13 October 2008

Nuclear autumn

Cosmos Online
The Cold War may be over, but even a relatively small nuclear exchange by rogue nations or terrorist groups could wreak havoc on global climate, threatening civilisation as we know it.

Credit: U.S. Department of Energy

In 1983, a team of scientists spearheaded by the late Carl Sagan became concerned about the long-term effects of a nuclear exchange between the U.S. and Soviet Union. They concluded that, devastating as they were, the bombs wouldn’t be what destroyed civilisation as we knew it.

That dubious honour, they calculated, belonged to the years-long ‘nuclear winter’ that would result from soot and smoke darkening the sun and plunging temperatures to below freezing across much of the globe.

With the end of the Cold War, the risk of all-out nuclear war receded, and so, it seemed, did the risk of nuclear winter. But the relief may have been premature, scientists say.

In a series of papers presented in San Francisco, at a December 2006 meeting of the American Geophysical Union (and subsequently published in Science), a group that included two members of the original nuclear-winter team (Richard Turco and Owen Toon) re-examined the effects of nuclear bomb blasts.

They found that even a small nuclear exchange between rising nuclear powers such as India and Pakistan would radically affect the Earth’s climate. The result wouldn’t be a full-fledged nuclear winter, but their simulations nevertheless showed that such a war would wreck the Earth’s ozone layer and initiate a global “nuclear autumn” that would cause years of crop failures, famine, and panic.

Booming cities

The threat stems from the fact that if you’re a small nuclear power with a serious grudge against a neighbour, you’re likely to use your arsenal primarily against cities. And it turns out that for this purpose, the monstrous megaton-scale bombs the U.S. and USSR once aimed at each other are “simply overkill,” says Turco, an atmospheric scientist at the University of California, Los Angeles.

The risk is compounded by the fact that everywhere, people are flocking to cities, many in politically unstable regions. “We’re growing megacities all over the world,” says Owen Toon, an atmospheric scientist at the University of Colorado at Boulder. “Tehran has 10 million people.”

These cities are so densely packed that the scientists estimate 20 million people could be killed outright in a war between India and Pakistan – even if each side used as few as 50 Hiroshima-sized bombs. That’s nearly as many people as died in World War II, and comparable to the number of casualties once predicted from some scenarios for an all-out war between the U.S. and the USSR.

Fueling the fire

After the bombs would come the fires. And when it comes to lighting giant fires, smaller bombs again turn out to be nearly as effective as their megaton cousins.

Based on what happened at Hiroshima, Turco says, one similar-sized airburst (about 15-kilotons) would ignite everything in a 13 km2 region. And that’s not even taking into account the fact that the fire would probably spread, since putting it out would be well-nigh impossible.

In a crowded city, a 13-km2 region has a lot to burn. Turco calculates that the average industrialised city has about 10 tonnes of combustible material per resident. In the developing world, it’s probably only half as much – but given these cities’ population densities, that’s a colossal amount of potential fuel.

To give even an inkling of the type of fire that would be ignited, Turco turns to the blaze that roared through San Francisco in the aftermath of the 1906 earthquake. When the earthquake occurred, author and journalist Jack London rushed to the city and watched in horror from a boat on San Francisco Bay.

Even though the day was dead calm, the rising flames drew air from all sides. “East, west, north, and south, strong winds were blowing upon the doomed city,” he wrote. “The heated air rising made an enormous suck. Thus did the fire of itself build its own colossal chimney through the atmosphere. Day and night this dead calm continued, and yet, near to the flames, the wind was often half a gale, so mighty was the suck.”

In other words, once started, the fires would fan themselves, gaining intensity and blasting a roaring column of smoke far into the sky. More than wood would burn. Gasoline tanks, asphalt shingles, automobile tires, and plastics would all contribute – important because all are extremely smoky.

All told, Turco calculates, an exchange in which both sides used 50 Hiroshima-sized bombs could easily produce five million tonnes of smoke.

A pall of smoke

To put that into perspective, Georgiy Stenchikov of Rutgers University, New Jersey, compared it to a giant forest fire that raged across 100,000 hectares of central Alberta in 2001.

But even that fire produced only 0.1 per cent as much as a 100-bomb nuclear war. To match the smoke from 100 Hiroshima-sized bombs, the fire would have needed to consume an area larger than California, all in a matter of hours.

Normally, smoke rises only a few thousand metres into the atmosphere, where it’s fairly easily removed by rain. Even the fires from Kuwaiti oil wells during the first Gulf War weren’t intense enough to penetrate the stratosphere.

But nuclear-bomb fires would be far more intense, and the resulting dense, black smoke would rise through the atmosphere in a two-step process.

First, the firestorms would produce vast amounts of heat – more than 1,000 times as much as the bomb blasts themselves – that would carry the smoke high into the air. Once there, it would spread in a dark blanket, not only blocking sunlight, but absorbing it.

This absorption of sunlight would heat the surrounding air, causing the soot to waft ever higher until it reached the stratosphere. The effect would be worst in southern countries, like India or Pakistan, where the sun is more intense.

Once in the stratosphere, the smoke would continue to rise. It would also be caught by high-altitude winds and whisked around the world. Within a month, a thin haze would girdle the planet. And the haze, Stenchikov added, would persist for years because it’s above the rainfall that would otherwise cleanse it out.

Lasting effects

When Mount Pinatubo erupted in The Philippines in 1991, it blasted enough sulfuric acid into the stratosphere to create a bright haze that not only caused spectacular sunsets, but also produced a noticeable dip in global temperatures.

Due to the nature of volcanic gases, the Mount Pinatubo effect lasted only a year. But the effect of the soot would be longer lasting and several times larger, says Luke Oman, another Rutgers scientist.

Overall, he and Alan Robock, also of Rutgers University, calculate that the average global temperature would dip by more than 1.1 °C and would stay below normal for a decade. Rainfall would also decline, by about 10 per cent.

That might not sound like a large temperature change, but it’s more than the cumulative effect of global warming since the dawn of industrial civilisation. It’s enough, Robock says, to have a radical effect on growing seasons in regions like Europe and the American Midwest.

Even countries that didn’t participate in the war could see their growing seasons reduced by 10 to 30 days – a major factor because farmers wouldn’t have planted the right crops for the new conditions.

Even in the tropics, the change would strain agricultural productivity. “This would not be nuclear winter,” Robock said, “but it would be a substantial disruption.”

“Global ozone hole”

There would also be increased exposures to damaging ultraviolet light, since soot absorbs heat, substantially increasing the temperature of the stratosphere. That alters conditions in the upper atmosphere enough to create what Toon calls a “global ozone hole,” from pole to equator.

Nobody knows whether an even smaller nuclear exchange would have similar effects. It would depend a lot on where the bombs are dropped, Turco says. And besides, the scientists don’t want to find themselves saying something on the order of: “15 bombs are OK, but 16 aren’t.”

What they will say is that many countries have the ability to make bombs. About 40 nations have materials on hand in their nuclear power plants to make from one to 10,000 bombs, Toon estimates, if they’re ever inclined to do so. “Fifty weapons is not a challenge.”

Turco sees diplomacy as the only solution. “We’ve lost the nonproliferation regime which was more or less holding,” he said, adding that we’re facing a future in which there could be many nuclear powers, “all as dangerous as the Soviet Union used to be.”

Richard A. Lovett is a writer based in the U.S. city of Portland, Oregon. He is a regular contributor to Cosmos.

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