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.”
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The post Experts: The key ‘unknowns’ of overshooting the 1.5C global-warming limit appeared first on Carbon Brief.
Experts: The key ‘unknowns’ of overshooting the 1.5C global-warming limit
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Drought and heatwaves occurring together – known as “compound” events – have “surged” across the world since the early 2000s, a new study shows.
Compound drought and heat events (CDHEs) can have devastating effects, creating the ideal conditions for intense wildfires, such as Australia’s “Black Summer” of 2019-20 where bushfires burned 24m hectares and killed 33 people.
The research, published in Science Advances, finds that the increase in CDHEs is predominantly being driven by events that start with a heatwave.
The global area affected by such “heatwave-led” compound events has more than doubled between 1980-2001 and 2002-23, the study says.
The rapid increase in these events over the last 23 years cannot be explained solely by global warming, the authors note.
Since the late 1990s, feedbacks between the land and the atmosphere have become stronger, making heatwaves more likely to trigger drought conditions, they explain.
One of the study authors tells Carbon Brief that societies must pay greater attention to compound events, which can “cause severe impacts on ecosystems, agriculture and society”.
Compound events
CDHEs are extreme weather events where drought and heatwave conditions occur simultaneously – or shortly after each other – in the same region.
These events are often triggered by large-scale weather patterns, such as “blocking” highs, which can produce “prolonged” hot and dry conditions, according to the study.
Prof Sang-Wook Yeh is one of the study authors and a professor at the Ewha Womans University in South Korea. He tells Carbon Brief:
“When heatwaves and droughts occur together, the two hazards reinforce each other through land-atmosphere interactions. This amplifies surface heating and soil moisture deficits, making compound events more intense and damaging than single hazards.”
CDHEs can begin with either a heatwave or a drought.
The sequence of these extremes is important, the study says, as they have different drivers and impacts.
For example, in a CDHE where the heatwave was the precursor, increased direct sunshine causes more moisture loss from soils and plants, leading to a drought.
Conversely, in an event where the drought was the precursor, the lack of soil moisture means that less of the sun’s energy goes into evaporation and more goes into warming the Earth’s surface. This produces favourable conditions for heatwaves.
The study shows that the majority of CDHEs globally start out as a drought.
In recent years, there has been increasing focus on these events due to the devastating impact they have on agriculture, ecosystems and public health.
In Russia in the summer of 2010, a compound drought-heatwave event – and the associated wildfires – caused the death of nearly 55,000 people, the study notes.
The record-breaking Pacific north-west “heat dome” in 2021 triggered extreme drought conditions that caused “significant declines” in wheat yields, as well as in barley, canola and fruit production in British Columbia and Alberta, Canada, says the study.
Increasing events
To assess how CDHEs are changing, the researchers use daily reanalysis data to identify droughts and heatwaves events. (Reanalysis data combines past observations with climate models to create a historical climate record.) Then, using an algorithm, they analyse how these events overlap in both time and space.
The study covers the period from 1980 to 2023 and the world’s land surface, excluding polar regions where CDHEs are rare.
The research finds that the area of land affected by CDHEs has “increased substantially” since the early 2000s.
Heatwave-led events have been the main contributor to this increase, the study says, with their spatial extent rising 110% between 1980-2001 and 2002-23, compared to a 59% increase for drought-led events.
The map below shows the global distribution of CDHEs over 1980-2023. The charts show the percentage of the land surface affected by a heatwave-led CDHE (red) or a drought-led CDHE (yellow) in a given year (left) and relative increase in each CDHE type (right).
The study finds that CDHEs have occurred most frequently in northern South America, the southern US, eastern Europe, central Africa and south Asia.
Threshold passed
The authors explain that the increase in heatwave-led CDHEs is related to rising global temperatures, but that this does not tell the whole story.
In the earlier 22-year period of 1980-2001, the study finds that the spatial extent of heatwave-led CDHEs rises by 1.6% per 1C of global temperature rise. For the more-recent period of 2022-23, this increases “nearly eightfold” to 13.1%.
The change suggests that the rapid increase in the heatwave-led CDHEs occurred after the global average temperature “surpasse[d] a certain temperature threshold”, the paper says.
This threshold is an absolute global average temperature of 14.3C, the authors estimate (based on an 11-year average), which the world passed around the year 2000.
Investigating the recent surge in heatwave-leading CDHEs further, the researchers find a “regime shift” in land-atmosphere dynamics “toward a persistently intensified state after the late 1990s”.
In other words, the way that drier soils drive higher surface temperatures, and vice versa, is becoming stronger, resulting in more heatwave-led compound events.
Daily data
The research has some advantages over other previous studies, Yeh says. For instance, the new work uses daily estimations of CDHEs, compared to monthly data used in past research. This is “important for capturing the detailed occurrence” of these events, says Yeh.
He adds that another advantage of their study is that it distinguishes the sequence of droughts and heatwaves, which allows them to “better understand the differences” in the characteristics of CDHEs.
Dr Meryem Tanarhte is a climate scientist at the University Hassan II in Morocco, and Dr Ruth Cerezo Mota is a climatologist and a researcher at the National Autonomous University of Mexico. Both scientists, who were not involved in the study, agree that the daily estimations give a clearer picture of how CDHEs are changing.
Cerezo-Mota adds that another major contribution of the study is its global focus. She tells Carbon Brief that in some regions, such as Mexico and Africa, there is a lack of studies on CDHEs:
“Not because the events do not occur, but perhaps because [these regions] do not have all the data or the expertise to do so.”
However, she notes that the reanalysis data used by the study does have limitations with how it represents rainfall in some parts of the world.
Compound impacts
The study notes that if CDHEs continue to intensify – particularly events where heatwaves are the precursors – they could drive declining crop productivity, increased wildfire frequency and severe public health crises.
These impacts could be “much more rapid and severe as global warming continues”, Yeh tells Carbon Brief.
Tanarhte notes that these events can be forecasted up to 10 days ahead in many regions. Furthermore, she says, the strongest impacts can be prevented “through preparedness and adaptation”, including through “water management for agriculture, heatwave mitigation measures and wildfire mitigation”.
The study recommends reassessing current risk management strategies for these compound events. It also suggests incorporating the sequences of drought and heatwaves into compound event analysis frameworks “to enhance climate risk management”.
Cerezo-Mota says that it is clear that the world needs to be prepared for the increased occurrence of these events. She tells Carbon Brief:
“These [risk assessments and strategies] need to be carried out at the local level to understand the complexities of each region.”
The post Heatwaves driving recent ‘surge’ in compound drought and heat extremes appeared first on Carbon Brief.
Heatwaves driving recent ‘surge’ in compound drought and heat extremes
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