You can now see an excellent visualization of global catastrophic risks estimates produced in the Ragnarök series here.

According to current IPCC estimates, unmitigated greenhouse emissions are likely to lead to global temperature increases of 2.6ºC-4.8ºC by 2100. If this happened, there’d likely be significant humanitarian harms, including more severe weather, food crises, and the spread of infectious diseases which would disproportionately affect the world’s worst off.

Moreover, the estimated humanitarian impacts of climate changes are likely to be highly nonlinear: marginal temperature increases are expected to cause more damage at already-increased temperatures (i.e. going from 3ºC to 4ºC is expected to be significantly worse than going from 1ºC to 2ºC).

According to some, there is also a non-negligible chance — perhaps around 10% — that unmitigated emissions will lead to global temperature increases even higher than 4.8ºC. More generally, estimates of temperature increases resulting from greenhouse emissions have a “fat” right tail, meaning that there is a low, but non-negligible chance of very high temperature increases. Hence, there is a non-negligible chance that unmitigated emissions may produce consequences which could be catastrophic for life on Earth.

For example, it has been argued that a 12ºC increase in mean global temperature—which is substantially outside the range considered plausible this century—would cause at least one day each year in the territories where half of all people live today to be hot enough to exceed human metabolic limits and cause tissue damage from hyperthermia after a few hours of exposure.

One way to reduce global temperatures quickly and cheaply is a form of climate engineering called Solar Radiation Management (SRM), which involves cooling the Earth by reflecting sunlight back into space. The most researched form of SRM involves injecting aerosols into the stratosphere. Most of the evidence so far suggests that ideal SRM deployment programmes would reduce overall damages relative to an un-engineered greenhouse world.

However, SRM brings its own risks. Of the currently known potential negative direct effects of SRM, only abrupt termination could plausibly bring about an existential catastrophe. If a very thick stratospheric veil were deployed and SRM was suddenly terminated and not resumed within a buffer period of a few months, then there would be very rapid and damaging warming. There might also be some reasons to expect the chance of at least one country to undertake climate engineering to be surprisingly high, especially when the relevant technologies are widespread, and the lack of global coordination permits each country to individually decide for or against the intervention.

In the headline question to this series, we defined a global catastrophe as a 10% decrease in the world population in any period of 5 years or less. This question resolves ambiguously if no such global catastrophe occurs. On the other hand...

If a global catastrophe happens before 2100, will it be principally due to human-made climate change, or the use of geoengineering as a deliberate large-scale intervention in the Earth’s climate system?

The question results positively if a human-made climate change or geoengineering catastrophe occurs that claims at least 10% in any period of 5 years or less before 2100. This catastrophe must be due to the direct effects of climate change that are generally believed very unlikely in a counterfactual world with little or no global warming but otherwise similar to ours. These include (but are not limited to) floods, extreme weather, the spreading of infectious disease, and the health effects of extreme heat. Finally, the effects due to the effects of the use of geoengineering that has been principally motivated to mitigate climate change risks, also count towards the population decline. Years are here defined as consecutive calendar years.