People have known for 200 years that there’s some link between sunspots and our climate. In 1800, the astronomer William Herschel didn’t need a climate model, he didn’t even have a calculator — yet he could see that wheat prices rose and fell in time with the sunspot cycle. Since then, people have noticed that rainfall patterns are also linked to sunspots.
Sunspots themselves don’t make much difference to us, but they are a sign of how weak or strong the sun’s magnetic field is. This massive solar magnetic field reaches out around the Earth, and it shields us from cosmic rays. Dr Henrik Svensmark has suggested that if more cosmic rays reach further down into our atmosphere, they might ionize molecules and help “seed” more clouds.
As it happens, this year, the sun has almost no sunspots, but for much of the late 20th Century, the solar magnetic field was extremely active. If the theory is right, an active field means a warming earth with fewer clouds. A quiet sun though, means a cooler earth with more clouds.
AGW replies: Lochwood and Frohlich showed the theory doesn’t fit rising temperatures after 1980.
Skeptics say: They used surface temperatures, not atmospheric ones (see the graph above). Cosmic rays correlate well with temperatures from weather-balloons. But thermometers on the surface are affected by things like car-parks, and air conditioners which are close to the sensors. All that Lockwood and Frohlich prove is that there’s no link between cosmic rays and air conditioners.
AGW replies: There’s no link with clouds and cosmic rays either.
Skeptics say: That’s only true if you look at the wrong kind of rays and the wrong kind of clouds. There’s a good correlation between high energy rays and low clouds.
The correlation between cosmic rays and temperature is much better over all time spans than that with carbon and temperature.
Sources: Reply to Lockwood and Frohlich Svensmark 2007. Falls in cosmic rays affect low clouds Svensmark 2009, see also Linkages between solar activity, climate predictability and water resource development, Alexander.
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In The Skeptics Handbook I, I was very careful to present a small target for critics. By the second booklet in the series, I had had a lot of requests to let people know about Svensmark’s theory. Also there are people out there who can’t let go of one idea until they have something else that could fit the bill. It’s important to let people know there are serious contenders out there that have empirical support (and it’s also just darn interesting).
This also important because it suggests a way that clouds could be a forcing in their own right, something that doesn’t sit well with the IPCC contention that clouds and water vapor are only feedbacks, and which Roy Spencer points out repeatedly.
With clouds covering 60% of the planet roughly, and reflecting massive amounts of sunlight before it can heat the surface, it would only take a small change in cloud cover to blow away any effect greenhouse gases might have.
I’ve always been impressed by the graph above. It has turning points that match, unlike the oft repeated simple rising line of CO2 and temperature (which only correlates from 1850 onwards in any case, and even then, only loosely).
I found The Chilling Stars by Nigel Calder (former editor of New Scientist) and Henrik Svesmark, is very interesting. It’s well written, easy to read, and a compelling case… that rare combination of cutting edge science in the hands of an excellent writer who is very familiar with the work. It covers the response of the scientific establishment, as well as the theory, the evidence, and the long history of the planet’s climate.
UPDATE – REFERENCES in full
- Svensmark, H. and Friis-Christensen, E.: Variation of cosmic ray flux and global cloud coverage – a missing link in solar-climate relationships, J. Atmos. Sol. Terr. Phys., 59, 1225–1232, 1997.
- Svensmark, H. (2007). Cosmoclimatology: a new theory emerges. Astronomy & Geophysics 48: 1.18-1.24. [PDF]
- Svensmark, H. 1998. Influence of cosmic rays on earth’s climate. Physical Review Letters 81: 5027-5030. [Discussion CO2Science]
- Svensmark, H., Bondo, T. and Svensmark, J. 2009. Cosmic ray decreases affect atmospheric aerosols and clouds. Geophysical Research Letters 36: 10.1029/2009GL038429. [Discussion CO2Science]
- Lockwood and Frohlich 2007, Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. II. Different reconstructions of the total solar irradiance variation and dependence on response time scale Proc. R. Soc. A, doi:10.1098/rspa.2007.1880, [Abstract, PDF]
- Svensmark, H. and Friis-Christensen, E. (2007) Reply to Lockwood and Fröhlich – The persistent role of the Sun in climate Forcing, Danish National Space Center, Scientific Report 3/2007 [PDF]