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In February 2007, depending on what newspaper you read, you might have seen an article detailing a "controversial new theory" of global warming.The idea was that variations in cosmic rays penetrating the Earth's atmosphere would change the amount of cloud cover, in turn changing our planet's reflectivity, and so the temperature at its surface.This, it was said, could be the reason why temperatures have been seen to be varying so much over the Earth's history, and why they are rising now.The theory was detailed in a book, The Chilling Stars, written by Danish scientist Henrik Svensmark and British science writer Nigel Calder, which appeared on the shelves a week after the Intergovernmental Panel on Climate Change (IPCC) had published its landmark report concluding it was more than 90% likely that humankind's emissions of greenhouse gases were warming the planet.In truth, the theory was not new; Dr Svensmark's team had proposed it a decade earlier, while the idea of a cosmic ray influence on weather dates back to 1959 and US researcher Edward Ney.The bigger question is whether it amounts to a theory of global warming at all.Small change
Over the course of the Earth's history, the main factor driving changes in its climate has been that the amount of energy from the Sun varies, either because of wobbles in the Earth's orbit or because the Sun's power output changes.Most noticeably, it changes with the 11-year solar cycle, first identified in the mid-1800s by astronomers who noticed periodic variations in the number of sunspots.If it varied enough, it could change the Earth's surface temperature markedly. So is it?"Across the solar cycle, the Sun's energy output varies only by about 0.1%," says Sami Solanki from the Max-Planck Institute for Solar System Research in Germany.
"When you look across much longer timescales, you also see changes only of about 0.1%. So just considering directly variations in energy coming from the Sun, this is not enough to explain the climatic changes we have seen and are seeing now."This is why scientists have been investigating mechanisms which could amplify the changes in solar output, scaling up the 0.1% variation into an effect that could explain the temperature rise of almost half a degree Celsius that we have seen at the Earth's surface in just the last few decades.One is Joanna Haigh from Imperial College, London, UK. She realised that although the Sun's overall energy output changes by 0.1%, it changes much more in the ultraviolet part of the spectrum."The changes in the UV are much larger, between 1% and 10%," she says."And that primarily has an impact in the stratosphere (the upper atmosphere) - UV is absorbed by ozone in the stratosphere and also produces ozone, and this warms the air."Using computer models of climate, Dr Haigh's team showed that warming in the stratosphere could change the way energy is distributed across the troposphere, the lower atmosphere, changing wind and weather patterns. But not by much."We found it might raise temperatures by a maximum of half to one Celsius in certain regions," she says. "But in terms of an impact on the global average temperature, it's small, maybe about 0.2C."Which is not enough to explain the warming that has occurred since the late 1970s.
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As in the days of Noah....
