Those skeptical of the theory of anthropogenic global warming often point to the sun as the source of modern era planetary warming. The reasoning is two-pronged. The argument goes that increases in solar activity increase the amount of heat received by the Earth. However, as this is insufficient to explain the recent warming trend, this is coupled with attributions to cosmic rays. As solar activity increases, that activity prevents cosmic rays from reaching the planet. The theory is that atmospheric ionization caused by the cosmic rays causes cloud formation. So, as the amount of cosmic rays reaching Earth decreases, cloud cover that reflects heat back into space decreases as well. As a result, the planet warms.
A new study published in the Institute of Physics’ Environmental Research Letters journal casts doubt on this cosmic ray theory. Researchers from the University of Durham and the University of Lancaster looked to validate the theory but found no correlation between a decrease in cosmic rays and a decrease in cloud cover. From the study’s abstract.
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 and so our conclusions are to doubt it. From the absence of corroborative evidence, we estimate that less than 23%, at the 95% confidence level, of the 11 year cycle change in the globally averaged cloud cover observed in solar cycle 22 is due to the change in the rate of ionization from the solar modulation of cosmic rays.
- Testing the proposed causal link between cosmic rays and cloud cover – Environmental Research Letters, T Sloan and A W Wolfendale (April 3, 2008)