The Role of Explosive Volcanism During the Cool Maunder Minimum

Understanding of the natural climate variability is crucial for evaluating the anthropogenic contribution to global warming. In particular, external forcing factors such as solar irradiation changes and aerosol forcing from explosive volcanism need to be captured accurately in order to detect and quantify the emerging signal. The short instrumental period limits our options to estimate the magnitude of external forcing through absence of the full range in magnitudes of the forcing factors as well as by lack of their low frequency representation. Thus, we are forced to use proxies to expand our record. Reconstructions of solar irradiance have often employed sunspot observations as a measure of solar activity. A striking feature has always been the Maunder Minimum, a multi-decadal period where the sunspots almost entirely disappeared. It is generally associated with reduced solar irradiance. Unusually cold conditions in Western Europe, especially during the late Maunder Minimum from 1675-1705, have often been used synonymous for the Little Ice Age. This link between the solar irradiance and temperatures during the Maunder Minimum has been applied for estimating either the magnitude of the low frequency solar irradiance changes while assuming a particular climate sensitivity, or conversely, to estimate the climate sensitivity assuming a magnitude of solar irradiance change. In doing so, other potential causes of the cool conditions were ignored. Interestingly, the climate conditions during the Maunder Minimum don't remain cold over the entire period but exhibit a number of very cold, pulse-like episodes of a few years length. Here, the role of explosive volcanism superposed on solar irradiance changes during the late Maunder Minimum is evaluated. Using the fully coupled NCAR Climate System Model different ice core based volcanic forcing series are applied and combined with solar irradiance reconstructions. Not only temporal radiative balance impacts of the forcings are analyzed but also the spatially characteristical evolution of the signals. These fingerprints are then verified by a series of high resolution proxy reconstructions of European and Northern Hemisphere climate. Through this comparison of model with proxy data we quantify the volcanic cooling during this period and highlight the danger of estimating the climate sensitivity when omitting other factors.