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Old threats never die, they fade away from our minds: nuclear winter

In 1983, scientists published a paper on nuclear winter. This boosted the death toll of all-out nuclear war from ‘only’ 200-500 million to the very real possibility of the complete extinction of the human race*. But some argued the report was alarmist, and there did seem to be some issues with the assumptions. So – a military phenomena that might cause megadeaths, possibly true but requiring further study, and a huge research defense budget that could be used to look into this critical phenomena and that was already spending millions on all aspects of nuclear weapons – can you guess what happened next?

Correct – the issue was ignored for decades. For over twenty years, there were but a tiny handful of papers on the most likely way we could end our own existence, and a vague and persistent sense that nuclear winter had been ‘disproved’. But in 2007, we finally had a proper followup – with the help of modern computers, better models and better observations, what can we now say? Well, that nuclear winter is still a major threat; the initial fear was right. Their most likely scenario was:

A global average surface cooling of –7°C to –8°C persists for years, and after a decade the cooling is still –4°C […]. Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about –5°C, this would be a climate change unprecedented in speed and amplitude in the history of the human race. The temperature changes are largest over land […] Cooling of more than –20°C occurs over large areas of North America and of more than –30°C over much of Eurasia, including all agricultural regions.

Also, precipitation would be cut in half and we’d lose most of the ozone layer. But there was a more worrying development: it also seems that a small-scale nuclear war could generate its own mini nuclear winter.

It’s important to understand that nuclear winter would not be a direct consequences of the nuclear explosions, but of the burning of our cities in the wake of the war (given enough heat, even roads and pavements will burn), generating clouds of very black smoke that rise into the stratosphere. The clouds do need to reach these heights: any lower and they’ll get rained out. This is what happened during the burning of the Kuwaiti oil wells in 1991: Carl Sagan, one of the fathers of the theory, predicted a nuclear winter-like scenario. But he wasn’t paying attention to the climate models: as they predicted, the local damage was severe, but the smoke didn’t reach the stratosphere, and global damage was avoided.

But for small scale nuclear exchange between Pakistan and India (the example used in the paper) the same models predict the smoke would reach the stratosphere. Their model used 100 Hiroshima-size bombs (less than 0.03% of the explosive yield of the current global nuclear arsenal), detonated on cities in close proximity. Because of the closeness, and the effect of the sun on the black smoke particles, enough would rise up to cause a mini nuclear winter lasting about decade. England, being surrounded by ocean, would be spared the worse effects, but we would still see solar luminosity fall by about a sixth, temperatures would fall by about 2 degrees (on a global scale, this is half way to an ice-age), and the growing season would be cut short by about a month – for inflexible crops, this would simply mean they wouldn’t grow. The effect would be worldwide, with reduced precipitation, ozone depletion and global famine (with an attendant collapse in global trade networks, possible new wars, the radiation, and, of course, all the devastations in India and Pakistan). Death tolls are hard to estimate, but they would certainly be large – and it would be a miserable decade for the survivors.

An initial report published in 1983, confirmed in 2007, about nuclear war while we are now facing a more and more peaceful era – this blog post doesn’t feel exactly topical, does it? Preventing nuclear war is no longer sexy, modern or cutting edge. But the threat is there, as it was yesterday and will be tomorrow – not talking about it doesn’t make it go away. Humans remain prideful, nationalistic, prone to misunderstanding each other. The probability of even a small nuclear conflict might be low, but the consequences would be so devastating that reducing this risk is probably one of the most efficient use of your resources if you want to help the world. The efficiency is probably even higher than the most efficient charities, but the numbers are hard to estimate – some people have great possibility of influencing these risks, others much less. And it’s not like there is an effective ‘give money to reduce risk of nuclear war’ charity waiting for donations (this fact in and of itself is very dispiriting). At least keep the issue in mind, share the word, and be ready to put your pressure and your vote in the right direction if the issue ever comes up.

*Edit: the extinction risks doesn’t come directly from the nuclear winter (some human groups will survive), but from the collapse of human society and fragmentation of the species into small, vulnerable subgroups, with no guarantee that they’d survive setbacks or ever climb back to a technological society.

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9 Comment on this post

  1. One interesting issue is whether treating nuclear war as causing tremendous damage rather than considering it to be the end of the world is helpful to spur action. Decision-makers are not terribly moved by existential risks, but losing elections do move them. If there is a chance that they will 1) survive, 2) have to face an angry electorate they might be more motivated to take mitigative action. The interesting aspect of the simulations is that they suggest that it might be in the interest of other countries to keep conflicts from going nuclear. That might be a wedge able to propel governments into reducing risks, especially if voters recognize their role in maintaining global security.

    1. Disagree about decision makers not being motivated by existential risks – it’s pretty clear the rapid, careful action that ended the Cuban missile crisis was motivated by this risk (even if the actual decisions shook down in a strange way… see Graham Allison’s Essence of Decision…). But it depends on the existential risk. And the perception of that risk. I can see why nation states might act as a global agent in the face of certain existential risks – meteor strike, invaders from Mars, pandemic – but why they would not in other cases, especially where the payoffs are not uniform for all players. [This is the case for climate change, even though the bulk of the evidence is that it is not an existential risk, merely a very serious one.] Political will does not scale with the magnitude of a problem, unless that scaling relationship holds for all (major) players. I think you’re much better off treating political will as an emergent property arising from the strategic dimensions of the problem, rather than an aggregate feature of the problem.

  2. Gorbachev claimed that nuclear winter models were part of his motivation for pursuing nuclear disarmament in the 1980s (e.g., (At the same time, Gorbachev increased funding for the Soviet biological weapons program, perhaps intending to substitute one form of deterrence for another.) Some charities that focus on preventing nuclear war: the Nuclear Threat Initiative (, the Bulletin of the Atomic Scientists (, Center for Arms Control and Non-Proliferation (, Wisconsin Project on Nuclear Arms Control (, academic think tanks such as the Managing the Atom ( at Harvard’s KSG, and several others listed here: Though I suspect that an organization like FHI, that takes a broad look at catastrophic risks, is more efficient.

  3. “the very real possibility of the complete extinction of the human race. ”

    I agree that nuclear risk has been neglected since the Cold War (although there have been significant efforts, e.g. by Warren Buffett, the Nuclear Threat Initiative, recent treaties, etc), and have been taking actions to get more work done on the topic. However, it should be made clear that the authors don’t claim that nuclear winter is likely to cause human extinction. They say that while casualties would be large they would be quite unlikely to wipe out humanity. Hunter-gatherer human ancestors survived massive volcano eruptions like Toba (and ice ages, as mentioned above), and we have some advantages today (technology, larger populations, shelters with large food stocks, potent fishing methods). It’s important to avoid overstatement as well as understatement.

    1. Yes, I should have been clearer that “the very real possibility of the complete extinction of the human race” referred to the risks of extinction caused by the collapse of human society and the fragmentation of the species into small, vulnerable, disconnected groups.

  4. Hi all – good discussion all around. In response to Carl’s point that humans have survived e.g. Toba: This reduces the probability that nuclear winter would cause human extinction, but it doesn’t render it impossible. Just because humans survived previous episodes doesn’t mean we’d necessarily survive this one. Maybe we survive, say, 90% or 99% (to make up numbers) of these events. Then there’s still a 10% or 1% (etc) chance of us going extinct. Also note that this time around, we’d be rebuilding without abundant supplies of certain natural resources such as fossil fuels. Potentially this could hinder our ability to rebuild advanced technological civilization, even if we do survive in a more limited form. On the other hand we’ll also have some useful artifacts and memories of the current civilization, which would be useful. This suggests investments we could make now to enhance the recovery, such as the Svalbard seed bank, of course in addition to whatever we can do to avoid the catastrophe in the first place.

  5. Also, regarding political viability, note that his activity on nuclear weapons was a main factor in Obama’s 2009 Nobel Peace Prize, followed by the 2010 “New START” treaty that the US ratified 71 to 26 in the Senate (350 to 96 in Russia).

  6. As real as this threat is…

    Why does this even have to happen? Is it really necessary? Is there any other reasonable reason other then greed and human nature? Why do we have to lead to our own destruction?

    This threat is indeed very real. I almost very positive it’s really going to happen one day if the countries wont work together to dispose of it entirely. This can happen in any given time. We simply need only one country to start this and the world world will collapse. I’m not even sure it that 1% people who survive are going to survive 10 years without the other 99%. All I see this will only lead to nothing but destruction. Even the one who started it most likely wouldn’t live to conquer anything.

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