There was a time in decades past when people assumed the sun played a leading role in climate change. After all, something was responsible for the mid-Holocene thermal maximum, the Roman Warm Period, the Medieval Warm Period and the Little Ice Age, and it wasn’t mankind. But along came the greenhouse gas theory and out went the sun, as well as some of the inconvenient parts of history including the Medieval Warm Period itself. The reason for tossing out the solar theory was that the IPCC decided its variations were too small on their own to account for global temperature changes. Which would be true, unless there is a mechanism that amplifies solar changes. A leading candidate was the Svensmark effect, by which small increases in solar energy suppress cloud formation, which boosts the initial warming. So far efforts to prove the Svensmark effect exists have met with limited success. But late last year phys.org reported on a new analysis of satellite data published in Nature Scientific Reports that found evidence for the link. The lead author, Henrik Svensmark, cautions that the results don’t explain away climate change, but they do put the sun back into the picture.
The Svensmark effect begins, not with direct heating by the sun, but with the constant shower of radiation in the form of cosmic rays from the galaxy, called Galactic Cosmic Rays (yes, really), which bombard the atmosphere and provide the energy that binds gas particles into larger and larger molecules that sometimes become aerosols large enough to form water droplets and from there to become clouds. Clouds play an enormous role in modulating the Earth’s climate, and it would only take a small reduction in cloud cover to cause warming on the scale of the 20th century trend. What might reduce cloud cover? One possibility is an increase in the solar wind that shields the atmosphere from cosmic rays. If the sun gets a bit brighter, it also blocks more cosmic rays, so the slight extra warmth from the sun is amplified by a slight reduction in cloud cover. That’s the Svensmark effect.
The trouble is that attempts to recreate it in laboratory settings failed to show a large enough change in nucleation rates to drive changes in global cloud cover. But Svensmark and his team decided to look at the real atmosphere to see if there was real-world evidence not being picked up in lab experiments. They noticed that occasional bursts of solar energy called Forbush events might, if the theory holds, be associated with temporary drops in cloud cover. Using detailed satellite records they examined cloud formation rates in the aftermath of the strongest Forbush events since 2000. And sure enough they found decreases in cloud cover on a scale large enough to cause a worldwide increase in solar energy absorption at the surface much larger than the initial increase in solar energy itself.
Svensmark notes that the effect only lasts for a few days so it isn’t enough on its own to drive climate change. But it does establish the validity of the mechanism. What would then matter for climate change is evidence if solar output grew over a long period of time, not just in a burst over a 5 day period. That, hints Svensmark, is what they are aiming at next.
“The solar effects in this study are too short-lived to have a lasting effect on the climate. They do, however, dramatize the cosmic ray-cloud mechanism that works more patiently on longer time scales. The hope is that this result will help rethink the long-term effect of solar activity and cosmic rays on climate,” says Henrik Svensmark.