The threat posed by climate change is sufficiently severe to justify action says a prestigious scientific body, the American Geophysical Union (AGU).

It calls for the development of various strategies, including reducing the emissions of gases, locking up carbon in vegetation or elsewhere, and adapting to the probable impacts, which could be "very disruptive".

The statement from AGU's governing body acknowledges the uncertainties still surrounding climate change, but calls for more research. It is cautiously worded, and echoes none of the more apocalyptic predictions of imminent disaster.

But it will encourage the many climatologists who argue that climate change - some of it caused by human activity - is already a reality.

The AGU restates as fact the accepted wisdom on global warming, that concentrations in the atmosphere of carbon dioxide (CO2) and other greenhouse gases have substantially increased because of the burning of fossil fuels and other human activities.

Encouraging vegetation could soak up CO2

It says these higher concentrations are predicted to persist, in some cases, for thousands of years.

But the statement says the detailed response of the earth and its atmosphere to this increase in gas concentrations is uncertain, because of the climate system's inherent complexity and natural variability.

The increase in global average surface temperatures over the last 150 years "appears to be unusual in the context of the last few centuries, but is not clearly outside the range of climate variability of the last few thousand years".

The AGU recognises, though, that the geological record of the distant past provides evidence of larger climate variations being associated with changes in atmospheric CO2.

And it says there is no known geological example in which such large amounts of CO2 have been injected into the atmosphere without simultaneous changes in other parts of the carbon cycle and the climate system.

This close coupling between atmospheric CO2 and climate "suggests that a change in one would in all likelihood be accompanied by a change in the other".

Alternative energy sources could reduce the use of fossil fuels

So the AGU says our present understanding "provides a compelling basis for legitimate public concern".

But although our knowledge of climate change has improved greatly in the last decade, there are still "significant scientific uncertainties". These include the probable local effects, the occurrence of very severe weather, and changes in clouds, rainfall, and ocean currents.

Because of these uncertainties, there is much public debate over how far greenhouse gases have caused or will cause climate change, and over how to react to it.

In a key passage, the statement says: "Science cannot be the sole source of guidance on how society should respond to climate issues. Nonetheless, scientific understanding based on peer-reviewed research must be central to informed decision-making."

The AGU wants more research into the extent of human influence on the climate, and urges scientists around the world to join the research and policy discussions.

Powerful call to act

The statement ends with a judicious yet powerful call to act, which could help to persuade sceptics of the reality of the threat.

"The world may already be committed to some degree of human-caused climate change, and further build-up of greenhouse gas concentrations may be expected to cause further change. The rapidity and uneven geographic distribution of these changes could be very disruptive."