PLANET-wide heating and cooling of the atmosphere during the 11-year sunspot cycle has been measured for the first time. Climate-change sceptics may seize on the findings as evidence that the sun鈥檚 variability can explain global warming 鈥� but mathematician Ka-Kit Tung says quite the contrary is true.
Tung and colleague Charles Camp, both of the University of Washington in Seattle, analysed satellite data on solar radiation and surface temperatures over the past 50 years, covering four-and-a-half solar cycles. They found that global average temperatures oscillated by almost 0.2 掳C between high and low points in the cycle, nearly twice the amplitude of previous estimates (Geophysical Research Letters, ).
The finding adds to the evidence that mainstream climate models are right about the likely extent of future human-generated warming, Tung says. It also effectively rules out some lower estimates in those models.
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鈥淭he findings provide important real-world evidence that climate-model predictions of global warming are correct鈥�
Joanna Haigh of Imperial College London, an expert on solar influences on climate, says this is the first time a statistically significant global temperature signal has been found for the 11-year cycle. 鈥淥ther people have previously found only regional responses,鈥� she says.
The 11-year cycle is the shortest of a number of known oscillations in the solar radiation reaching the Earth. Tung says the sun is currently at a low point in the 11-year cycle. Unless other influences like volcanic eruptions or El Ni帽os intervene, we can expect strong warming of the atmosphere in the next five years, as an upturn in the cycle reinforces human-generated warming.
Tung says his findings provide important real-world evidence that climate model predictions of global warming are correct. For instance, they show that the temperature changes are two to three times as strong in polar regions. On the face of it this is surprising, because the variation in solar radiation is greatest in the tropics. But Tung says 鈥渋t reinforces the idea of melting ice as an amplification mechanism in the climate-change models鈥�.
What will excite climate scientists most is that Tung and Camp are the first to measure directly how a given change in the amount of heat energy in the atmosphere translates into a change in temperature. Researchers call this the 鈥渃limate sensitivity鈥� of the atmosphere, and it is a vital factor in climate research because it determines how fast the planet will warm as a result of climate change.
All previous estimates of climate sensitivity have been based on models. Now Tung and Camp have used real figures to show that an extra 0.9 watts per square metre of heating at the Earth鈥檚 surface, at the height of the 11-year cycle, produces an immediate warming of the atmosphere of 0.16 掳C.
In an as yet posted on Tung鈥檚 website, he and Camp say this shows that a doubling of carbon dioxide levels in the atmosphere would cause a warming of between 2.3 and 4.1 掳C within about a year. They say this makes the lower estimates of some models of climate change 鈥渦nlikely鈥�.
This immediate warming, Tung stresses, is almost certain to be an underestimate of the overall effect of greenhouse gases, because extra warming is delayed due to the deep ocean heating up only slowly. 鈥淏ut our findings give a lower bound to the atmosphere鈥檚 climate sensitivity that we have not had before.鈥�
Climate modeller Peter Cox from the University of Exeter, UK, says Tung has shown, without recourse to climate models, that a doubling of carbon dioxide would cause at least 2 掳C of warming, 鈥渨hich is considered by many to be the threshold of dangerous climate change.鈥�
