NASA still reluctant to admit that the Sun might affect Earth’s climate

Last week, NASA held a conference on solar activity, showing the results of Ulysses’ third and last orbit around the sun (1). “The sun cycles between periods of great activity and lesser activity. Right now, we are in a period of minimal activity that has stretched on longer than anyone anticipated,” explained Ed Smith, NASA’s Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena, California. “When the results of the third scan were compared with observations from the previous solar cycle, the strength of the solar wind pressure and the magnetic field embedded in the solar wind were found to have decreased by 20 percent. The field strength near the spacecraft has decreased by 36 percent.”

As someone trained at finding patterns in data (and trained at recognizing when patterns are spurious), I couldn’t help wondering if the sun’s current low level of activity might help explain the sharp drop in global temperatures over the last year. And whether the exceptionally strong solar activity during the last decades of the previous century (2) might have had something to do with the sharp increases in global temperatures during those decades.

Of course I am not the first to suspect that variations in the sun’s activity might have an influence on our planets climate. Already in 1878, the English economist William Stanley Evans suggested that the sunspot cycle influenced the business cycle through its effect on crop productivity (3). As more information accumulated and the technical methods of the analysis of cyclical data improved, people started testing the relationships between sunspots, Earth’s climate, and economic activity on Earth. In 1934, Carlos Garcia-Mata and Felix I Shaffer published a long article in The Quarterly Journal of Economics on “Solar and Economic Relationships” (4).

Figure 1 shows the sunspot series from 1750 to 2008 subjected to one of those fancy technical methods, the Morley Wavelet Transform. Time is read on the horizontal axis (on top) and frequency is read at the vertical axis (2^x months), with cycles of 11, 22 and 44 years highlighted. Sunspot intensity is indicated by colors, with red being high intensity and blue low intensity. The well-known 11 year sunspot cycle is very clear. There is also a strong cycle of about 85 years (2¹⁰ months), and some weaker cycles in between.

Sunspot cycles are counted at the 11 year frequency. The last cycle is Cycle 23, which is noticeably weaker than the previous 5 cycles. Currently we are at a low point of solar activity as indicated by everything being blue from top to bottom. This has only happened once before, just before the Dalton Minimum in the beginning of the 19^th century.

Figure 1: A Morlet Wavelet Transform of Smoothed Sunspot Numbers

Source: http://wattsupwiththat.files.wordpress.com/2008/09/wavelet_ssn.png

Sunspot cycles are not entirely regular, but vary in length between 10 and 12 years, depending on the sun’s activity level. When the sun is very active (strong magnetic field), the cycles get shorter, and when the sun is relatively quiet, they get longer.

A 1991 Science paper by Friis-Christensen and Lassen showed that there is a close inverse relationship between sunspot cycle length and Northern Hemisphere land temperatures over the 1860-1985 period (5). See Figure 2.

Figure 2: Sunspot cycle length and temperature on Earth

Source: (http://www.sciencemag.org/cgi/content/abstract/254/5032/698)

Until recently, nobody really understood how the sunspot cycle could affect climate on Earth, since the direct effects of variations in irradiance are clearly too small to drive climate variations on Earth.

However, in 2007, the Danish physicist Henrik Svensmark, together with science writer Nigel Calder, published The Chilling Stars: A New Theory of Climate Change, which puts forth a theory which does explain the links. Basically, when the sun is very active, it’s magnetic field is strong, which helps shield the Earth from cosmic rays. Cosmic rays are central to cloud formation on Earth, so when the sun is active, there are fewer clouds. Since clouds have a net cooling effect on Earth’s climate, fewer clouds will imply higher temperatures.

The theory corresponds very well to observation, with cool periods coinciding with periods of low solar activity and more cloud cover. It also explains events that the theory on CO₂-induced global warming has trouble explaining, such as the cooling period from 1945 to 1970, and the cooling of Antarctica amidst general warming.

The theory is understandably unpopular among the Anthropogenic Global Warming (AGW) crowd, as it leaves little room for a CO₂ effect on global temperatures.

My guess is that if you put together the two theories (Svensmark and AGW), you will be able to model climate much more accurately than if you rely on just one of them, and the predictions of the combined model would be much less dramatic future warming than current IPCC models suggest.

Will the Sun affect Earth’s climate? Leave your reply below.

Lykke Andersen is the Director of the Center for Economic and Environmental Modeling and Analysis (CEEMA) at INESAD.

(1) NASA: Ulysses Reveals Global Solar Wind Plasma Output at 50-Year Low.
(2) In 2004 we learned that “sunspots have been more active the last 70 years than it has for the previous 8000 years.” (New Scientist, 27 October 2004).
(3) Jevons, W.S. (1878) “The Periodicity of Commercial Crises and its Physical Explanation.” Paper read at the meeting of the British Association on August 19, 1878.
(4) Garcia-Mata, C. & F. I. Shaffner (1934) ” Solar and Economic Relationships: A Preliminary Report.” The Quarterly Journal of Economics, 49(1): 1-51.
(5) Friis-Christensen, E. & K. Lassen (1991) “Length of the Solar Cycle: An Indicator of Solar Activity Closely Associated with Climate.” Science, Vol. 254. no. 5032, pp. 698 – 700.