperature because it means less of the sun’s heat is reflected back into space.”

Dr Kaitlin Naughten, sea ice modeller at British Antarctic Survey, said:

“Antarctica is largely protected from atmospheric warming by the westerly winds which surround the continent. This means that Antarctic sea ice doesn’t respond directly to global warming averaged over the whole planet, but rather to changes in these winds. Climate change is affecting the winds, but so is the ozone hole and short-term cycles like El Nino. The sea ice also responds to the level of ocean mixing, which is affected by meltwater from the Antarctic Ice Sheet. 

“Until 2014, the total effect of all these factors was for Antarctic sea ice to expand. But in 2014, something flipped, and the sea ice has since declined dramatically. Now scientists are trying to figure out exactly why this happened.

“The story is much simpler in the Arctic, where the atmosphere has quickly warmed and the sea ice has declined in response. Even so, the decrease in Antarctic sea ice since 2014 has far outpaced anything seen in the Arctic.”

Dr Jack Landy, Lecturer & Research Fellow, University of Bristol, said:

“Prior to 2014, Antarctic sea ice was growing at a rate four times lower than Arctic sea ice was declining. Now, after a few years where Antarctic sea ice coverage has dropped off rapidly, the trend since 1979 is just +1% per decade. This is about 8 times lower than the rate of sea ice decline in the Arctic and therefore does not have the same implications for global climate change.

“It should be noted that the ocean around the Western Antarctic Ice Sheet, where ice sheet melt in response to warming climate has been focused, has actually experienced declining sea ice coverage since 1979. This fits the overall climate warming trend.

“Across the entire Southern Ocean, fluctuations in sea ice area over periods of a few years now appear to be getting stronger and stronger, with the Antarctic sea ice cover becoming increasingly pliable by winds and ocean currents.

“It’s crucial to acknowledge the fantastic 40-year satellite dataset that allows us to investigate these Antarctic sea ice trends, and dramatic recent changes hammer home the importance of maintaining such a record into the future.”

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

“There has been a lot of discussion about how to explain Antarctic sea ice trends, which are clearly different to those observed in the Arctic. However, the basic explanation here is that Arctic ocean vs Southern ocean conditions and influences are completely different, so a comparison between them is probably unwise and unfair.

“There are some possible ways to explain how Antarctic sea ice remains relatively unchanged: First is ocean conditions – The mixing of waters around the southern ocean, and the lack of north-south transfer of heat to the Antarctica (both different in the Arctic) can keep Antarctic ocean conditions cold, irrespective of ocean warming elsewhere; Another is the ozone hole, which may have acted to increase cold katabatic winds from the continent to the ocean surface, thus cooling it. Thus, it has been proposed, if/when the ozone hole is fully mended we might expect to see Antarctic sea ice decline.”

Dr Miguel Ángel Morales Maqueda, Senior Lecturer in Oceanography, Newcastle University, said:

“This is not the first paper to identify a weak positive trend in sea ice extent and/or a slight cooling trend in sea surface temperatures in the Southern Ocean around Antarctica. As far as I know, the causes for this somewhat unexpected trend are not yet entirely understood. However, it is important to realise that, contrary to naïve ideas, the Arctic and Southern Ocean/Antarctic climates are very different and, therefore, their response to climate variability and climatic changes are expected to be equally different. In the Southern Ocean, for example, winds have a much stronger control over sea ice growth and decay than in the Arctic. Circumpolar winds around Antarctic, for example, cause large enough heat losses in summer to keep surface temperatures low and to preclude surface sea ice melting. These winds have intensified in the last three decades, and so this might be a mechanism to explain the unexpected surface trends. In any case, one thing that Parkinson’s excellent article is most certainly not is a counterargument to global climatic change.”

Prof Peter Wadhams, Professor of Ocean Physics, University of Cambridge, said:

“The explanation is given in my book “A Farewell to Ice” (Penguin, 2017), p166-167. It is that the circumpolar belt of west winds over the Southern Ocean has been growing steadily in strength. Ice floes are blown eastward by the direct force of the wind stress on their surface, but as they move they experience the Coriolis force tending to turn them to the left, i.e. giving them a northward component of motion. The faster they move, the greater the northward force. So although the ice floe reaches a latitude where the warmer atmosphere will melt it, its northward speed will take it further north than previously before this happens. This theory originated with Jinlun Zhang of the University of Washington.”

Prof Chris Rapley, Professor of Climate Science, University College London (UCL), said:

“Unlike the Arctic Ocean where the maximum sea ice extent is constrained by the coastline that surrounds it, the Antarctic sea ice is unbounded. The upshot is that the location of the outer ice boundary is determined by a multiplicity of factors that include the location of the fast flowing Antarctic Circumpolar Current (ACC), which circles the Southern Ocean, the strength and direction of the winds that blow offshore from the Antarctic continent / ice sheet,  the quantity of ice formed by the low temperature air blowing over the ocean surface, and the temperature and circulation of the ocean beneath. The result is the area and volume of Antarctic sea ice, and the location of it’s outer boundary, are determined by a combination of factors and processes, all of which are affected by climate change – (and also it so happens by the Ozone Hole, which has affected the strength and location of the circum-Antarctic winds and the ACC). A modest increased in extent over recent decades thus in no way falsifies the fact that the world is warming. It simply demonstrates that in a complex, interconnected system, counter-intuitive outcomes can occur – at least for a while. The US social scientists Jay Forrester, writing in the 1970s noted on ‘causality’: “The human mind is not (well) adapted to interpreting how (complex) systems behave. Such systems belong to the class of multi-loop nonlinear feedback systems. In the long history of evolution, it has not been necessary until very recent historical times for people to understand complex systems, and we are not very good at it” . We have a tendency to seek simplistic explanations of cause and effect, when in reality the situation is much more complicated and nuanced.”

Dr David Rippin, glaciologist at the University of York’s Department of Environment and Geography, said:

“The Arctic ocean is deep and very sensitive to climate warming, and so as the ocean warms, sea ice melts. As this new paper points out, the situation in Antarctica is different.  

“Here, we have a large landmass surrounded by ocean. For some time, the way that the winds moved around the continent, pushed sea ice away from Antarctica. This had the effect of opening up more ocean that could then also be frozen.  

“This had the effect of allowing more ocean to freeze during the winter, and so Antarctic sea ice expanded. A number of years ago, these large-scale wind circulation patterns changed so that they no longer pushed sea ice that formed close to the continent outwards, and so the mechanism that previously allowed sea ice in Antarctica to apparently be expanding ceased.  

“Now, it is being impacted by the fact that the ocean has warmed – and is warming – and so the amount of sea ice is shrinking. Sometimes the extent of the expansion of Antarctic sea ice has been used wrongly as an argument against global warming. The southern oceans are very much warming, and this is having a big impact there. This warming is now also impacting the amount of sea ice around the Antarctic continent.”

Dr Julie Jones, Senior Lecturer in Climate Science at the University of Sheffield’s Department of Geography, said:

“There is a strong link between changes to winds around the Antarctic continent and the distribution of Antarctic sea ice over recent decades, which has led to an overall increase in the area of sea ice until recent years.

“Although the extent of the sea ice has increased, the Antarctic Ice Sheet itself has been melting, partly as a result of changes to these winds. These wind changes can be related to human activity. That melting from the ice sheet has made a significant contribution to global sea level rise, which threatens communities around the world.”

Prof Julienne Stroeve, Professor of Polar Observation & Modelling, University College London (UCL), said:

“This study supports our understanding that sea ice varies considerably from year to year in the Antarctic, thereby weakening any long-term trends.

“Large year-to-year variability of sea ice in the Antarctic outweighs slight positive trends.

“While the Antarctic is experiencing larger year-to-year variability than what we observe in the Arctic, the Arctic has pronounced long-term trends towards less sea ice in all calendar months.”

* ‘A 40-y record reveals gradual Antarctic sea ice increases followed by decreases at rates far exceeding the rates seen in the Arctic’ by Claire L. Parkinson was published in PNAS at 8pm UK time on Monday 1 July 2019

Declared interests

None received.