Climate forcing by changing solar radiation

By how much does changing radiation from the sun influence the earth's clim ate. presently and in the recent past, compared with other natural and anth ropogenic processes? Current knowledge of the amplitudes and timescales of solar radiative output variability needed to address this question is descr ibed from contemporary solar monitoring and historical reconstructions. The 17-yr observational database of space-based solar monitoring exhibits an I l-yr irradiance cycle with amplitude of about 0.1%. Larger amplitude solar total radiative output changes-of 0.24% relative to present levels-are esti mated for the seventeenth-century Maunder Minimum by parameterizing the var iability mechanisms identified for the Il-yr cycle, using proxies of solar and stellar variability. The 11- and 22-yr periods evident in solar activit y proxies appear in many climate and paleoclimate records, and some solar a nd climate time series correlate strongly over multidecadal and centennial timescales. These statistical relationships suggest a response of the clima te system to the changing sun. The correlation of reconstructed solar irrad iance 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 fo rcing may have contributed about half of the observed 0.55 degrees C surfac e 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 equ ilibrium 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 irrad iance reduction, in general agreement with the preindustrial parameterizati on. But attributing a significant fraction of recent climate warming to sol ar forcing presents serious ambiguities about the impact of increasing gree nhouse gas concentrations whose radiative forcing has been significantly la rger than solar forcing over this time period. Present inability to adequat ely specify climate forcing by changing solar radiation has implications fo r policy making regarding anthropogenic global change, which must be detect ed against natural climate variability.