Climate forcing by changing solar radiation

By how much does changing radiation from the sun influence the earth's climate. presently and in the recent past, compared with other natural and anthropogenic processes? Current knowledge of the amplitudes and timescales of solar radiative output variability needed to address this question is described from contemporary solar monitoring and historical reconstructions. The 17-yr observational database of space-based solar monitoring exhibits an Il-yr irradiance cycle with amplitude of about 0.1%. Larger amplitude solar total radiative output changes-of 0.24% relative to present levels-are estimated for the seventeenth-century Maunder Minimum by parameterizing the variability mechanisms identified for the Il-yr cycle, using proxies of solar and stellar variability. The 11- and 22-yr periods evident in solar activity proxies appear in many climate and paleoclimate records, and some solar and climate time series correlate strongly over multidecadal and centennial timescales. These statistical relationships suggest a response of the climate system to the changing sun. The correlation of reconstructed solar irradiance and Northern Hemisphere (NH) surface temperature anomalies is 0.86 in the pre industrial period from 1610 to 1800, implying a predominant solar influence. Extending this correlation to the present suggests that solar forcing may have contributed about half of the observed 0.55 degrees C surface warming since 1900 and one-third of the warming since 1970. Climate model simulations using irradiance reconstructions provide a tool with which to identify potential physical mechanism for these implied connections. An equilibrium simulation by the Goddard Institute for Space Studies GCM predicts an NH surface temperature change of 0.51 degrees C for a 0.25% solar irradiance reduction, in general agreement with the preindustrial parameterization. But attributing a significant fraction of recent climate warming to solar forcing presents serious ambiguities about the impact of increasing greenhouse gas concentrations whose radiative forcing has been significantly larger than solar forcing over this time period. Present inability to adequately specify climate forcing by changing solar radiation has implications for policy making regarding anthropogenic global change, which must be detected against natural climate variability.