An empirical model of global climate – Part 1: Reduced impact of volcanoes upon consideration of ocean circulation

Abstract

Observed reductions in Earth's surface temperature following explosive volcanic eruptions have been used as a proxy for geo-engineering of climate by the artificial enhancement of stratospheric sulfate. Earth cools following major eruptions due to an increase in the reflection of sunlight caused by a dramatic enhancement of the stratospheric sulfate aerosol burden. Significant global cooling has been observed following the four major eruptions since 1900: Santa María, Mount Agung, El Chichón, and Mount Pinatubo, leading IPCC (2007) to state "major volcanic eruptions can thus cause a drop in global mean surface temperature of about half a degree Celsius that can last for months and even years". We use a multiple linear regression model applied to the global surface temperature anomaly to suggest that exchange of heat between the atmosphere and ocean, driven by variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC), has been a factor in the decline of global temperature following these eruptions. The veracity of this suggestion depends on whether the Atlantic Multidecadal Oscillation (AMO) truly represents a proxy for the strength of the AMOC and the precise quantification of global cooling due to volcanoes depends on how the AMO is detrended. If the AMO is detrended using anthropogenic radiative forcing of climate, we find that surface cooling attributed to Mount Pinatubo, using the Hadley Centre/University of East Anglia surface temperature record, maximizes at 0.15 °C globally and 0.35 °C over land. These values are about a factor of 2 less than found when the AMO is neglected in the model and quite a bit lower than the canonical 0.5 °C cooling usually attributed to Pinatubo. The AMO had begun to decrease prior to the four major eruptions, suggesting that exchange of heat between the atmosphere and ocean due to variations in the strength of the AMOC drives the climate system, rather than responds to volcanic perturbations. The satellite record of atmospheric temperature from 1978 to present and other century-long surface temperature records are also consistent with our suggestion that volcanic cooling may have been over estimated by about a factor of 2 due to prior neglect of ocean circulation. Finally, a regression using AMO simulates pre-WWI cooling and WWII warming of global temperature particularly well, supporting the possibility that variations in the strength of the AMOC have truly exerted influence on global climate.