In Reasons and Persons, philosopher Derek Parfit wrote:
I believe that if we destroy mankind, as we now can, this outcome will be much worse than most people think. Compare three outcomes:
2. A nuclear war that kills 99% of the world’s existing population.
3. A nuclear war that kills 100%
2 would be worse than 1, and 3 would be worse than 2. Which is the greater of these two differences? Most people believe that the greater difference is between 1 and 2. I believe that the difference between 2 and 3 is very much greater… If we do not destroy mankind, these thousand years may be only a tiny fraction of the whole of civilized human history.
The ethical questions raised by the example have been much discussed, but almost nothing has been written on the empirical question: given nuclear war, how likely is scenario 3?
The most obvious path from nuclear war to human extinction is nuclear winter: past posts on Overcoming Bias have bemoaned neglect of nuclear winter, and high-lighted recent research. Particularly important is a 2007 paper by Alan Robock, Luke Oman, and Georgiy Stenchikov: “Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences.” Their model shows severe falls in temperature and insolation that would devastate agriculture and humanity’s food supply, with the potential for billions of deaths from famine in addition to the direct damage.
So I asked Luke Oman for his estimate of the risk that nuclear winter would cause human extinction, in addition to its other terrible effects. He gave the following estimate:
The probability I would estimate for the global human population of zero resulting from the 150 Tg of black carbon scenario in our 2007 paper would be in the range of 1 in 10,000 to 1 in 100,000.
I tried to base this estimate on the closest rapid climate change impact analog that I know of, the Toba supervolcanic eruption approximately 70,000 years ago. There is some suggestion that around the time of Toba there was a population bottleneck in which the global population was severely reduced. Climate anomalies could be similar in magnitude and duration. Biggest population impacts would likely be Northern Hemisphere interior continental regions with relatively smaller impacts possible over Southern Hemisphere island nations like New Zealand.
Luke also graciously gave a short Q & A to clarify his reasoning: