Scientists have long suspected that the sun affects climate on Earth, but that connection has proved hard to pin down. Researchers recently demonstrated that the 11-year cycle of solar activity influences weather in the tropical Pacific Ocean. Even then the exact cause remained obscure, since the sun’s brightness varies by just one-tenth of a percent. Two studies from 2009 are filling in the gaps.
In August an international team led by Gerald Meehl, a climatologist with the National Center for Atmospheric Research, announced that the sun’s outsize influence results from its combined effects on our atmosphere and oceans. When the sun is at its most intense, ozone in the stratosphere absorbs more ultraviolet energy, making areas near the equator warmer than usual. The added heat changes wind patterns, bringing more rain to the western tropics. At the same time, the extra sunlight causes more evaporation off the ocean, which adds to downpours in the western tropics. Simulations that modeled just one of these effects failed to match the real world. Meehl saw that the two mechanisms “feed off each other, producing a stronger response than either can alone.” His results should help climatologists predict monsoons in Asia and overall climate in North America and might someday allow them to estimate seasonal rainfall years in advance.
Meanwhile, Henrik Svensmark of the Technical University of Denmark and his colleagues are exploring a broader climate impact of solar activity. He believes that cosmic rays—energetic subatomic particles from outer space—help seed cloud-forming water droplets in the lower atmosphere. During peak solar activity, eruptions from the sun spew out huge clouds of plasma that shield Earth from those cosmic rays. After examining cloud cover and cosmic ray fluxes, Svensmark concluded that declines in cosmic rays lead to fewer clouds, implying that an active sun could lead to warmer surface temperatures. Following the strongest solar eruptions, he found that the sky lost 7 percent of its cloud water. Many scientists doubt the significance of these cosmic ray effects, but Svensmark sees the question as ripe for investigation. “The sun is doing natural experiments on Earth’s atmosphere, giving us the opportunity to test these ideas,” he says.