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Experts: The key ‘unknowns’ of overshooting the 1.5C global-warming limit Last week, around 180 scientists, researchers and legal experts gathered in Laxenburg, Austria to attend the first-ever international conference focused on the controversial topic of climate “overshoot”. This hypothesised scenario would see global temperatures initially “overshoot” the Paris Agreement ’s aspirational limit of 1.5C, before they are brought back down through techniques that would remove carbon dioxide from the atmosphere. (For more on the key talking points, new research and discussions that emerged from the three-day conference, see Carbon Brief’s full write-up of the event.) On the sidelines of the conference, Carbon Brief asked a range of delegates what they consider to be the key “unknowns” around overshoot. Below are their responses, first as sample quotes, then, in full: Dr James Fletcher : “Yes, there will be overshoot, but at what point will that overshoot peak? Are we peaking at 1.6C, 1.7C, 2.1C?” Prof Shobha Maharaj : “There are lots of places in the world where adaptation plans have been made to a 1.5C ceiling. The fact is that these plans are going to need to be modified or probably redeveloped.” Sir Prof Jim Skea : “There are huge knowledge gaps around overshoot and carbon dioxide removal.” Prof Kristie Ebi : “If there is going to be a peak – and, of course, we don’t know what that peak is – then how do you start planning?” Prof Lavanya Rajamani : “To me, a key governance unknown is the extent to which our current legal and regulatory architecture…will actually be responsive to the needs of an overshoot world.” Prof Nebojsa Nakicenovic : “One of my major concerns has been for a long time…is whether, even after reaching net-zero, negative emissions can actually produce a temperature decline.” Prof Debra Roberts : “For me, the big unknown is how all of these areas of increased impact and risk actually intersect with one another and what that means in the real world.” Prof Oliver Geden : “[A key unknown] is whether countries are really willing to commit to net-negative trajectories.” Dr Carl-Friedrich Schleussner : “This is a bigger concern that I have – that we are pushing the habitability in our societies on this planet above that limit and towards maybe existential limits.” Dr Anna Pirani : “I think that tracking global mean surface temperature on an overshoot pathway will be an important unknown.” Prof Richard Betts : “One of the key unknowns is are we going to continue to get the land carbon sink that the models produce.” Prof Hannah Daly : “The biggest unknown is whether countries can translate these global [overshoot] pathways into sustained domestic action…that is politically and socially feasible.” Dr Andrew King : “[W]e still have a lot of uncertainty around other elements in the climate system that relate more to what people actually live through.” Dr James Fletcher Former minister for public service, sustainable development, energy, science and technology for Saint Lucia and negotiator at COP21 in Paris. The key unknown is where we’re going to land. At what point will we peak [temperatures] before we start going down, and how long will we stay in that overshoot period? That is a scary thing. Yes, there will be overshoot, but at what point will that overshoot peak? Are we peaking at 1.6C, 1.7C, 2.1C? All of these are scary scenarios for small island developing states – anything above 1.5C is scary. Every fraction of a degree matters to us. Where we peak is very important and how long we stay in this overshoot period is equally important. That’s when you start getting into very serious, irreversible impacts and tipping points. Prof Shobha Maharaj Adjunct professor at the University of Fiji and a coordinating lead author for Working Group II of the IPCC’s seventh assessment First of all, there is an assumption that we’re going to go back down from overshoot. Back down is not a given. And secondly, we are still in the phase where we are talking about uncertainty. Climate scientists don’t like uncertainty. We are not acknowledging that uncertainty is the new normal… But because we’re so bogged down in terms of uncertainties, we are not moving towards [the issue of] what we do about it. We know it’s coming. We know the temperatures are going to be high. But there is little talk about the action. The focus seems to be more on how we can understand this or how we can model this, but not what we do on the ground. Especially when it comes to adaptation planning – [and around] how does this modify whatever the plans are? There are lots of places in the world where adaptation plans have been made to a 1.5C ceiling. The fact is that these plans are going to need to be modified or probably redeveloped. And no one is talking about this, especially in the areas that are least resourced in the world – which sets up a big, big problem. Sir Prof Jim Skea Chair of the Intergovernmental Panel on Climate Change (IPCC) and emeritus professor at Imperial College London ’s Centre for Environmental Policy There are huge knowledge gaps around overshoot and carbon dioxide removal. As it’s very clear from the themes of this conference, we don’t altogether understand how the Earth would react in taking carbon dioxide out of the atmosphere. We don’t understand the nature of the irreversibilities and we don’t understand the effectiveness of CDR techniques, which might themselves be influenced by the level of global warming, plus all the equity and sustainability issues surrounding using CDR techniques. Prof Kristie Ebi Professor of global health at the University of Washington ‘s Center for Health and the Global Environment There are all kinds of questions about adaptation and how to approach effective adaptation. At the moment, adaptation is primarily assuming a continual increase in global mean surface temperature. If there is going to be a peak – and of course, we don’t know what that peak is – then how do you start planning? Do you change your planning? There are places, for instance when thinking about hard infrastructure, [where overshoot] may result in a change in your plan – because as you come down the backside, maybe the need would be less. For example, when building a bridge taller. And when implementing early warning systems, how do you take into account that there will be a peak and ultimately a decline? There is almost no work in that. I would say that’s one of the critical unknowns. Prof Lavanya Rajamani Professor of international environmental law at the University of Oxford I think there are several scientific unknowns, but I would like to focus on the governance unknowns with respect to overshoot. To me, a key governance unknown is the extent to which our current legal and regulatory architecture – across levels of governance, so domestic, regional and international – will actually be responsive to the needs of an overshoot world and the consequences of actually not having regulatory and governance architectures in place to address overshoot. Prof Nebojsa Nakicenovic Distinguished emeritus research scholar at the International Institute for Applied Systems Analysis and executive director of The World In 2050. One of my major concerns has been for a long time – as it was clear that we are heading for an overshoot, as we are not reducing the emissions in time – is whether, even after reaching net-zero, negative emissions can actually produce a temperature decline…In other words, there might be asymmetry on the way down [in the global-temperature response to carbon removal] – it might not be symmetrical to the way up [as temperature rise in response to carbon emissions]. And this is really my major concern, that we are planning measures that are so uncertain that we don’t know whether they will reach the goal. The last point I want to make is that I think that the scientific community should, under all conditions, make sure that the highest priority is on mitigation. Prof Debra Roberts Honorary professor at the University of KwaZulu-Natal , coordinating lead author on the IPCC’s forthcoming special report on climate change and cities, board chair of the Red Cross Red Crescent Climate Centre and co-chair of Working Group II for the IPCC’s sixth assessment Well, I think coming from the policy and practitioner community, what I’m hearing a lot about are the potential impacts that come from the exceedance component of overshoot. What I’m not hearing a lot about is the responses to overshoot and their impacts – and how those impacts might interact with the impacts from temperature exceedance. So there’s quite a complex risk landscape emerging. It’s three dimensional in many ways, but we’re only talking about one dimension and, for policymakers, we need to understand that three dimensional element in order to understand what options remain on the table. For me, the big unknown is how all of these areas of increased impact and risk actually intersect with one another and what that means in the real world. Prof Oliver Geden Senior fellow and head of the climate policy and politics research cluster at the German Institute for International and Security Affairs and vice-chair of IPCC Working Group III [A key unknown] is whether countries are really willing to commit to net-negative trajectories. We are assuming, in science, global pathways going net negative, with hardly any country saying they want to go there. So maybe it is just an academic thought experiment. So we don’t know yet if [overshoot] is even relevant. It is relevant in the sense that if we do, [the] 1.5C [target] stays on the table. But I think the next phase needs to be that countries – or the UNFCCC as a whole – needs to decide what they want to do. Dr Carl-Friedrich Schleussner Research group leader and senior research scholar at the International Institute for Applied Systems Analysis I’m convinced that there’s an upper limit of overshoot that we can afford – and it might be not far outside the Paris range [1.5C-2C] – before human societies will be overwhelmed with the task of bringing temperatures back down again. This [societal limit] is lower than the geophysical limits or the CDR limit. The impacts of climate change and the challenges that will come with it will undermine society’s abilities to cooperatively engage in what is required to achieve long-term temperature reversal. This is a bigger concern that I have – that we are pushing the habitability in our societies on this planet above that limit and towards maybe existential limits. We may not be able to walk back from it, even if we wanted to. That is a big unknown to me. I’m convinced that there is an upper limit to how much overshoot we can afford, and it might be just about 2C or a bit above – it might not be much more than that. But we do not have good evidence for this. But I think these scenarios of going to 3C and then assuming we can go back down – I have doubts that future societies grappling with the impacts of climate change will be in the position to embark on such an endeavour. Dr Anna Pirani Senior research associate at the Euro-Mediterranean Center on Climate Change (CMCC) and former head of the Technical Support Unit for Working Group I of the IPCC I think that tracking global mean surface temperature on an overshoot pathway will be an important unknown – how to take account of natural variability in that context, to inform where we are on an overshoot pathway and how well we’re doing on it. I think, methodologically, that would prove to be a challenge. The fact that it occurs over many, many years – many decades – and, yet, we sort of think about it as a nice curve. We see these graphs that say “by the 2050s, we will be here and we’ll start declining and so on”. I think that what that actually translates to in the evolution of global surface temperatures is going to be very difficult to measure and track. Even how we report on that, internationally, in the UNFCCC [UN Framework Convention on Climate Change] context and what the WMO [World Meteorological Organization] does in terms of reporting an overshoot trajectory, that would be quite a challenge. Prof Richard Betts Head of climate impacts research in the Met Office Hadley Centre and professor at the University of Exeter One of the key unknowns is are we going to continue to get the land carbon sink that the models produce. We have got model simulations of returning from an overshoot. If you are lowering temperatures, you have got to reduce emissions. The amount you reduce emissions depends on how much carbon is taken up naturally by the system – by forests, oceans and so on. The models will do this; they give you an answer. But we don’t know whether they are doing the right thing. They have never been tested in this kind of situation. In my field of expertise, one of the key [unknowns] is how these carbon sinks are going to behave in the future. That is why we are trying to get real-world data into the models – including through the Amazon FACE project – so we can really try and narrow the uncertainties in future carbon sinks. If the carbon sinks are weaker than the models think, it is going to be even harder to reduce emissions and we will need to remove even more by carbon capture and removal. Prof Hannah Daly Professor of sustainable energy at University College Cork We know ever more about the profound – and often irreversible – damages that will be felt as we overshoot 1.5C. Yet we seem no closer to understanding what will unlock the urgent decarbonisation that remains our only way to avoid the worst impacts of climate change. Global models can show, on paper, what returning temperatures to safer levels after overshoot might look like. The biggest unknown is whether countries can translate these global pathways into sustained domestic action – over decades and without precedent in history – that is politically and socially feasible. Dr Andrew King Associate professor in climate science at the University of Melbourne I think, firstly, can we actually achieve net-negative emissions to bring temperatures down past a peak? It’s a completely different world and, unfortunately, it’s likely to be challenging and we’re setting ourselves up to need to do it more. So I think that’s a huge unknown. But then, beyond that, I think also, whilst we’ve built some understanding of how global temperature would respond to net-zero or net-negative emissions, we still have a lot of uncertainty around other elements in the climate system that relate more to what people actually live through. In our warming world, we’ve seen that global warming relates to local warming being experienced by everyone at different amounts. But, in an overshoot climate, we would see quite diverse changes for different people, different areas of the world, experiencing very different changes in our local climates. And also definitely worsening of some climate hazards and possibly reversibility in others, so a very different risk landscape as well, emerging post net-zero – and I think we still don’t know very much about that as well. Overshoot: Exploring the implications of meeting 1.5C climate goal ‘from above’ Global temperature | 08.10.25 State of the climate: 2025 on track to be second or third warmest year on record Global temperature | 29.07.25 Guest post: Why 2024’s global temperatures were unprecedented, but not surprising Global temperature | 18.06.25 Mapped: How climate change affects extreme weather around the world Attribution | 18.11.24 The post Experts: The key ‘unknowns’ of overshooting the 1.5C global-warming limit appeared first on Carbon Brief . https://www.carbonbrief.org/experts-the-key-unknowns-of-overshooting-the-1-5c-global-warming-limit/