By Alex Kirby
BBC News Online environment correspondent
The principal cause of recent climate change is not cosmic rays but human activities, a group of scientists says.
Clouds' role in climate change remains contested
They say an article last year linking cosmic rays and changes in temperature was "scientifically ill-founded".
They say the authors' methods were open to doubt and their conclusions wrong, surprising experts with their claims.
In Eos, the journal of the American Geophysical Union, the 11 Earth and space scientists insist that greenhouse gases remain the chief climate suspect.
In the climate mainstream
They say the most important physical processes are well understood, and model calculations and data analyses both conclude the human contribution to the global warming of the 20th Century through increased emissions of carbon dioxide (CO2) and other gases was dominant.
The authors of the Eos article - Cosmic Rays, Carbon Dioxide And Climate - are from Canada, France, Germany, Switzerland and the US.
The research by Nir Shaviv, an astrophysicist, of Hebrew University in Jerusalem, and Jan Veizer, a geologist, of the University of Ottawa and Ruhr University in Germany, was published in July 2003 in the Geological Society of America's journal GSA Today.
COSMIC RAYS AND CLOUDS
The Sun's magnetic field and solar wind shield the Solar System from cosmic rays (very energetic particles and radiation from outer space)
Changes in solar activity will affect the performance of the shield and how many cosmic rays get through to Earth
Theory suggests cosmic rays can "seed" clouds. Some satellite data have shown a close match between the amount of cloud cover over Earth and the changing flux in cosmic rays reaching the planet
It said the Earth's climate was profoundly affected by cosmic rays, high-energy particles from outer space, which normally cool the Earth's surface by helping clouds to form.
But increased solar activity lessens the cosmic rays reaching the Earth, and Shaviv and Veizer suggested this blocking effect had been the dominant cause of global warming over the past century.
No ground given
They said cosmic ray changes accounted for at least 66% of the temperature variation during that period.
The Eos authors, led by Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, Germany, say the paper by Shaviv and Veizer was "incorrect and based on questionable methodology".
The Eos team says humans affect climate more than clouds
They say the data on cosmic rays and temperature so far in the past are extremely uncertain.
They argue that the authors' reconstruction of ancient cosmic rays is based on only 50 meteorites, and say most other experts interpret their significance in a very different way.
Arguing that Shaviv and Veizer had in places adjusted the data, "in one case by 40 million years", the Eos team says they did not show any correlation between cosmic rays and climate.
And even if their analysis had been methodologically correct, it says, their work applied to time scales of several million years, while the current climate warming has occurred during just a hundred years, for which completely different mechanisms are relevant.
Dr Shaviv told BBC News Online: "The article in Eos raises general claims without
substantiating them with any actual evidence. The few more specific arguments that they bring are simply flawed and easily refuted."
Professor Veizer told BBC News Online: "It's a long story, and the whole issue is politically driven.
"We stand by what we said, that there is a correlation between the cosmic ray flux and the temperatures we calculated, though on the details we can disagree."