Climate and the solar cycle
Chilling out in the winter sun
Stratospheric changes can lead to nasty cold snaps
THOSE unconvinced—and those seeking to unconvince others—of the reality of man-made global warming often point to the changeable behaviour of the sun as an alternative hypothesis. A new study showing how the severity of winters in Europe, and warming in the Arctic, might be linked to changes in solar activity might seem to add to this case. In itself, it does not, for the heat (or, rather, the cold) in question is being redistributed, not retained. But it does point to two other lessons about climate change: that hard data on the factors which affect it are sometimes difficult to come by; and that computer models of the climate can be quite impressive tools for working out what is going on.
The sun’s activity waxes and wanes on an 11-year cycle, and over this cycle the amount of ultraviolet (UV) light the sun emits changes a lot more than does the total amount of energy. The stratosphere, the part of the Earth’s atmosphere which does most to absorb UV, might thus be expected to be particularly sensitive to the cycle.
In a paper just published in Nature Geoscience, Sarah Ineson of Britain’s Meteorological Office and her colleagues compared the way that the Met Office’s new and putatively improved climate model dealt with winters at times of high UV and at times of low UV, using data on the amount of ultraviolet the sun gives off that were collected by a satellite called SORCE.
Dr Ineson found that at low UV levels the stratosphere in the tropics was cooler, because there was less UV for it to absorb, which meant the difference in temperature between the tropical stratosphere and the polar stratosphere shrank. That changed the way the atmosphere circulated, and as those changes spread down into the lower atmosphere they made it easier for cold surface air from the Arctic to come south in winter, freezing chunks of northern Europe. These conditions looked similar to those seen in the past two cold European winters—which occurred at a time of low solar activity. The Arctic itself, in models and in real life, was warmer than usual, as were parts of Canada. In contrast, northern Europe, swathes of Russia and bits of America were colder.
Why had this solar effect not been seen before? To some extent it had. Earlier modelling of a period of prolonged low solar activity in the 17th and 18th centuries showed similar patterns. That models of today’s climate had not was, in part, because they used much lower estimates of the amount of UV variation over the solar cycle than those derived from the SORCE data, the most precise to be taken from a satellite looking at the sun. It may just be that working with more realistic data made the model work better.
This does not mean the question is settled. Some scientists suspect the SORCE data may be exaggerating the sun’s variability, and if they were revised the link might go away. There are other theories around seeking to explain the recent cold winters, too. Improving predictions of future cold winters on the basis of this work, as the researchers say they would like to do, may thus prove hard.
But though global warming has made people look to models as predictors of the future, that is not their strongest suit. Something they can do much better is look at what happens when a variable such as UV is altered, compare that with the data, and thus gain insight into the mechanisms by which climate works. This new research provides a good example of what such an approach can achieve.
stratospheric adj. 同温层的, 平流层的 alternative adj. 非此即彼的; 供选择的 stratosphere n.同温层 putatively adv. 推定地 circulate v. 循环 exaggerate v. 夸张, 夸大 mechanism n. 机械