Solar Variation - Solar Variation and Climate

Solar Variation and Climate

See also: Radiative forcing and Climate sensitivity

Both long-term and short-term variations in solar activity are hypothesized to affect global climate, but it has proven extremely challenging to directly quantify the link between solar variation and the earth's climate. The topic continues to be a subject of active study.

As discussed above, there are three suggested mechanisms by which solar variations may have an effect on climate:

• Solar irradiance changes directly affecting the climate ("Radiative forcing"). This is generally considered to be a minor effect, as the amplitudes of the variations in solar irradiance are much too small to have significant effect absent some amplification process.
• Variations in the ultraviolet component. The UV component varies by more than the total, so if UV were for some (as yet unknown) reason having a disproportionate effect, this might explain a larger solar signal in climate.
• Effects mediated by changes in cosmic rays (which are affected by the solar wind) such as changes in cloud cover.

Early research attempted to find a correlation between weather and sunspot activity, mostly without notable success. Later research has concentrated more on correlating solar activity with global temperature.

Crucial to the understanding of possible solar impact on terrestrial climate is accurate measurement of solar forcing. Unfortunately accurate measurement of incident solar radiation is only available since the satellite era, and even that is open to dispute: different groups find different values, due to different methods of cross-calibrating measurements taken by instruments with different spectral sensitivity. Scafetta and Willson found significant variations of solar luminosity between 1980 and 2000. But Lockwood and Frohlich find that solar forcing has declined since 1987.