by BARRY WIGMORE

Last updated at 22:36 13 September 2007

Global warming is a natural event and the effects are not all bad, two respected researchers claimed yesterday.

Authors Dennis Avery and Fred Singer looked at the work of more than 500 scientists and argue that these experts are doubtful the phenomenon is caused by man-made greenhouse gases.

Climate change is much more likely to be part of a cycle of warming and cooling that has happened regularly every 1,500 years for the last million years, they say.

And the doom and gloom merchants, who point to the threat to the polar bear from the melting North Pole, are wrong, the authors say.

Even if our climate is changing, it is not all bad, they suggest, because past cold periods have killed twice as many people as warm periods. Mr Avery said: "Not all of these researchers who doubt man-made climate change would describe themselves as global warming sceptics but the evidence in their studies is there for all to see.

"Two thousand years of published human histories say that the warm periods were good for people.

"It was the harsh, unstable Dark Ages and the Little Ice Age that brought bigger storms, untimely frost, widespread famine, plagues and disease."

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Mr Singer said: "We have a greenhouse theory with no evidence to support it, except a moderate warming turned into a scare by computer models whose results have never been verified with real-world events.

"The models only reflect the warming, not its cause."

The most recent global warming was between 1850 and 1940, the authors say, and was therefore probably not caused by man-made greenhouse gases.

Historical evidence of the natural cycle includes a record of floods on the Nile going back 5,000 years; Roman wine production in Britain in the first century AD; and thousands of museum paintings that portray sunnier skies during what is called the Medieval Warming, and more clouds during the Little Ice Age.

The authors looked at a raft of studies which, they claim, undermine the "scare-mongering" by those blaming man for destroying the planet.

In the current warming cycle, they say there is evidence that storms and droughts have been fewer and milder; corals, trees, birds, mammals and butterflies have adapted well; and sea levels are not rising significantly.

Mr Avery is a fellow of the Hudson Institute, an independent U.S. thinktank that tends to side with big business.

He was a senior agricultural analyst at the State Department when Ronald Reagan was president. Mr Singer is a climate physicist.

The pair spent months analysing scientific reports for their book, Unstoppable Global Warming: Every 1,500 Years, to counter claims made by former U.S. Vice President Al Gore in his film An Inconvenient Truth.

They argue that variations in the Sun's radiation have far more influence on our climate than humans.

Mr Singer said: "This can all be explained by the Sun's activity."

He added: "The number of the Sun's cosmic rays hitting the Earth affect the number of low, cooling clouds that reflect solar heat back into space, amplifying small variations in the intensity of the Sun."