The Science of Nuclear Winter: What We Know and What We Don't Know about the Consequences of a Nuclear War | Drell Lecture 2026 with Brian Toon

About the Event

Estimates of fatalities in a nuclear war range from none to Earth’s population. We can confidently state if there is never a nuclear explosion, there will be no fatalities. But what is the most probable loss? Fatalities from explosions in populated areas could be in the hundreds of millions, exceeding those from fallout radiation by a factor of ten. However, fatalities from environmental changes and the breakdown of society, which together may collapse global agriculture, could exceed those from blast by another factor of ten, approaching the population of the planet. Fatalities from blast, burns and prompt radiation are relatively easy to compute, but those from environmental changes are difficult to predict. We know that physically similar, but more extreme, environmental insults occurred 66 million years ago due to an asteroid impact and caused the extinction of about 75% of the known species. Toon will go through the chain of complex issues that need to be addressed to compute the environmental changes, their uncertainties and their impacts on humans: likely targets; fuel loads; fire ignition; smoke production, composition, lofting and rainout; climate response to stratospheric smoke; and agricultural responses to climate changes. A recent U.S. National Academy of Science panel recommended that multiple agencies fund research to reduce the uncertainties. For example, new satellite based global measurements of building types and volumes can provide improved fuel loads. Measurements of smoke from burning buildings, now non-existent, could determine smoke composition. New aircraft data on smoke from wildfires could inform lofting. However, no federal agency has developed a research program, just as they have avoided doing for the past 40 years. Thousands of scientific papers have been written about the extinction of the dinosaurs, but tens about our possible fate from a nuclear conflict. NASA is searching for asteroids that might hit us and planning how to stop them. But nuclear stockpiles are increasing, more countries seek nuclear weapons, and new and more terrifying weapons are being built.

About the Speaker

Brian Toon studies radiative transfer, aerosol and cloud physics, and parallels between Earth and planets. He led NASA airborne missions studying volcanic clouds, ozone holes, and cloud-climate impacts. Toon published about 400 refereed scientific papers, and co-authored Earth in Flames. He received the American Physical Society's Leo Szilard Award for work on nuclear winter and was recognized by the UN Environmental Program for contributions to the Nobel Peace Prize winning IPCC reports. Additionally, he won the American Geophysical Union’s Roger Revelle Medal, the American Meteorological Society’s Carl-Gustaf Rossby Medal, and the 2022 Future of Life Award for reducing the risk of nuclear war.