Global warming in the context of the Little Ice Age

Understanding the role of volcanic and solar variations in climate change is important not only for understanding the Little Ice Age but also for understanding and predicting the effects of anthropogenic changes in atmospheric composition in the twentieth century and beyond. To evaluate the significance of solar and volcanic effects, we use four solar reconstructions and three volcanic indices as forcings to an energy-balance model and compare the results with temperature reconstructions. Our use of a model representing the climate system response to solar and volcanic forcings distinguishes this from previous direct comparisons of forcings with temperature series for the Little Ice Age. Use of the model allows us to assess the effects of the ocean heat capacity on the evolution of the temperature response. Using a middle-of-the-road model sensitivity of 3°C for doubled CO₂, solar forcings of less than 0.5% are too small to account for the cooling of the Little Ice Age. Volcanic forcings, in contrast, give climate responses comparable in amplitude to the changes of the Little Ice Age. A combination of solar and volcanic forcings explains much of the Little Ice Age climate change, but these factors alone cannot explain the warming of the twentieth century. The best simulations of the period since 1850 include anthropogenic, solar, and volcanic forcings.