School of Physical and Chemical Sciences Te Kura Matū Seminar Series

Impacts of volcanic eruptions, geoengineering, and nuclear war on climate and the ozone layer


Michael J. Mills, Erskine Visitor with the Atmospheric Group from


National Center for Atmospheric Research, Boulder, CO

Time & Place

Wed, 09 Oct 2019 12:00:24 NZDT in West 701, Level 7

All are welcome


In order to identify and fully understand the impacts of industrial emissions on climate and ozone loss, we must understand sources of natural variability in modulating long-term changes. Explosive volcanic eruptions can perturb levels of stratospheric aerosol, which can both cool the Earth’s surface and enhance stratospheric ozone loss on time scales of several years. A new generation of space-born and ground-based observations has highlighted the previously overlooked role of small-to-moderate volcanic eruptions in enhancing stratospheric aerosol. New capabilities of global chemistry-climate models to calculate stratospheric aerosol properties from volcanic emissions is providing new insights into the effects of such eruptions on reducing the rate of global warming as well as delaying the recovery of stratospheric ozone. Calculations using a volcanic sulfur dioxide (SO2) emissions inventory in NCAR’s Whole Atmosphere Community Climate Model (WACCM) with detailed sulfur chemistry and a prognostic stratospheric aerosol scheme show that small-to-moderate eruptions have offset 30% of the warming from CO2 emissions since 2000. During the same era, model calculations show that eruptions have periodically expanded loss in the seasonal Antarctic ozone hole. Only by identifying and removing the impacts of such eruptions can we now conclude that stratospheric ozone recovery from anthropogenic halogens is currently emerging.

The cooling effect of volcanic eruptions has inspired proposals to offset global warming via artificial enhancement of stratospheric aerosol. WACCM has been used to evaluate such geoengineering schemes, incorporating intelligent feedback algorithms to optimize multiple climate targets. The results show potential for reducing global warming, but raise risks to the ozone layer, to rapid warming if geoengineering were terminated, and to warfare over control of the Earth’s climate.

Nuclear war represents the greatest threat to humanity. The hundreds of thousands or millions of immediate casualties in the theaters of war would only be the beginning. For hours after bombs explode in modern cities, firestorms would build for hours, turning cities into fuel for their own destruction. Millions of tonnes of smoke would then rise into the stratosphere and circle the globe, blocking sunlight and wiping out the protective ozone layer. Calculations using WACCM to model the effects of such smoke produced the surprising results that a nuclear war between new nuclear states, such as India and Pakistan, using much less than 1% of the current global nuclear arsenal, could produce climate change unprecedented in recorded human history, global-scale ozone depletion, and widespread famine.


Dr. Michael Mills of the National Center for Atmospheric Research (NCAR) is an Erskine visitor teaching PHYS319/PHYS419 Atmosphere, Ocean and Climate Dynamics for the remainder of the term. 

Mike previously visited Christchurch as a graduate student in 1992, in transit to and from McMurdo Station in Antarctica, where he took spectroscopic observations of stratospheric chlorine molecules in the ozone hole. 

His research focuses on stratospheric aerosol, including impacts of volcanic eruptions, nuclear winter, and potential climate intervention (geoengineering).

He is NCAR’s community liaison for the Whole Atmosphere Community Climate Model (WACCM), which is the high-top atmosphere component of the Community Earth System Model (CESM). His work in that role involves support to researchers using WACCM outside of NCAR, development of new model capabilities, and running coupled climate simulations for CMIP6 and projections of future climate change.