Solving an ice age mystery with a million year old ice core

International moves are afoot to drill an ice core in Antarctica containing a 1 million year record of the Earth’s climate. The project is one of four recommended by the International Partnerships in Ice Core Sciences – a recently formed group of ice core climate researchers from 12 nations, including Australia. A 1 million year record is significant because it would provide information such as climatic temperature and atmospheric greenhouse gas concentrations over a period of fundamental change that occurred in the ice age cycle some 900 000 years ago.

For about 2.5 million years, ice ages had come and gone in a regular 41 000 year cycle. But about 900 000 years ago, something in the Earth’s climate system changed and the cycles mysteriously switched to an ice age every 100 000 years (see figure). These glacial cycles are linked to small variations in the Earth’s orbit around the sun, which produce changes in the amount of solar radiation reaching the Earth. However, these changes in solar radiation are too small to directly produce the large temperature changes needed to move in and out of ice ages. Rather, a range of natural ‘amplifying’ phenomena (feedback mechanisms, thresholds and resonance) in the Earth’s climate system must be driving the switch. We need to understand these amplifiers to better understand the climate system and make realistic predictions of future natural or human-induced changes.

Our knowledge of long-term climate variability comes from sources such as ice cores and ocean sediment cores that store data in layers that can be accessed by drilling. Sediment cores provide records that extend back millions of years but, so far, the oldest ice core is that drilled at Dome C during the European Project for Ice Coring in Antarctica (EPICA) – dating back 804 000 years.

Recent data from a sediment core suggests that the ice age cycle switch may have been due to a change in the carbon dioxide (CO2) concentration in the atmosphere. Because CO2 is a greenhouse gas that helps the atmosphere trap solar heat,a change in CO2 will alter the global heat balance and so could alter the amount of heat necessary to trigger a jump into or out of an ice age. Ice cores contain samples of the atmosphere in bubbles, which can be analysed to determine atmospheric composition. Currently, CO2 levels in the atmosphere are higher than they have been for at least 450 000 years. A long ice core record of atmospheric composition would therefore allow us to directly test the role of greenhouse gases in causing the mysterious ice age period switch.

Drilling a 1.2 million year ice core is a large undertaking and would most likely be carried out by an international consortium. Drilling is likely to start around 2011, but before then we need to identify a suitable site (or sites – since two cores are necessary to confirm that the deepest part of the record is a true record of climate and not distorted by uneven ice flow over the bedrock hills). The Aurora Basin, 700 km inland of Casey station, has ice sheet properties that may be suitable (see box). In 2008-09, we propose to drill a 600 m pilot ice core over Aurora Basin to investigate ice sheet properties in the area. This pilot core will also provide a climate record midway between the inland Dome C site (where the 800 000 year core was drilled) and the coastal Law Dome site (where a 90 000 year core was retrieved by Australian Antarctic Division glaciologists between 1989 and 1993),allowing us to compare the different climate records.

The pilot core project proposal has already gained international interest, with the offer of a Danish ice core drill. At this stage, international participation depends on commitments of other national programmes and also on our ship and air transport options. Fortunately, Aurora Basin is within convenient reach of our new CASA 212- 400 aircraft from Casey. The advent of the Australian-Antarctic Airlink will also greatly facilitate the project and encourage collaboration.

VIN MORGAN, Antarctic Climate and Ecosystems Cooperative Research Centre and Ice, Oceans, Atmosphere and Climate Programme, AAD

Predicting the age of ice cores

The age of cores from an ice sheet depends primarily on the rate of snow accumulation and the ice thickness. For example, at Dome C the current accumulation rate is 2.7 cm of ice per year and the ice thickness is 3136 m. The bottom of the Dome C ice core obtained during EPICA is 804 000 years old. This age is greater than that obtained simply by dividing the depth by the surface accumulation, because the annual ice layers get thinner with depth, due to ice flow. There are areas of Antarctica where accumulation rates are even lower (resulting in thinner ice layers), and there are areas where the ice thickness is over 4000 m. One of these deep areas is Aurora Basin. Here, accumulation rates vary from about 10 cm at the coastal boundary to less than 2.5 cm at the inland edge. With a similar accumulation rate to the Dome C site, and with some 900 m more ice, we anticipate that an ice core recording 1–1.2 million years of climate history will be obtainable