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.