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.