The 1.5 Degree Temperature Target is a Dead Man Walking

On the consequences of policy-based evidence in climate politics

That’s the limit. Source: Vox.

Since the early 1990s, climate policy has focused on a number of different metrics around which to set policy goals, including emissions cuts (Kyoto 1997), atmospheric carbon dioxide concentrations (Copenhagen 2009) and global average temperature change from pre-industrial times (Paris 2015). In an earlier post I described the arbitrary and non-scientific origins of the 2 degree Celsius temperature target of the Paris Agreement (2015).

In today’s post I explain the origins of its companion, the 1.5 degree Celsius target that has in many respects eclipsed the 2C target in public discussions of climate. Most scientists in climate know that despite its prominent role in political exhortation the 1.5 C target is already obsolete. In fact, it has always been.

Here is how that happened.

According to climate scientist James Hansen, in 2007 environmental Bill McKibben contacted him for Hansen’s advice on what to call his new climate advocacy organization:

“In 2007, the environmentalist and writer Bill McKibben began bugging me, very politely, to either confirm 450 parts per million [ppm carbon dioxide in the atmosphere] as the appropriate target level of carbon dioxide in the atmosphere or else to define a more appropriate one. He was developing a Web site to draw attention to this target limit and was thinking of calling it 450.org.”

At the time, Hansen was working on a paper in which he and colleagues argued:

“We suggest an initial objective of reducing atmospheric CO2 to 350 ppm, with the target to be adjusted as scientific understanding and empirical evidence of climate effects accumulate.”

For Hansen, 350 ppm was never a target not to be exceeded, since the world was already well past that when he wrote the paper, but a target for policy makers to seek to return to by the end of the century.

McKibben and colleagues took Hansen’s advice and established 350.org in 2007. That year, he acknowledged that there world was already well past 350 ppm carbon dioxide, and that it was an aspirational target not to avoid hitting but to aim to return to in the future, much as Hansen had argued:

We're already at 383 parts per million, and it's knocking the planet off kilter in substantial ways. So, what does that mean? . . . Does that mean we're doomed? Not quite. Not any more than your doctor telling you that your cholesterol is way too high means the game is over. Much like the way your body will thin its blood if you give up cheese fries, so the Earth naturally gets rid of some of its CO2 each year. We just need to stop putting more in and, over time, the number will fall, perhaps fast enough to avert the worst damage.

In promoting his new organization worldwide, McKibben partnered with the Tällberg Forum, a Swedish foundation, to promote global advocacy of 350 ppm as a new target for climate policy. Their stated objective was to influence the upcoming major climate conference in Copenhagen at the end of 2009.

350 ppm goes global in 2008.

On June 23, 2008 the Tällberg Forum published the full page advertisement above in the Financial Times, the International Herald Tribune and the New York Times with more than 150 signatories. In the days that followed days the Tällberg Forum hosted an international conference to promote the proposed target, which was attended by United Nations Secretary General Kofi Annan. Their campaign was a resounding success.

The nerdy concept of 350 parts per million of carbon dioxide in the atmosphere had gone global, despite the fact that there was essentially no science behind it, just Hansen’s non-peer-reviewed draft paper and a bunch of sign-ons to a newspaper ad. Well done!

World leaders at the disastrous Copenhagen climate conference in 2009. Source: Vanity Fair.

In 2009, ahead of the year-ending Copenhagen climate conference, the U.N. Framework Convention on Climate Change (FCCC) was still calling for a climate target of 450 ppm carbon dioxide, which they equated to a global temperature rise of 2.0-2.4 degrees Celsius by 2100, and necessitating emissions cuts of 50% by 2050. But at the Copenhagen conference that December, which ended quite badly, small island states in particular emphasized the 350 ppm carbon dioxide target as a way to gain leverage in the negotiations.

The desire to keep a global coalition together in the face of dissention at Copenhagen helped to give the 1.5 C target a political foothold in order to keep many countries engaged in the negotiations. As one summary of the Copenhagen conference explained:

“Many countries supported a goal of keeping temperature increase below 2˚C above pre-industrial levels, with AOSIS [Alliance of Small Island States] underlining 1.5˚C and Bolivia 1˚C.”

Acknowledging 1.5C was not about science, but realpolitik.

Because climate policy target setting was shifting from carbon dioxide concentrations targets to global temperatures, to keep pace, the discourse needed to change as well. So 450 ppm became recast as 2 degrees Celsius and 350 ppm became 1.5 degrees Celsius, even though 350 ppm had never really been studied and the previous IPCC assessment associated 450ppm with temperature outcomes of 1.4C to 3.1C.

These adoption of a temperature target over a carbon dioxide concentration target was reflected in the final text of the Copenhagen Accord which defined the long-debated concept of “dangerous anthropogenic interference” in terms of 2C:

“To achieve the ultimate objective of the Convention to stabilize greenhouse gas concentration in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system, we shall, recognizing the scientific view that the increase in global temperature should be below 2 degrees Celsius, on the basis of equity and in the context of sustainable development, enhance our long-term cooperative action to combat climate change.”

A nod to the small island states and their call for a 1.5 degree Celsius target was also included, with the Accord briefly expressing a commitment to consider strengthening the target to 1.5 degrees Celsius by 2015.

Notice what happened here – 350 ppm had long since been passed and was intended as a 2100 target, whereas the world had yet to hit a temperature increase of 1.5 degrees Celsius which became recast as a contemporary guardrail. As the language changed, so too did the underlying meaning.

Goal posts = moved.

The U.N. FCCC always recognized that the 1.5 C target was much more grounded in political aspirations than in science, noting in 2015, for instance, while following up on its commitment to reconsider the target:

“while science on the 1.5C warming limit is less robust, efforts should be made to push the defence line as low as possible.”

The political motivation is understandable (and seems to also have been appreciated by the IPCC), but it should not be confused with evidence-based policy.

A lot of campaigning, lobbying and diplomatic negotiations ensued (e.g., see here and here, for example). The 1.5 C target was adopted as an aspirational target under the Paris Agreement, but in much that followed, the rhetoric that has accompanied it has elevated it into an actual policy goal that many take seriously (such as President Biden). However, at the time it was enshrined in the Paris Agreement, many experts viewed the target as an impossibility.

However, the scientific community —rather than loudly proclaim that the aspirational targets were exactly that and practically unreachable — followed the political demand for scientific justification after the fact to legitimize the impossible goal as an actual target to guide policy.

One recent analysis concluded that politics led science:

“Before the Paris Agreement, the 1.5C limit was outside the range of explored pathways. It was deemed unrealistic, and thus irrelevant (Livingston & Rummukainen, 2020; van Beek et al., 2022). After the Paris Agreement, pathways compatible with 1.5C became widespread in the literature.”

Integrated Assessment (IA) modelers dutifully produced fanciful scenarios that achieved 1.5 C outcomes in their models, and for the most part (at least early on) did not express their reservations about what they were doing (though later, criticisms became much more common):

“IAM teams developed pathways compatible with 1.5C to meet the demand, relying on temporary overshoot and negative emissions, but also expanding the range of mitigation options they considered (van Beek et al., 2022, p. 198). Back in 2017, one modeler explained that when he started, for him “2C was impossible, but as I kept seeing it, it has become something very common. The same thing is happening with 1.5C even if there are many arguments against it”

Central to the scientific legitimization of the 1.5C target was the creation of hypothetical “negative emissions technologies” in the IA models. The FCCC requested that the Intergovernmental Panel on Climate Change prepare a special report on 1.5C. The IPCC’s positive and uncritical response further helped to legitimize the political status of the 1.5 C target, even as many researchers continued to doubt its feasibility. Arguably, the IPCC took on a greater advocacy focus in its 1.5C report, asking and answering leading questions such as: “Why is it necessary and even vital to maintain the global temperature increase below 1.5°C versus higher levels?”

In an excellent paper on these dynamics van Beek and colleagues write (emphases added):

“This [tailoring of science to meet political demand] reveals an empirical example of ‘calibrating’ the model analysis in view of relevance: despite the personal conviction of realism of some of the modellers at the time, modelling efforts were redirected from exploring 2 °C pathways to those limiting warming to 1.5 °C. The alternative would have been to say that the 1.5 °C goal was infeasible according to modelling results. However, this would disregard small island states (interview 5, IPCC co-chair). In fact, if the IPCC would have concluded that the 1.5 °C was unrealistic, Paris negotiators might even have had to go back to the negotiation table (interview 22, COP21 negotiator). On the other hand, the shift from 2 °C to 1.5 °C implied faster emissions reduction, in which the rapidly appearing 1.5 °C scenario literature relied on NETs to an even more significant degree (interview 2,3,6,15). As explained by one modeller: “I am not more confident that we can reach it, but I am more confident that we can model it. […] we would never have to say it would not be achievable, we just put more negative emissions in” (interview 18).”

Rather than deliver “uncomfortable knowledge” to policy makers in the FCCC and around the world, the IPCC community (or more accurately, an important subset of that community) decided to play along with the charade that 1.5C was actually a meaningful policy target, as opposed to an aspirational political ambition.

As a consequence, today the world sits in an awkward place. The 1.5C target will not be met. The renegotiation of the Paris Agreement that was avoided in the past decade will not be avoidable forever – though it is possible that a return to the 2C target may prolong such a reconsideration for a decade or more (1.5C? Never mind!).

In 2023, we are now seeing academic papers asking questions like the following:

“How did an almost impossible target become the point of reference for climate action? How does it maintain its legitimacy despite the incompatibility between ambitions and actions that it makes evident?”

It remains to be seen whether climate advocates will decide that the attention paid to the 1.5C target over the past 14 years represented an inefficient detour from more productive paths to decarbonization, or if the shared willing suspension of disbelief in hopes of gaining political currency moved things along. Longtime readers here will know my views on this question – expert advisory bodies like the IPCC should always play things straight, and resist the temptation to provide or promote policy-based evidence, even if the alternative is uncomfortable knowledge.

Telling policy makers what they don’t want to hear may not always be what they want, but for effective policy to result, it may be exactly what they need.