Volcanoes, solar variability and climate change over the past 600 years

Recent work investigating climate variability since 1400 has confirmed that previous centuries experienced significantly cooler temperatures than the twentieth century. Possible causes of the colder and warmer episodes over this time period include volcanic aerosols, solar variations, anthropogenic changes, and internal variability of the climate system. We need to understand these causes in order to assess possible greenhouse warming in the twentieth century and to predict plausible climate scenarios for the future.; To evaluate the significance of volcanic aerosols as causes of climate change, we need an index of past volcanic forcing. Because the existing indices are unsatisfactory, I have compared data from ice cores and combined them to form an ice core-based volcanic index, the IVI. I used this and other volcanic indices, along with several published reconstructions of past solar variability, to force an upwelling-diffusion energy-balance model. While the results from solar forcings are not large enough to match the reconstructed temperature changes for the past 600 years, the volcanic forcings are clearly sufficient to produce the required variability. Comparison of the results to reconstructions of temperature show some significant relationships, but the correlations vary strongly between time periods and between different versions of the same forcing. Contrary to the results of Lean et al. (1995), my results show that the likely climate responses to volcanic forcings in the LIA are at least as well-correlated to temperature reconstructions as are responses to solar forcings. Combinations of volcanic and solar forcings provide more consistent explanations of temperature variations over the last 600 years, accounting for up to 64% of variability from 1610 on.