Antarctic ice and the global climate system
Planet Earth is a natural greenhouse. Some naturally occurring atmospheric trace gases, called greenhouse gases, permit incoming solar radiation to reach the Earth's surface but restrict the outward flow of infrared radiation. Carbon dioxide and water vapour absorb this outgoing infrared energy and re-radiate some of it back to ground level. This greenhouse effect is essential to most life on Earth. Without it the average temperature of the surface would be a frigid minus 18°C, rather than about 14°C as it is today.
But the concentration of greenhouse gases in the atmosphere, especially carbon dioxide, has been increased by human combustion of fossil fuels, exacerbated by deforestation. Since the Industrial Revolution began, carbon dioxide levels have risen from 280 parts per million by volume (ppmv) to 370 ppmv, and are reliably predicted to reach double pre-industrial levels in the second half of this century. Humans have also added other greenhouse gases such as methane, CFCs, and nitrous oxide to the atmosphere. The combined effect of these additional gases will be a rise in global temperatures, predicted by climate models to be 1°C to 4°C, by the end of the 21st century.
Global warming will not be uniform over the earth because of the complex interactions within and between oceans, atmosphere, land surface, clouds, biological systems and ice and snow. Some of the largest changes are predicted to occur at high latitudes. Exposed ocean or bare earth caused by the loss of ice and snow cover through melting will result in increased absorption of solar energy, which in turn will further reduce ice and snow cover leading to an amplified effect – a positive feedback. Against this, however, is an increase in heat fluxes from the ocean to the atmosphere – a negative feedback – caused by a decrease in sea ice.
A central objective of Australia's Antarctic program is to understand the role of Antarctica in the global climate system. This requires us to study Antarctic processes contributing to the climate system, determine the response of the Antarctic to climate change and seek evidence of past and present change in the region. Many important Antarctic climate-related processes involve ice. Ice and snow (the 'cryosphere') are important components of climate, with snow in particular limiting absorption of solar energy at the surface through its high reflectivity ('albedo'). Freezing of water and melting of ice involve latent heat exchange, and snow and ice on land or sea inhibit heat transfer. The water volume stored in ice sheets and glaciers is a major factor in considering sea level change. Ice and snow also provide evidence of past change from the ice core climate record and visual evidence of ongoing change due to melt.
All these factors make it important to understand the role of ice and snow in the climate system, a need recognised in the recent establishment of a new international research initiative, Climate and Cryosphere within the World Climate Research Programme. The Australian Antarctic glaciology program contributes to this program with cryosphere studies taking in Antarctic sea ice, the continental ice sheet including ice core climate records, subantarctic glaciers and abrupt change.
The extent of Southern Hemisphere sea ice (frozen sea) varies seasonally by a factor of five, from a minimum of 3–4 million km2 in February to a maximum of 17–20 million km2 in September. When the ice forms it ejects salt to the ocean, destabilising the water column and deepening the surface mixed layer. It can also influence formation of the global oceans' deep and bottom water and help drive overturning ocean circulation. Sea ice moved by wind and currents, as it melts, deposits freshwater onto the ocean surface to stabilise the water column. Sea ice has a dramatic effect on the physical characteristics of the ocean surface, modifying surface radiation balance due to its high albedo and influencing the exchange of momentum, heat, and matter between atmosphere and ocean. Through these effects, sea ice plays a key role in the global heat balance. A retreat of sea ice associated with climate warming could have global consequences through various feedback processes.
The Antarctic ice sheet
Antarctica's ice sheet covers 12.4 million km2. It comprises 25.7 million km3 of ice or 70% of the world's freshwater, which if melted would raise the sea level by nearly 65 m. Mass is continually added to the ice sheet from snowfall, and removed via melt and iceberg calving, particularly from ice shelves. Any change in the ice sheet's 'mass budget' caused by imbalance between these mass input and output terms affects sea level. However, with present Antarctic data a 20% imbalance, corresponding to about 10 cm of sea-level change per century, cannot be detected with confidence. The ice sheet is not a single dynamic entity, but comprises different drainage systems with both surface mass balance and dynamics responding differently to changing conditions. We need to be able to estimate the sensitivity of the mass budget to climate change before we can estimate Antarctica's future contribution to sea level change.
Most of the ice lost from the ice sheet comes from fast-flowing, wet-based outlet glaciers and ice streams, much of which passes through floating ice shelves. Up to 40% of the Antarctic coastline is composed of either large ice shelves in coastal embayments such as Filchner-Ronne, Ross and Amery or fringing shelves on the periphery of the ice sheet such as the West, Shackleton and Larsen shelves. Since ice shelves are floating on ocean waters at the freezing point, even a small change in ocean temperature (induced perhaps by changed ocean currents) can significantly affect the shelves' basal melt rate and cause them to thin much more quickly than rising air temperature. Ice shelves are already floating, which means their disintegration will by itself have no measurable impact on global sea level, but their depletion may lead to increased drainage of grounded ice 'buttressed' by the shelves which may cause sea-level rises.
Ice core records of past climate
Antarctica's ice sheet stores the Earth's longest and most representative record of atmospheric composition and temperature in times past. The ice sheet's layers of ice and snow, accumulated over tens or even hundreds of thousands of years, form a natural archive of global environmental information, accessible by drilling into the ice to sample past surface deposits. Analyses of the ice and the material trapped in it allow records to be made of both natural and man-made environmental variations over the time period during which the ice sheet has accumulated. Deep ice cores have yielded evidence of major interrelated climate and cryosphere fluctuations in glacial-interglacial cycles. Accurate information on local, regional and global climate change and potential changes in ice sheet surface elevation are available from ice cores.
Like mountain glaciers in most parts of the world, subantarctic glaciers have been noticeably retreating over the past 50 years or more. Retreat of non-polar glaciers has contributed to sea level rise over the past century while also providing clear evidence of a changing climate. On Heard Island, for example, the Brown Glacier has decreased in area by 33% and in volume by 38% over the past 50 years.
Palaeoclimate records from ice and ocean sediment cores show evidence of abrupt and widespread past climate changes – particularly, it seems, during periods of transition from one climate regime to another over glacial-interglacial cycles. While the causes and mechanisms of such rapid changes are by no means clear, a variety of roles have been suggested for ice sheets, glaciers and sea ice. These include effects of rapid glacial discharge and decomposition with a rise to melting point of basal ice temperature, massive iceberg discharge into the ocean delivering freshwater capable of modifying the overturning circulation of the global ocean, and changing sea ice formation causing change in brine release to the ocean. Greenhouse warming and other human alterations of the climate system may increase the possibility of large and abrupt regional or global climatic events.
A startling illustration of how abrupt some processes are is the recent rapid collapse of the Larsen B Ice Shelf on the eastern side of the Antarctic Peninsula in February and March 2002, when 3250 km2 of ice 200 m thick disintegrated over a few weeks (see satellite image). The break-up of this shelf into thousands of small icebergs is totally different from the normal episodic calving of giant icebergs from the front of ice shelves. Temperatures around the Antarctic Peninsula have risen by 2.5°C over the past 50 years. It is hypothesised that water from large surface melt ponds that formed on the ice shelf as a result of this warming forced open cracks and crevasses to completely fracture the shelf. The ice of the shelf was already floating, so the collapse has no measurable effect on sea level, and direct impacts are believed to be mostly local. However, a similar collapse of some other shelves could bring a significant increase in glacial discharge from the ice sheet.
Glaciology Program Leader,
Australian Antarctic Division