Antarctic Sea Ice

Guest post by Dr Jonathan Day, Department of Meteorology, University of Reading

What is going on with the Antarctic sea ice?

March 2017 was an interesting month for sea ice. Both northern and southern hemispheres experienced record breaking low extents for the time of year. The extent of Arctic sea ice reached the maximum area of its seasonal cycle on March 7th coming in at 14.42 million km2. This was a fraction below the previous record, set in 2015 and is in line with what we expect to see in a warming climate. Meanwhile the other side of the planet Antarctic sea ice continues to confound expectations.

Increasing Antarctic sea ice

Over the last 38 years the area covered by sea ice in Antarctica has been increasing slightly in all seasons, leading to record high conditions reported in 2015. This is not what one would expect in a warming climate. However, this year has gone in completely the other direction and on March 3rd the all-time record minimum of 2.11 million km2 was announced, about 25% below normal. So what does this all mean and why was the sea ice increasing despite global warming?

Firstly, one year, even a record breaker, doesn’t tell us a lot more than we knew before. We know that the magnitude of year-to-year variability of sea ice in Antarctica is very high compared to the long term trend [Fig 1].

Figure 1. February monthly mean Antarctic sea ice extent from NSIDC.

There are a number of competing theories as to why the ice has been increasing and these can be split into two categories:

  1. Changes associated with human activities;
  2. Natural variability.

Human activities causing changes in Antarctic sea ice

In the first category, physically plausible mechanisms have been proposed that link human activities associated with the creation of the ozone hole1 and increased runoff from the Antarctic ice sheets2 (land ice) to increased sea ice. However, different studies have come to different conclusions regarding the magnitude of these effects.

Natural variability in Antarctic sea ice

The second category relates to climate variability from natural causes. For example we know that the major modes of climate variability such as the El Nino Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) project strongly onto Antarctic sea ice variability. In addition, climate models and observations suggest that there may be modes of variability which act on multi-decadal timescales, although understanding of such modes is currently limited3.

Signal-to-noise ratio in sea ice changes

In order to detect the influence of climate change we need the signal caused by man-made changes to be large compared to natural variability. We can measure this ratio in climate model experiments and express it as a signal-to-noise ratio4. Climate models suggest that this ratio is small in the Southern Ocean compared to other parts of the world, therefore the signal of change may be drowned by the noise of variability [see the low values around Antarctica in Fig 2].

Figure 2. multi-model mean CMIP5 simulated change in air temperature over the 21st century divided by the simulated amplitude of natural variability – the signal-to-noise ratio (from Ed Hawkins).

Another line of evidence is that sea ice and temperature trends in the Southern Ocean changed sign in the 1970s for no apparent reason. The climate was generally warming from 1950-1978 and the cooling thereafter5 [Fig 3]. To me this is highly suggestive of natural multi-decadal variability, rather than a forced change6, but the jury is still out.

Figure 3. Southern Ocean SST and sea ice trends from HadSST, for the periods 1950-1978 (left) and 1979-2014 (right) and the zonal mean of both (middle) from Fan et al. (2014). Sea ice concentration is not available for the 1950-1978 period.

Is a signal starting to emerge?

Although one low year is not enough to tell if the sign of the trend is changing it is may be a sign that the climate change signal is starting to emerge from the noise of natural variability.

Will we enter another ice age?

There are a number of web and news articles around surrounding the question of whether or not we will enter another ice age. Many of these questions arise from the idea that a collapse or significant melting of the Greenland Ice Sheet will produce enough fresh water to shut down the global thermohaline circulation, dropping us into a new ice age in the next 10,000 years.

Continue reading

Antarctic Peninsula has strong sensitivity to surface warming

The Antarctic Peninsula is warming very rapidly, about six times the global average[1-3]. There has been a 95% increase in positive degree day sums since 1948[4]. Glaciers in the region are accelerating, in response to frontal thinning and recession[5]. In addition, ice shelves are collapsing[6], glacier fronts are retreating[7]. The causes for much of these changes has often been attributed to ocean forcing, with warm ocean waters melting these glaciers from below[8-11]. However, while ocean forcing may dominate further south, such as at Pine Island Glacier, a few recent papers have highlighted the importance of surface processes and surface melt induced by warmer surface air temperatures and longer melt seasons, specifically on the Antarctic Peninsula. Continue reading

Just published: most comprehensive review ever of the glaciation of Antartica

A major new review of the last glaciation of the entire Antarctic Ice Sheet has just been published by Quaternary Science ReviewsThe special issue of the journal includes a suite of review papers involving an international team of experts regarding the last glaciation of the entire Antarctic Ice Sheet. This review, which comprises six review papers and an overview paper in a special issue of Quaternary Science Reviews, is now complete and all papers have been accepted for publication. As this is the most important, up to date and inclusive review ever to be attempted for the glaciation and recession of the Antarctic Ice Sheet, it represents a major step forward in our understanding of palaeo ice-sheet dynamics, provides a benchmark against which future research needs can be identified and highlighted, and provides a compilation of data unlike anything seen before, which can be used to test and calibrate numerical ice-sheet models.

Continue reading

In defence of reticence

Occasionally, comments on this website call me reticent. I think that this is because I try not to let my personal opinions cloud my professional, scientific judgement. I am proud to be reticent. I always try to be informative, to give values of uncertainties and ranges and assessments of confidence. I try to present both sides of the story, while always relying on peer-reviewed papers published in reputable scientific journals. I try to let the evidence speak for itself. Continue reading

Patagonian Ice Sheet thinning during a changing climate

J.Boex, C. Fogwill, S. Harrison, N.F. Glasser, A. Hein, C. Schnabel and S. Xu.  Rapid thinning of the Late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies. Scientific Reports 3: 2118, p. 1-6

Download the PDF

The Patagonian Ice Sheet

Patagonian mountains east of the North Patagonian Icefield. Credit: Stephen Harrison

Patagonian mountains east of the North Patagonian Icefield. Credit: Stephan Harrison

This recent open-access paper in the new journal Science Communications, which is part of the Nature group, has demonstrated that the during the deglacial period (~19,000 years ago), the Patagonian Ice Sheet in South America responded rapidly in response to changing precipitation patterns and warming during the last deglaciation. The fact that the Patagonian Ice Sheet responded so quickly to changes in precipitation and temperature has vivid implications for the current, and future, behaviour of the current North Patagonian Icefield  and South Patagonian Icefield. We already know that the shrinkage of the North and South Patagonian ice fields was faster over the last decade or so than at any point in the last couple of centuries. Understanding on a broader scale how these sensitive, high-latitude ice masses are dependent on small changes in atmospheric circulation means that we will better be able to predict the future behaviour of these ice sheets. Reconstructing rates of ice-sheet decay since the Last Glacial Maximum means that we can better assess the mechanisms of climate change (including changing wind patterns) during a major climate transition. Continue reading

George VI Ice Shelf

Holt, T.O., Glasser, N.F., Quincey, D. and Siegfried, M.R., 2013. Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 7: 797-816.

George VI Ice Shelf

George VI Ice Shelf, Alexander Island

George VI Ice Shelf, Alexander Island, showing ice flowing onto the ice shelf from both the Antarctic Peninsula and Alexander Island

Alexander Island and George VI Ice Shelf is an area I’m particularly interested in (see our project details), and the ice shelf is worth investigating for several reasons. For a start, it’s unusual, being trapped between the mainland and Alexander Island, and secondly, because it’s right on the -9°C mean annual air temperature isotherm (like a contour, but of mean annual air temperatures).  Some people have argued that this mean annual air temperature is the critical threshold above which ice shelves may dramatically collapse, which has implications for accelerated flow of glaciers and ice-sheet thinning. Ice shelves are also susceptible to warming from below by warm currents penetrating onto the continental shelf. So, research into this important part of the peninsula is always welcome. Holt and colleagues have just completed a study (open access) that investigates the response of George VI Ice Shelf to environmental change (i.e., oceanic and atmospheric temperature variations), and offer an assessment as to its future stability (Holt et al., 2013). Continue reading

Antarctic ice shelves – the hidden villain

Sea ice and ice shelves

What is sea ice? Sea ice is frozen sea water; it perennially expands and contracts during each year’s winter and summer. Amongst the sea ice are icebergs calved from tidewater glaciers and ice shelves. Melting sea ice does not contribute directly to sea level rise (ice floats and displaces the same volume of water), but sea ice is important because it enhances climate warming. It changes the reflectivity of the sea water, reflecting lots of sunlight back (it has a high albedo), and is therefore an important component of the climate and cryospheric (icey) system.
Continue reading