Evaluating sun-climate relationships since the Little Ice Age

From the coldest period of the Little Ice Age to the present time, the surface temperature of the Earth increased by perhaps 0.8 degrees C. Solar variability may account for part of this warming which, during the past 350 years, generally tracks fluctuating solar activity levels. While increases in greenhouse gas concentrations art: widely assumed to be the primary cause of recent climate change, surface temperatures nevertheless varied significantly 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 warming evident in the paleoclimate record from 1650 to 1790, assuming that climate 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 irradiance. Thus, while solar Variability likely played a dominant role in modulating climate during the Little Ice Age prior to 1850, its influence since 1900 has become an increasingly less significant component of climate change in the industrial epoch. It is unlikely that Sun-climate relationships can account for much of the warming since 1970, not withstanding recent attempts to deduce long term solar irradiance fluctuations from the observational data base, which has notable occurrences of instrumental drifts. Empirical evidence suggests that Sun-climate relationships exist on decadal as well as centennial time scales, but present sensitivities of the climate system 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-induced ozone changes in the atmosphere, and speculated charged particle mechanisms whose pathways and sensitivities are not yet specified. (C) 1999 Elsevier Science Ltd. All rights reserved.