Ancient rocks provide clues to early warming of Earth

A greenhouse gas now considered very likely behind global warming may have helped save the Earth from freezing early in the planet's history, according to an analysis of ancient rocks from the Eastern shore of Hudson Bay.

A team of scientists said they have found evidence of high concentrations of carbon dioxide in the atmosphere from samples of rocks left over from the Precambrian period almost four billion years ago.

While a report last week from the Intergovernmental Panel on Climate Change said greenhouse gases fromhuman activities such as the burning of fossil fuels is 90 per cent certain to be behind today's hotter temperatures and rising sea levels, levels of the gas in ancient times might actually have helped the planet.

The high levels of the greenhouse gas may have helped keep surface temperatures above freezing at a time when the sun was 25 per cent fainter than today, they said.

"We now have direct evidence that Earth's atmosphere was loaded with CO2 early in its history, which probably kept the planet from freezing and going the way of Mars," said Stephen Mojzsis of the University of Colorado, one of four researchers behind the study.

Nicolas Dauphas, an assistant professor in Geophysical Sciences at the University of Chicago, offered a more cautious interpretation of the data.

"Our study shows the greenhouse gas that could have sustained surface temperatures above freezing 3.75 billion years ago may have been carbon dioxide," said Dauphas.

The two scientists and Nicole Cates of the University of Colorado and Vincent Busigny, now of the Institut de Physique du Globe in Paris, will present their data in the Feb. 28 issue of the journal Earth and Planetary Science Letters.

The grey-white rocks in the village of Inukjuak on Hudson Bay's Purpose Cove are among the oldest-known in the Earth's history, having been discovered by a team of Canadian scientists in 2001.

Dauphas and his team of researchers analyzed the iron content of the rocks and found hints of iron carbonates, which can only be formed with the help of higher levels of carbon dioxide than are found today in the atmosphere.

Dauphas speculated the greenhouse gas would have played an important early role as a thermostat in support of life on Earth.

Colder temperatures increase ice caps and decrease the chemical interaction between the land and atmosphere, which in turn would cause carbon dioxide to accumulate in the atmosphere and increase the greenhouse effect, he said. Warmer temperatures should, theoretically, cause more chemical weathering, trapping gases like carbon dioxide in the atmosphere.