Solar irradiance is a measure of the energy output of the Sun and is determined by processes internal to the Sun. While there is good evidence that changes in atmospheric greenhouse gases are closely correlated with global surface temperature on timescales of millennia to hundreds of millennia, the evidence for the correlation between solar irradiance and surface temperature is limited to the last few centuries.

Solanki and Fligge found a high correlation between solar irradiance since 1750 and Earth's global surface air temperature. (However, they also reported that surface air temperature had increased by 0.2 C since the late 19th century, whereas solar irradiance had risen by a much smaller amount.) Following the medieval warm period (950 to 1250 C.E.) there were periods of intense cold in Europe from the 14th to the 19th centuries which collectively have been dubbed the Little Ice Age. This regional cooling was restricted to Europe and North America and there was only modest cooling of the entire Northern Hemisphere during the period.

Figure The yearly averaged sunspot number for a period of 400 years (1610-2010). SOURCE: NASA Marshall Space Flight Center.

Several causes for this cooling have been suggested. For instance, it was noticed that during the period between 1550 and 1700 there was very low solar activity as measured by the number of sunspots. This is referred to as the Maunder minimum. Some researchers have attempted to use forecasts of future solar activity to predict the beginning of a new "little ice age" beginning in the middle of the 21st century.


Solar variability

Solar variability, as this variation in the intensity of the sun's radiation is called, has long been known to follow cycles. In addition to well-known known 11-year cycle during which the sun's irradiance varies by only about a tenth of a percent, there are also longer term larger cycles in solar variability. Direct observations of sunspot numbers are available for the past four centuries and sunspot numbers can be reconstructed for the paleoclimate from proxies such as the concentration of radiocarbon in tree rings or isotopic beryllium in ice cores.

Figure Satellite measurement of the total solar irradiance (TSI) compared to sunspot activity. SOURCE: Greg Kopp, University of Colorado.

Based on measurements of the strength of sunspots Matthew Penn and William Livingston of the National Solar Observatory have found evidence that sunspot activity is decreasing and that the sun could be on the threshold of a mini-Maunder event right now. This is supported by the current 11-year solar cycle being the weakest in more than 50 years and the next cycle beginning in 2020 is projected to be even weaker.

Solar irradiance has only been directly observed with modern high accuracy instrumentation since 1978. Several researchers have attempted to reconstruct solar variability prior to 1978. Solanki and Fligge point out that it is possible to reconstruct the irradiance with some reliability from today to around 1874, and with lower accuracy back to the Maunder minimum. Some researchers have gone even further and attempted to reconstruct solar variability for the past 7,500 years. However, there is a problem arising from the decreasing amount and accuracy of the relevant data for reconstructing irradiance with age. In AR4 the IPCC pointed to serious discrepancies between reconstructions using different proxies and on this basis state that prior to 1750 reconstructions of solar activity are considered highly unreliable.

Figure Comparison of two reconstructions (thin solid line and dashed line) of total solar irradiance with northern hemisphere land temperature (thick solid line). The hatched area between the irradiance curves gives a rough indication of the uncertainty in the reconstructions. SOURCE: Solanki and Fligge

Some evidence has been found that suggests that prior to 1975 the increase in irradiance anticipates the rise in temperatures which is compatible with a causal relationship between solar irradiance and climate.

Figure Total solar irradiance and global surface temperature. SOURCE: Georgios Florides et al.

Since 1975 the evidence shows that global surface temperature has been rising faster than solar irradiance. The growing divergence between changes in solar irradiance and global surface temperature has been ascribed to the relatively rapid growth in the contribution of greenhouse gases to warming.

Forecasting solar variability

Solar models have been constructed to try to forecast changes in solar irradiance decades in advance. For example, Valentina Zharkova of Northumbria University has developed a solar model which forecasts that the decade from 2030-2040 will see a significant reduction in solar activity which will lead to the properties of a ‘Maunder minimum’, resulting from solar conditions last seen 370 years ago. However, at present there is very low confidence in projections of future long term solar variability.


Long term climate change, extending over centuries to hundreds of millennia, has been ascribed to solar variability, variations in atmospheric greenhouse gases, cosmic rays, volcanic eruptions, and aerosols. While research has shown that cosmic rays, volcanic eruptions and aerosols do not correlate highly with long term changes in global surface temperature, there is evidence that solar variability and greenhouse gases do. Comparing solar irradiance with northern hemisphere surface temperature reveals significant correlation from about 1750 through to 1975. Since 1975 there is growing correlation of atmospheric greenhouse gas concentrations and a weakening correlation with the variability in solar irradiance. Based on the evidence over the past several decades the climate impact of changes in solar irradiance are smaller in magnitude than the increase in warming due to rising greenhouse gases. Therefore, the little ice age projected by some researchers for the middle of the 21st Century is seen as unlikely.


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