Houghton Lecture Series – B.D. Santer (UCLA)

Volcanic Effects on Climate: From El Chichón to “Moderate” Early 21st Century Eruptions

This lecture was motivated by a close encounter with Mount St. Helens in April 1980. Since then, I have had a long-standing fascination with volcanic effects on climate.

As in the case of anthropogenic signal identification, identifying volcanic effects on climate is a signal-to-noise (S/N) problem. This S/N problem is complicated by the fact that the surface and tropospheric
cooling signals of the two largest recent volcanic eruptions (El Chichón in 1982 and Pinatubo in 1991) were partly obscured by short-term warming induced by two separate El Niño events. Standard multiple linear regression approaches can be affected by this fortuitous “ENSO masking” of volcanic cooling. An iterative regression method partly resolves the problem of collinearity between volcanic signals and ENSO variability, yielding more reliable estimates of the true cooling signals induced by El Chichón and Pinatubo. Such noise removal issues are important to consider when analysts compare smoothed, multi-model average volcanic cooling signals from the Coupled Model Intercomparison Project (CMIP) with volcanic signals estimated from the single noisy realization of observations.

Following the pioneering 2011 Science paper by Solomon et al., considerable scientific attention was focused on the climatic impact of a succession of “moderate” post-2000 volcanic eruptions. When ENSO variability is removed from observations, signals of these “moderate” eruptions are statistically identifiable in a wide range of different climate variables. The eruptions increased the stratospheric aerosol optical depth (SAOD), leading to negative volcanic radiative forcing over the early 21st century. Models participating in phase 5 of CMIP neglected this negative volcanic forcing, thus introducing a systematic warming bias relative to observations. One important lesson learned from this work is that such forcing errors can have non-negligible impact on inferences regarding consistency between modeled and observed temperature trends. This is evident from numerical experiments in which the same climate model is run with both CMIP5 and CMIP6 volcanic forcing.

Several recent modeling studies rely on simulations with prescribed volcanic emissions of SO2 rather than on simulations with prescribed stratospheric distributions of volcanic aerosols. Such studies allow analysts to consider whether information on total column volcanic aerosol burdens yields more confident volcanic signal detection than use of stratospheric aerosol burdens alone. Use of SO2-driven simulations may be particularly important for detecting the climate signals of high-latitude eruptions and of effusive volcanic eruptions with aerosol burdens that are primarily confined to the troposphere.

About the Series
Supported through the Houghton Fund, Houghton Lecturers are distinguished visitors from outside MIT invited to spend a period of time, ranging from a week to several months, as scientists-in-residence within the EAPS Program on Atmospheres, Oceans and Climate (PAOC). During their stay it is customary for each lecturer to offer a short-course or a series of lectures on some topic of wide interest.

For questions and Zoom meeting details,  contact Daisy Caban: dmcaban@mit.edu