Volcanic eruptions and climate: A data and model intercomparison

Explosive volcanism can release large amounts of particles and gases into the atmosphere. Sulfuric acid droplets in the lower stratosphere are the primary substance interacting with the radiative fluxes over many months and possibly years. Because of their sub-micron size, they are more efficient at scattering incoming shortwave radiation from the sun back into space than absorbing and trapping longwave radiation from the earth. This results in a negative impact on the earth energy balance causing a general cooling below the aerosol layer. The magnitude of the cooling depends mostly on the amount of radiatively active aerosol particles as well as the duration of the perturbation. The cooling signal is largest in the upper troposphere through feedbacks with a slowed hydrologic cycle. At the surface, heat release, mostly from the oceans, can buffer some of the cooling.; A combined approach using both observations/proxy data and a state-of-the-art coupled General Circulation Model (GCM) to analyze the volcanic effects on climate can help in our understanding of the possible range of responses. Here, the most recent large eruption of Mt. Pinatubo (June 1991) was used to verify the implementation of the aerosol parameterization in the radiation code of the GCM. From there, an analysis of the volcanic contribution since 1870 A.D. was performed. A simple way of describing the spatial aerosol distribution is presented. In general, only a handful of eruptions were found large enough to significantly perturb the radiative balance of the earth. These few events caused a global climate signal, which is clearly detectable against the background noise of internal variability of the climate system. Next to the influence of isolated events, model simulations confirm earlier suggestions that temporally closely spaced large events can cause a further cooling in climate before the system can recover. Thus, explosive volcanism must be regarded as an important player in decadal to multi-decadal natural climate variations. In case of the 20th-Century, volcanic cooling in the last decades could have offset any possible warming due to increased solar irradiation. The potential volcanic role in other important time periods in the past must be studied, including the cooling in decades generally solely attributed to the sun. Proxy records, in particular tree rings, point to a potentially large role of explosive volcanism in the past.; Additionally to the radiative effects, increased atmospheric flow at high latitudes, particularly in winter, is the result of dynamical responses to changes in meridional temperature gradients in the lower stratosphere through heating in the aerosol layer. This effect helps to orchestrate the spatial distribution of the climate signal for several years after the eruption. Currently, no clear influence on other internal modes of variability, such as El Niño, could be unanimously confirmed in both observations/proxy reconstructions and the model simulations. But more work is needed, as better proxy climate data for earlier large events get available.