| Summary: | Understanding natural causes of climate change is vital to evaluate the relative impacts of human pollution and land surface modification on climate. Nevertheless, assessment of the contribution of natural causes to past climate change has been limited by limitations of the existing forcing indices. This dissertation has investigated one of the most important natural causes of climate change, volcanic eruptions, by developing a volcanic forcing index using 54 ice core records from both the Arctic and Antarctica. The extensive collection of ice core data reduces errors inherent in reconstructions based on small number of cores, which enables us to obtain much higher accuracy in both detection of events and quantification of the radiative effects. I extracted volcanic signals by applying a high-pass loess filter to each ice core record and examining peaks that exceed twice the 31-yr running median absolute deviation. I then studied the spatial pattern of volcanic sulfate deposition on Greenland and Antarctica, and combined this knowledge with a new understanding of stratospheric transport of volcanic aerosols to produce a forcing index as a function of month from 501 to 2000 CE, latitude in 10° bands, and height from 9 to 30 km at 0.5 km resolution. This index is the longest and most advanced volcanic forcing index of the type. It eliminates or minimizes many aspects of problems previous reconstruction had with the ice core records. I forced an energy balance model with this new volcanic forcing index, together with solar and anthropogenic forcing, to simulate the large scale temperature response over the past 1500 years. The results agree well with instrumental observations for the past 150 years and reasonably well with proxy records for the entire period. Through better characterization of the natural causes of climate change, this new data set will lead to improved prediction of anthropogenic impacts on climate. Previous work suggested the 15th century Kuwae eruption might have been a two-phase event occurred ...
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