These categories describe two low-probability but
The risk assessment stems from the objective stated in the 2015 Paris Agreement regarding climate change that society keep average global temperatures "well below" a 2°C (3.6°F) increase from what they were before the Industrial Revolution.
Even if that objective is met, a global temperature increase of 1.5°C (2.7°F) is still categorized as "dangerous," meaning it could create substantial damage to human and natural systems. A temperature increase greater than 3°C (5.4°F) could lead to what the researchers term "catastrophic" effects, and an increase greater than 5°C (9°F) could lead to "unknown" consequences which they describe as beyond catastrophic including potentially existential threats. The specter of existential threats is raised to reflect the grave risks to human health and species extinction from warming beyond 5° C, which has not been experienced for at least the past 20 million years.
The scientists term warming probability of five percent or less as a "low-probability high-impact" scenario and assess such scenarios in the analysis "Well Below 2°C: Mitigation strategies for avoiding dangerous to catastrophic climate changes," which will appear in the journal Proceedings of the National Academy of Sciences on Sept. 14.
Ramanathan and Xu also describe three strategies for preventing the gravest threats from taking place.
"When we say five percent-probability high-impact event, people may dismiss it as small but it is equivalent to a one-in-20 chance the plane you are about to board will crash," said Ramanathan. "We would never get on that plane with a one-in-20 chance of it coming down but we are willing to send our children and grandchildren on that plane."
Read more at: https://phys.org/news/2017-09-climate-classification-account-potential-existential.html#jCp
