How Sunlight Controls Climate: Scientific American

Small changes in the sun's brightness can have big impacts on our planet's weather and climate. And now scientists have detailed how that process might work, according to a new study published August 28 in Science.

For decades some scientists have noted that certain climate phenomena—warmer seas, increased tropical rainfall, fewer clouds in the subtropics, stronger trade winds—seem to be connected to the sun's roughly 11-year cycle, which causes ebbs and flows in sunspots that result in variations in solar output.

That variation is roughly equal to 0.2 watt per meter squared—far too little to explain, for instance, actual warming sea-surface temperatures. A variety of theories have been proposed to explain the discrepancy: ozone chemistry changes in the stratosphere, increased sunlight in cloudless areas, even cosmic rays. But none of these theories, on its own, explains the phenomenon.

Now, using a computer model that pairs ozone chemistry with the fact that there are fewer clouds in the subtropics when the sun is stronger, climate scientist Gerald Meehl of the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and colleagues have reproduced all the observed cyclical climate phenomena as sunlight waxed and waned in intensity over the course of the last century. "Even though [sunlight variability] is a very small number on a global average, regionally or locally it can be much bigger," Meehl explains. Changes to stratospheric ozone chemistry and cloud cover in the subtropics "kind of add together and reinforce each other to produce a bigger amplitude of this small solar forcing signal," he says.

If the model is correct, the mechanism works like this when the sun is at maximum strength: Ozone in the tropical stratosphere traps slightly more heat under the increased ultraviolet sunlight, warming its surroundings and, in turn, allowing increased ozone production. (Warmer temperatures make it easier for ultraviolet light to break up O2 molecules, thereby allowing the resulting free oxygen ions to hook up with other molecules of their kind to create ozone.) That ozone also warms and the cycle continues, resulting in roughly 2 percent more ozone globally. But this change also begins to affect the circulation of the stratosphere itself, which then alters the circulation in the lowest layer of the atmosphere, known as the troposphere, by reinforcing certain wind patterns that then affect the weather we experience.

via How Sunlight Controls Climate: Scientific American.