In 1982 the relationship between solar activity and global temperature came into question. A paper by Felix Sulman, Head of the Bioclimatology Unit at Hebrew University in Jerusalem, entitled "Short and Long Term Changes in Climate" discussed a correlation between sunspot cycling and global temperature, however, at the time the precise relationship remained unclear. Five years later, scientist George Reid discovered that there was a 7-year lag between sunspot number and the observed temperature record. Research continued, and the debate became heated, as scientists argued over the extent to which solar variability influenced global temperature. Robert Jastrow et all reported in a 1990 paper that satellite records indicate fluctuations in the solar constant over the ten-year period from 1978 to 1988, with a decrease of about 0.1% between 1978 and 1985 and a slight increase from 1985 to 1988. While a change of 0.1% is not significant enough to explain the observed 0.5 deg/C temperature increase that has occurred since 1880, it is possible that a larger fluctuation has occurred over the longer time scale, thus bringing about more drastic global temperature change. It is estimated that a 1% increase in solar activity would be significant enough to cause the observed temperature increase over the past 100 years, and a 2% increase would match the increase in temperature predicted to occur with rising greenhouse gases by the middle of the twenty-first century (Jastrow 1990).
The debate accelerated when Danish scientists Eigel Friis-Christensen and Knud Lassen reported that solar activity did not correspond with fluctuations in sunspot number, but rather solar cycle length (Friis-Christensen and Lassen 1991). By using a different proxy, they argued that there is a remarkably high correlation between the solar cycle length and the temperature record over the past 130 years, with short solar cycles corresponding with periods of increased solar irradiance and long cycles corresponding with decreased solar activity. Observations show a striking correlation between Northern Hemisphere temperature trends and solar output, as the period from 1900 to 1940 corresponds with increased solar output, as does the period since 1970. The cooling trend, as observed during the period from 1940 to 1970 is matched with a period of reduced solar irradiance. This new parameter not only indicated a remarkably high correlation coefficient between solar activity and temperature (on the order of 0.95), but it also eliminated the problem of the 7-year lag encountered by Reid.
While these findings do offer strong support for the correlation between solar activity and temperature, the evidence remains strictly circumstantial, as the mechanism behind solar activity and climate variability remains unclear. Moreover, some scientists maintain that fluctuations in solar irradiance are not significant enough to bring about the temperature changes observed over the past 100+ years (Kerr 254). Such is the position of British scientists' Kelly and Wigley, who maintain that changes in the solar constant are not responsible for the observed fluctuations in temperature, and that since the middle of the 19th century the influence of increased anthropogenic greenhouse gases is the main cause of increased temperature trends. Greenhouse Warming advocate James Hansen agrees, stating that changes in the solar constant will not be significant enough to counter any human-induced global warming.
The IPCC stance with respect to the issue is a conservative one. While the panel's 1990 report states that changes in the output of energy from the sun do have an effect on climate, as determined by the 11-year solar cycle, its most recent report states that the role of solar output remains uncertain. The panel simultaneously stresses the significance of increasing CO2 concentrations on climate, thus indicating that a great deal more needs to be learned about solar activity before it is considered as a major factor in forcing climate change.
More recently, well known contrarian Fred Singer has reported on the role of solar irradiance in forcing climate change. Singer counters the standard scientific stance that changes in solar activity are not significant enough to have an effect, stating that solar variability is much greater in ultraviolet regions, thus having a significant effect on ozone levels and consequently a major impact on atmospheric circulation. In addition, he maintains that variations in solar activity can influence climate indirectly due to increased solar wind (Singer 1998).