Oldest Antarctic ice poses climate puzzle

Allen Hills Ice Core. Courtesy COLDEX.

Scientists have managed to analyse air trapped in ice-core samples from Antarctica that are as old as 3 million years, including the first ever samples from the Pliocene, which ended about 2.6 million years ago when the ice ages began. Some of the new samples may be up to 6 million years old, which more than doubles the previous record for Earth’s oldest ice.

The results were obtained as part of the COLDEX initiative – the US Center for Oldest Ice Exploration – which reported its latest findings at the European Geosciences Union General Assembly in Vienna.

The story they tell of past climates is unexpected. Analysis of the Pliocene air trapped in the ice shows that the Earth was several degrees warmer than today, even though carbon dioxide (CO2) levels were similar to those today. Further analysis suggests that CO2 levels were low in the late Pliocene, and did not change very much between 2.7 million and 1 million years ago, the time when the Pliocene ended and the ice ages began.

Source: Wikipedia

Previous ice cores only extended back 800,000 years, but scientists have realised that so-called “blue ice” obtained in the Allan Hills district can go back much further.  In recent years the COLDEX team have recovered ice as old as 2.7 million years, including an analysis of greenhouse gases in air bubbles up to 1.5 million years old.

Preliminary data presented at the conference suggests that Pliocene CO2 levels were probably about as high as today’s elevated level, some 425 parts per million (ppm).

The researchers say that the CO2 data is important because of an unexplained climate shift that commenced 1.2 million years ago. At this time, ice ages became longer and deeper, changing from 40,000-year cycles to deeper 100,000-year ones. Some scientists have speculated the reason was that CO2 levels fell, allowing greater ice growth. This new data casts doubt on that picture.

Each year scientists return to Antarctica to seek older ice, especially from the geologically complex Allan Hills ice folds. The greenhouse gas data in this ancient ice is throwing up surprises, perhaps indicating our limited knowledge of the interplay between CO2 and climate.