Evaluating sun-climate relationships since the Little Ice Age

From the coldest period of the Little Ice Age to the present time, the surf ace temperature of the Earth increased by perhaps 0.8 degrees C. Solar vari ability may account for part of this warming which, during the past 350 yea rs, generally tracks fluctuating solar activity levels. While increases in greenhouse gas concentrations art: widely assumed to be the primary cause o f recent climate change, surface temperatures nevertheless varied significa ntly during pre-industrial periods, under minimal levels of greenhouse gas variations. A climate forcing of 0.3 W m(-2) arising from a speculated 0.13 % solar irradiance increase can account for the 0.3 degrees C surface warmi ng evident in the paleoclimate record from 1650 to 1790, assuming that clim ate sensitivity is 1 degrees C W-1 m(-2) (which is within the IPCC range). The empirical Sun-climate relationship defined by these pre-industrial data suggests that solar variability may have contributed 0.25 degrees C of the 0.6 degrees C subsequent warming from 1900 to 1990, a scenario which time dependent GCM simulations replicate when forced with reconstructed solar ir radiance. Thus, while solar Variability likely played a dominant role in mo dulating climate during the Little Ice Age prior to 1850, its influence sin ce 1900 has become an increasingly less significant component of climate ch ange in the industrial epoch. It is unlikely that Sun-climate relationships can account for much of the warming since 1970, not withstanding recent at tempts to deduce long term solar irradiance fluctuations from the observati onal data base, which has notable occurrences of instrumental drifts. Empir ical evidence suggests that Sun-climate relationships exist on decadal as w ell as centennial time scales, but present sensitivities of the climate sys tem are insufficient to explain these short-term relationships. Still to be simulated over the time scale of the Little Ice Age to the present is the combined effect of direct radiative forcing, indirect forcing via solar-ind uced ozone changes in the atmosphere, and speculated charged particle mecha nisms whose pathways and sensitivities are not yet specified. (C) 1999 Else vier Science Ltd. All rights reserved.