Modelling Armageddon: The effects of nuclear weapons on climate

nuclear weapons on climate
© Siarhei Nosyreu

Owen B. Toon, Alan Robock & Richard P. Turco turn our thoughts towards modelling Armageddon in terms of the effects of nuclear weapons on climate

Nuclear weapons were built to deter international aggression. But there are currently more than 13,000 nuclear warheads worldwide, with about 92% in Russia and the United States – far more weapons than are needed for deterrence. Indeed, both the U.S. and Russia have sufficient warheads on hand – roughly 2,000 each allowed under existing treaties – to attack every city of 100,000 or more residents in both countries with an average of eight nuclear bursts, each with an explosive yield far exceeding that of the Hiroshima bomb.

At Hiroshima, the immediate blast and developing firestorm devastated more than twelve square kilometres (4.6 sq. miles) in the central city. A second bomb produced a similar result in Nagasaki. These small nuclear weapons, carried by single bombers, caused as much damage and death as the intensive incendiary bombing raids on Dresden, Tokyo and other cities, involving hundreds of aircraft deployed in multiple waves.

But what might happen today in an escalating nuclear conflict?

The outcome would almost certainly be described as Armageddon. In 1978, Congress last called on the Department of Defense to produce an unclassified analysis of the effects of nuclear war for a range of scenarios. In response, the Office of Technology Assessment1 estimated that an attack involving thousands of warheads aimed at military targets in the U.S. would kill up to 150 million Americans – two-thirds of the total population – while not even accounting for the added death and destruction caused by mass fires that would certainly be ignited.

And a few years later, it was recognised that smoke generated in nuclear conflagrations could threaten global humanity with a ‘nuclear winter’2. In this dark premonition, massive emissions of black smoke would block sunlight from reaching Earth’s surface, leading to frosts and subfreezing temperatures across continental landmasses for years, crippling any residual agricultural activity, and ending in mass starvation across the world.

In the 1980s, international civil protests against the nuclear arms buildup, which had approached 70,000 weapons, were common. Ultimately, facing the growing threat of global destruction and nuclear winter, Presidents Ronald Reagan and Mikhail Gorbachev in 1987 presciently took the first steps toward the reduction and possible elimination of nuclear weaponry by banning missiles under the INF Treaty having ranges between 500 and 5,500 km (310-3,410 miles). Every American and Russian President since that agreement has further reduced the inventory of nuclear warheads – to about 20% of the peak numbers. Moreover, the explosive yields of later generations of nuclear weapons have decreased significantly. Nevertheless, nuclear war remains the most significant existential threat to human civilization, even as it has receded from public consciousness.

The complex problem of nuclear war

The actual outcomes of nuclear war cannot be tested in any reasonable manner, and this has been one of the primary factors supporting the concept of deterrence – that is, uncertainty. However, the complex problem of nuclear war impacts can still be analysed based on statements on nuclear policies, knowledge of nuclear forces, and research into nuclear effects. Such studies can be carried out using modelling tools now available, which have been applied to investigate similar issues – such as global warming due to greenhouse gas emissions, worldwide cooling related to large volcanic eruptions, and the environmental impacts of intense wildfires and firestorms, among other natural and man-made events. Specifically, in the case of nuclear war, one needs to develop scenarios for how such a war might start and evolve, the weapons that might be deployed, and the possible targets and their characteristics.

Obviously, all of these factors could never be determined precisely beforehand. Even so, enough work has been done – exhaustively in many areas – to define the boundaries of the problem. For example, the damages inflicted by a nuclear blast and prompt radiation are relatively well understood from more than 500 above-ground weapons tests conducted from the 1940s through the 1960s. Also known are the incendiary effects of nuclear bursts, but these are less well known partly because testing was not as focused on this aspect of nuclear weapons. Understanding the climate changes that might result from smoke produced by nuclear fires requires knowledge of the types and amounts of fuel consumed, the quantities and forms of black carbon (the highly absorbing component of smoke) generated, and the fraction of that black carbon deposited in the upper atmosphere, where it could reside for years.

As each of these questions is slowly being resolved, the overall uncertainty in the outcome of nuclear war is steadily being reduced. New evidence, for example, from large forest fires in Canada (2017) and Australia (2020) reveals that smoke from intense fires is readily lofted into the stratosphere. There, without rain to remove it, smoke can be tracked by satellites over many months, and its distribution analysed using the same models employed to predict global changes following a nuclear war, including nuclear winter. The behaviour of the wildfire smoke mimics that predicted for massive smoke injections expected from nuclear-ignited firestorms, including alterations in wind patterns and air temperatures3. Moreover, newer simulations of historical volcanic eruptions also indicate similar effects – such as the “year without a summer” that followed the Tambora eruption of 1815 – and which was noted by Ronald Reagan as a reason to fear a nuclear winter.

Recently, we employed these same models to simulate the possible outcome of a regional-scale nuclear conflict between India and Pakistan, perhaps triggered by their continuing disputes over Kashmir. Remarkably, even such a limited use of nuclear weapons could lead to the coldest global temperatures recorded in the last 1,000 years4, resulting in a reduction in worldwide crop yields by tens of percent5.

The existence of nuclear weapons

It is generally agreed that the only legitimate reason for maintaining substantial arsenals of nuclear weapons is to avoid war. Nevertheless, the existence of such weapons and the robust means to use them, implies that global nuclear conflict cannot be dismissed as impossible or even improbable. And unlike global warming, which has been decades and centuries in the making, nuclear warfare could play out in mere hours or days. But the consequences would last years, shatter civilization, and create climatic anomalies comparable to a sudden ice age, leading to deprivation on a global scale6, possibly affecting billions of individuals. Leadership must do everything in its power to ensure that nuclear weapons can never be used, including actionable treaties on arms reductions, national policies to increase weapon security, and international diplomacy to stop and reverse nuclear proliferation.



1 Office of Technology Assessment. 1979. The Effects of Nuclear War, Office of Technology Assessment, Washington, DC., NTIS order #PB-296946.

2 Turco, R. P., Toon, O. B., Ackerman, T. P., Pollack, J. B., and Sagan, C. 1983. Nuclear winter: Global consequences of multiple nuclear explosions. Science, 222, 1283-1292. doi:10.1126/science.222.4630.1283.

3 Yu. P., et al. 2019. Black carbon lofts wildfire smoke high into the stratosphere to form a persistent plume. Science, 365 (6453), 587-590. doi:10.1126/science.aax174.

4 Toon, O.B., et al. 2019. Rapidly Expanding Nuclear Arsenals in Pakistan and India Portend Regional and Global Catastrophe. Science Advances, 5:eaay5478. doi:10.1126/sciadv.eaay5478.

5 Jägermeyr, Jonas et al. 2020. A regional nuclear conflict would compromise global food security. Proc. U.S. National Acad. Sci.117, 7071-7081.doi 10.1073/pnas.1919049117.

6 Coupe, J., Bardeen, C. G., Robock, A., & Toon, O. B. 2019. Nuclear winter responses to nuclear war between the United States and Russia in the Whole Atmosphere Community Climate Model  Version 4 and the Goddard Institute for Space Studies ModelE. J. Geophys. Research, 124. doi.10.1029/2019JD030509.


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