Sun's warming influence 'under-estimated'
By BBC News Online science editor Dr David Whitehouse
28 Nov 2000
Opinion is divided over the Sun's impact on climate change
The weather observations, made almost daily since 1795, comprise the longest climate archive available for a single site in Ireland.
Dr John Butler, the astronomer in charge of the project, told BBC News
Longer is better
The observations at Armagh began in 1795, a few years after the observatory was founded. Temperature, pressure and, later, rainfall have been measured every day with the exception of a period around 1825. In all that time, the Armagh meteorological instruments have been moved only about 20 metres.
Armagh Observatory's weather archive
When analysed, the data allow the average temperature at Armagh to be calculated to an accuracy of 0.1 deg C per decade. Eventually the entire data set will be placed on the internet.
"It's quite apparent from our data that global warming, of about a degree C, has been taking place for at least a hundred years," Dr Butler told BBC News Online.
Shorter is warmer
The researchers point out that the mean average temperature at Armagh seems to be related to the length of the Sun's activity cycle. This cycle is on average 11 years in duration but it can vary a few years either way.
"We have found that it gets cooler when the Sun's cycle is longer and that Armagh is warmer when the cycle is shorter," said Dr Butler. Scientists cannot yet fully explain how natural variations in the Sun's brightness and activity may affect the Earth's climate. While the Sun is about 0.1% brighter during shorter cycles the effect is not enough to account for the observed warming trend.
"But the Sun's activity does affect the flux of cosmic rays, high-energy
Average temperatures in Armagh appear to correlate with solar activity
Low clouds cool the Earth by reflecting more solar radiation back into
It may be that changing cloud cover has caused global warming over the past century or so. However, Dr Butler is cautious about this issue: "There is currently very little evidence for a low-altitude cloud reduction over the past century. But there is some evidence for a global increase in total cloud."
"I suspect that the greenhouse lobby have under-estimated the role of solar variability in climate change," he added. "However I am not in favour of polluting the atmosphere, for whatever reason."
an emissary solution