CO2 at two million year high

Atmospheric carbon dioxide is at its highest level for more than two million years, scientists have revealed.

For most of that time concentrations of the greenhouse gas have been relatively stable, a new study has shown.

Only recently has there been a "spike" in CO2 levels, coinciding with the start of the industrial age.

The findings suggest that the Earth is going through a period unique in its history - at least for the last 2.1 million years. It means scientists will have to search back further in time to find conditions comparable with those driving modern-day climate change.

Studying previous climate change trends can help experts understand what is happening now, and what is likely to occur in the future. Scientists had thought that decreasing levels of carbon dioxide in the atmosphere caused a major change in the Earth's glacial cycle some time between 1.2 million and 500,000 years ago.

Previously, the planet underwent an ice age roughly every 41,000 years. The timing of ice ages is believed to be controlled mainly by the Earth's orbit and tilt, which affects how much sunlight falls on each hemisphere.

After the transition, the cycles of freezing and warming grew to 100,000 years. At the same time, ice sheets covered more of the Earth's surface than they had for several million years - a change too great to be explained by orbital variation alone.

One likely reason for the change was believed to be lowering levels of carbon dioxide in the atmosphere. But the new research, based on a study of tiny sea creatures called foraminifera, has discounted this theory.

Scientists estimated historic carbon dioxide levels by analysing the chemical make-up of foraminifera shells preserved in sediment cores drilled from the ocean floor off west Africa. Measurements of the element boron in the shells gave an indication of sea water acidity when the creatures were alive. This in turn provided a marker of carbon dioxide concentrations.

The research, led by Dr Barbel Honisch, from Columbia University, New York, is reported in the journal Science. It shows that peak carbon dioxide levels over the last 2.1 million years averaged 280 parts per million. Today, CO2 levels are 38% higher at 385 parts per million. Commenting on the findings Professor Richard Alley, a US glaciologist at Pennsylvania State University, said: "We know from looking at much older climate records that a large and rapid increase in CO2 in the past (about 55 million years ago) caused large extinction in bottom-dwelling ocean creatures, and dissolved a lot of shells as the ocean became acidic. We're heading in that direction now."