Nuclear winter from a Pakistan-India war could kill 2 billion » Yale Climate Connections

Military clashes between nuclear-armed India and Pakistan over the past few weeks have once again raised the sobering question: What would a “limited” nuclear war do to the global climate? The answer is not reassuring. Research over the past decade has found that such a conflict would be capable of causing a catastrophic global nuclear winter, and recent work predicts that over 2 billion people could be killed — with famines and diseases in the aftermath eventually killing hundreds of millions more.

The extraordinary dangers of global nuclear war

In the 1980s and early 1990s, a series of scientific papers published by Soviet and Western scientists (including prominent scientists Carl Sagan, host of the PBS “Cosmos” TV series, and Nobel Prize winner Paul Crutzen) laid out the dire consequences to the global climate of a major nuclear exchange between the U.S. and Soviet Union. The nuclear explosions would send massive clouds of dust high into the stratosphere, blocking so much sunlight that a nuclear winter would result, they said. Global temperatures would plunge 20-40 degrees Celsius for months and remain 2-6 degrees Celsius lower for one to three years. Up to 70% of the Earth’s protective stratospheric ozone layer would be destroyed, allowing huge doses of ultraviolet, or UV, light to reach the surface. This UV light would kill much of the marine life that forms the basis of the food chain, resulting in the collapse of fisheries and the starvation of the people and animals that depend on it. The UV light would also blind huge numbers of animals, who would then wander sightless and starve. The cold and dust would create widespread crop failures and global famine, killing billions of people who did not die in the nuclear explosions.

The nuclear winter papers were widely credited with helping lead to the nuclear arms reduction treaties of the 1990s, as it was clear that we risked catastrophic global climate change in the event of a full-scale nuclear war.

Even a ‘limited’ nuclear would kill billions

But even a limited nuclear war between India and Pakistan is a catastrophic threat to Earth’s climate. A landmark 2008 paper by Brian Toon of the University of Colorado, Alan Robock of Rutgers University, and Rich Turco of UCLA, “Environmental Consequences of Nuclear War,” concluded that a war between India and Pakistan using 50 Hiroshima-sized weapons with a 15-kiloton yield on each country, exploded on cities, would immediately kill or injure about 45 million people.

And a 2014 paper led by Michael Mills of the National Center for Atmospheric Research, “Multidecadal global cooling and unprecedented ozone loss following a regional nuclear conflict,” found that the final toll would be global — and astronomically higher.

Mills and his co-authors used an Earth system climate model including atmospheric chemistry, ocean dynamics, and interactive sea ice and land components to investigate a limited nuclear war where each side detonates 50 15-kiloton weapons — just 30% of the existing 340 or more warheads India and Pakistan are estimated to have. These urban explosions were assumed to start 100 firestorms. Firestorms are self-feeding fires that suck air into themselves and generate immense columns of rising smoke that lofts into the stratosphere, where it spreads globally. The model predicted the smoke would block enough of the sun’s energy to reduce the global average temperature by 1.25 degrees Celsius for three to four years and by more than 0.5 degree Celsius for a decade.

The effects would be similar to what happened after the greatest volcanic eruption in history, the 1815 Tambora eruption in Indonesia. The cooling from this eruption triggered the infamous Year Without a Summer in 1816 in the Northern Hemisphere, when killing frosts disrupted agriculture every month of the summer in New England, creating terrible hardship. Exceptionally cold and wet weather in Europe triggered widespread harvest failures, resulting in famine and economic collapse.

However, the cooling effect of that eruption only lasted about three years. Cooling from a limited nuclear exchange would cause five to 10 consecutive Years Without a Summer and more than a decade of significantly reduced crop yields. Killing frosts would reduce growing seasons by 10-40 days per year for five years at midlatitudes. Global precipitation would fall 6% during the first five years and be reduced by 4.5% 10 years later, resulting in a crippling increase in regional droughts. Over the Asian monsoon region, including the Middle East, the Indian subcontinent, and Southeast Asia, annual rainfall would fall by 20-80%, so that even the “winner” of the nuclear war between India and Pakistan would experience devastating famine from the failure of the life-giving monsoon rains.

Destruction of ozone would lead to another global calamity. As smoke in the stratosphere absorbed sunlight, the stratosphere would heat by 30 degrees Celsius (54°F). In the hot stratosphere, chemical reactions would destroy ozone, causing global ozone losses of 20-50% over populated areas. Ultraviolet light would increase by 30-80% over the midlatitudes, likely causing widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems.

The latest research

The most recent research has strengthened these conclusions. Catastrophic forest fires in Canada in 2017 and Australia in 2019 and 2020 lofted massive quantities of smoke into the stratosphere. These events have allowed researchers to test their models of what a nuclear war might do.

A 2022 paper led by Lili Xia of Rutgers University, “Global food insecurity and famine from reduced crop, marine fishery and livestock production due to climate disruption from nuclear war soot injection,” used state-of-the-art climate, crop, and fishery models to determine the impact of a nuclear war on human survival.

“In a nuclear war, bombs targeted on cities and industrial areas would start firestorms, injecting large amounts of soot into the upper atmosphere, which would spread globally and rapidly cool the planet,” the authors wrote. “Such soot loadings would cause decadal disruptions in Earth’s climate, which would impact food production systems on land and in the oceans.”

They estimated over 2 billion people would die from a “limited” nuclear war between India and Pakistan. The 100 nuclear weapons used in such a war are only about 0.8% of the world’s total nuclear arsenal of over 12,000 warheads, and the authors estimated that over 5 billion might die from a wide-scale global nuclear war between the U.S. and Russia.

In a 2023 article for the journal Public Health Policy, Andreas Vilhelmsson and Seth Baum implored experts and institutions in public health to study the potentially cataclysmal health impacts of nuclear winter more thoroughly: “Given the global scope of nuclear winter, there should be participation from public health experts and institutions from around the world.”

The bottom line: Preventing nuclear war is crucial to protecting the future of humanity.