Sun Cycles and Climate Change

The record of sunspot activity (in terms of number of sunspots) over the last 250 years clearly illustrates the a dominant 11 year cycle in sunspot number (From: Science@NASA ).
11-year Sunspot Cycles
Ever since the astrophysicist John Eddy pointed out in 1976 that the "Little Ice Age" occurred within a period of reduced sunspot activity, the search has been on for evidence that a variable Sun controls climate. The original idea was that sunspot cycles influence climate on an 11-year cycle, a concept explained in a book by Hoyt and Schatten (1997), The Role of the Sun in Climate Change .

Sunspots are huge magnetic storms that show up as darker regions on the sun’s surface. They tend to occur in cycles, with the number and size reaching a maximum approximately every 11 years. During periods of maximum sunspots the sun emits more energy (about 0.1 percent more) than during periods of sunspot minimums. Apparently bright spots that form around the sunspots radiate more energy, thus offsetting the effect of the dark spots. Note, however, that the energy difference is very small, amounting to approximately one tenth of the total effect that increasing levels of greenhouse gases have had on warming the atmosphere to date.

Solar Output & Climate
Although many theories have been proposed to linked sunspot cycles to climate change, none have been proven. Scientists now believe that the intensity of sunspot cycles is an indicator of the overall brightness of the sun, which changes on cycles of a century and does have an influence on climate. Research by Dr. Judith Lean, a solar physicist at U.S. Naval Research Laboratory in Washington, D.C. and colleagues noted a strong correlation between solar output and temperatures since 1610. Notice that a period during the “Little Ice Age”, from the 17th to early 18th centuries called the “Maunder Minimum,” was characterized by a Sun that was 0.25% dimmer than it is now. This change goes well beyond the 0.1% dimming ascribed to the 11-year sunspot cycle, so that a climate impact becomes much more probably. In addition, Lean assumes that the change in UV output from the Sun must have been 6 times larger than that of visible light (a fact which, if true, holds interesting implications for the history of the ozone layer).

Solar input reconstruction (green) graphed with estimated surface temperatures from the past (blue) and measured northern hemisphere temperatures (red, dashed). Solar irradiance was relatively more important in forcing temperature until about 1800; by comparison only 1/3 of the temperature increase in past 150 years is attributed to solar forcing. Volcanic eruptions resulted in a major dip in temperature from 1800 to 1850.
The Last 25 Years
Lean’s study found that "solar forcing may have contributed about half of the observed 0.55°C surface warming since 1860 and one third of the warming since 1970". However, lest we take unwarranted comfort from the fact that the Sun seems most important and anthropogenic warming is less than originally estimated, keep in mind that if the Sun controls substantial climate fluctuations by changing its brightness by only 0.25%, a change of more than 1 percent in “virtual brightness” (from trace greenhouse gases like CO2 and CH4) could have a considerably greater impact. The fact is we do not know for sure which will have the greater effect, but it is well to remember that the reconstruction of sunspots, their relationship to solar energy output, and the link to overall background brightness are areas of science that are still changing. Thus, solar contributions may be much less (or a somewhat more) than those currently estimated. In any case, the conclusion that can be taken from this discussion is that the warming since 1975 is outside the range of a purely solar effect and may safely be ascribed to a strong anthropogenic component.