Several weeks ago, the first results of CERN's Cosmics Leaving OUtdoor Droplets (or CLOUD) experiment were revealed. This experiment was designed to study the chemical factors behind cloud formation and, more particularly, to study the influence of galactic cosmic rays on the Earth's climate through the media of aerosols and clouds. Naturally, the results were mangled by all the usual suspects, which variety of response prompted several of the scientists involved in CLOUD to attempt a few on-the-fly debunkings in the comments section of Real Climate. One, Jeffrey Pierce of Dalhousie, has returned with a more extensive discussion. Considering the subject matter, it is a model of clarity. Here's his conclusion re the "clear-sky hypothesis", which is the one pushed by climate change skeptics as most likely driver behind AGW:
In summary, the clear-sky hypothesis is driven by 5-20% changes in ion formation rates in the troposphere. These ion changes would need to drive changes in cloud cover by several percent to account for reported correlations. While uncertainties in processes remain, it appears unlikely to me (and most other scientists working on aerosol-cloud interactions who’ve shared their thoughts on this hypothesis with me) that this mechanism will be strong enough to greatly change clouds. I would not go so far to say that the case is closed on this mechanism, but if it is to be important there must be some amplification factor in one (or more) of the questions described above that we are currently unaware of. Thus, it will be exciting to see what the future CLOUD experiments (or other controlled experiments) show regarding questions #3 and #4.
And here is his more general conclusion:
While reported observed correlations between cosmic rays and clouds are suggestive of effects of cosmic rays on clouds, cosmic rays rarely change without other inputs to the Earth system also changing (e.g. total solar irradiance or solar energetic particle events, both also driven by changes in the sun, but distinct from cosmic rays). Thus, we must understand the physical basis of how cosmic rays may affect clouds. However, it is clear that substantially more work needs to be done before we adequately understand these physical connections, and that no broad conclusions regarding the effect of cosmic rays on clouds and climate can (or should) be drawn from the first round of CLOUD results. Finally, there has been no significant trend in the cosmic ray flux over the 50 years, so while we cannot rule out cosmic-ray/cloud mechanisms being relevant for historical climate changes, they certainly have not been an important factor in recent climate change.
So there you have it.