UK scientists say only a combination of natural and human causes can explain the Earth's warming during the 20th Century.

They combined data on greenhouse gas emissions, ozone and sulphate aerosol levels, solar variations, and volcanic aerosols in different versions of a state-of-the-art climate model.

Natural causes, they found, mattered more early in the century, and human-induced factors during the present warming.

They say their work increases their confidence in predictions of human contributions to future warming.

The scientists, from the Hadley Centre for Climate Prediction and Research, part of the UK Met Office, and the Rutherford Appleton Laboratory, report their findings in the magazine Science.

Explanation sought

They write: "A comparison of observations with simulations of a coupled ocean-atmosphere general circulation model shows that both natural and anthropogenic factors have contributed significantly to 20th Century temperature changes.

The researchers say human factors matter more now . . .

"More than 80% of observed global mean temperature variations and more than 60% of 10- to 50-year land temperature variations are due to changes in external forcings."

Global mean temperature near the Earth's surface has been increasing at 0.2 degrees Celsius a decade over the last three decades.

A comparable rise occurred between 1910 and 1945, with a lull then until the mid-1970s.

Until now climate simulations had found it hard to explain this earlier warming, having ignored many natural factors.

"For the first time," the Met Office says, "this new research combines the most important human and natural factors in one climate model."

The authors say the simulations they made in their model incorporate changes in greenhouse gases, including carbon dioxide and methane, changes in tropospheric and stratospheric ozone, and changes in sulphur emissions.

"When we include both anthropogenic and natural forcings", they write, "our model successfully simulates not just the observed global mean response, but also some of the large-scale features of the observed temperature response.

"We conclude that both anthropogenic and natural factors are required to account for 20th Century near-surface temperature change.

"The model successfully simulates large-scale temperature changes over the 20th Century.

Consistent findings

"However, it does not capture observed changes in the Atlantic in the early part of the century, nor does it simulate the rise in the North Atlantic Oscillation index observed over the last three decades.

"It may be that the model does not sufficiently resolve the stratosphere, or that there are deficiencies in generating or responding to sea surface temperature variations."

Despite the uncertainties, they conclude: "The overall large-scale pattern of observed near-surface temperature change over the 20th Century is consistent with our understanding of the combined impacts of natural and anthropogenic forcings.

. . . but natural ones did earlier

"Natural forcings were relatively more important in the early-century warming, and anthropogenic forcings have played a dominant role in warming observed in recent decades.

"External forcings appear to be the main contributors controlling near-surface decadal-mean temperature changes on global and continental land scales.

Credibility demonstrated

"Our successful hindcast of large-scale temperature changes over the 20th Century increases our confidence in predictions of the anthropogenic contribution to future temperature changes."

Dr Peter Stott, who led the research team, said: "This model is still not perfect but, by successfully simulating past temperature changes, it demonstrates the credibility of our climate predictions.

"These show that the current rate of warming of two to three degrees Celsius per century is likely to continue over the coming decades.

"However, there is much more work to do if we are to provide the predictions needed to assess the impacts of climate change on individual countries."