A powerful new analytic technique has found that although cosmic rays can lead to year-on-year temperature variations, they are not the cause of climate change. Predictably, the findings are already being distorted by climate change denial websites.
Supernovae, gamma ray bursts and the intense conditions at the center of galaxies can acceleratate protons and atomic nucleii to close to the speed of light. These spread through the universe and some collide with the Earth's atmosphere, where they ionize the air. For over a century, we have known that ionizing radiation assists in the process of cloud formation. Since clouds in the lower troposphere reflect sunlight, it has been a credible theory that cosmic radiation might make the Earth cooler than it otherwise would be.
More recently, a few scientists and many more non-scientists have seized on cosmic radiation as an alternative explanation to greenhouse gas emissions for rising global temperatures. This theory has always had a fatal flaw – it relies on a drastic fall in cosmic radiation hitting the Earth's atmosphere over the last thirty years, something we know has not been occurring.
However, atmospheric scientists have been genuinely interested in the question of what effect a change in radiation might have. While it has been credibly proposed that a fall in cosmic radiation would warm the planet, some research has suggested that various complicating factors cancel this out.
Cosmic rays entering the solar system change frequency only slowly, but the solar magnetic field can block rays from entering the Earth's atmosphere. Consequently, the strength of the wind is inversely related to the number of impacting rays.
The latest research, published in Proceedings of the National Academy of Sciences, concludes that changes in the number of cosmic rays hitting the Earth, driven by the solar cycle, do indeed contribute to year-to-year changes in global temperature. That is, in years where the sun is most active, and cosmic rays are therefore least likely to reach the atmosphere, average temperatures increase (after allowing for other influences), while the reverse applies when the solar activity is low.
"Our results suggest weak to moderate coupling between cosmic rays and year-to-year changes of [global temperature]," the authors write. "However, we find that the realized effect is modest at best, and only recoverable when the secular trend in [global temperature] is removed."
Similar results have been produced before, but this paper used a new statistical technique known as convergent cross mapping (CCM) to make the conclusion more robust. CCM is used to compute specific correlations between two time series variables in order to tease apart the coupled variables, distinguishing them from situations where one variable determines the other without being affected itself.
The findings are not in the least surprising. Cosmic ray-induced warming would have a very different fingerprint from the ones we see, which closely match those expected from greenhouse gasses. Moreover, solar activity has shown a slight declining trend over the last 35 years, the period in which we have had satellites to take accurate measurements. It would be strange indeed if this was causing an effect in the opposite direction.