February 6, 2012

The Sun: O Inconstant Star!

Filed under: Climate Forcings, Solar

As solar activity declines and rate of global warming follows suit, it is natural to wonder whether the two are in some manner related.

Science is all over the map on this one—and is hardly the “settled” stuff our greener friends want us to believe. One school holds that there is little-to-no detectable relationship between solar changes and surface temperatures, while another holds that there is a strong influence and that a projected period of low solar activity over the next several decades will offset much of the anthropogenic greenhouse-gas induced warming. Of course, there are also gradations in between these poles of opinion.

Case 1 : A Large Solar Influence

One school of thought for which there is a growing scientific literature holds that solar variability is amplified through various mechanisms, can and does have a large and controlling influence of the earth’s temperature and, in fact, can largely explain the temperature rise over the 20th century. Further, there is growing evidence suggesting that the solar activity level (and output) will be on average lower during the 21st century than it was during the second half of the 20th century—a situation which would indicate significantly less warming than projected by climate models.

The grandfather of solar influence was MIT’s estimable Hurd C. Willett, who developed solar-based long range forecasts for the Weather Bureau (today’s National Weather Service) in the 1930s. Willett’s scientific fame was assured when a prediction he made 40 years in advance came true—that the Great Salt Lake, which was shrinking, would expand dramatically in the 1980s. He also told Midwestern farmers to persevere through the severe drought of the mid-1950s (which was bigger than the Texas drought of 2011) because it would be followed by two decades of plentiful yields. That, too was correct.

Studies of solar influence went into eclipse when Willett retired, after being awarded the American Meteorological Society’s highest honor, which came to be known as the Carl Gustav-Rossby Medal for excellence in research. Long range forecasters became intrigued with El Niños, Pacific and Atlantic temperature oscillations, and other factors that now drive season-in-advance forecasting.

That all changed in 1991 with a the publication of a paper in Science by Eigel Friis-Christansen and Knud Lassen showing a remarkable relationship between the length of the well-known solar sunspot cycle, which varies from its 11-year average, and Northern Hemisphere mean surface temperature. Since then, a number of researchers have attempted to explain this result, with some novel theories emerging, especially in the last decade.

The problem, which we note below in “Case 2”, is that the observed changes in incoming solar radiation aren’t large enough to have much direct thermal effect—and yet we can see that the Little Ice Age, when surface temperatures were about a 1.5°C colder than present, was concurrent with a complete dearth of sunspots for sixty years. This period is known as the Maunder Minimum and will come up again in this discussion.

So some mechanism has to be hypothesized and defended that “amplifies” small solar changes. Primary among those proposed is the sun’s influence on the number of cosmic rays which strike the earth’s atmosphere. Cosmic rays are energetic particles from deep space which, when they pass through the earth’s atmosphere, are thought to create cloud condensation nuclei (CNN) as they interact with various constituents of the atmosphere. CNN are tiny aerosol particles upon which water vapor can condense upon and start the formation of low clouds, which are known to contribute to a net cooling of surface temperatures.

This hypothesis is generally attributed to Henrik Svensmark of the National Space Institute of the Technical University of Denmark.

It goes like this. The more active the sun is, the more often “coronal mass ejections” occur. These resemble explosions on the sun which cast out clouds of plasma that sweep past the earth and provide a temporary shield from cosmic rays. The fewer cosmic rays there are, the fewer cloud condensation nuclei are created and fewer clouds are produced. The fewer clouds in the lower atmosphere, the more the sun shines on the earth’s surface and the hotter it becomes. The opposite happens when solar activity is low—a situation which ultimately leads to a cooling influence under this hypothesis.

Svensmark has laid out observational evidence of this chain of events in a paper that he and his colleagues published in the journal Geophysical Research Letters in 2009 and he is currently involved in the CLOUD (Cosmics Leaving OUdoor Droplets) experiment being conducted at the European Center for Nuclear Research (CERN) which aims to gain more insight through laboratory simulations of these processes. Svensmark describes the progress being made in the CERN CLOUD study and how it impacts his hypothesis in a recent interview, stating “I welcome the CLOUD results. They basically confirm our own experimental results since 2006, and does so within a larger variation of parameters” and while admitting that “of course there are many things to explore” that “the cosmic-ray/cloud seeding hypothesis is converging with reality.”

Another researcher supporting a cosmic ray influence on climate is Nir Shaviv of the Hebrew University in Jerusalem. Shaviv was one of the 16 signers of the recent Wall Street Journal piece “No Need to Panic About Global Warming” which called for restraint on climate change issues. In a research paper that garnered a lot of attention a few years ago, Shaviv, along with colleague Ján Veizer, demonstrated a link between cosmic ray flux (as the solar system moved through our Milky Way galaxy) and the timing of ice ages on earth. While Shaviv’s original work focused on geologic timescales, he believes that cosmic ray fluctuations can affect climate over shorter periods as well. He believes that solar variability has had a significant impact on the global temperature warmings that occurred twice in the 20th century. Factoring out the solar influence lessens the influence of greenhouse gas increases, and, Shaviv calculates that the climate sensitivity (how much the global temperature will change from a doubling of atmospheric carbon dioxide concentration) is only about 1°C—a value that lies below the low end of the UN’s climate sensitivity range of 2°C-4.5°C.

A lot more information on Nir Shaviv’s calculations and findings can be found at his blog site ScienceBits.com. There, he too comments on the recent result from the CERN CLOUD experiment. Here is an excerpt from his post titled “The CLOUD is clearing”:

The CLOUD collaboration from CERN finally had their results published in Nature, showing that ionization increases the nucleation rate of condensation nuclei. The results are very beautiful and they demonstrate, yet again, how cosmic rays (which govern the amount of atmospheric ionization) can in principle have an affect on climate.

Also, significant effort in describing a large solar influence on the earth’s recent temperature history has been done by Duke University’s Nicola Scafetta. In past work, Scafetta has calculated that as much as half of the warming of the 20th century was caused by changes in the solar output, and more recently, has determined that up to 60% of the increase in global temperature since 1970 has an explanation that lies with a combination of solar variability and planetary alignments (which may act to influence solar variability). Scafetta provides a less technical discussion of this recent study here.

By using the combination of natural occurring cycles that he identified (including solar variability), Scafetta can make projections of the evolution of climate into the future. Figure 1 shows how he sees things to come compared with the U.N.’s vision of the future. Scafetta foresees little additional global warming for the next 2-3 decades and an overall warming between now and the end of the 21st century of between 0.3°C and 1.2°C—values that are about 1/3rd of the U.N.’s projected rise.

Figure 1. A comparison between Scafetta’s model (light blue) and the projections made by the U.N. More information about this graph, and the science that lies behind it can be found here).

Case 2: Little-to-No Influence

The idea that solar variability exerts little-to-no influence on the global average surface temperature is based upon several lines of reasoning.

The first is that the difference in the amount of total incoming radiation from the peak of the well-known 11-yr solar sunspot cycle to the trough of the cycle is very low, only about one-quarter of Watt per square meter at the earth’s surface. Depending on the climate sensitivity to incoming radiation that you prefer, this works out to a change in the global average temperature of maybe a tenth of a degree Celsius, give or take a few hundredths of a degree. Detecting such a small “signal” amidst other forms of climate “noise” (such as El Niño, volcanoes, and a myriad of circulation patterns) becomes rather challenging.

The second, is that over a period spanning several solar cycles (several decades), the direct correlation between the solar variability and global temperature variability (after accounting for volcanoes and El Niño/La Niña cycles) is basically zero ( it even switches signs from time to time). This means that knowing what the sun is doing gives you little information as to what the global temperatures are doing. But notice the use of the word “direct”. In Case #1 the mechanism is “indirect” with the sun modulating cloud formation via cosmic rays and not timed precisely with the more common measures of solar output (e.g. sunspot counts).

However, if your analysis is confined to last two of solar cycles, then it appears as if a decline/rise in solar output over the course of the 11-yr cycle is tied to a decline/rise in global temperatures. Such a correlation leads to the conclusion that declining solar output over the past decade has been, in part, responsible for the contemporaneous slowdown in the rate of global warming—accounting for maybe 0.05 to 0.1 degree of cooling over the course of past 10 years or so. This explanation is currently en vogue with respect to the obvious lack of strong warming in almost fifteen years. It is interesting to note that a solar explanation was largely absent (and in fact was pretty much pooh-pooed) by this same group of people during earlier periods when the warming rate was more to their liking.

A string of papers in the scientific literature have reported that even over the time period of the past several centuries that the influence of solar variability on the earth’s average temperature has been slight. For example, Judith Lean and David Rind found that, although they could identify a persistent solar signal in the temperature record during the past century, the signal was small and little-changed over the course of the past 100 years. In other words, solar variability could not explain the observed warming trend. And another just-published paper by Gifford Miller and colleagues even makes the case that the cold period known as the Little Ice Age, long thought to have been the result of an extended period of low solar output, was primarily caused by a concurrence of large volcanic eruptions and feedback processes resulting therefrom. Currently, that paper is an outlier in the field and time will tell whether or not it is correct.

So in very general terms, what buoys the little-to-no solar variability influence reasoning is that straightforward empirical analyses trying to relate solar changes (both directly observed and inferred from proxies such as sunspots) to changes in the global temperature (both directly observed and inferred from proxies) fail to find a large direct influence on the latter from the former.

Even within the “little-influence” community, though, the science is not settled.

Bottom Line

In the scientific literature there is to be found a wide range of opinions regarding the magnitude of the impact of the variability in the activity of the sun on the earth’s average temperature. As such, solar influence on the climate will remain a major topic of inquiry and study.

One thing that many researchers agree upon is that the sun is showing signs of heading into a prolonged period (~several decades or more) of relative inactivity that is reminiscent of other quiescent periods within the past several centuries such as the Dalton Sunspot Minimum or even the Maunder Sunspot Minimum—which most scientists link with the Little Ice Age, a period so cold that the annual winter fairs were held on the Thames river ice in London, something that could only be done by boats today. The degree to which a future lessening of solar activity will reduce the earth’s average temperature could factor significantly into how the climate responds to increasing greenhouse gas concentrations as the developing world develops—and how much the developed world tries to intervene.

Remember this: in a multifactor problem like climate change, surprises are to be expected and greeted as such. One example from today’s news is obvious. Five years ago, science held that we were “running out” of natural gas. Then the revolution in horizontal drilling along with shale fracturing revealed a massive source of domestic energy that can lower emissions of carbon dioxide while keeping energy prices low (as opposed to expensive and unreliable solar and wind power). No one except a few on the fringe thought such a revolutionary change was possible. But they were right. The longer one practices science, the more surprises are experienced. Keep an open mind on climate change and the sun.


Friis-Christensen, E., and K. Lassen, 1991. Length of the Solar Cycle: An Indicator of Solar Activity Closely Associated with Climate, Science, 254, 698–700, doi:10.1126/science.254.5032.698.

Lean, J.L., and D. H. Rind, 2008. How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters, 35, L18701, doi:10.1029/2008GL034864

Miller, G.H., et al. 2012. Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks. Geophysical Research Letters, 39, L02708, doi:10.1029/2011GL050168

Scafetta, N., and B. J. West, 2006. Phenomenological solar contribution to the 1900-2000 global surface warming, Geophysical Research Letters, 33, L05708, doi:10.1029/2005GL025539

Scafetta, N., 2012. Testing an astronomically based decadal-scale empirical harmonic climate model versus the IPCC (2007) general circulation climate models. Journal of Atmospheric and Solar-Terrestrial Physics, in press, doi: 10.1016/j.jastp.2001.12.005

Shaviv, N.J., and J. Veizer, 2003. Celestial driver of Phanerozoic climate? GSA Today, July 2003, 4-10, http://www.sciencebits.com/ClimateDebate/

Svensmark, H., T. Bondo, and J. Svensmark, 2009. Cosmic ray decreases affect atmospheric aerosols and clouds. Geophysical Research Letters, 36, L15101, doi:10.1029/2009GL038429.

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