Globalwarming awareness2007

Modeling studies reported in the IPCC Third Assessment Report (TAR) did not find that changes in solar forcing were needed in order to explain the climate record for the last four or five decades. These studies found that volcanic and solar forcings may account for half of the temperature variations prior to 1950, but the net effect of such natural forcings has been roughly neutral since then. In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be eight times larger than the change in forcing due to increasing solar activity over the same period.

Since the TAR, some studies (Lean et al., 2002, Wang et al., 2005) have suggested that changes in irradiance since pre-industrial times are less by a factor of 3 to 4 than in the reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean, 2000.). Other researchers (e.g. Stott et al. 2003) believe that the effect of solar forcing is being underestimated and propose that solar forcing accounts for 16% or 36% of recent greenhouse warming. Others (e.g. Marsh and Svensmark 2000) have proposed that feedback from clouds or other processes enhance the direct effect of solar variation, which if true would also suggest that the effect of solar variability was being underestimated. In general the level of scientific understanding of the contribution of variations in solar irradiance to historical climate changes is “very low”.

The present level of solar activity is historically high. Solanki et al. (2004) 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. Solanki concluded based on their analysis that there is a 92% probability that solar activity will decrease over the next 50 years. In addition, researchers at Duke University (2005) have found that 10–30% of the warming over the last two decades may be due to increased solar output [11]. In a review of existing literature, Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period.