"We started the work when a group of scientists made the claim that global warming was mainly due to cosmic rays," Terry Sloan of the University of Lancaster told environmentalresearchweb. "We tried to find evidence to corroborate their claim. So far we have found no definite evidence of an effect. The effect, if present, cannot make a contribution of more than 10% of the global warming seen in the 20th century, otherwise we would have detected it."

Together with Arnold Wolfendale of the University of Durham, Sloan compared data on the rate of cosmic rays entering the atmosphere, which can be used as a proxy for solar activity, with the record of global temperatures going back to 1955. Their aim was to quantify the effect that solar activity – whether directly or through cosmic rays – might have had on global temperatures.

The researchers found a small correlation between cosmic rays and global temperatures occurring every 22 years. However, the change in cosmic-ray rate lagged behind the change in temperatures by between one and two years, suggesting that the cause is a more direct one – changes in solar irradiance. Variations in solar activity are known to alter the cosmic-ray rate one or two years down the line. By examining the size of the oscillations, Sloan and Wolfendale found that less than 14% of the global warming seen since 1955 could be attributable to solar activity.

This is not a reason to discard the IPCC findings and we should continue to respect their recommendations
Terry Sloan

Cosmic rays ionize the atmosphere. Some scientists believe that water droplets can condense on the resulting ions and aerosol particles, assisting cloud formation. Sunspot activity, which ebbs and flows on an 11-year cycle, decreases the cosmic-ray flux by increasing the solar wind – charged particles emitted by the Sun. The solar wind's greater magnetic field then deflects away some of the cosmic rays that would otherwise hit Earth from elsewhere in the galaxy.

After reviewing the literature, Sloan and Wolfendale also concluded that the palaeontological evidence for a correlation between cosmic rays and the global temperature is weak and confused. What's more, as modern correlations between cosmic rays and cloud cover exist only for small portions of the globe, they can only explain a fraction of the climate change seen in the 20th century.

A further literature review revealed that while Svensmark et al. found a link between cosmic ray rate and low-level cloud cover in solar cycle 22, the following solar cycle did not show this same relationship. What's more, the size of the dip in cloud cover in solar cycle 22 did not vary with magnetic latitude, as should have been the case. And the cosmic-ray rate decreased during the first half of the 20th century when the rate of increase in mean global surface temperature was small, but changed only slightly during the second half of the century, when temperature increased rapidly. Analysis of this data indicates that the contribution of the changing cosmic-ray rate is less than 10% of the 20th century rise in temperature. Similarly, any connection between changing cosmic rays and clouds cannot contribute more than 10% of warming.

According to Sloan and Wolfendale, CERN's CLOUD experiment identified one ionization-sensitive process that could influence cloud formation but it only contributes a small fraction to the overall aerosol production rate in the low-level atmosphere. The team also looked without success for increases in cloud cover associated with the Chernobyl nuclear accident (which increased the atmospheric ionization rate), in regions of India that have large radon concentrations, after the ionization burst from the large ground-level event produced by the Sun in 1989, and after nuclear-weapon tests in the 1950s and 60s.

"The work implies that we cannot rely on cosmic rays and solar activity as a significant source of global warming, as some people claim, i.e. this is not a reason to discard the IPCC findings and we should continue to respect their recommendations," said Sloan.

The team reported its study in Environmental Research Letters (ERL).

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