Some kind of overshoot of 1.5˚C is increasingly being seen as inevitable by scientists and policymakers.
This new study, published in the peer-reviewed journal Nature, cautions against “overconfidence” in such a scenario when the dangers are not fully appreciated.
An overshoot could trigger impacts that last hundreds if not thousands of years, or cross “tipping points” that prompt large and unrepairable changes in earth’s climate system, the scientists warn.
It could mean the thawing of permafrost and peatlands, carbon-rich landscapes that would release huge volumes of planet-heating greenhouse gases if lost.
And sea levels could rise an additional 40 centimeters (16 inches) if 1.5˚C is exceeded for a century, the authors said, an existential difference for vulnerable low-level island nations.
Taken together, the world’s existing pledges for climate action would result in nearly 3˚C of warming by 2100, according to the UN.
To limit warming to 1.5˚C, emissions must be at net zero by 2050, which means balancing the amount of carbon dioxide produced against the amount humanity can remove from the atmosphere via technology.
This process, known as carbon removal, would need to be massively scaled up to pull global temperatures back down in the event of an overshoot, something that is far from guaranteed.
“We cannot be confident that temperature decline after overshoot is achievable within the timescales expected today,” the authors wrote.