On the Hard Reality that the 1.5 Degree Scenario is Out of Reach

We now know that the likelihood of keeping the increase in global temperatures to 1.5 C is not materially different to zero. Yet we continue to talk of the 1.5 target in public discourse as if it were the reference scenario. I believe that as a specialist community, we allow this misleading public discourse and even ourselves contribute to its prevalence. I’d like to offer some thoughts on this and the consequences.

First, the science and the math behind my opening statement. A recent paper quantifies what we already sensed: given where global carbon emissions are today, we would need an immediate and precipitous drop to stay with the 1.5 degree carbon budget. The Remaining Carbon Budget “for a 50% chance of keeping warming to 1.5 °C is around 250 GtCO2 as of January 2023, equal to around six years of current CO2 emissions.” This means that, to achieve 1.5 degrees, net carbon emissions would need to decrease linearly starting now to reach zero in 2034. [Lamboll, R.D., Nicholls, Z.R.J., Smith, C.J. et al.Assessing the size and uncertainty of remaining carbon budgets. Nat. Clim. Chang. (2023). ]

To say that this is not likely would be a mis-statement. In reality, “there are no social or technical scenarios in the scientific literature that even describe how that would be possible,” says Joeri Rogelj of Imperial College London, the author of the paper cited above. [New Scientist, 30 October 2023] No serious projection of energy emissions, to take only those, is remotely compatible with this trajectory. National policies are, in aggregate, not even directionally potentially compatible with such a scenario.

We are therefore not at the stage of ascribing a probability to a 1.5 degree scenario. From where we stand today, there is no such scenario identified. And we need to remember that even if we identified the scenario and achieved it, it would only give a probability of meeting a temperature increase objective as the dynamics of any temperature outcome are stochastic. Hence the pretty straightforward conclusion that the probability of the 1.5 degree scenario is not materially different to zero.

These are new findings, but the underlying reality is not new. Yet we continue to hear everywhere – not least in the communications material of the international organizations – how important it is that we act now to keep global warming to 1.5 degrees. It is therefore perfectly reasonable that the general public continues to have the understanding that there is a 1.5 degree scenario and that achieving it is within reach in order to avoid widespread catastrophic effects of climate change.

We know that were good reasons historically to have communicated on the basis of a scenario that only had a sliver of probability.

The argument given in Paris in 2015 was that if we start giving the idea that 2 degrees is “acceptable” we will be less likely to get meaningful policy action from countries. We wanted to drill home the scientific reality established by the IPCC: 1.5 degrees is the level at which we can hope to avoid the most catastrophic effects of climate change. 1.5 degrees, and nothing more, must be the standard.

I believe there is another, deeper reason, that has to do with cultural factors. The political arena has not accepted that catastrophic climate change is the reality of our future. When we speak on those terms we come across as alarmist and extremist and become marginalized. Of course, some of us take that risk, and do indeed become labeled extremists. So the majority makes the calculation that it is better to adhere to a watered-down message that we know is not just misleading but factually incorrect in order to maintain our position in public discourse and hopefully influence outcomes in a positive way. It’s a bit of a Faustian bargain. 

I’d like to submit that we have reached a point where that stance – putting aside ethics – is simply counterproductive. 

Instead of increasing the likelihood of action, presenting a 1.5 degree increase as a realistic scenario today does not increase the likelihood of action, it decreases it. It gives the impression that what was presented as urgent in Paris in 2015 wasn’t in the end that urgent and is probably not that urgent now. It facilitates the idea that the consequences of inaction are probably not as bad as doomsayers say and that we still have time to think of something.

It also has an impact on all sorts of decision-makers across the spheres of government and business, from central bankers to city managers. Decision-makers who do not directly read the scientific material can reasonably infer that someone somewhere is working on a 1.5 degree scenario that still has a good probability of happening. Allowing the world to think that way facilitates this sort of middle-of-the-road consensus which ultimately leads to inaction on both mitigation and adaptation. 

This risk seems to be borne out. A 2021 paper found that we are collectively “betting on the best case”. The majority of studies take 1.5 or 2 degrees scenarios as the reference case. We focus on the best cases, with low probability, and severely neglect the more likely but more adverse scenarios:  “Temperatures of 3 °C or above account for around two-thirds of the probabilistic mass for 700 ppm, but just over 10% of mentions.” [Betting on the Best Case: Florian U Jehn et al 2021 Environ. Res. Lett. 16084036]

The lack of research on what is most likely is concerning. This is why an international team of researchers led by the University of Cambridge called for a special report of the IPCC on catastrophic climate change in order to “bring into focus how much is at stake in a worst-case scenario)” in order to galvanize research and inform the public. [Climate Endgame: Exploring catastrophic climate change scenarios: Luke Kemp et al 2022 Proceedings of the National Academy of Sciences, Vol. 119 | No. 34]

The continued predominance of a reference scenario that is effectively ruled out is causing serious harm. We should stop. 

We can try to avoid being alarmist, but we need to be factual: 1) We are certain to experience a global temperature increase greater than 1.5 degrees and widespread catastrophic effects of climate change; 2) Everything we do to eliminate greenhouse gas emissions from our economy as fast as possible will have significant effect to reduce the magnitude of these catastrophic effects; and 3) We need to focus much more on adaptation and resilience of our physical environment and of our economies, particularly systemic and cascading risks. 

Letting people believe there is still a get out of jail card within reach is not helping us move forward. As a community of people knowledgeable in these matters, we need to do everything we can to ensure journalists and politicians have themselves understood the reality of the situation so we can engage in public debate with at least the best possible shared understanding of the reality of the situation.