Cosmic ray theory of global warming gets cold response

Danish theorist’s latest paper overstates the effects of solar activity in climate change, critics say. Tim Wallace reports.

Cosmic rays contribute to global warming, according to one group of researchers. Not so much, argue others.
Cosmic rays contribute to global warming, according to one group of researchers. Not so much, argue others.
Heritage Images / Getty Images

For those who want an explanation for global warming that lets fossil fuels off the hook, the work of Danish physicist Henrik Svensmark has long burned brighter than the midday sun on a cloudless day.

For two decades, Svensmark, of the Danish National Space Institute (DTU Space) at the Technical University of Denmark, has propounded a theory of “cosmoclimatology”, which holds that cosmic rays and sunspots are the real drivers of climate change.

His latest published research – a paper on cloud formation in Nature Communications that he claims is “the last piece of the puzzle explaining how particles from space affect climate on Earth” – has received plenty of uncritical media attention.

Britain’s Daily Express has warned archly of the imminent possibility of another Ice Age and The Australian newspaper has reported the results as meaning the impact of solar activity on the climate is up to seven times greater than climate models suggest.

Other scientists are less impressed. “The press release goes way beyond what the paper actually shows,” says Steven Sherwood, an expert in atmospheric climate dynamics at the University of New South Wales, in Sydney, Australia.

Svensmark’s theory in a nutshell is this: cosmic rays are atomic fragments – mostly nuclei – blown into space from exploding stars that constantly bombard the Earth. When they enter the atmosphere, their electric charge helps form clusters of molecules – aerosols – that in turn act as seeds, or nuclei, for water droplets to condense around, creating clouds.

More cosmic rays means more ‘cloud condensation nuclei’ (CCN), more clouds, and a colder climate. Fewer rays means a warmer climate.

Which is where the sun comes in. At times of high solar activity, signified by higher numbers of sunspots, our own star’s magnetic field helps shield the planet from cosmic rays, meaning less cloud formation and thus higher temperatures. When the sun is ‘quiet’, there is more ionisation in the atmosphere, meaning more clouds and a cooler climate.

Svensmark’s new paper – the “last piece of the jigsaw” – co-authored by Martin Bødker Enghoff, also at DTU Space, Nir Shaviv, at the Hebrew University of Jerusalem, and Jacob Svensmark at the University of Copenhagen – demonstrates in the lab that cosmic ray ionisation can lead to greater cloud formation than previously believed.

The paper then argues that the result “should be incorporated into global aerosol models, to fully test the atmospheric implications”.

Scientists involved in related research, however, doubt the new findings make much difference to accepted climate models.

“The authors need to quantify the effects in an atmospheric model rather than just speculating,” says Ken Carslow, of the University of Leeds, UK, who has also studied potential links between cosmic rays and aerosol formation as part of CERN’s Cosmics Leaving Outdoor Droplets (CLOUD) experiment. “It’s a tiny effect and previous studies suggest it will not be important,” he states.

Terry Sloan, of the University of Lancaster, UK, whose own research has calculated the contribution of cosmic rays at less than 10% of the global warming seen in the 20th century, is also dubious. He points out that other atmospheric “impurities”, such as dust and salt particles, play more important roles as cloud-condensing nuclei.

“The effects [of ionisation] are too small to measure except in the dust- and impurity-free atmosphere such as in their experiments,” Sloan says. “Dust in the atmosphere plays a much bigger part in cloud formation.”

Steven Sherwood concurs. The paper itself, he notes, only suggests the result “may be relevant in the Earth’s atmosphere under pristine conditions”. Even if things do work in the real world the same way as in a laboratory, cloud growth due to ions would only make up “several per cent” of the total.

“Several per cent ain’t much, and the real atmosphere is not pristine,” Sherwood says. While the new research has shown that cosmic rays can produce particles big enough to seed clouds, that was never “the real problem” with Svensmark’s ideas. A bigger issue is the number of such particles, which “would be negligible compared with the background aerosol and the aerosol humans are adding by burning things, tilling soil, etc.”

“If clouds were affected by cosmic rays,” he adds, “they would have been affected a hundred times more strongly by human air pollution, and the world would have cooled over the past century, rather than warmed.”

Tim Wallace is a contributor to Cosmos Magazine
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