By Alex Kirby
BBC News website environment correspondent
A stark warning of the probable effects of global warming in Europe has been given by a UK climate research group.
Heatwaves could become routine
Scientists at the Met Office's Hadley Centre say the 2003 European heatwave, the hottest ever recorded, could within just 60 years pass as "unusually cool".
They cannot yet reliably estimate the risk of a Gulf Stream collapse, but say it would mean "significant" cooling.
The researchers say 2003 was the third warmest year on record, about 0.8C hotter than just over a century ago.
The Hadley Centre for Climate Prediction and Research is one of the world's leading scientific groups studying what a warming world will be like.
Into the heat
Its report, Uncertainty, Risk And Dangerous Climate Change, is published as the countries which have signed the Kyoto Protocol, the global climate treaty, meet in Argentina.
The report says last year's European heatwave, the most intense since records began, caused more than 15,000 extra deaths.
The authors say they estimate man-made climate change has already doubled the risk of such heatwaves.
They investigate one scenario prepared by the Intergovernmental Panel on Climate Change, which envisages medium to high emissions of greenhouse gases.
On that basis, they predict that by the 2040s more than half of all European summers are likely to be warmer than 2003's. They add: "By the 2060s, a 2003-type summer would be unusually cool."
But the report says things could turn out very differently: "While climate is expected to change gradually over the course of the century, there are some components of the climate system which could change abruptly.
"There are also concerns that some processes may have a trigger point which, once exceeded, will make the changes inevitable, no matter how much we reduce the emissions subsequently."
It looks at the thermohaline circulation, the system of ocean currents that carries heat from the tropics to higher latitudes to keep them warmer than they would otherwise be.
Chill out? Nice chance, if the predictions are right
If this circulation, which influences a largely wind-driven North Atlantic surface current known as the Gulf Stream, shuts down, the report says, the whole of the northern hemisphere is predicted to cool, "leading to large impacts".
It says there is "a significant possibility" that the melting of the Greenland ice sheet could be triggered in the next few centuries.
The report adds that there is concern that the ice might never return to its present volume, even if atmospheric carbon dioxide were reduced to pre-industrial levels.
The authors say 2003's global average surface temperature was nearly 0.8C above that at the end of the 19th Century, making it the third warmest since instrumental records began 143 years ago.
They write: "The 10 warmest years have occurred since 1990, including each year since 1997. Since 1975, the land has warmed at approximately twice the rate of the oceans."
In a separate study, UK and US astronomers have again raised the possibility that the Sun's indirect effects may have had a bigger impact on the Earth's climate than is generally recognised.
Their analysis suggests there is a strong link between low-level cloud formation and changes in the amount of cosmic rays - high-energy space particles - hitting the atmosphere.
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
Solar activity is very directly correlated to this cosmic ray flux, and some scientists suspect the impacts can somehow seed clouds, altering the Earth's ability to either reflect or retain the Sun's radiation (although the actual mechanism is not known).
The UK-US team tell the Journal of Atmospheric and Solar-Terrestrial Physics that a simple model constructed to investigate the cloud and cosmic ray link could "explain a significant part of the global warming over the past century, but not all".
It is a controversial idea, with many climate scientists arguing that greenhouse gases have been by far the dominant force pushing the Earth on to a sharp warming trend over the past 150 years.
Enric Pallé, John Butler and Keran O'Brien say emissions of gases such as carbon dioxide may be responsible for the significant warming for which their model cannot account.