Sun's warming influence 'under-estimated'

Opinion is divided over the Sun's impact on climate change

By BBC News Online science editor Dr David Whitehouse

Scientists at Armagh Observatory claim a unique weather record could show that the Sun has been the main contributor to global warming over the past two centuries.

I suspect that the greenhouse lobby have under-estimated the role of solar variability in climate change

Dr John Butler

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 Online: "We can see global warming taking place over the past two centuries that suggests that changes in the Sun are at least partially responsible."

The data will confuse some climate experts who argue that the influence of changes in the Sun on rising temperatures has already been studied, and discounted, as a major cause of global warming.

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.

The Armagh Observatory's weather archive spans two centuries

"What makes the data so useful is that the site of the observatory has not changed all that much in 200 years," said Dr Butler. "Other weather stations have been engulfed by towns and cities that make the long-term reliability of their data questionable."

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 particles from deep space, that strike our atmosphere," said Dr Butler.

Consequently it has been suggested that because cosmic rays are the main source of ionisation in the Earth's atmosphere they may have an influence on cloud formation.

Average temperatures in Armagh appear to correlate with solar activity

In general, the more cosmic rays that reach the Earth, the more low cloud there is. However, a higher solar activity leads to lower cosmic ray flux and reduced low cloud.

Low clouds cool the Earth by reflecting more solar radiation back into space, so a drop in the amount of low cloud contributes to global warming.

High cloud does the opposite and tends to warm the Earth by reflecting more of the Earth's infra-red radiation back to the ground.

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."