FOLLOWING earlier studies1-6, attention has recently been directed again to the possibility that long-term solar irradiance variations, rather than increased greenhouse gas concentrations, have been the dominant cause of the observed rise in global-mean surface temperature from the mid-nineteenth century to the present. Friis-Christensen and Lassen7 report a high correlation (0.95; ref. 8) between the variable period of the '11-year' sunspot cycle and the mean Northern Hemisphere land surface temperature from 1865 to 1985. The Marshall Institute report9 concludes that '...the sun has been the controlling influence on climate in the last 100 years, with the greenhouse effect playing a smaller role." Here we explore the implication that such putative solar irradiance variations would have for global warming. Our results provide strong circumstantial evidence that there have been intercycle variations in solar irradiance which have contributed to the observed temperature changes since 1856. However, we find that since the nineteenth century, greenhouse gases, not solar irradiance variations, have been the dominant contributor to the observed temperature changes.
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