September 1, 2009
One of the more persistent climate change myths is that any warming we’ve been experiencing here on Earth is because of sunspots, not increasing amounts of greenhouse gases in our atmosphere. Of course, the Sun is an important factor in climate, and changes in solar output are suspected to be behind large climate events such as the Little Ice Age. But how the Sun can have an effect that big has been a bit of a mystery for scientists; changes in the amount of energy put out by the Sun are not enough on their own to account for the magnitude of the effects on Earth.
In a new study in Science, Gerald Meehl of the National Center for Atmospheric Research and colleagues contend that two mechanisms work together to produce the changes seen when the sunspot cycle hits its peak and there is a small increase in the amount of ultraviolet radiation produced by the Sun.
With the “bottom up” mechanism, the extra solar energy results in more water being evaporated from the ocean, causing fewer clouds to form in the subtropics and more solar energy to reach the ocean, creating a feedback loop.
With the “top down” mechanism, the extra solar energy causes changes in the upper atmosphere that result in changes in precipitation in the tropics.
The two mechanisms reinforce each other by boosting the rising of tropical air that is driven by evaporation, Meehl [explained to Science]. “That’s the key commonality,” he says. “That amplifies things.”
The result is an equatorial eastern Pacific that is cooler and drier than usual, similar to a La Nina event, and the peak of the sunspot cycle could thus work to enhance a La Nina event or dampen El Nino. So variations in solar activity can drive changes in the weather. But that doesn’t mean solar activity is to blame for global warming, as Meehl and his colleagues note:
This response…cannot be used to explain recent global warming because the 11-year solar cycle has not shown a measurable trend over the past 30 years.
Climate change skeptics—you’ve been warned.
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