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India-Pakistan Nuclear War Would Have Dire Impact on Global Climate

map of loss biological production from nuclear simulation
Global biological productivity is projected to fall two years after a nuclear war between India and Pakistan. | Cheryl S. Harrison, Lili Xi and Nicole Lovenduski

A nuclear war between India and Pakistan — which share a long history of conflicts — would not only result in 50 to 125 million direct fatalities but could jeopardize the entire planet, causing sharp drops in global temperatures and precipitation that could devastate the world's food supply, according to a new study published October 2 in Science Advances.

The authors evaluated a simulated nuclear war scenario for the year 2025 between the two countries based on advice from policy and military experts. They find that if Pakistan attacks urban targets in 2025 with 150 nuclear weapons and if India responds with 100 nuclear weapons, depending on the size of the weapons, ranging from 15-kiloton to 100-kiloton, smoke from burning cities would release 16 to 36 teragrams of black carbon into the atmosphere, blocking out sunlight and cooling the global surface by 2 to 5°C (3.6 to 9°F).

Global average precipitation would drop by 15% to 30% and the rate at which plants store energy as biomass would decline by 15% to 30% on land and by 5% to 15% in oceans, threatening mass starvation.

Although Pakistan attacks first in this scenario, the researchers do not think Pakistan would be more likely than India to initiate conflict and they expect the results would be similar in both cases. Russia and the United States still possess by far the most nuclear warheads, at 6,850 and 6,550, respectively. Pakistan and India are two of seven additional countries with nuclear capabilities.

Neither Pakistan nor India is likely to initiate a nuclear conflict without significant provocation. India has declared that it will not use nuclear weapons first except in response to an attack with biological or chemical weapons, while Pakistan has declared that it would only use nuclear weapons if it were attacked first or if it could not suppress an invasion by conventional means.

However, since British India was divided into India and Pakistan in 1947, the countries have engaged in four wars, often over the disputed region of Kashmir. Two months ago, India stripped political autonomy from the part of Kashmir it controls, with the head of Pakistan-controlled Kashmir recently warning that the border dispute could escalate to nuclear conflict.

Owen B. Toon, a professor of atmospheric and oceanic sciences at the University of Colorado, and his colleagues note that India and Pakistan are locked in a nuclear arms race and may expand their nuclear weapon arsenals from between 140 and 150 warheads by current estimates to between 200 and 250 warheads by the year 2025, raising concerns that a future conventional war between them could turn nuclear. These mounting tensions motivated the researchers to study a worst-case scenario conflict between the two nations.

"It is disconcerting to think the actions of Indian and Pakistani generals could possibly start a war that could impact the entire world more than any previous world war," said Toon. "We hope India and Pakistan will use this information to stop building their nuclear arsenals and to start solving the Kashmir problem through negotiation."

Previous research by Toon and colleagues, among other papers, helped the United Nations pass the Treaty on the Prohibition of Nuclear Weapons in 2017, the first legally binding international agreement that would prohibit nuclear weapons if ratified by 50 nations. No states with nuclear weapons have signed, and the treaty must still be ratified by 18 more nations before it can go into effect.

In their new study, the researchers calculated the amount of smoke lofted into the upper troposphere (the lowest layer of Earth's atmosphere, extending 6 to 10 kilometers or 3.7 to 6.2 miles above the ground) by assuming that the area subject to fire ignition for a 15-kiloton nuclear explosion is the same as that observed in the 1945 atomic bombing of Hiroshima, Japan and using a recent population database to calculate how much fuel would burn in this area.

They acknowledge that the model contains several uncertainties, mainly regarding the amount of smoke created by burning cities — the primary cause of climate change after a nuclear conflict — and the quantity of black carbon lofted into the atmosphere. Black carbon traps more heat than any other component of particulate matter. While the researchers believe it would be best to treat smoke as a mixture of black and organic carbon, they omitted organic carbon because its interactions with the atmosphere and dispersal are too complex for the model to simulate at this time.

The researchers conducted climate and plant productivity simulations using a Community Earth System Model similar to one used to simulate the climate following the asteroid impact that contributed to the death of the dinosaurs. As with the projected aftermath of nuclear war, the asteroid would have blanketed the atmosphere in sun-blocking particles that cooled the planet. Although the model suggests drops in cooling and precipitation would happen all over the world, it anticipates the Northern Hemisphere would be affected to a greater degree than the Southern Hemisphere.

Alan Robock, a climatologist at Rutgers University and author on the study, hopes the research will provide a sobering warning about the consequences of nuclear conflict.

"I hope this will shock the world into realizing how increasing nuclear arsenals make the world more dangerous rather than less," said Robock. "We have been very lucky that in the 74 years since the first nuclear war that there has not been another. Negotiation is the only way to solve disagreements, and nuclear weapons are only weapons of mass genocide."

[Credit for associated image: Gerd Altmann from Pixabay]