Solar variation

Solar variation

Express varied opinions concerning the cause of global warming. Some say only that it has not yet been ascertained whether humans are the primary cause of global warming; others attribute global warming to natural variation;increased or cosmic rays. The consensus position is that solar radiation may have increased by 0.12 W/m² since 1750, compared to 1.6 W/m² for the net anthropogenic forcin The TAR said, "The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades." The AR4 makes no direct assertions on the recent role of solar forcing, but the previous statement is consistent with the AR4's figure 4.

A few studies claim that the present level of solar activity is historically high as determined by sunspot activity and other factors. Solar activity could affect climate either by variation in the Sun's output or, more speculatively, by an indirect effect on the amount of  Solanki and co-workers suggest that solar activity for the last 60 to 70 years may be at its highest level in 8,000 years; Muscheler et al. disagree, suggesting that other comparably high levels of activity have occurred several times in the last few thousand years Muscheler et al. Solanki et al. concluded "that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades," and that "at the most 30% of the strong warming since then can be of solar origin. concluded that "solar activity reconstructions tell us that only a minor fraction of the recent global warming can be explained by the variable Sun."

Another point of controversy is the correlation of temperature with

and  reject the claim that the warming observed in the global mean surface temperature record since about 1850 is the result of solar variationsLockwood and Fröhlich conclude that "the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified.