Since the telescope was invented in 1610, people have been regularly recording differences in the appearance of the sun. Although some observations were made earlier, it was really the telescope that allowed consistent sightings of dark blotches on the sun's surface. It was not until 1843, however, that Heinrich Schwabe, an amateur astronomer, declared that these sunspots appeared to come and go in a regular 11-year cycle.
Following Schwabe's announcement, there were many attempts at making correlations with the sunspot cycles. Everything from the economy to crop yields as well as weather was linked to 11-year cycles. But when the statistics were closely examined, there was little evidence that these apparent correlations were based in reality.
It was not until 1976 that a paper published in Science by John A. Eddy of Saginaw Valley State University renewed interest in the sun-climate relationship with a comprehensive analysis of many different historical records of solar observation. Once this evidence had been synthesized, it became much clearer that there were robust correlations between the temperature and sunspots.
From 1645 to 1715, a period known as the Maunder Minimum, there were virtually no sunspots observed, indicating a "quiet" period in the sun's activity. This period coincides with the height of a time known as the Little Ice Age, which was a period of lower temperatures in Europe and perhaps globally.
Image: Mt. Wilson Observatory
Perhaps the most important development in the solar-climate link came in 1978. At that time, satellites that could measure the Total Solar Irradiance (TSI) received by the Earth from the Sun without atmospheric disturbance were launched. TSI is shown to be directly related to these "activity" cycles that Schwabe noticed 150 years ago. At the maxima of these cycles, there are more sunspots (which are magnetic phenomena that decrease total irradiance), but new instruments show that these dark sunspots are more than compensated for by bright areas on the sun, called faculae. Therefore, the overall irradiance increases in correspondence with higher numbers of sunspots.
The satellite data have now been able to capture two complete cycles (21 and 22). When Richard C. Willson of the Jet Propulsion Laboratory and Columbia University's Center for Climate Systems Research analyzed the data in the September 26, 1997 issue of Science, he noticed an increase in TSI of 0.036 percent from the previous minimum in 1986.
There are certain issues regarding the accuracy of the satellite data. When the Active Cavity Radiometer Irradiance Monitor (ACRIM I) satellite ended its mission, there was a delay in launching ACRIM II, which meant that data from the Earth Radiation Budget Satellite (ERBS) satellite data had to be used during the intervening period. ERBS produces different absolute values from its measurements and is not able to calibrate itself as well as the ACRIM satellites. Still, it seems as though the precision of the data is sufficient to believe this increase in the minimum values.
But identifying this evident increase in minimum values as a trend provokes concern. Obviously it is difficult to suggest that this trend is real when there are only two minima ever measured in this way. Also if we are to relate this finding to climate variations, we have to be aware that the warming trend in the surface temperature goes far beyond the last two decades. Therefore, we must try to extend the solar record to assess whether its activity is indeed increasing at the minima of the cycles (and its irradiance is also increasing) and to assess its potential influence on the climate.
The solar record has been extended by the use of the historical sunspot records already mentioned. It is also believed that solar activity correlates to the amounts of isotopes such as 14C and 10Be, which can be found in tree rings and ice cores respectively. In addition, the analysis of ionized calcium emission (an indication of solar magnetic activity) can produce TSI estimates. Therefore estimates of the solar irradiance can be made from the analysis of these different proxies and comparison to the climate records can be undertaken.
In 1995, Judith L. Lean of the Naval Research Laboratory and David Rind of NASA Goddard Institute for Space Studies made correlations between solar irradiance and the temperature curve since 1610. Their solar estimates were based on a number of different proxies and the temperature was taken from the Bradley and Jones Northern Hemisphere record. These results produced a correlation from 1610 to 1800 of 0.86, suggesting a predominant solar influence in the pre-industrial period. The authors also estimated that roughly half the observed warming from 1860 to the present could be attributable to the irradiance increase.
Of the 0.55 degree Celsius warming since 1860, 0.36 degrees Celsius have occurred since 1970, and the solar irradiance can only account for less than a third of this rise. This fact indicates that some other influence, or "forcing" or several different forcings, are becoming more influential in controlling the temperature change. It is thought that CO2 would be the most likely candidate in this industrial and post-industrial era.
Recent studies of global warming have necessitated a more comprehensive effort to quantify the natural climate variability so that the residual change may be attributed to the anthropogenic emissions of greenhouse gases. This attempt at quantification of the many different forces effect on the climate has re-emphasized the complexity of the climate system and the simultaneous interaction of many influences.
It is clear that the solar irradiance may indeed account for some of the temperature increases recorded over the last several decades. However, as the atmospheric CO2 rises--due to the almost exponential increase in emissions from industrial sources--the influence of solar variability on the Earth's climate will most likely decrease, and its relative contribution will be far surpassed by "greenhouse" gases.