Climatologists and astronomers speaking at the American Association for the Advancement of Science meeting in Philadelphia say the present warming may be unusual - but a mini ice age could soon follow.

The sun provides all the energy that drives our climate, but it is not the constant star it might seem.

Careful studies over the last 20 years show that its overall brightness and energy output increases slightly as sunspot activity rises to the peak of its 11-year cycle.

And individual cycles can be more or less active.

The sun is currently at its most active for 300 years.

That, say scientists in Philadelphia, could be a more significant cause of global warming than the emissions of greenhouse gases that are most often blamed.

The researchers point out that much of the half-a-degree rise in global temperature over the last 120 years occurred before 1940 - earlier than the biggest rise in greenhouse gas emissions.





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They have also studied other sun-like stars and found that they spend significant periods without sunspots at all, so perhaps cool spells should be feared more than global warming.

The scientists do not pretend they can explain everything, nor do they say that attempts to reduce greenhouse gas emissions should be abandoned. But they do feel that understanding of our nearest star must be increased if the climate is to be understood.