The Sun's in our eyes

December 1, 2000

Could the natural variability of the Sun be fooling climatologists into underestimating the human impact on the atmosphere? Stuart Clark investigates.

At an altitude of 100km lies a natural boundary between Earth and space. Above it, in the thermosphere, the Sun exerts most of its influence on the remaining traces of atmosphere. If you want to study phenomena up here, you would seek research funds from the Particle Physics and Astronomy Research Council. Far below the boundary, in the troposphere, is where weather takes place. If you want to study things down here, you go to the Natural Environment Research Council. But where do you go if you suspect that astronomical events influence climate change?

Researchers who want to study the way that high altitude change propagates downwards find themselves stumbling into what they sardonically term the ignorosphere or the fundopause. Mike Lockwood, of the Rutherford Appleton Laboratory and University of Southampton, is only too familiar with this twilight zone for grants.

Professor Lockwood is one of a growing number of scientists who see clear evidence that the Sun affects the weather. However, few details of those interactions are understood. "Some of those unknowns have arisen because the subject crosses the boundary between the two funding bodies," Lockwood says.

"There is no doubt some areas of this science have become a bit of a backwater because that is where the division of the two areas of responsibility happens to lie."

To address this problem, the two research councils recently organised a one-day workshop at Birkbeck College, London, to allow scientists from both sides to discuss their work and look for areas of overlap.

The Sun is an inconstant generator. It passes through a regular 22-year sunspot cycle, with evidence for longer periods of change superimposed on this cycle. Indeed, the Sun's influence is one candidate for an unusually clement period in the Earth's history known as the "medieval warm period". Between the 9th and 14th centuries, global temperatures rose to between 0.5 and 1ºC higher than at present. Tree-ring analysis shows this high was not associated with a rise in atmospheric carbon dioxide, the main suspect for possible global warming in the industrial era.

The finger of suspicion also points towards the Sun for a temperature rise early in the 20th century. To understand these effects, scientists would like to focus on how solar variability affects cloud cover. Although there are a number of putative mechanisms, all cross the "fundopause".

Myles Allen, a scientist at the Rutherford Appleton Laboratory and the University of Oxford, is part of a consortium working on a design for the Albedo Solar Terrestrial Interactions Experiment, also known as Asterix. It is designed to characterise Earth's response to changes in the Sun's output.

"The idea is to have an instrument that looks both at the Sun and at the Earth so that you can see an event on the Sun being reflected back from the Earth. This gives you a measure of the way the Earth's ability to reflect energy (which is governed by its cloud cover) responds to changes in solar activity on a wide range of timescales," Allen says.

As yet, the mission is searching for nothing more than funding. Although Allen has an inkling that current models underestimate the effect of the Sun, he is quick to point out it is not the principal reason for the current warming.

"We have seen an acceleration in warming over the past 30 years. If you look at the history of solar irradiance, that flattened out in the latter part of the 20th century. So it is very difficult to explain the observed acceleration in warming solely in response to changes in solar irradiance," Allen explains.

Nevertheless, humanity cannot simply ignore it. Even though its effects have been superseded by artificial emissions of carbon dioxide, the Sun's past influence may be causing climatologists to underestimate just how fast greenhouse gas emissions are taking hold of our world.

Lockwood has a gut feeling. "If we allow for the solar contribution properly, we would be left with a man-made contribution that was later in its onset than previously thought, but steeper. In other words, that term is rising more rapidly than we thought. So there is a danger that the solar variability has disguised just how severe the man-made changes have been over the past 20 or 30 years," he says. Hence, the urgency in the scientific community to explore the Sun's role in global warming.

Sue Horne, space-based facilities coordinator at the PPARC, says scientists have to start this work immediately for the UK to make the biggest possible impact. "We think this is such an important programme that it should be funded." To that end, the PPARC is looking to meet the cost from its settlement in last week's spending announcement for the research councils.

Stuart Clark is director of public astronomy education at the University of Hertfordshire and author of Life on Other Worlds and How to Find It .

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