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Galactic Cosmic Rays and Climate: Forbush Puts Kibosh on Theory

by Bill Chameides | February 8th, 2010
posted by Erica Rowell (Editor)

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While the Sun is the primary driver of Earth’s temperatures, subtle changes in the Sun’s output related to sunspots do not account for global warming ( But can they explain the stock market’s movements?

A new study finds no evidence of a connection between galactic cosmic rays and cloudiness — and, presumably, the stock market.

We all know that the greatest driver of global temperatures is the Sun. All things being equal, if the Sun heats up, we get warmer, and if it cools down, we get cooler. But what about more subtle changes in the Sun, like those associated with the solar cycle?

The solar cycle, you will recall, refers to the ups and downs in sunspots that happen every 11 years or so. Solar scholars have been counting sunspots for millennia, and for more than a century scientists have speculated about a possible connection between sunspots and climate.

Speculating on the Speculation Over Sunspots and Climate

Such speculation is not without any basis because it seems that just about everything has followed the rise and fall of sunspots at one time or another. What do I mean by “everything”? Well, things that obviously relate to climate like rainfall, El Ninos, and agriculture. But also things whose connection to climate is less obvious, such as the Dow Jones Industrial Average and the length of women’s skirts [pdf]. (Other “connections” here.)

If these statistical relationships are valid and truly reflect a cause-and-effect relationship, they suggest that the solar cycle is having a profound, perhaps even insidious influence on our lives. However, many scientists remain unconvinced, suspecting that the apparent statistical correlations are just fortuitous.

For example, if economic downturns tend to occur every 11 years or so, we’d expect to find times of resonance between solar cycles and economic indicators. It’s also true that there’s a little inconsistency in the nature of the correlations that have been reported. For instance, Theodore Modis (in a paper [pdf] published in Technological Forecasting & Social Change) found that “GDP peaks precede the sunspot peaks [by about 2 years] in a rather orderly manner,” while Michael Mandeville found (in a non-reviewed paper here [pdf]) just the opposite: a “strong connection between major recessions and the peaks of the sunspot cycles.”

Since a statistical correlation does not mean there is a cause-and-effect relationship, proponents of the sunspot-climate connection have been challenged to come up with a mechanistic explanation.

The simple explanation based on an increase in Sun’s total output over a solar cycle seems unlikely. The variation in the Sun’s output from its maximum to its minimum is only about 0.1 percent and can explain a temperature variation of only ~ 0.2 degrees Celsius. (Read more on solar cycles and climate here and here; forcing agents underlying climate change; solar cycle length, greenhouse forcing and global climate; global warming and the Sun.)

To explain these correlations, an amplifying mechanism is needed.

The hypothesis connecting cosmic rays and clouds has been the subject of a lively, sometimes contentious scientific debate. A new paper finds no existence of a correlation.

Enter Danish physicist Henrik Svensmark and his cosmic ray/cloud connection (CRC) hypothesis.

The Cosmic Ray/Cloud Connection Hypothesis

The hypothesis holds that the penetration of cosmic rays into the Earth’s atmosphere triggers cloud formation that cools the Earth by reflecting the Sun’s rays. During times in which the Sun is active, the hypothesis, if correct, works in the opposite way, such that:

  1. an increase in sunspots would lead to increased ejection of energetic particles from the Sun in the form of the solar wind,
  2. the increased solar wind would shield the Earth from cosmic rays,
  3. fewer cosmic rays would mean fewer clouds,
  4. fewer clouds would mean less reflection of the Sun’s rays, and
  5. thus, higher temperatures.

To support his hypothesis, Svensmark has reported laboratory experiments that show a tendency for particle formation by energetic particles and statistical studies showing a nice correlation between cloudiness and sunspot number over a little more than a single solar cycle.

Other scientists have countered with their own studies of cloud variations from solar cycle to solar cycle finding no evidence of a correlation, which has been countered by others … and so forth. (Further reading here, here, and here.)

New Paper Finds No Evidence Supporting CRC Hypothesis

Adding to the controversy is a new paper published in Geophysical Research Letters by Jasa Calogovic of the Hvar Observatory in Zagreb, Croatia, and colleagues. Rather than focus on decadal variations over a solar cycle, the authors looked at the effects of sudden drops in cosmic rays known as Forbush decreases. These events, named after physicist Scott Forbush, are caused by a sudden ejection of mass from the Sun’s corona, which in turn causes an approximately week-long decrease in cosmic rays.

The authors analyzed the six largest Forbush decreases between 1989 and 2001 along with global cloud cover data from the International Cloud Climatology Satellite Project (ISSCP) and found no evidence of a correlation.

So, is this the end of the Svensmark hypothesis? I suspect not. This has been the subject of a lively, sometimes contentious debate. I am sure we will hear more pros and cons in the coming years. And so I have no advice for your stock portfolio.

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