By Quirin Schiermeier

During the warm periods between recent ice ages, temperatures in Antarctica reached substantially higher levels than scientists had previously thought. This conclusion, based on ice-core studies, implies that East Antarctica is more sensitive than it seemed to global warming.

Previous estimates suggested that peak temperatures during the warmest interglacial periods -- which occurred at around 125,000, 240,000 and 340,000 years ago -- were about three degrees higher than they are today. But a team led by Louise Sime of the British Antarctic Survey in Cambridge, UK, concludes that Antarctica was actually around six degrees warmer.

The team based its analysis on ratios of oxygen and hydrogen isotopes in ice cores drilled in East Antarctica. Warmer air temperatures cause more water containing the heavier isotopes oxygen-18 or deuterium to evaporate from the surrounding ocean. Once it falls on inland Antarctica and packs into ice, it gives climate researchers a proxy of local temperatures at that time.

Climate reconstructions usually assume a simple linear relationship between these isotope ratios and temperature. But Sime's team says that although that relationship holds up for the cold glacial periods, it does not work so well during the warmer interglacials.

The scientists measured the isotope ratios in three ice cores from across East Antarctica, each of which dates back to at least 340,000 years ago. They then compared those results with predicted isotope distributions derived from a global climate model.

They found that higher average temperatures were required to reconcile observations and model experiments. "The available evidence only fits together if we assume peak temperatures around six degrees above current values," says Sime. "We didn't expect this at all."

The team believes that the relationship between temperature and the isotopic composition of water vapour changes as climate warms. For example, the isotopic signatures of ice cores depend on the seasonal distribution of precipitation. A change in the time of year when most snow falls could lead to biases in temperature reconstruction, says Sime, whose team reports its findings in Nature.

Six degrees of trouble

Unlike the rapidly warming Antarctic Peninsula, mainland Antarctica has so far been relatively resilient to climate change. But if past responses to warming are a guide to the future, this could change. "We don't know if the present state of the climate system might allow for a six-degree warming in East Antarctica," says Sime, "but it is not impossible."

At the height of the last interglacial, when greenhouse-gas levels were roughly similar to current values, the global sea level was 4-6 metres higher than it is today. Temperatures in East Antarctica would still have been too cold to cause any glacier melting there. But because the surrounding ocean would have been warmer, and stabilizing sea ice less abundant, the massive East Antarctic ice sheet may have contributed to higher sea levels by flowing more quickly towards the ocean.

The findings are impressive, says Gerhard Kuhn, a marine geologist at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, and Germany's representative in ANDRILL, an international sediment-drilling project in Antarctica.

Although scientists know that the tools for reconstructing past climates at polar latitudes are far from perfect, he says, pinning down the relationship between isotope ratios and temperature is essential.

Sime adds that scientists still know too little about what happened in Antarctica during warm climates similar to ours. "We'd be better off if we did."