A world-renowned geologist has published a study suggesting carbon dioxide may not have a huge effect on climate change. The implications of Jan Veizer's work has sent shockwaves through the climate research community.

It has long been theorised that carbon dioxide is the driving force behind the greenhouse effect and global warming. But Veizer's research identified long periods in the Earth's geological past when elevated levels of carbon dioxide were actually associated with a drop in average temperature.

He says he doesn't want to jeopardize the environmental agenda, but he thought it better to be honest than to conceal the results.

Veizer, who works at the University of Ottawa and at Ruhr University in Germany, examined fossils from a primitive clam that has existed for almost 500 million years. When these organisms were alive, the composition of the atmosphere left its traces in their shells and skeletons. Using this data, the researchers were able to reconstruct the Earth's climate over time.

Levels of carbon dioxide taken from this picture were then fed into a computer climate model. The computer then generated the expected temperatures for those time periods.

During two periods, the actual and expected temperatures didn't match. Veizer does admit the computer model could be wrong, but if it's not, he says the numbers point to a diminished role for carbon dioxide in global warming.

He says he thinks carbon dixide and other greenhouse gases may not drive climate change, but simply amplify change set off by some other factor. He thinks that factor may be the sun and solar radiation.

Others disagree

His critics say Veizer's readings may be accurate, but that other factors need to be considered before carbon dioxide can be discounted as a cause of global warming.

Andrew Weaver, one of Canada's leading climate modelers says the Earth of 400 million years ago was significantly different and can't be compared fairly to our situation today.

Weaver points out that during the ice ages Veizer's numbers point to, all the land was concentrated arount the South Pole, and radiation from the sun was lower than it is today.

Others say the fact that carbon dioxide levels have increased at an unprecedented rate during the past few decades cannot be properly addressed by Veizer's study.