Ice core evidence for 20% decline in Antarctic sea ice extent since the 1950s

Antarctica remains an under-explored region of the planet, despite the fact that some expeditioners take a trip ‘south’ for granted. It is a difficult place to get to today and was obviously even harder in the past, limiting the amount of reliable scientific data.

One area where this limit is felt is in the sparsity and shortness of climate records, which are needed to interpret recent changes and to test the longer-term applicability of climate models. Key climate parameters from the Antarctic region are the extent of sea ice, and its potential response to climate change.

Antarctic sea ice forms an icy covering on the south seas each year — more than doubling the size of Antarctica and has often been termed the ‘frozen skin of the Southern Ocean’ (Nicol and Allison, American Scientist 85, 426-439, 1997). While it often hinders Antarctic operations, it is a vital component in the global climate system. It plays a major role in climate through its influence on heat exchange between ocean and atmosphere, it assists the formation of Antarctic Bottom Water (through brine rejection) which sinks to the depths of the ocean subsequently driving global ocean circulation, and it provides essential ecosystem support right through the food chain from microbes, phytoplankton, and krill, to penguins, seals, and whales. Sea ice also acts as a sensitive indicator of climate change ... or does it?

The general scientific consensus is that global climate is warming, and this warming should be seen in Antarctica. However, the picture is complicated, and the Antarctic continent cannot be viewed as a single climate regime. Warming is seen in the Peninsula region, and through most of coastal Antarctica, but the interior is generally cooling — most likely as a result of changes in atmospheric circulation.

Most climate models predict that sea ice should respond to ocean and atmospheric warming with a resulting decrease in sea ice extent. Records of whaling ship locations (de la Mare, Nature 389, 57 (1997) and penguin population survival tend to suggest Antarctic sea ice may be declining — however satellite records of sea ice extent tell a different story.

The advent of satellite imagery since the mid-1970s enabled scientists to remotely ‘view’ Antarctica and measure the extent of sea ice. This data shows little or no change, or possibly even an increase in sea ice extent since the 1970s. So what is happening? Is Antarctic sea ice extent increasing or decreasing? The answer is both! The effect seen depends on the timescale we are considering.

The problem is that there were no reliable records of sea ice extent prior to the mid-1970s, to look at long-term trends.

Enter the phytoplankton. These single celled algae live in and around the sea ice and release chemical signals to the atmosphere which gets trapped in snowfall on the continent. This is locked away in the Antarctic ice sheet, waiting for glaciologists to drill ice cores and analyse them to reveal the hidden message from the phytoplankton. This message takes the form of methanesulphonic acid or MSA.

MSA is produced from oxidation in the atmosphere of dimethylsulphide, which is itself produced by certain species of phytoplankton and in the Southern Ocean. The distribution of these species is intimately associated with sea ice. Through analysis of ice cores from Law Dome, we have discovered that the amount of MSA in the core is related to the maximum extent of sea ice in that region. This is because in years where there is more sea ice (greater northerly extent) there is more phytoplankton activity following sea ice decay and more MSA production.

In a recent paper in the journal Science, (Curran et al., November 14, 302, 1203, 2003) we have calibrated the ice core MSA record against satellite records of sea ice extent since the mid 1970s (see Figure 1). Annual MSA concentrations significantly correlate with maximum sea ice extent around the whole of Antarctica, with the highest correlation in the region (80–140°E) surrounding Law Dome.

This calibration enables us to use the MSA record from the Law Dome ice core as a tool to investigate past sea ice extent. So what happened to sea ice extent prior to the 1970s and the satellite era?

The MSA record from the top 150m of the Law Dome ice core gives a ‘proxy’ for sea ice extent over the period 1840 to 1995. The two main findings presented in the paper are the discovery of these persistent, high-amplitude, decadal fluctuations and the dramatic decrease in sea ice extent over the last 50 years. The high variability explains why satellite trends are confusing. Detection of long-term change is masked by large fluctuations from decade to decade and it is these decadal fluctuations that have produced apparent short-term increases in the satellite data. The large reduction of the northerly extent of sea ice in the region south of Australia (80–140°E) of 1.5 degrees of latitude equates to a 20% decrease since the 1950s.

While this research indicates the Antarctic sea ice is decreasing, we cannot say how unusual these changes might be over longer timescales from centuries to millennia or more. What we have is a better understanding of the past history of sea ice and this provides another piece in the puzzle of understanding the role of Antarctica in the global climate system. Work is continuing to look deeper in the ice core, further back in time, to provide more knowledge of past sea ice changes and climate variations. This will improve our ability to predict future changes, guide policy and adapt as necessary.

Mark Curran & Tas van Ommen, Glaciology Program, AAD & ACE CRC