Should We Set 1.7°C as a New Global Warming Limit?

When you tell a child to “stay far from the cliff’s edge,” how close can they get before you call them back? This dilemma is currently perplexing climate scientists: the risk of exceeding our global commitment to keep warming below 1.5°C above pre-industrial levels is increasing. Once we step into the danger zone, what consequences will follow?

“The government is aiming for a 1.5°C target, but understanding what this means in a world that’s already above that threshold is not straightforward,” says Robin Lambor from Imperial College London. “It would be beneficial if discussions became more defined and specific regarding the actual objectives we seek.”

These national objectives stem from the International Paris Agreement signed in 2015, which serves as a vague starting point for defining climate actions. The agreement officially commits to “pursuing efforts” to limit warming to 1.5°C, while also striving to keep increases “well below” 2°C. Yet, how do we define “below”?

“The wording of the long-term temperature goal in the Paris Agreement is both a solution and a challenge,” notes Jori Rogelgi from Imperial College London. “It provided common ground for nations to agree upon, but it also allows for considerable interpretation.”

Rogelj worries that if the phrase “down sufficiently” regarding the 2°C limit isn’t clarified soon, there’s a risk that it might be accepted as a new benchmark. Many scenarios projecting 2°C provide only a 50% chance of success, meaning that by targeting this limit, we could potentially overestimate our safety.

To address this uncertainty, Rogelj and Lamboll emphasize that international consensus is crucial for interpreting these terms accurately. They argue that if the Paris Agreement pledges to keep temperatures below 2°C, most people don’t foresee a significant chance of overshooting that promise.

Currently, one model predicts a 66% likelihood of staying below 2°C, while another claims a 90% chance. “People struggle with probabilities,” explains Lambor. “The difference between a 66% and a 90% chance is significant.”

This variation arises from differing assumptions within various scenarios; stricter emission control measures are more likely to remain under the 2°C threshold. The authors argue that identifying peak temperatures—expected to be the highest before mitigation measures cool the atmosphere—better captures the variations among scenarios and helps establish clearer climate goals.

In ongoing research, Rogelj and Lamboll explored four 2°C climate model scenarios and calculated the median peak temperatures necessary to remain below 2°C with 66%, 83%, and 90% probabilities, respectively. For instance, one scenario suggests that to maintain a 66% chance of staying under the limit, the temperature should peak at approximately 1.83°C, while a 90% success rate necessitates a peak of 1.54°C.

When examining all models, the authors argue that it’s essential to promise a temperature significantly below 2°C to achieve an 83% chance of remaining under that threshold.

Other researchers echo this conclusion. Gottfried Kirchengast and Moritz Pichler from Graz University in Austria recently suggested a limit of 1.7°C, aligning with predictions from the Intergovernmental Panel on Climate Change and offering an 83% chance of staying below 2°C—indicating that 1.7°C is the peak temperature cap for “well below 2°C.”

“A 1.5°C threshold serves as a clear guide. Determining 1.7°C will serve as another vital boundary well below 2°C,” observes Kirchengast. This newly established warming threshold will aid policymakers in calculating remaining emission budgets and planning their transition strategies accordingly.

How daunting is this goal? Given current policies, limiting warming to 1.7°C is indeed very ambitious when tracking global warming projected to reach 2.6°C by the century’s end; however, it’s not entirely out of reach. The most optimistic scenarios suggest a stabilization at 1.9°C if all nations fully meet their climate commitments, according to recent UN evaluations. To meet a 1.7°C goal, exceeding existing promises is essential.

Yet, even as some scientists propose that “well below” 2°C translates to a peak temperature around 1.7°C, many oppose formalizing targets beyond 1.5°C.

There’s still much we don’t understand about the climate system. Carl-Friedrich Schleussner from the Berlin Institute for Climate Science warns of considerable uncertainty regarding the Earth’s sensitivity to greenhouse gas emissions, indicating that the planet might warm more rapidly than anticipated. “We should be cautious not to overestimate our knowledge,” he warns. Setting a specific temperature target “could imply we have a clear trajectory, but that’s not the reality,” he adds.

Instead, Schleussner urges governments to take accountability for failing to meet the 1.5°C target by calculating their “carbon debt” accrued since surpassing that threshold. “Unless we establish accountability for not limiting warming to 1.5°C, the Paris Agreement isn’t achieving its objectives,” he says.

Vulnerable countries, especially small island nations, have fought to cement the 1.5°C target within the Paris Agreement and may resist any attempts to recalibrate global climate ambitions. Ilana Seid, the UN Ambassador for Palau and chair of the Small Island Developing States (AOSIS), states that rising sea levels and threatened coral reefs due to warming beyond 1.5°C pose existential threats to her country.

“For AOSIS, the standard is 1.5°C. That’s our unyielding position,” says Seid. “There’s a critical reason we stand firm at 1.5°C…everything else is just noise.”

Natalie Unterstell, a former UN climate negotiator in Brazil now with the climate policy think tank Thalanoa, asserts that shifting towards a global target of warming below 1.7°C “signals to governments and markets that failure is permissible.”

“If you change your goals mid-game, only lobbyists and special interests will benefit. This risks diluting political will, muddling public messaging, and normalizing climate degradation,” she explains. “Presently, new temperature targets create the cognitive dissonance that fossil fuel interests thrive on.”

“The 1.5°C limit isn’t merely symbolic; it represents billions of lives at stake,” states Unterstell. “If anything, this moment demands an escalation of our actions, not a relaxation of our targets.”

Beyond the ethical implications of adopting new global targets, she notes that concretizing 1.7°C will be exceptionally challenging under the UN climate framework, which relies on a rulebook governing the Paris Agreement requiring unanimous support from all over 200 member states—a feat unlikely to be achieved at the upcoming COP30 Summit in Belem, Brazil. However, the Brazilian presidency will face pressure to extract robust climate commitments from polluting nations and address the “ambition gap” between 1.5°C and current warming projections.

But should this discussion be framed as a competition between 1.5°C and a newly proposed, slightly more lenient goal? For Rogelj, the aim of limiting warming to 1.5°C remains a fundamental global target, despite the possible introduction of new temperature thresholds. “The target of 1.5°C continues to exist,” he affirms. “This is because the objective to ‘pursue efforts’ to limit warming to 1.5°C is still intact, even above that level.”

At the conception of the Paris Agreement in 2015, a limit of 1.5°C was seen as ambitious yet attainable. Most climate models have since eroded to the point where they no longer represent a viable path to this goal without “overshooting.” Temperatures have been above 1.5°C for decades, yet technologies such as carbon capture are posited to bring us back below this threshold by century’s end. Clarifying the exact meaning of being “well below 2°C” doesn’t negate the target of 1.5°C but rather establishes a higher bar for warming in a world that overshoots and aims to ultimately revert to that level, according to Rogelj.

Now, the policymakers must ask themselves: if 1.5°C serves as our safety line and 2°C marks the cliff’s edge, just how close should we dare to approach?