Is it time to aim for 1.7°C as the new limit for global warming?

If you had told a child to stay “far” with a cliff edge, how much could the edge slip before starting to cry so that he turns around? This is the question of confusing climatologists at the moment, because it seems almost certain that we are going to violate the global commitment to limit warming to no more than 1.5 ° C above the pre-industrial levels. While we are entering the danger area, what is the next step?

“Governments have set targets of 1.5 ° C” I think it would be very useful for people to begin to speak much more concise and concretely on the real objectives they have in mind. “

These national objectives are derived from the Paris International Agreement, which was signed in 2015 and is a vague starting point to define the climatic limits. The text officially commits the countries to “continue the efforts” to limit warming to 1.5 ° C and maintain any increase in temperature to “well below” from 2 ° C. But how much “well below”?

“The wording of the long -term temperature objective of the Paris Agreement is both a solution and a challenge,” explains Jeri Rogelj, also at the Imperial College in London. “The solution is that it was the wording that countries could accept. The challenge is that he leaves room for interpretation. ”

Rogelj fears that less that the sense of “well below” of 2 ° C will be clarified – and quickly – there is a risk that the world will simply take 2 ° C as a new line in the sand. The problem is that many scenarios to reach 2 ° C provides only 50% of success, which means that the aim of this line could still be widely exceeded.

To remedy this, Rogelj and Lamboll point out that international agreements are generally necessary to use a normal understanding of words. So, if the Paris agreement promises to maintain temperatures much less than 2 ° C, the pair says that most people would not expect a significant risk of exceeding to comply with this promise.

However, over things, two model scenarios could both pretend to limit warming within 2 ° C, but one could offer only 66% chance of staying below the limit, while the other offers a 90% chance. “People are not very good at managing the probabilities,” says Lamboll. “A 66% chance and a 90% chance are incredibly different things.”

This uncertainty stems from different hypotheses underlying the scenarios, those requiring a stricter control of emissions having better chances to stay below 2 ° C. The pair claims that the top temperature – the most of the world is likely to warm up before all the mitigation measures come into play to reduce temperatures – better captures the differences between the scenarios and therefore provides a clearer climatic.

In unpublished work, Rogelj and Lamboll have evaluated four climate model scenarios of 2 ° C, calculating for each the median cutting edge temperature necessary to remain less than 2 ° C with 66%, 83% and 90% of opportunities. For example, a scenario shows that for 66% chance of staying below the limit, temperatures should peak at around 1.83 ° C, but for a chance of 90%, they would need to peak at 1.54 ° C.

Looking at all the models, the pair concludes that, to offer the world an 83% chance of staying below 2 ° C of warming – a fair representation, they say, of the promise to remain “well below” of the threshold – the median temperature cannot peak beyond 1.63 to 1.67 ° C, the beach given in all models.

Other researchers come to the same conclusion. GOTTFRIED KIRCHENGAST and MORITZ PICHLER, both at the University of Graz in Austria, recently proposed 1.7 ° C as top temperature limit to keep us “well below 2 ° C”, because it conforms to an intergovernmental panel on the projections of climate change which give 83% chances of staying below 2 ° C.

“The 1.5 ° C is a clear guard rail. [Defining] 1.7 ° C would “well below 2 ° C” a light guard rail “, explains Kirchengast. This new “upper limit” of warming would help political decision-makers to calculate their remaining emission budgets and their plan transition paths accordingly, he argues. “Politics needs these directives.”

To what extent would this target be difficult to reach? Limiting warming to 1.7 ° C is certainly extremely ambitious, since current policies put the world on the right track for 2.6 ° C of warming by the end of the century, but it is not completely fanciful. The most optimistic scenario, assuming that each country diligently holds all its climatic promises, suggests that global warming would stabilize at 1.9 ° C by the end of the century, according to a recent United Nations assessment. Using 1.7 ° C would require going beyond the existing promises.

But even if some scientists start to regroup around the idea that “well below” 2 ° C in fact means a top temperature limit of around 1.7 ° C, many people are opposed to the codification of a post-1.5 ° C.

We still do not understand the climate system enough to be convinced that we can aim for such specific warming levels, explains Carl-Friedrich Schleussner to the climate analysis of the Berlin climate. There is still a considerable uncertainty about the sensitivity of earth systems to greenhouse gas emissions, which could mean that the planet will warm up much faster than expected. “We have to be careful not to be too confident,” he warns. Set a specific temperature goal “transmits the message we know exactly where we are going, which is not the case,” he says.

Instead, Schleussner believes that the emphasis should be placed on the holding of governments responsible for any failure of the target of 1.5 ° C, for example by calculating the “carbon debt” accumulated by nations because they exceed this warming threshold. “Unless we can establish responsibility not to limit warming to 1.5 ° C, I think we fail in the Paris Agreement,” he said.

Low -income countries, in particular the small island states that have fought for the inclusion of the temperature objective of 1.5 ° C in the Paris Agreement, are also likely to fiercely resist any attempt to recalidate the ambition of the global climate to a new target. Ilana Seid, Palauan ambassador to the UN and president of the Alliance of Small island States (AOSIS), a UN negotiation block, says that the increase in sea level and the death of the coral reefs expected to warming greater than 1.5 ° C are an existential threat to the countries it represents.

“For AOSIS, the number is 1.5 ° C. This is our rallying call,” explains Seid. “There are important reasons for us to be below or at 1.5 ° C, and this is where we stick to it … Everything else is only a distraction.”

Natalie Unterstell, a former United Nations Climate negotiator for Brazil who is now in the Talanoa climate policy, says that a transition to the adoption of a global objective of not more than 1.7 ° C of warming “reported to the governments and the markets that failure is acceptable”.

“The displacement of goal posts while we are still in the game only helps latecomers and lobbyists. He fractures political will, confuses public messaging and risks normalizing climate insufficiency, “she said. “A new temperature target would now precisely create the type of cognitive fog on which the interests of fossil fuels count.”

“The limit of 1.5 ° C is not only a symbolic threshold but a line of life or death for billions,” explains Unterstell. “So, if something is the time to double the action, not to demarize our objectives.”

In addition to ethical concerns concerning a decision to adopt a new global objective, practically by speaking, it would be extremely difficult to codify 1.7 ° C in the United Nations climate system, she underlines, requiring a reopening of the rules of rules governing the Paris Agreement and the unanimous support of all 200 member states and more. It is unlikely that this is an objective at the next COP30 summit in Belem, Brazil, later this year, although the Brazilian presidency is under pressure to extract the more daring climatic plans of the polluting nations at the top to fill “the space of ambition” between 1.5 ° C and the current warming trajectories.

But should this debate be supervised as a competition between 1.5 ° C and a new slightly less strict objective? For Rogelj, limiting warming to 1.5 ° C will remain a global durable key target, even if a new temperature objective is also introduced. “1.5 ° C will never die, the target will remain,” he says. “It is because the target is to” continue its efforts “to limit warming to 1.5 ° C

When the Paris Agreement was drawn up in 2015, the limitation of warming to 1.5 ° C was ambitious but achievable. Now, few climate models show a realistic route to achieve this goal without at least a little “overcoming” – temperatures exceeding 1.5 ° C for a few decades before being brought back below the limit by the end of the century, using technology such as carbon capture. The movement to clarify the precise meaning of “well below 2 ° C” is not necessarily to provide a replacement target for 1.5 ° C, but to fix a higher temperature threshold for warming in a scenario where the world exceeds, then bring back the warming to the limit of 1.5 ° C, explains Rogelj.

The question for political decision-makers is as follows: if 1.5 ° C is the safety line and 2 ° C is the edge of the cliff, how far in the danger zone should we be ready to move away?

https://www.youtube.com/watch?v=nfmzPAO5WOW