It seems that worrying about nuclear war is back in fashion. It's not just Britain's quietly growing debate on Trident. It's also the increased sabre rattling from Russia and China, the ongoing risk of conflict in the Middle East and, of course, North Korea.
At the Project for Study of the 21st Century, we thought it might be a good idea to quantify some of these worries. So we polled 50 of the best national security experts we could find from around the world on what they thought the risks were.
On average, their answers pointed to a 6.8 percent chance of a catastrophic nuclear war in the next 20 years killing more people than the Second World War (80 million, at upper estimates). The chance of a variety of small wars between major nations, both nuclear and conventional, however, were rather higher.
The results were wide-ranging enough to tell us that even the experts have some very different ideas about how likely things are to go wrong. As Professor Sir Lawrence Freedman, emeritus professor at King’s College London, told the launch of our report this week, all anyone can really say for sure is that the risk is higher than zero – and that it seems to be rising.
From the survey, though, it is possible to roughly extrapolate which cities should worry most. Here are our conclusions.
1) Major Indian and Pakistani cities
India versus Pakistan, the poll suggests, is seen by far the most likely potential great power conflict with a 40 percent chance of war and a 9 percent chance of nuclear exchange.
Given the two nations' history of rivalry, that's not entirely surprising. Military blog War on the Rocks last week labelled the danger a "pink flamingo", an obvious risk that was simply being ignored – essentially, the opposite of a truly unforeseeable or unpredictable "black swan" danger.
The good news, for what it's worth, is that India and Pakistan have shown reasonable restraint in the past, at least in the sense that they have fought several limited conventional wars along their borders without resorting to nuclear action.
Should they ever do so, though, the results would be catastrophic. The population of New Delhi alone is close to 25m. And both India and Pakistan teem with megacities that sometimes seemingly struggle to function even on a good day.
According to the highly respected Federation of American Scientists, India and Pakistan probably have 110-130 warheads each. Should war ever come to South Asia, it's pretty clear it could rapidly become the worst thing to ever happen in the world ever.
2) Cities in the Middle East
So far this century, cities in the Middle East have had a pretty rough deal. Baghdad was torn apart by sectarian violence after the US invasion; Aleppo, Damascus and the other cities of Syria have fared even worse.
Despite this year's nuclear deal between Iran and the world's major powers, our experts still saw a 27 percent risk of some kind of conflict between Tehran and its enemies: either the US, Israel, the Gulf states, or all of the above. (For those of you wondering, we defined "war" as several days of ongoing conflict between state military forces including at least 100 deaths.)
The risk of a nuclear exchange, they estimated, was roughly 6 percent over the next 20 years.
For now, of course, Israel is the only suspected nuclear weapons state in the region, with an estimated 80 or so warheads (although it has never officially confirmed it). Should Iran go back on its pledge not to build a nuclear bomb, however, experts believe Saudi Arabia and perhaps others might look to go down the same route.
3) Anywhere in range of North Korea
When it comes to fears about North Korea, location is everything.
The secretive communist dictatorship conducted its first nuclear test in 2006., and the most recent in 2013. For now, however, most intelligence experts and governments believe it has not yet developed the capability to put those warheads on missiles (although the country occasionally suggests that it does).
Should it ever achieve that, it already has rockets judged capable of hitting Japan, much of China and, of course, anywhere and everywhere in South Korea. In the longer run – no one really knows how long that might be – it also seems keen to build rockets capable of reaching further, at least to Hawaii and ultimately the US West Coast.
The nearest part of North America to North Korea is Alaska. The closest major cities are Vancouver and Seattle.
A visitor passes a picture of the mushroom cloud over Hiroshima when it was bombed in 1945. Image: Getty.
The good news is the North Korean missiles have a habit of exploding early in mid flight. The Federation of American Scientists estimate the current North Korean arsenal is less than 10 devices.
The bad news for South Korea, though, is that North Korea still retains enough heavy artillery to be able to do catastrophic damage to the Seoul-Incheon metropolitan area – the world's fifth-largest city, according to Demographia, home to some 23 million people – with conventional artillery alone.
For really terrifying nuclear arsenals, though, one has to look much closer to home.
More than a quarter of a century after the fall of the Berlin Wall, Europe is still home to more than half the world's nuclear weapons.
The largest European arsenal by far – and the second largest in the world – belongs to Russia, with an estimated 1780 deployed strategic nuclear weapons (the US has roughly 1900, the Federation of American Scientists says). France has another 290, the United Kingdom around 150.
Moscow has slightly more nuclear warheads in reserves than Washington does, FAS believes, giving it a total inventory of 7500 against 7200 for the United States
For most of the 21st century so far, no one particularly worried about those weapons: the Cold War was over and Europe was supposed to have outgrown such petty things as war.
Last year's Ukraine crisis, however, has rather thrown that into question. In its confrontations with its neighbours and the broader West, Russia has been very keen to remind the rest of the world just how potentially dangerous it is. If nothing else, the sheer size of its arsenal makes defending against it all but impossible.
The greatest risk of confrontation, most experts believe, would be over the Baltic states – once part of the USSR, now part of the European Union and NATO. A suspected Russian cyber attack on Estonia in 2007 crippled many of its essential systems. Now, NATO hopes a robust – if limited – military presence including US, British and other NATO forces will stop Russia getting too enthusiastic in its pursuit of a region which still houses large numbers of Russian speakers.
As well as its nuclear arsenal, Western defence experts say Russia has also deployed conventional Iskander rockets towards its western borders; these could wreak havoc with road junctions and urban centres across the Baltic states, Poland and parts of the Nordics. Russia has been keen to signal things might not stop there, though: according to senior Western officials, a 2013 Russian "counterterrorism" exercise dubbed "zapad" ended with simulating a nuclear missile launch at Warsaw.
The risk isn’t that Russia would launch a sneak attack: the western nuclear arsenal is large enough that that the retaliation would be similarly devastating. The real risk danger is miscalculation.
Our PS21 panel, incidentally, estimated a 21 percent chance NATO and Russia would fight at least a limited conventional war in the next 20 years, with a 4 percent chance it might go nuclear.
So again, it's probably fine.
5) China and its neighbours
On the other side of the world, things are scarcely more uplifting. A rising China is clearly unnerving its neighbours.
As in India and Pakistan, the region contains some truly enormous cities where even a single nuclear detonation could inflict casualties on an unprecedented scale. Of the world's 20 largest cities, all but six are in Asia, almost all in countries that could reasonably be dragged into a regional conflict.
Our panel estimated the risk of US fighting China as 14 percent, with a 2 percent chance of nuclear conflict. The prospect of Japan winding up fighting China, with or without the US, was put slightly higher at 19 percent, again with a 2 percent chance of going nuclear. (Japan doesn't currently have a nuclear weapons program but experts say it could probably build one in a hurry if it believed it needed it.)
The good news, though, is that the number of nuclear weapons in the region is much lower than Europe. China has a stockpile of perhaps 260, FAS says: that’s less than France.
Well, maybe not quite everywhere. It's actually reassuringly difficult to imagine circumstances under which one of the existing nuclear power's would target a major African or Latin American city such as Lagos or São Paulo.
Still, though, with our panel predicting a 17 percent chance that a nonstate group would detonate a nuclear device in the next 20 years, it’s clear that no one is entirely safe.
They also saw a 38 percent chance of a state and a 48 percent chance of a nonstate group carrying out a cyber attack over the same period that killed more than 100 people. Presumably, the likely targets for such attacks would include the world's most prestigious cities: New York, London, Washington DC, Paris and elsewhere. But it's very difficult to know.
Even more importantly, it is perhaps difficult to know what to do even if you assume these risks are real. In most cases, they remain distant enough prospects – and almost impossible to defend against – that one might end up concluding that it's barely worth bothering.
Peter Apps is on secondment from Reuters as executive director for the Project for Study of the 21st Century (PS21). For more details, click here.