Why climate change is not caused by the sun

Posted: 19 March 2007

Fresh controversy over the true cause of climate change has been sparked in Britain by the recent Channel 4 documentary provocatively entitled 'The Great Global Warning Swindle'. This argued that the main cause is not human behaviour but the activity of the sun. Here Dr Richard Betts, of the UK Meteorological Office's Hadley Centre, explains why the climate change now being experienced could not have been caused by the sun.

"Although the sun can play a part in climate change, there is overwhelming scientific evidence that it is not the cause of the climate warming that we have seen over recent decades. "There have been proposed two mechanisms for how the sun might might be influencing climate. The first is that changes in the amount of solar radiation given off by the sun might be to blame. Solar irradiance has been monitored continuously for the last 28 years, and although an 11-year cycle has been well-established, no significant long-term trend has been detected over that period. The warming trend in global temperatures over recent decades therefore cannot be explained by changes in solar irradiance, simply because there has been no overall change in solar irradiance over that time. "Earlier records show an increase in solar irradiance in the early part of the Twentieth Century, followed by a brief decrease before the current period of no significant trend. Global temperatures also underwent a warming until around 1940 and a subsequent cooling until around 1950, but then warming set in again and became more rapid. Computer models of climate suggest that solar irradiance changes may well have been a significant cause of climate change until the mid-Twentieth Century. However, the correlation between solar irradiance and global temperature breaks down after the 1960s as temperatures rose while solar irradiance did not. "Over tens and hundreds of thousands of years, the amount of energy received by the Earth from the sun has changed because of changes in the Earth's orbit and the tilt of its axis. These changes led to the coming and going of ice ages in the past and are part of a natural cycle of climate change. However, these changes take many thousands of years and are therefore much too slow to account for climate changes seen over a few decades. "The other hypothesis is that changes in cosmic rays associated with solar cycles might affect cloud cover and hence influence climate. However, while cloud cover was reported to be correlated with cosmic ray fluxes some time ago, this correlation has broken down as more years of data have become available. The available data therefore do not support the cosmic ray hypothesis." Dr Betts was talking to The Ecologist magazine (March 2007). His response is reproduced here with permission. The Ecologist online can be accessed www.theecologist.org/home.asp