Global warming in the context of the Little Ice Age

Understanding the role of volcanic and solar variations in climate change i s important not only for understanding the Little Ice Age but also for unde rstanding and predicting the effects of anthropogenic changes in atmospheri c composition in the twentieth century and beyond. To evaluate the signific ance of solar and volcanic effects, we use four solar reconstructions and t hree volcanic indices as forcings to an energy-balance model and compare th e results with temperature reconstructions. Our use of a model representing the climate system response to solar and volcanic forcings distinguishes t his from previous direct comparisons of forcings with temperature series fo r the Little Ice Age. Use of the model allows us to assess the effects of t he ocean heat capacity on the evolution of the temperature response, Using a middle-of-the-road model sensitivity of 3 degrees C for doubled CO2, sola r forcings of less than 0.5% are too small to account for the cooling of th e Little Ice Age. Volcanic forcings, in contrast, give climate responses co mparable in amplitude to the changes of the Little Ice Age. A combination o f solar and volcanic forcings explains much of the Little Ice Age climate c hange, but these factors alone cannot explain the warming of the twentieth century. The best simulations of the period since 1850 include anthropogeni c, solar, and volcanic forcings.