

How Sunlight Controls Climate

New computer models begin to suggest how changes in the sun's strength might change weather patterns
earth and sun

Courtesy of NASA

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.

Meanwhile, the increased radiance during the solar max also adds slightly more heat to the ocean in areas that are already relatively cloudless because of sinking, cooler air. That produces a little more evaporation, which is carried by the trade winds back into the tropics where it comes down again as increased rainfall, but also helps strengthen the upward convection that causes the subtropical cloudless skies. That, in turn, further increases downward pressure back in the subtropics, resulting in even fewer clouds—again roughly 2 percent less clouds over these parts of the Pacific. "You basically spin up this whole system," Meehl says.

But the model did not exactly reproduce real-world conditions. Whereas sea-surface temperatures in the actual eastern Pacific typically decline by roughly 0.8 degree Celsius under a stronger sun, the model could only replicate about 0.6 degree C of cooling. Nor did the model predict changes where they actually occur on the planet. Other factors are likely at work, Meehl says, and even the best computer model can only begin to approximate the complexity of the actual climate.

Right now, the sun is stuck in a period of extremely low sunspot activity, not unlike the "Maunder Minimum" that may have been responsible for the Little Ice Age that cooled Europe in the late 17th century as well as the fall of imperial dynasties in China. And, for the latter half of the 20th century, the sun's output remained relatively constant as global temperatures rose—ruling out our star itself as the direct source of global warming.

Nevertheless, the research begins to explain the physical mechanisms by which changes in the sun's radiance can have outsized impacts on the planet. And that means that the next uptick in the solar cycle, and thereby the sun's brightness, might bring La Niña conditions—unusually cold surface waters—in the equatorial Pacific. "Whenever it happens," Meehl predicts, "chances are it would behave like a weak La Niña–like pattern."

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