An alternative hypothesis is that recent warming may be the result of variations in solar activity.Stott and colleagues have suggested that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases. Another paper suggests that the Sun may have contributed about 45–50 percent of the increase in the average global surface temperature over the period 1900–2000, and about 25–35 percent between 1980 and 2000.In 2006, Peter Foukal and colleagues found no net increase of solar brightness over the last 1,000 years. Solar cycles led to a small increase of 0.07 percent in brightness over the last 30 years. This effect is too small to contribute significantly to global warming.The general view is that the combined effect of the two main sources of natural climate forcing, solar variation and changes in volcanic activity, probably had a warming effect from pre-industrial times to 1950 but a cooling effect since.
An increase in solar activity should warm the stratosphere, whereas an increase in greenhouse gases should produce cooling there.The observed trend since at least 1960 has been a cooling of the lower stratosphere.Reduction of stratospheric ozone also has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.
Svensmark and colleagues have proposed another hypothesis related to solar activity, which is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.Another paper found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed this criticism of their hypothesis.A 2007 paper found that in the last 20 years there has been no significant link between changes in cosmic rays coming to Earth and cloudiness and temperature.
Svensmark and colleagues have proposed another hypothesis related to solar activity, which is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.Another paper found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed this criticism of their hypothesis.A 2007 paper found that in the last 20 years there has been no significant link between changes in cosmic rays coming to Earth and cloudiness and temperature.
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