The Climate of the Past 10,000 Years and the Role of the Sun

Abstract

Solar effects on climate are considered for the past 300 years and the past 10,000 years. A box-upwelling-diffusion climate model is used to evaluate the effects of irradiance variations on global-mean temperature. The model shows how periodic solar variations have differing effects on climate according to the period of the forcing oscillations. More rapid fluctuations show greater damping and smaller lags between forcing and response: for a 10-year cyclic forcing the response is only 13–23% of the equilibrium response, while the corresponding fraction for an 80-year cycle is 39–59%. A solar diameter-irradiance relationship in which irradiance is related to the time derivative of diameter is considered and shown to provide a better fit to observations than the conventional theory. The potential climatic effects of cyclic, radius-related irradiance changes are evaluated giving a range of global-mean temperature fluctuations of 0.2–0.3°C over the past 300 years. For the past 10,000 years, the recent glacial chronologies of Röthlisberger (1986) are compared with the ¹⁴C anomaly curve of Stuiver et al. (1986). The agreement between times of major ¹⁴C anomaly and times of globally-advanced glaciers (i.e. cool summers) is shown to be statistically significant. The implied reduction of solar irradiance during times of maximum century-time-scale ¹⁴C anomaly such as the Maunder Minimum is shown to be around 6 Wn^-2, equivalent to a net radiative forcing change of about 1 Wm^-2 at the top of the troposphere. If another major ¹⁴C anomaly began early in the 21st century, the associated solar perturbation would be of considerable importance, but still insufficient to fully offset the projected warming due to future greenhouse gas concentration increases.