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An empirical model of global climate ? Part 1: A critical evaluation of volcanic cooling

by T Canty, N R Mascioli, M D Smarte, R J Salawitch
Atmospheric Chemistry and Physics ()

Abstract

Observed reductions in Earth's surface temperature following explosive\nvolcanic eruptions have been used as a proxy for geoengineering of\nclimate by the artificial enhancement of stratospheric sulfate. Earth\ncools following major eruptions due to an increase in the reflection\nof sunlight caused by a dramatic enhancement of the stratospheric\nsulfate aerosol burden. Significant global cooling has been observed\nfollowing the four major eruptions since 1900: Santa Mar�a, Mount\nAgung, El Chich�n and Mt. Pinatubo, leading IPCC (2007) to state\n"major volcanic eruptions can, thus, cause a drop in global mean\nsurface temperature of about half a degree Celsius that can last\nfor months and even years". We use a multiple linear regression model\napplied to the global surface temperature anomaly to suggest that\nexchange of heat between the atmosphere and ocean, driven by variations\nin the strength of the Atlantic Meridional Overturning Circulation\n(AMOC), has been a factor in the decline of global temperature following\nthese eruptions. The veracity of this suggestion depends on whether\nsea surface temperature (SST) in the North Atlantic, sometimes called\nthe Atlantic Multidecadal Oscillation, but here referred to as Atlantic\nMultidecadal Variability (AMV), truly represents a proxy for the\nstrength of the AMOC. Also, precise quantification of global cooling\ndue to volcanoes depends on how the AMV index is detrended. If the\nAMV index is detrended using anthropogenic radiative forcing of climate,\nwe find that surface cooling attributed to Mt. Pinatubo, using the\nHadley Centre/University of East Anglia surface temperature record,\nmaximises at 0.14 �C globally and 0.32 �C over land. These values\nare about a factor of 2 less than found when the AMV index is neglected\nin the model and quite a bit lower than the canonical 0.5 �C cooling\nusually attributed to Pinatubo. This result is driven by the high\namplitude, low frequency component of the AMV index, demonstrating\nthat reduced impact of volcanic cooling upon consideration of the\nAMV index is driven by variations in North Atlantic SST that occur\nover time periods much longer than those commonly associated with\nmajor volcanic eruptions. The satellite record of atmospheric temperature\nfrom 1978 to present and other century-long surface temperature records\nare also consistent with the suggestion that volcanic cooling may\nhave been over estimated by about a factor of 2 due to prior neglect\nof ocean circulation. Our study suggests a recalibration may be needed\nfor the proper use of Mt. Pinatubo as a proxy for geoengineering\nof climate. Finally, we highlight possible shortcomings in simulations\nof volcanic cooling by general circulation models, which are also\nbeing used to assess the impact of geoengineering of climate via\nstratospheric sulfate injection

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