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GCR’s, Clouds, and Climate, Oh My!

July 31st, 2006 by Sean Davis · No Comments

If you were at a BBQ with a group of atmospheric scientists, and wanted to raise some hackles, there are several subjects that are notoriously easy to get people riled up… …Talk of Hockey Sticks, Inhofe, Crichton, Coulter, and you’re sure to get people going… …Bring up the effects of galactic cosmic ray’s (GCR’s) on climate, and you’re sure to lure some group of otherwise peaceful scientists into a gloves-off throwdown!

I haven’t seen any papers on this subject in a while, but just this week in GRL a new paper was published by Luis Eduardo Antunes Vieira and Ligia Alves da Silva of the Instituto Nacional de Pesquisas Espaciais, Brazil, entitled “Geomagnetic modulation of clouds effects in the Southern Hemisphere Magnetic Anomaly through lower atmosphere cosmic ray effects”.

As a bit of background, the basic ideas behind the potential GCR-climate connection is that GCR’s increase low cloud cover via increased CCN production (via increased atmospheric ionization), which acts as a cooling effect on the climate. There are several reasons that the GCR-climate connection has been controversial. The good folks over at Realclimate.org have several posts on the topic (here, here, and here), and I don’t really feel jumping into the fray and taking sides on this issue.

My impression of this topic is that some (on both sides of the “climate war”) are using it to wage a proxy battle over climate policy. From the AGW contrarian standpoint, the existence of a GCR-climate connection could “prove” that the global warming is of solar, and not anthropogenic, origin, and therefore mandate that nothing be done about GHG emissions. I’ve oversimplified the logic of course, but one could also imagine a desire to disprove a GCR-climate connection as a means to promote action on climate change. Again, this is my impression, and I don’t want this blog to be another proxy ground for the climate wars. …Readers interested in the climate wars would be much better off following one of the links to another site.

Given that this issue is a red herring to some, I’d rather focus on what is new, unique, interesting, or controversial about this paper. …So what does the paper actually say?
Several previous (and perhaps flawed) studies have noted a correlation between globally-averaged low cloud cover and GCR intensity (and are cited in this paper). Whereas these papers have focused primarily on the cloud effects due to the 11-year solar cycle modulation of GCR intensity, this paper focuses on cloud effects due to a region of anomolously weak magnetic field (and correspondingly high GCR intensity) known as the Southern Hemisphere Magnetic Anomaly (SHMA), or South Atlantic Anomaly (SAA). A picture of the SAA, from Wikipedia, is shown below.
Vieira and da Silva show that the correlation coefficient between the magnetic field (a proxy for GCR intensity) and LW net flux anomalies decrease from the inner core of the SAA towards the outer edge. Their figure 5 (here, for JGR subscribers) is the most convincing in the paper, to me. Overall, this seems to be an interesting result, and may prove useful in understanding the GCR-cloud connection. I don’t feel particularly qualified to assess the importance of this paper, but do have a couple of questions and comments…
First, why did they only look at the South equatorial Pacific for their study? This region is subject to significant ENSO related variability, which must be separated from any variability caused by magnetic field anomaly. They only partially address their reasoning for choosing this area by saying

The patterns in the Atlantic are more complex and seem to be biased by meteorological systems over the South America.

So they threw out all of the Atlantic data based on these grounds. A better justification for this should be given in my mind. I’m willing to give them the benefit of the doubt on that, but still, what about the Pacific North of 0°?
Also, I’m curious about what the implications are for the potential solar-cycle modulation of cloud effects? I suspect that the change in GCR flux between the inner and outer core of the SAA is much greater than the solar cycle (I don’t have data to back this up, so please point me to the literature if I’m wrong!), and even in the data presented here the effects of things like El Niño dominate variations in cloud fluxes. So if one were to extrapolate the effects seen here to the types of GCR variations expected from the solar cycle, how large is this effect? Obviously this is a difficult question, and is beyond the scope of a GRL article. But it is one to keep in mind in assessing the implications of this paper…

Tags: climate · cosmic rays · satellite

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