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, 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.