Climate tipping points depend on net-zero targets: study

The higher that global average temperatures climb, and the longer they stay at such heights, the greater the odds that potentially devastating climate tipping points will be triggered, a new study finds.

Why it matters: The research makes clear how every tenth of a degree of warming matters, and decisions made to cut emissions now could reverberate for centuries.

Zoom in: The new study, published Thursday in Nature Communications, finds that it is already likely that the world will exceed the Paris climate target of 1.5°C above preindustrial levels through 2100.

The period during which this threshold, or the less stringent 2°C target, is exceeded is known as temperature "overshoot."
The study is novel for seeking to quantify how different warming scenarios change the risk of triggering known tipping points.
These include the potential for a rapid shutdown of a vital Atlantic Ocean circulation that transports heat into northern latitudes, and the sudden, rapid and irreversible melting of large portions of the Greenland and Antarctic ice sheets, which would dramatically raise sea levels and inundate coastal cities.

Friction point: Climate scientists are aware that such tipping points exist, given evidence for their occurrence in historic climate data, such as tree rings, ice cores and coral samples.

Climate history is full of instances of non-linear change that takes place astonishingly swiftly.
But climate scientists disagree on whether and when they could be set off, with debates taking place in the scientific literature and at conferences over how sensitive Earth's climate is to the human-caused buildup of greenhouse gases.
There is, however, near-universal agreement that tipping points exist, and once they are set into motion, they may not be possible to reverse.

What they found: The study, by a group of researchers based in Europe, found that if current climate policies are followed, there is a 45% chance of triggering one or more tipping points by the year 2300, even if temperatures are brought back below the 1.5°C target.

The tipping points they consider include the ocean circulation example, as well as rapid loss of ice sheets, and the destabilization of the Amazon rainforest.
The study found the risk of setting off these climate change tripwires increases with every tenth of a degree Celsius of warming above 1.5°C and climbs especially quickly if warming exceeds 2°C compared to preindustrial levels.
Long-term tipping point risks can best be minimized, the study shows, if emissions are sharply reduced in the next decade and reach "at least net zero" by 2100.

Yes, but: Current policies of countries worldwide are not aligned with the 1.5°C target and instead would yield at least between 2°C and 3°C of warming by 2100.

Between the lines: Ultimately, the shorter the length of the overshoot and the lower the long-term level of climate change, the lower the odds of setting off a cascade of potentially catastrophic climate impacts, the study shows.

"The risks of overshoot can be minimised if warming is swiftly reversed," the study finds. "However, this would require rapid employment of appropriate mitigation measures."
Such actions would include carbon removal technologies, which are attracting considerable investment today but have yet to work at scale.
The study shows that stabilizing the climate at 1.5°C above preindustrial levels over the long term would still carry considerable risks and that temperatures would need to come down further, to about 1°C above preindustrial, to sufficiently limit the risks of tipping points.

What they're saying: The study authors make clear that the 1.5-degree target, which most experts believe is already infeasible due to the continued rapid pace of emissions today, should be viewed as a guardrail, not an end goal.

"A global mean temperature increase of 1.5°C is not 'safe' in terms of planetary stability but must be seen as an upper limit," the study states.
Lead author Tessa Möller told Axios that even a temporary exceedance of the 1.5°C temperature limit "significantly increases the risk of crossing key climate tipping points."
"The findings emphasize that immediate and substantial emissions cuts are essential in the next decade to avoid these tipping points and prevent long-term damage to the climate," Möller said in an email.