The Sun Again Intrudes on Earth's Decadal Climate Change

Most climatologists have learned to be skeptical about apparent links between the sun's variability and Earth's climate. Again and again, researchers have uncovered plausible correlations, but the evidence usually crumbled under closer scrutiny. And nobody had come up with a convincing mechanism to explain how tiny changes on the sun might change climate on Earth. But suspicious associations between sun and climate keep cropping up. Now, two such correlations--a 22-year climate cycle recorded in glacial sediments and the tracing of an 11-year cycle from the stratosphere into the lower atmosphere--may be robust enough to give the sun-climate link a touch more respectability.

"There's more and more evidence of something pretty distinct at this time scale," says statistical climatologist Michael Mann of the University of Virginia in Charlottesville, a discoverer of the ice age cycle. "I've always been a skeptic" of sun-climate connections, he says, "and I remain a little skeptical, but we can't dismiss it as a statistical anomaly." Paleoceanographer Theodore Moore of the University of Michigan, Ann Arbor, confirmed the 22-year climate cycle in the same glacial record, and he too remains on the skeptical side. "It's hard for me to say where [that cycle] is coming from," he says, but "it's a fertile area for research,"

Geologists Tammy M. Rittenour and Julie Brigham-Grette of the University of Massachusetts, Amherst, and Mann found their 22-year cycle, along with shorter cycles, buried in the layered bottom sediments of now-vanished New England lakes. About 15,000 years ago, sediment-laden waters poured into the lakes from melting glaciers. The warmer a summer's weather, the more ice melted and the more sediment washed into lakes, so the thickness of each annual layer of sediment reflects the temperature during that melt season. Rittenour and her colleagues analyzed 4000 years' worth of layer thicknesses and found statistically significant periodicities falling between cycle lengths of 3 and 5 years. In their 12 May Science paper (p. 1039), they attributed those climate fluctuations to the long-range influence of an ancient El Niño. Superimposed on these fluctuations, the researchers identified a 22-year cycle of varying layer thickness.

The 22-year oscillation (actually 22.2 ± 0.2 years), which the researchers described in the same paper, provoked little attention, as it was relegated to a few lines of text and a figure label. But 22 is a significant number to scientists looking for sun-climate links. It's twice the 11-year period at which sunspot abundance and, much less dramatically, solar brightness vary, and it equals the length of the cycle in which the sun flips its magnetic poles back and forth. When an 11- or 22-year periodicity shows up in climate records, suspicion falls on the sun, although it's never been clear exactly how a feeble change in solar brightness or the flipping of the sun's magnetic field would trigger measurable climate change.

"It was a little awkward when we found" the 22-year period, says Mann. "We weren't looking for it. Our immediate guess was that this is from chance sampling variations, that it's a fluke, but we tested its robustness. The thing just holds up. It's a real feature. It's not a dominant signal, but it's always there." Moore doesn't doubt it's there, but he would be very hesitant to say that sunspots are behind any 11- or 22-year climatic periodicity. "I'm cured of that," he says. "We should be very open-minded about what that [periodicity] means." Another, perhaps more palatable, possibility is oscillations inherent in the oceans, he says, that could swing climate to and fro much as one ocean oscillation appears to do on longer, multidecadal time scales of 40 to 80 years (see main text).

The link between sun and climate would be strengthened if, rather than just pointing out solarlike climate periodicities, researchers could demonstrate that their 11- or 22-year climate variations were in step with the variations on the sun. Last year, climate modeler Drew Shindell of NASA's Goddard Institute for Space Studies in New York City and his colleagues showed how, in their model at least, feeble variations of solar output over the 11-year sunspot cycle could gain leverage in the stratosphere and even propagate temperature changes down to the surface (Science, 9 April 1999, pp. 234 and 305). Meteorologists Karin Labitzke of the Free University of Berlin and Harry van Loon of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, had shown that something was letting stratospheric temperatures vary in time with the sunspot cycle, but they hadn't been able to trace solar cycle effects into the underlying troposphere, where weather and climate reside (Science, 4 August 1995, p. 633).

Now, van Loon and Dennis Shea of NCAR have crunched the latest set of temperature data since 1958 and found an 11-year variation of several tenths of a degree in the Northern Hemisphere troposphere that was in step with sunspots over four solar cycles. The effect decreases toward the surface when it is averaged around a latitude band spanning the hemisphere, van Loon notes, but he has yet to comb the data for possible effects varying from place to place along latitude bands. Sun-climate effects "is still a topic that's much alive," he says.

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Volume 288, Number 5473, Issue of 16 Jun 2000, p. 1986.
Copyright © 2000 by The American Association for the Advancement of Science.