Sun-Climate Connection

Up next — particle physicist Jasper Kirkby talks about some possible links between cosmic rays, clouds and global climate change. Jasper Kirkby, a particle physicist at CERN — the European Organization for Nuclear Research — in Switzerland.

Kirkby wonders if a more active sun leads to a warmer Earth. When the sun is most active magnetically, its solar wind might be strong enough to blow cosmic rays out of Earth’s path. Fewer cosmic rays might mean fewer earthly clouds. And fewer clouds would let in more sunlight — so Earth would get slightly warmer.

The theory hinges on whether, as some believe, cosmic rays do indeed help create clouds over Earth. Researchers have evidence that they do — but not everyone agrees. So Kirkby and others have proposed an experiment.

Jasper Kirkby: There are several particle accelerators in the world that could imitate cosmic rays and they have the advantage over natural cosmic rays in that you can turn them on and off . . .

The idea is to let cosmic rays strike air in a particle accelerator — under the same conditions found in the atmosphere. Will the stuff of clouds — cloud nuclei and water droplets — appear? If so, this strengthens the theory that cosmic rays help create clouds — and gives a sense of how strong the effect is on our climate.

Kirkby and others are organizing a conference for later this year to develop a plan for carrying out the experiment.

A few years ago, Danish researchers announced the discovery that the amount of cloud cover over Earth is lowest when the sun is most active — in other words, when the sun is at the peak of its 11-year sunspot cycle. It’s thought that at the same time, fewer of the sun’s cosmic rays reach Earth’s atmosphere. That’s because the more active the sun, the stronger the solar wind — and this blows more of the cosmic rays out of the inner solar system. When cosmic rays hit Earth’s atmosphere, they create charged particles which can help form clouds. So fewer cosmic rays mean fewer clouds and fewer clouds allow more sunlight to reach Earth. And this leads to a warmer Earth. This warming effect might be as large as the effect of greenhouse gases we release into the atmosphere.

But not everyone agrees with the analysis of the Danish researchers.

As far as I understand it from talking to Jasper Kirkby, the fact that cosmic rays instigate cloud formation is uncontroversial. Likewise, the fact that they have some effect on cloud cover is also uncontroversial. What is controversial is the correlation between the amount of sunspot activity during the 11-year solar cycles (and resulting cosmic ray density variation) and the amount of cloud cover on earth. This theory was initially laid out by Henrik Svensmark and Eigil Friis-Christensen in the Journal of Atmospheric and Solar-Terrestrial Physics in 1997. Kirkby says the controversy arose because conclusions in that paper were based on amalgam satellite data, which many scientists feel is insufficient for such a claim. They also feel that there are unaccounted-for factors, such as human-produced greenhouse gasses, that could confound the correlation.

Some excerpts from an interview with Dr. Kirkby:

“In 1997 or so, these couple of Danish meteorologists published a paper which showed that the cloud cover on earth goes up and down following this so-called sunspot cycle. Now during the sunspot cycle, the cosmic rays go up and down as well. Roughly speaking, when the sun is magnetically active, that is when there are a lot of spots on the sun, that means that the solar wind is very strong and has the effect of sweeping the rays out and preventing them from getting to the inner part of the solar system, which is where earth is. And so, roughly speaking, at the peak of the sunspot cycle, the cosmic ray density on earth is 15% less than it is when there are no spots on the sun. And it’s just due to this steady windÉ the solar wind it’s called, pushing the cosmic rays away.

“These changes in the clouds – the authors claimed that they fitted or followed very closely the changes in cosmic rays. So they actually proposed that the cosmic rays were somehow actually affecting the clouds. And if that were true, this would be a possible mechanism by which very small changes on the sun could have very big effects on the climate because clouds have a very big effect on the climate. Let’s just look at the size of the effect. Roughly speaking, if you took an average snapshot of the earth, 65% percent of the earth would be covered by clouds. It’s a rather big figure. What this paper showed, was that over the solar cycle, that amount of cloud varies by about 5%. It turns out that [clouds] have a net cooling effect. So, this variation was equivalent to one or one and a half watts per square meter. Just to put that in context, that’s essentially the same amount as all of the carbon dioxide that mankind has thrown up. That’s what its been calculated to add in terms of heat load to the earth’s radiation budget.

“This paper suggests there could be a connection between them. Now, there was a very strong reaction to the paper because – for a variety of reasons and many of those have been addressed by improved satellite data subsequently. But it’s still controversial. It’s still not at all clear whether there is a real effect on the clouds but the paper certainly has raised the whole question as to what might be the effect of cosmic rays on clouds. Are there any possible mechanisms? And the answer is there are some possible ways by which they could affect clouds. Namely, they could affect the so-called cloud condensation nuclei É But the actual experimental knowledge of that is very, very poor and that’s what actually led to our idea to go to a laboratory situation where you can control all the various conditions and actually try to look at the microphysics – the actual interaction of cosmic rays with these actual physical processes that would then affect clouds.”

More Notes:

According to Kirkby, scientists have long intuited that solar activity was having an effect on cloud formation and climate and in recent years there has been a large amount of theory published about the cosmic ray/cloud/climate connection. However, there seem to be few studies that document how and to what extent this happens. So, it’s not only that these scientists need to review or get better satellite data — its generally agreed that it’s not enough. They also need/want to conduct a wide variety of experiments to get a picture of the exact mechanisms by which cosmic rays might influence condensation nuclei and clouds in the atmosphere, and how _much_ of a cloud forming effect cosmic rays have. As I understand it, that will help them determine more definitively whether cosmic ray density actually _directly_ correlates with cloud cover in the atmosphere.

According to “CLOUD Proposal: A study of the link between cosmic rays and clouds with a cloud chamber at the CERN PS” (published April 24, 2000), “In 1997 Svensmark and Friis-Christensen [1] announced a surprising discovery that global cloud cover correlates closely with the galactic cosmic ray intensity, which varies with the sunspot cycle. Although clouds retain some of the Earth’s warmth, for most types of cloud this is more than compensated by an increased reflective loss of the Sun’s radiation back into space. So more clouds in general mean a cooler climate Ñ and fewer clouds mean global warming. The Earth is partly shielded from cosmic rays by the magnetic disturbances carried by the solar wind. When the solar wind is strong, at the peak of the 11-year sunspot cycle, fewer cosmic rays reach the Earth. The observed variation of cloud cover was only a few per cent over the course of a sunspot cycle. Although this may appear to be quite small, the possible long-term consequences on the global radiation energy budget are not.

The article continues, “Beyond its semi-periodic 11-year cycle, the Sun displays unexplained behavior on longer timescales. In particular, the strength of the solar wind and the magnetic flux it carries have more than doubled during the last century [2]. The extra shielding has reduced the intensity of cosmic rays reaching the Earth’s atmosphere by about 15%, globally averaged. This reduction of cosmic rays over the last century is independently indicated by the light radioisotope record in the Greenland ice cores. If the link between cosmic rays and clouds is confirmed it implies global cloud cover has decreased during the last century. Simple estimates indicate that the consequent global warming could be comparable to that presently attributed to greenhouse gases from the burning of fossil fuels.”

The article continues, “These observations suggest that solar variability may be linked to climate variability by a chain that involves the solar wind, cosmic rays and clouds. The weak link is the connection between cosmic rays and clouds. This has not been unambiguously established and, moreover, the microphysical mechanism is not understood. Cosmic rays are the dominant source of ions in the free troposphere and stratosphere and they also create free radicals. It has been proposed [3]Ð[5] that ions may grow via clustering to form aerosol particles which may ultimately become cloud condensation nuclei (CCN) and thereby seed clouds. Recently a search for massive ions in the upper troposphere and lower stratosphere was started by MPIK-Heidelberg[6] using aircraft-based ion mass spectrometers. Preliminary results indeed indicate the presence of massive positive and negative ions. In addition to their effect on aerosol formation and growth, cosmic rays may also possibly enhance the formation of ice particles in clouds [7].”

The article continues, “We therefore propose to test experimentally the link between cosmic rays and clouds and, if confirmed, to uncover the microphysical mechanism. We propose to make the measurements under controlled laboratory conditions in a beam at the CERN Proton Synchrotron (PS), which provides an adjustable source of “cosmic rays.” The experiment, which is named CLOUD (Cosmics LeavingOUtdo or Droplets), is based on a cloud chamber that is designed to duplicate the conditions prevailing in the atmosphere. To our knowledge, cloud chamber data under these conditions have never been previously obtained.”

The aim of the CLOUD experiment was more explicitly stated in, “Beam Measurements of a CLOUD (Cosmics Leaving OUtdoor Droplets) Chamber, by Jasper Kirkby (February 24, 1998). Kirkby writes, “However a direct link between cosmic rays and cloud formation has not been unambiguously established. We therefore propose to experimentally measure cloud (water droplet) formation under controlled conditions in a test beam at CERN with a CLOUD chamber, duplicating the conditions prevailing in the troposphere. These data, which have never been previously obtained, will allow a detailed understanding of the possible effects of cosmic rays on clouds and confirm, or otherwise, a direct link between cosmic rays, global cloud cover and the Earth’s climate. The measurements will, in turn, allow more reliable calculations to be made of the residual effect on global temperatures of the burning of fossil fuels, an issue of profound importance to society.”

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