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This week at the AAAS : Fierce wind from space warms the Earth

点击量:   时间:2017-09-09 01:01:04

By Jeff Hecht THE solar wind may be the missing link that explains why temperatures on Earth vary with the 11-year cycle of solar activity, Brian Tinsley of the University of Texas at Dallas told the AAAS. When the Sun is at its most active and its surface is freckled with many sunspots, the Earth becomes slightly warmer. “You can correlate global mean temperatures over the last 400 years with the sunspot cycle,” says Tinsley. Explaining why this should be so has proved difficult. While the Sun is slightly brighter at the sunspot peak, this is not enough to account for a warmer Earth. This brightening is stronger at ultraviolet wavelengths, but little of this radiation penetrates the stratosphere, so it cannot easily explain global temperature changes. Tinsley told the AAAS that the answer may lie in the flux of charged particles rushing from the Sun in the solar wind. This causes the flux of galactic cosmic rays with an energy of a billion electronvolts to vary by a factor of two over the solar cycle. Each of these high-energy particles produces a cascade of lower-energy ions as it rips through the atmosphere. As the flux of ions through the atmosphere increases, says Tinsley, more become trapped within clouds. At the cloud tops the charged particles trigger the freezing of supercooled droplets. Ice crystallisation releases latent heat into the atmosphere, and thins the clouds so they reflect less solar radiation back into space. At low latitudes, cloud clearing means more solar energy reaching the surface. Towards the poles thinner clouds let more heat radiate out into space at night. Tinsley suggests that the effects on cloud cover could also disrupt the jet stream, causing regional climate shifts. Because the release of latent heat is probably the greatest of these effects,