Do Cosmic Rays Explain Global Warming?

A common argument against man-made climate change is that cosmic rays influence cloud cover, cosmic rays are down, so clouds are down, and the Earth is warmer. What does the data say?

Cosmic Rays, Cloud Cover, and You

The Earth is under attack from outer space. The space weapon is energetic particles from the sun and other sources. These particles are called "cosmic rays". In principle they could be dangerous, but almost all of them are absorbed by the Earth's atmosphere, or are swept up by the Earth's magnetic field (these particles end up being channeled to the poles where they cause the Northern and Southern Lights when they hit air).

What do cosmic rays have to do with clouds? An energetic particle moving through air can knock electrons off of molecules. The resulting ions then act as nucleation points (effectively small attractive chunks of stuff) for water droplets. Get enough water droplets and you have a cloud. The connection to climate comes from the fact that clouds reflect solar energy back out to space (a cooling effect) and thermal radiation from the Earth back down (a blanket-like warming effect). Which effect wins is tricky to tell, but we are told that on average fewer clouds means a warmer Earth. The possibility that cosmic rays could affect weather and climate was first made in 1959 by Edward Ney. It has recently been revived by Henrik Svensmark who is director of the Center for Sun-Climate Research at the Danish Space Research Institute.

It all sounds plausible, but is it true? Early papers by Svensmark and others examined the rate of cosmic rays impinging Earth and compared this with cloud cover. They found a strong correlation between the two quantities (an see example, or another) and hence concluded that cosmic rays could indeed be seeding clouds. Also, since there were fewer cosmic rays than in the past, this could also explain global warming. This caused a bit of a stink since most climate scientists believe that cosmic rays have a minimal impact on the climate.

How exactly is the incidence of cosmic rays measured? Its not so simple, there are many kinds of particles and cosmic rays tend to be absorbed by the atmosphere. One way to measure them is to detect neutrons at high altitudes. Neutrons are neutral and so can plow through air molecules more effectively, and of course, higher altitude means less air to get through. There are a handful of spaceward pointing neutron detectors around the world, one of them is in Climax, Colorado. This detector has been collecting data since 1953. Some of this data is shown in the figure below.

The top curve shows the number of neutrons detected per minute in the Climax detector; the bottom curve is a measure of global cloud cover. What Svensmark noticed was the strong correlation between the curves before 1996 (when he was writing). You can see how the neutron count went down from 1987 to 1990 and a similar drop in cloud cover over the same period. Similarly, neutron counts went up from 1990-ish to 1996, as did cloud cover. Case closed?

Not so fast, as you can see, new data does not support Svensmark's claims. Counts and clouds went down from 1997 to 2001, but from 2001 to the present neutron counts have gone up steadily while cloud cover has gone down. This does not look good for the cosmic cloud crowd. Of course we would like a more thorough analysis of the data. Many people have provided this. I quote below from one study by T. Sloan and A. Wolfendale:

A decrease in the globally averaged low level cloud cover, deduced from the ISCCP infrared data, as the cosmic ray intensity decreased during the solar cycle 22 was observed by two groups. The groups went on to hypothesize that the decrease in ionization due to cosmic rays causes the decrease in cloud cover, thereby explaining a large part of the currently observed global warming. We have examined this hypothesis to look for evidence to corroborate it. None has been found...