Introduction: Mount Pinatubo as a test of climate feedback mechanisms

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The June 15, 1991 Mount Pinatubo eruption was a large but relatively shortlived shock to the Earth’s atmosphere. It thus provided an excellent opportunity to study the workings of the climate system, to test climate models, and to examine the impacts of climate change on life. The largest eruption of the 20th Century inspired a large amount of research on the connection between volcanic eruptions and the Earth’s atmosphere in the 12 years since that eruption, as exemplified by the chapters in this book. Here several additional examples of our new understanding of these connections are presented. While the global cooling after Pinatubo was not surprising, the observed winter warming over Northern Hemisphere continents in the two winters following the eruption is now understood as a dynamic response to volcanically produced temperature gradients in the lower stratosphere from aerosol heating and ozone depletion, and to reduced tropospheric storminess. Interactions of the Quasi-Biennial Oscillation of tropical stratospheric winds with the climate system are also now better understood by examining their role in the Pinatubo response. We have more confidence in the sensitivity of climate models used for attribution and projection of anthropogenic effects on climate because the strength of the water vapor feedback has been validated with Pinatubo simulations. The response of the biosphere to the Pinatubo eruption also illustrates its sensitivity to climate change and clarifies portions of the carbon cycle. Death of coral in the Red Sea in the winter of 1991–1992 and an unusually large number of polar bear cubs born in the summer of 1992 were two responses to the characteristic winter and summer temperature responses of the climate system. This strengthens our concern about negative impacts of global warming on polar bears and other wildlife. Enhanced vegetation growth from more diffuse and less direct solar radiation took more carbon dioxide out of the atmosphere than normal, temporarily reducing the observed long-term increase in carbon dioxide. Continued research on the Pinatubo eruption and its aftermath will undoubtedly enhance our understanding of the climate system.