Metaculus Help: Spread the word
If you like Metaculus, tell your friends! Share this question via Facebook, Twitter, or Reddit.
Ragnarök Question Series: if a global climate disaster occurs by 2100, will the Earth's human population decline by 95% or more?
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.
This question is part of the Ragnarök Question Series. Please have a look at the other questions and contribute your insights, analyses, and factorizations, especially on the questions on what might happen if a global catastrophe occurs (for which we are currently short on predictions):
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.
Metaculus help: Predicting
Predictions are the heart of Metaculus. Predicting is how you contribute to the wisdom of the crowd, and how you earn points and build up your personal Metaculus track record.
The basics of predicting are very simple: move the slider to best match the likelihood of the outcome, and click predict. You can predict as often as you want, and you're encouraged to change your mind when new information becomes available.
The displayed score is split into current points and total points. Current points show how much your prediction is worth now, whereas total points show the combined worth of all of your predictions over the lifetime of the question. The scoring details are available on the FAQ.
Note: this question resolved before its original close time. All of your predictions came after the resolution, so you did not gain (or lose) any points for it.
Note: this question resolved before its original close time. You earned points up until the question resolution, but not afterwards.
This question is not yet open for predictions.
Metaculus help: Community Stats
Use the community stats to get a better sense of the community consensus (or lack thereof) for this question. Sometimes people have wildly different ideas about the likely outcomes, and sometimes people are in close agreement. There are even times when the community seems very certain of uncertainty, like when everyone agrees that event is only 50% likely to happen.
When you make a prediction, check the community stats to see where you land. If your prediction is an outlier, might there be something you're overlooking that others have seen? Or do you have special insight that others are lacking? Either way, it might be a good idea to join the discussion in the comments.