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Last Updated: Wednesday, 10 December, 2003, 17:05 GMT
Humans' 10,000-year warming habit
Black, BBC
By Richard Black
BBC science correspondent, in San Francisco

Clouds, AP
Human influence on climate is hotly debated
Humans have been warming the Earth's climate for the last 10,000 years, US scientist William Ruddiman claims.

The University of Virginia professor says agriculture has put greenhouse gases in the atmosphere, pushing up temperatures by about one Celsius.

This, he claims, has broadly balanced the cooling that should have come from a natural reduction in the Sun's heat reaching Earth over the same period.

The professor has presented his ideas to the American Geophysical Union.

The AGU is holding its annual autumn meeting here in San Francisco.

Natural cooling

Over timescales of thousands of years, the Earth goes through a natural cycle of warmer and colder periods, driven by changes in heat coming from the Sun.

Professor Ruddiman has now calculated that if the Earth had followed its natural cycle over the last 10,000 years, it should have got steadily colder.

It has not because, he believes, human activities have been keeping the temperature steady.

"What should have happened with the natural climate is it should have cooled substantially," he told BBC News Online.

"And instead humans just started adding greenhouse gases at a rate which cancelled most, but not all, of that natural cooling; and so it's a combination of a natural cooling mostly cancelled by a human warming."

Fast warming

The birth and development of agriculture is the key, and it substantially changed the nature of the land and its interaction with the atmosphere.

Our ancestors started adding the greenhouse gas carbon dioxide principally by cutting down trees for farming. Methane concentrations - another potent greenhouse contributor - started to rise with wet farming of rice.

Professor Ruddiman, of the Department of Environmental Sciences at Virginia, believes this 10,000-year warming added almost a degree Celsius to the global average temperature.

This though is a radical departure from existing theories about climate change and will inevitably be debated by other researchers.

But there is supporting evidence, and it is consistent with what we know about deforestation and farming today.

And it does not alter the assertion that almost a further degree Celsius has been added over the last century alone, mainly through our dependence on fossil fuels.

The broad body of scientific opinion is of the view that the world is now warming faster than at any time in recorded history.

Sophisticated ways

Other research here at the AGU meeting suggests changes in climate brought into existence the civilisations of southern Mesopotamia, widely regarded as the birthplace of modern western society.

In northern Mesopotamia, rain-fed agriculture had been practised with success for 1,000 years when it was brought to a shuddering halt by a sudden reduction in rains 8,200 years ago, according to Professor Harvey Weiss from Yale University.

Professor Weiss told the AGU that this sudden drying out necessitated a change to irrigated fields.

But that was impossible in northern Mesopotamia where the Tigris and Euphrates rivers, which bordered Mesopotamia, ran in channels below the level of the agricultural land.

It was possible, however, in southern Mesopotamia, which is why - so the argument goes - great societies emerged there characterised by art, literature and sophisticated social structures.

Professor Weiss said it was something of an irony that natural changes in climate made modern society possible, whereas society was now changing the climate in ways which threatened its existence.

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