Is limiting the temperature increase to 1.5°C still possible?

Abstract

It is always possible to find arguments to make 1.5°C forever possible, but they increasingly diverge from reality. It is time to admit that the world will cross 1.5°C and the likelihood of returning below 1.5°C via overshoot is slim. But, failing on 1.5°C does not mean the world has failed. The Paris Agreement is about balancing risks in a ‘well below 2°C’ world, and that level of ambition remains achievable despite a less certain climate outcome. We no longer need scenarios with impossibly steep emission declines to 1.5°C, but more nuanced country-level scenarios integrating national circumstances and feasible ambitions. This is harder for researchers, but more confronting for policy makers. Crossing 1.5°C is not a time to give up but a time to acknowledge our failures and find a new hope moving forward.

The worst nightmare at the end of a presentation is the question: ‘Is limiting the temperature increase to 1.5°C still possible?’. The speaker squirms and gauges who is listening, and ultimately, provides an overly diplomatic answer weighing up numerous conflicting judgements. The question is open to interpretation and is scientifically, politically, and socially charged. The answer will win or lose friends. I suspect the audience already knows the answer, they presumably only seek joy in watching the speaker squirm.

My strategy is to not answer the question, but rather, let the audience weigh up the value judgements themselves. Let's start with the observations of where we are.

The latest IPCC assessment says the world has warmed 1.09 [0.95 to 1.20]°C in the period 2011 to 2020 compared to 1850 to 1900 (IPCC, 2023). More recent updates find that the world has warmed 1.19 [1.06 to 1.30]°C in the period 2014 to 2023 (Forster et al., 2024), 1.43 [1.32 to 1.53]°C in 2023 across several datasets (Forster et al., 2024), and 1.54 ± 0.06°C in 2023 according to one dataset (Rohde, 2024). To complicate matters, the world may have already warmed as much as 0.2°C before 1850 relative to the true preindustrial (Schurer et al., 2017). The rate of warming has also increased in the last decade (Forster et al., 2024).

Within this sea of numbers, have we crossed 1.5°C already or is it inevitable? We currently lack clear definitions of what crossing 1.5°C means (Betts et al., 2023), but the day that we are splitting hairs over whether 1.5°C has been crossed, is essentially the day it has been crossed.

If the world has not crossed 1.5°C then one could argue that it is possible to avoid crossing it. However, as scientists, we should understand that the socioeconomic system driving the temperature increase has inertia.

The world emitted record amounts of CO₂ emissions (Friedlingstein et al., 2023) and greenhouse gases (UNEP, 2023) in 2023. Even though the rate of increase has slowed in the last decade, emissions are still increasing. Energy-related CO₂ emissions are expected to slightly decline by 2030 based on stated policies (IEA, 2023), but these emissions rapidly deplete the remaining carbon budget to keep below 1.5°C. Global warming is driven by cumulative CO₂ emissions, referred to as the carbon budget, and the remaining carbon budget for 1.5°C from the start of 2024 is 200 GtCO₂ (Forster et al., 2024). This will be depleted in just five years if emissions continue near current levels; noting, one year has already passed.

To stop the accumulation of CO₂ in the atmosphere and further global warming, CO₂ emissions need to go to zero, accompanied by substantial declines in non-CO₂ greenhouse gas emissions. This makes it possible to rephrase the question: ‘Is it possible to get to zero CO₂ emissions emitting less than 200 GtCO₂’? If that was a straight line from 2024, zero CO₂ emissions would be reached around 2034. That is in the future, but is it possible to get from record-high CO₂ emissions to zero CO₂ emissions in the space of a decade?

To get to zero CO₂ emissions, the CO₂ emissions from coal, oil, gas, and cement would have to go to near zero or utilise Carbon Capture and Storage (CCS), and the remaining emissions need to be offset by Carbon Dioxide Removal (CDR). Deforestation would go to or near zero. Non-CO₂ emissions from agriculture would also have to drop by around one-third. In parallel, this fossil-based energy system would need to be replaced by clean energy sources over the same time frame to deliver food, shelter, and mobility, and allow billions to gain higher living standards. Even if this was technically possible in the space of a decade, of which I have my doubts, the state of world politics is nowhere near even pondering such a step change.

But, there remains two get-out-of-jail cards

Most scenarios consistent with 1.5°C have a peak and decline profile: they first cross 1.5°C before returning below 1.5°C at a later date, known as temperature overshoot (Geden and Löschel, 2017). Overshoot scenarios are not that different to scenarios that avoid overshoot (Riahi et al., 2022). Overshoot scenarios require all the same elements of a scenario that stays below 1.5°C – zero CO₂ emissions with reductions in non-CO₂ emissions – but the speed and timing determine the climate outcome. Put differently, there is no scenario where the world acts slowly and finds a pathway to 1.5°C. This is because climate is a cumulative problem: each year the world emits 40 GtCO₂, then 40 GtCO₂ needs to be removed back from the atmosphere. Delay is extremely costly, especially when 1.5°C is excruciatingly close.

The IPCC finds that CDR is a necessary technology to achieve net zero CO₂ emissions (IPCC, 2023). This does not mean CDR is also a savour to allow overshooting 1.5°C. First and foremost, CDR is needed just to get to net zero CO₂ and, if necessary, net zero GHG emissions. In addition to that, CDR could play a role in achieving overshoot scenarios (returning warming to below 1.5°C), but the scales involved are immense. Hypothetically, if the world holds CO₂ emissions flat until 2030, five years hence, and then goes to net zero in the space of 10 years (by 2040), there will be an overshoot of a 1.5°C carbon budget by more than 200 GtCO₂, which would take decades to remove in the most optimistic CDR scenarios. Recalling, this CDR is additional to what is needed to achieve and maintain net zero CO₂ or GHG emissions. There is no escaping the reality: all 1.5°C scenarios, with and without overshoot, require dramatic short-term emission reductions (Riahi et al., 2022).

There is another get-out-of-jail card: Solar Radiation Management (SRM). The way in which this could be deployed will remain a contentious area for debate for some time, maybe forever, but it does offer the potential to relatively cheaply and quickly offset some warming. For it to be most functional, it is again likely that the world will need to reduce emissions somewhat in line with a 1.5°C pathway, to avoid perpetual deployment of SRM (Baur et al., 2023) and risks to other ecosystems, such as ocean acidification. SRM and CDR in the context of overshoot are not solutions if the world does not reduce emissions: they are parts of the solution when the world rapidly reduces emissions.

Is limiting the temperature increase to 1.5°C still possible? Yes, if you make optimistic assumptions on the current temperature, assume the world can transform away from fossil fuels in 5 to 10 years while replacing them with clean alternatives, and have immense faith in CDR and/or SRM at scale. As I said once before: Yes, but only in a model.

A new hope

The fundamental challenge with 1.5°C is that we are basically at 1.5°C. That is the reality. Perhaps a more challenging and relevant question is what temperature level is possible to avoid?

The relationship between temperature and cumulative CO₂ emissions says that 200 GtCO₂ leads to about 0.1°C of warming, which is five years of current emissions. Was 1.5°C possible if we started mitigating in 2020 (five years of current emissions) or 2015 (10 years of current emissions)? Maybe, one could have made a much stronger argument back then. Even if 1.5°C is nigh impossible, it remains that 1.6°C (+200 GtCO₂) or 1.7°C (+400 GtCO₂) are all ambitious but possible, albeit with higher climate impacts than 1.5°C. They will not be possible if emissions continue near current levels.

The conversation needs to shift away from ‘is 1.5°C possible’ towards ‘what is possible’ given our current circumstances. This necessarily requires weighing up different types of risks. It is not easy. The tendency to benchmark 1.5°C as possible are scenarios that implement a global carbon price uniformly applied in all countries, rich and poor, and all sectors, with perfect and least cost deployment of new technologies and retirement of old technologies. Integrated Assessment Models (IAMs) have lured us down this path with a palatable outcome (1.5°C) but using totally unrealistic assumptions on technological, political, and social grounds. These scenarios could be seen as the maximum possible ambition in a utopia, but they are unlikely very representative of how the world will mitigate.

To gauge what is possible, we need to have a much deeper conversation about what scenarios are useful. We need to move beyond the current utopia thinking of assuming the world can instantly shift from record emissions to a cost-optimal 1.5°C pathway overnight. As 1.5°C is passed, there is the risk modellers will just make more elaborate overshoot pathways and continue the roadshow of impossibly steep emission declines while, in reality, emissions continue to rise and climate impacts worsen.

Perhaps unintentionally, 1.5°C has been the greatest oversell of the climate community (activists and scientists), given the Paris Agreement clearly states ‘well below 2°C … pursuing efforts to … 1.5°C…, recognizing that this would significantly reduce the risks and impacts of climate change’. The focus on 1.5°C may have shifted the Overton Window, certainly in terms of net zero pledges, but perhaps not in terms of emission reductions. We will never know. It is long overdue to shift focus back to the wording of the Paris Agreement and its risk framing. We scientists need to learn how to communicate the risks and the benefits of mitigating. Using 1.5°C as a clarion call for climate action has possibly served its purpose. Now is the time to shift to a more risk-based approach based on an ever-evolving starting point.

The world has failed at 1.5°C, but this does not mean the world has failed. While crossing 1.5°C is a disappointing milestone, it does give the opportunity to reframe the problem. Staying aligned with the Paris Agreement ‘well below 2°C’ ambition is still very possible. A new generation of scenarios with impossibly steep emission declines is no longer useful. Rather, a mosaic of more nuanced country-level emission pathways that align with national circumstances is more useful, even if harder and more complex for researchers and more confronting for policymakers. Crossing 1.5°C is not the time to give up, but the time is to roll up our sleeves and work harder.