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, our planet would become a more precarious place, but it will likely remain mostly habitable.
However, according to some, there is also a non-negligible chance 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.
One explanation is that increases in global mean temperatures might have substantial self-reinforcing feedbacks that could place us on a "Hothouse Earth" pathway. These feedback processes include permafrost thawing, decomposition of ocean methane hydrates, increased marine bacterial respiration, and loss of polar ice sheets accompanied by a rise in sea levels and potential amplification of temperature rise through changes in ocean circulation.
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.
In an earlier question in the Ragnarök question series, I asked 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?, where a global catastrophe was defined as a 10% or more reduction in human population in 5 years or less.
If a global climate catastrophe occurs by 2100, will the Earth's human population decline by more than 95% relative to the pre-catastrophe population?
The question resolves ambiguous if a global climate catastrophe that reduces the human population by at least 10% does not occur.
It resolves ambiguous if this catastrophe is primarily due the effects on the climate system of a nuclear war.
It resolves positive if such a catastrophe does occur, and the global population is less than 5% of the pre-catastrophe population. It does also not matter how the Earth's population declines, this might be due to mass fatalities, or mass immigration to a different planet, as long as this decline is highly unlikely on a counterfactual Earth in which the climate disaster did not occur.
It resolves positive if the 95% decline in population is primarily due the effects on the climate system of the use of geoengineering that has been principally motivated to mitigate climate change risks.
The question resolves negative if a global climate catastrophe occurs that claims at least 10% (in any period of 5 years or less) but the post-catastrophe population does not fall 95% or more relative to the pre-catastrophe population at any point within 25 years of the catastrophe.
This question is part of the Ragnarök Question Series. Check out the other questions in the series:
Also, please check out our questions on whether a global catastrophe will occur by 2100, and if so, which?:
All results are analysed here, and will be updated periodically.