Published online by Cambridge University Press: 12 April 2016
Using present global warming and paleoclimatic records from climatically sensitive regions as a frame of reference, we infer that global temperature changes did not exceed ±0.5°C during the current interglacial or ±2.0°C during the last glacial period. In order to completely explain such fluctuations by solar irradiance changes, all solar and terrestrial factors must be optimized. Since this is unlikely, we conclude that solar forcing of pre-anthropogenic climate change is a significant and perhaps dominant factor but other processes must also be significant. Solar irradiance changes alone cannot result in the temperature decrease required to change from interglacial to glacial as suggested by Opik (1965).