Most climatologists now agree that Earth is getting warmer — records suggest surface temperatures have climbed about 1.3°F (0.7°C during the last century. But there's less agreement on whether this rise has been triggered by an increase in atmospheric greenhouse gases, a change in the Sun's output, or both.
Careful spacecraft measurements reveal that Sun's total energy output does fluctuate in step with its 11-year-long activity cycle, but it's a tiny blip: just 0.2 watt per square meter, compared to the solar "constant" of 1,366 W/m² reaching the top of Earth's atmosphere. Part of the problem has been trying to understand how a variation of just 0.01% could possibly affect our climate. (Yea, yea, I know all about a butterfly flapping its wings — but that's chaos theory, not direct cause and effect.)
Now a team of climate modelers led by Gerald Meehl (National Center for Atmospheric Research) believe they've found the key to how small solar variations can be amplified enough to alter our climate. In Science for August 28th, they argue that it's a consequence of two factors acting together.
First, the "top down" effect: When the Sun waxes during solar maximum, its ultraviolet output varies by as much as a few percent. This extra energy gets absorbed by the stratosphere, which warms up and creates more ozone (O&sub3;). The warmer stratosphere, primarily over the tropics, alters the circulation lower down in the troposphere and, in particular, more rainfall at latitudes farther from the equator.
Second, the "bottom up" effect: When stronger sunlight passes through relatively cloud-free tropical regions, the ocean surface warms up. It's slight, only about 0.2°F, but enough to cause more water to evaporate. This extra moisture gets carried by trade winds poleward to the Intertropical Convergence Zones, where it rains like crazy.
Both of these factors have been known for some time, but neither seems capable of driving climatic change. The warmer oceans, for example, would seem to require a cyclic change of 0.5 W/m². And while you can see how the two effects might be linked, curiously no single atmospheric model had incorporated both of them — until Meehl's team came along. They actually needed three computer models to draw out the connection. One analyzed interactions between sea-surface temperatures and the lower atmosphere, while a second simulated the stratosphere's ozone response.
It was the third model, which combined features of the other two for the first time, that revealed how the tropical Pacific's response during solar maxima mimicked the actual climatic observations. "The Sun, the stratosphere, and the oceans are connected," Meehl comments in an NCAR press release, "in ways that can influence events such as winter rainfall in North America."
Meehl and his team will have to wait a while to see how their predictions pan out during coming solar maximum, and even then it's likely to be a weak peak. The Sun appears hopelessly stuck in its deep, lasting minimum. According to Texas amateur Tom Fleming, who heads a small but dedicated group of solar observers, the Sun has been completely spotless for 22 weeks so far this year.