Updated: Feb. 25, 2011; 11:40 p.m. EST

WASHINGTON – Even a small nuclear exchange could ignite mega-firestorms and wreck the planet’s atmosphere.

New climatological simulations show 100 Hiroshima-sized nuclear bombs – relatively small warheads, compared to the arsenals military superpowers stow today – detonated by neighboring countries would destroy more than a quarter of the Earth’s ozone layer in about two years.

Regions closer to the poles would see even more precipitous drops in the protective gas, which absorbs harmful ultraviolet radiation from the sun. New York and Sydney, for example, would see declines rivaling the perpetual hole in the ozone layer above Antarctica. And it may take more than six years for the ozone layer to reach half of its former levels.

Researchers described the results during a panel Feb. 18 at the annual meeting of the American Association for the Advancement of Science, calling it “a real bummer” that such a localized nuclear war could bring the modern world to its knees.

“This is tremendously dangerous,” said environmental scientist Alan Robock of Rutgers University, one of the climate scientists presenting at the meeting. “The climate change would be unprecedented in human history, and you can imagine the world ... would just shut down.”

To defuse the complexity involved in a nuclear climate catastrophe, Wired.com sat down with Michael Mills, an atmospheric chemist at the National Center for Atmospheric Research, who led some of the latest simulation efforts.

'It’s pretty clear this would lead to a global nuclear famine.'__Wired.com: __In your simulation, a war between India and Pakistan breaks out. Each country launches 50 nukes at their opponent's cities. What happens after the first bomb goes off?

__Michael Mills: __The initial explosions ignite fires in the cities, and those fires would build up for hours. What you eventually get is a firestorm, something on the level we saw in World War II in cities like Dresden, in Tokyo, Hiroshima and so on.

Today we have larger cities than we did then – mega cities. And using 100 weapons on these different mega cities, like those in India and Pakistan, would cause these firestorms to build on themselves. They would create their own weather and start sucking air through bottom. People and objects would be sucked into buildings from the winds, basically burning everything in the city. It’ll burn concrete, the temperatures get so hot. It converts mega cities into black carbon smoke.

Wired.com: I see – the firestorms push up the air, and ash, into the atmosphere?

Mills: Yeah. You sometimes see these firestorms in large forest fires in Canada, in Siberia. In those cases, you see a lot of this black carbon getting into the stratosphere, but not on the level we’re talking about in a nuclear exchange.

The primary cause of ozone loss is the heating of the stratosphere by that smoke. Temperatures initially increase by more than 100 degrees Celsius, and remain more than 30 degrees higher than normal for more than 3 years. The higher temperatures increase the rates of two reaction cycles that deplete ozone.

Wired.com: And the ozone layer is in the stratosphere, correct?

Mills: OK, so we live in the troposphere, which is about 8 kilometers [5 miles] thick at the poles, and 16 km [10 miles] at the equator.

At the top of the troposphere, you start to encounter the stratosphere. It’s defined by the presence of the ozone layer, with the densest ozone at the lowest part, then it tails off at the stratopause, where the stratosphere ends about 50 km [30 miles] up.

We have a lot of weather in the troposphere. That’s because energy is being absorbed at the Earth’s surface, so it’s warmest at the surface. As you go up in the atmosphere it gets colder. Well, that all turns around as you get to the ozone layer. It starts getting hotter because ozone is absorbing ultraviolet radiation, until you run out of ozone and it starts getting colder again. Then you’re at the mesosphere.

How Nukes Gobble Up Ozone

When we talk about ozone, we’re talking about the odd oxygen family, which includes both ozone (O 3 ) and atomic oxygen (O). Those two gases can interchange rapidly within hours.

Ozone is produced naturally by the breakdown of molecules of oxygen, O 2 , which makes up 20 percent of the atmosphere. O 2 breaks down from ultraviolet solar radiation and splits it into two molecules of O. Then the O, very quickly, runs into another O 2 and forms O 3 . And the way O 3 forms O again is by absorbing more UV light, so it’s actually more protective than O 2 .

Ozone is always being created and destroyed by many reactions. Some of those are catalytic cycles that destroy ozone, and in those you have something like NO 2 plus O to produce NO plus O 2 . In that case, you’ve gotten rid of a member of the odd oxygen family and converted it to O 2 . Well, then you’ve got an NO which can react with ozone and produce the NO 2 back again and another O 2 . So the NO and NO 2 can go back and forth and in the process one molecule can deplete thousands of molecules of ozone.

It’s a similar process to chlorofluorocarbons, Those are the larger molecules that we’ve manufactured that don’t exist naturally. They break down into chlorine in the stratosphere, which has a powerful ozone-depleting ability. *—Michael Mills

*

__Wired.com: __Where do the nukes come in? I mean, in eroding the ozone layer?

__Mills: __It’s not the explosions that do it, but the firestorms. Those push up gases that lead to oxides of nitrogen, which act like chlorofluorocarbons. But let’s back up a little.

There are two important elements that destroy ozone, or O 3 , which is made of three atoms of oxygen. One element involves oxides of nitrogen, including nitrogen dioxide, or NO 2 , which can be made from nitrous oxide, or N 2 O – laughing gas.

The other element is a self-destructive process that happens when ozone reacts with atomic oxygen, called O. When they react together, they form O 2 , which is the most common form of oxygen on the planet. This self-reaction is natural, but takes off the fastest in the first year after the nuclear war.

In years two, three and four, the NO 2 builds up. It peaks in year two because the N 2 O, the stuff that’s abundant in the troposphere, rose so rapidly with the smoke that it’s pushed up into the stratosphere. There, it breaks down into the oxides like NO 2 , which deplete ozone.

__Wired.com: __So firestorms suck up the N 2 O, push it up into the stratosphere, and degrade the ozone layer. But where does this stuff come from?

__Mills: __N 2 O is among a wide class of what we call tracers that are emitted at the ground. It’s produced by bacterias in soil, and it’s been increasing due to human activities like nitrogen fertilizers used in farming. N 2 O is actually now the most significant human impact on the ozone, now that we’ve mostly taken care of CFCs.

__Wired.com: __You did similar computer simulations in the past few years and saw this ozone-depleting effect. What do the new simulations tell us?

__Mills: __Before, we couldn’t look at the ozone depletion’s effects on surface temperatures; we lacked a full ocean model that would respond realistically. The latest runs are ones I’ve done in the Community Earth System Model. It has an atmospheric model, a full-ocean model, full-land and sea-ice models, and even a glacier model.

We see significantly greater cooling than other studies, perhaps because of ozone loss . Instead of a globally averaged 1.3-degree–Celsius drop, which Robock’s atmospheric model produced, it’s more like 2 degrees. But we both see a 7 percent decrease in global average precipitation in both models. And in our model we see a much greater global average loss of ozone for many years, with even larger losses everywhere outside of the tropics.

I also gave this to my colleague Julia Lee-Taylor at NCAR. She calculated the UV indexes across the planet, and a lot of major cities and farming areas would be exposed to a UV index similar to the Himalayas, or the hole over the Antarctic. We’re starting to look at the response of sea ice and land ice in the model, and it seems to be heavily increasing in just a few years after the hypothetical war.

Wired.com: What would all of this do to the planet, to civilization?

Mills: UV has big impacts on whole ecosystems. Plant height reduction, decreased shoot mass, reduction in foliage area. It can affect genetic stability of plants, increase susceptibility to attacks by insects and pathogens, and so on. It changes the whole competitive balance of plants and nutrients, and it can affect processes from which plants get their nitrogen.

Then there’s marine life, which depends heavily on phytoplankton. Phytoplankton are essential; they live in top layer of the ocean and they’re the plants of the ocean. They can go a little lower in the ocean if there’s UV, but then they can’t get as much sunlight and produce as much energy. As soon as you cut off plants in the ocean, the animals would die pretty quickly. You also get damage to larval development and reproduction in fish, shrimp, crabs and other animals. Amphibians are also very susceptible to UV.

A 16 percent ozone depletion could result in a 5 percent loss in phytoplankton, which could result in a 7 percent loss in fisheries and aquaculture. And in our model we see a much greater global average loss of ozone for many years; the global average hides a lot.

Wired.com: This doesn’t sound very good at all.

__Mills: __No, as we said it’s a real bummer. It’s pretty clear this would lead to a global nuclear famine.

You have the inability to grow crops due to severe, colder temperatures and also the severe increases in UV light. You have the loss of plants and proteins in the oceans, and that leads to widespread food shortages and famine (PDF).

The winner, Frank Zauft. Photo: Volkswagen

Wired.com: There have been thousands of nuclear tests. Why hasn’t this already happened?

Mills: We’re not talking about direct impacts of the explosions themselves, but the firestorms that result when you detonate these in cities. Most tests were in deserts or atolls or space or underground.

Wired.com: When you talk nuclear reductions, you’re wading into political territory. As a scientist, how do you handle that?

Mills: The response to this from the policy community has been rather underwhelming. We know, from what both Gorbachev and Reagan have said in anecdotes, that these kinds of studies had a big impact on thinking at the time. People started realizing nuclear war was not something you can win. You’d just destroy the whole planet.

That led to some of the dramatic reductions we saw in the original START treaty, but we still have the ability to basically destroy the planet with one-tenth of 1 percent of the world’s current arsenals.

By the way, there’s nobody really funding these kinds of studies. All of us here are doing these on our own time. You can’t get grants to do this kind of research. It’s puzzling to me.

Wired.com: What would you like to see happen?

Mills: We’d all like to see much more dramatic reductions in the number of nuclear weapons we’re seeing proposed in the new START treaty, and the SORT treaty under the Bush administration. These just seem like refinements, in which the number of weapons is reduced, but each airplane counts as one weapon that can carry multiple bombs. So we might not be seeing any reductions.

Wired.com: Should nations have any nukes?

Mills: How many times do you need to explode a nuclear weapon in your enemy’s capital to deter them? I think just once. But given the consequences, I don’t think it’s reasonable to have any.

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