Back in the 1990s, Danish physicist Henrik Svensmark and colleagues began publishing studies arguing that the sun's influence on the climate is amplified by so-called galactic cosmic rays (it’s not a sci-fi weapon, they’re actually called that). When the sun gets brighter, greater solar wind shields the atmosphere from cosmic rays that constantly bombard the atmosphere, which suppresses cloud formation, amplifying the warming effect. Or so the theory goes. It has been hotly debated ever since, especially since it might account for a lot of 20th century warming and thus leave less to blame on CO2. But it has been difficult to get enough data to test the theory. Via Benny Peiser we learn that Japanese researchers took a different approach by studying historical winter monsoon patterns. Their conclusion: Svensmark is right.
Mainstream climate researchers agree that variations in the sun affect the world's temperatures. But they say the changes are too small to explain global warming, even though solar output did go up since the 1700s. Something would have to amplify the effect to make it a rival to greenhouse gases. That something may be clouds. As physicist William van Wijngaarden explained in our "simple physics" video, no one really knows what clouds will do.
Svensmark proposed that a key ingredient to cloud formation is ionizing radiation in the atmosphere which constantly bombards us from deep in the galaxy. We are partly shielded from it by solar wind. If the sun dims a bit, the shield weakens, more cosmic rays hit the atmosphere, and clouds form more readily, cooling the surface. Likewise, a stronger sun means less cloud cover. This way, small fluctuations in solar output can cause big changes in surface temperature.
Up to now the theory has been debated using limited satellite and experimental data, but the evidence wasn't strong enough one way or the other. The new study from Japan gets around this by looking at indirect evidence over a long geological interval. During the last so-called geomagnetic reversal, cosmic ray intensity in the atmosphere went way up and stayed up for 5,000 years. At the same time, dust layers near the Gobi desert related to the winter monsoon thickened, which happens when the monsoon intensifies. The authors concluded cloud cover had to form an "umbrella effect" over that period. They also found evidence that temperatures in the region dropped by several degrees.
The lead author, Prof. Masayuki Hyodo, concluded this way: “This phenomenon has never been considered in climate predictions due to the insufficient physical understanding of it. This study provides an opportunity to rethink the impact of clouds on climate. When galactic cosmic rays increase, so do low clouds, and when cosmic rays decrease clouds do as well, so climate warming may be caused by an opposite-umbrella effect. The umbrella effect caused by galactic cosmic rays is important when thinking about current global warming as well as the warm period of the medieval era.”
Keep an eye on this one.