Warming 'very likely' human-made

Climatic changes seen around the world are "very likely" to have a human cause, the Intergovernmental Panel on Climate Change (IPCC) will conclude.

By "very likely", the IPCC means greater than 90% probability.

The scientific body has spent this week finalising its positions on key issues, notably forecasts of sea level rise, as it prepares to publish a major report.

But a new study released on the eve of publication suggests its previous reports may have been too conservative.

Research by an international group of scientists suggests that temperatures and sea levels have been rising at or above the maximum rates proposed during the last IPCC report in 2001.

The first part of the Panel's 2007 assessment will be formally released on Friday morning in Paris.

Now, the Panel concludes, it is at least 90% certain that human emissions of greenhouse gases rather than natural variations are warming the planet's surface.

"This report closes the doors to those who were able to detract from the issue, and puts an end to the notion of uncertainty and doubt about man's role in climate change," commented United Nations Environment Programme executive director Achim Steiner.

On key scientific questions - how much the Earth's surface will warm by, and how high sea levels will rise - the experts have been locked in discussion.

It is widely believed they will predict an average global temperature rise by the end of the century of between about 2C and 4.5C, without ruling out higher numbers completely.

On sea level, there is a more fundamental debate. Computer models of climate do not generally include water coming into the oceans as icecaps melt; so should the IPCC exclude this from its calculations, or estimate the effect of a process which scientists do not understand well, but which could have a big impact?

There is likely to be some backing for a theory which has proved highly controversial in recent years, with the IPCC set to conclude that it is likely - meaning a greater probability than 66% - that rising temperatures have contributed to more powerful tropical storms in some areas of the world.

The actual average level, measured by tide gauges and satellites, has risen faster than the IPCC predicted it would due to simple thermal expansion of seawater, and at the maximum which the scientific body considered possible if its estimates for the melting of land-based glaciers and ice sheets were included.

"Our intention with this study was to demonstrate how temperatures and sea levels have actually gone up," said study leader Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research in Germany, "because 'climate sceptics' have cast doubt on IPCC projections and whether they are exaggerated."

The group suggests that if anything, the IPCC has been conservative.

The most obvious cause of the discrepancy - a faster-than expected rise in carbon dioxide concentrations - is ruled out, as the observed CO2 rise has mimicked model projections fairly well.

Another explanation could be that the effect on temperature of increasing carbon dioxide levels in the atmosphere - the factor called "climate sensitivity" - is higher than scientists had believed when they built their computer models.

Alternatively it could be natural climate variability.

"We cannot conclude the exact cause," commented Professor Rahmstorf, "but there is no doubt that for this period, the climate has been changing faster than the IPCC predicted."

Together with his co-researchers on this project, Professor Rahmstorf is in Paris for the IPCC discussions, which will on Friday morning end with the release of summary conclusions on the current state of climate science, drawing on the work of thousands of researchers.

The full climate science report will be released later in the year, as will other IPCC chapters looking at the probable impacts of climate change, options for adapting to those impacts, and possible routes to reducing emissions of greenhouse gases