Questions and answers following the CMCC Lecture, “Overshoot: Challenges and Choices After We Exceed 1.5°C,” focused on topics such as carbon dioxide removal, nuclear energy, negative emissions, climate risks, and CO2 and CH4 emissions. Andy Reisinger addressed questions from the lecture audience.

Andy Reisinger’s lecture “Overshoot: challenges and choices after we exceed 1.5°C” was very well attended and generated a great deal of interest among those who followed it. So much so that in the sixty minutes dedicated to the lecture, it was not possible to answer all the questions posed to the speakers through the chat.

To address any remaining questions, CMCC has collected all the comments received during the webinar and organized them into thematic questions, which we have presented to the lecture speaker. Here Below Andy Reisinger’s answers.

One of the topics that most animated the discussion following the lecture was CDR – Carbon Dioxide Removal. Considering how these technologies are currently expensive and energy-demanding, what role can CDR play in the perspective of Overshoot scenarios?

A core requirement for achieving a decline of global warming levels back to 1.5 degrees will be to achieve sustained globally net negative CO₂ emissions. This will require large-scale carbon dioxide removal. CDR is therefore an absolute requirement to achieve a decline in global temperature, whether we like it or not.

But yes, all CDR technologies come with some trade-offs, whether it be energy demand, or demand for land or water. The most widely practiced CDR ‘technology’ today is afforestation, but this is also not without trade-offs, as it requires land for trees to be planted on, and stable governance to ensure those trees remain standing in the face of climate impacts in the form of wildfires and droughts, and of course, competing demands for this land.

Being able to upscale CDR will therefore depend on a portfolio approach, so that the most suitable or least problematic solution can be deployed in different contexts.

But CDR will not be able to deliver net-negative emissions on its own. A critical prerequisite is to reduce gross emissions as far as possible, since otherwise we will need large amounts of CDR just to counterbalance remaining emissions to reach net-zero. Reaching net-negative emissions relies on minimizing residual, gross emissions as well as upscaling CDR.

In some cases, such reductions of gross emissions can also deliver important synergies with CDR. For example, a key concern with afforestation or bioenergy combined with carbon capture and storage (BECCS) to achieve CO₂ removals is that this could jeopardise food security and destroy native ecosystems. A dietary shift towards less reliance on extensive ruminant meat production would reduce reliance on that land for food production and enable greater rates of afforestation on degraded land without jeopardizing food security or compromising existing native ecosystems. However, none of those options are ‘easy’, they will require concerted efforts with equity front of mind.

What role does the Overshoot theme have in UNFCCC negotiations and under the Paris Agreement? 

The Paris Agreement does not explicitly mention overshoot, nor does it rule it out. To some extent, very little changes with an exceedance of 1.5 degrees since the goal of limiting warming to ‘well’ below 2 degrees remains feasible, and reaching globally net zero greenhouse gas emissions during the second half of the 21^st century (and as soon as possible) still the critical objective.

However, past Conferences of the Parties have placed particular emphasis on the 1.5-degree element of the long-term temperature goal, but have not yet explicitly addressed the fact that we are about to shoot past this limit. Passing this limit will likely increase an emphasis on loss and damage and raise demands for adaptation finance and other support; it will likely also increase debates about responsibility for failure to limit warming to that level. It is far from clear that passing the 1.5-degree limit will in itself spur greater action on mitigation.

Claims for what actions are still consistent with 1.5 degrees will change: once we are past that limit, ‘keeping 1.5 degrees alive’ will require a long-term goal and plan to bring temperature back down again, which means to achieve sustained net-negative CO₂ emissions and potentially even net-negative GHG emissions. That realization will be a critical step in collective action, including in corporate action alongside the Paris Agreement. Globally net-negative emissions can only be achieved if front-runner countries start to explicitly adopt long-term net-negative targets in their Low-Emissions Development Pathways, and develop credible policies and plans to achieve those.

For countries to take this step, many will rely on an explicit shared understanding in the UNFCCC that a decline in global temperature is indeed a global objective, facilitated by e.g. a structured expert dialogue on overshoot and the consequences that this brings for collective and individual country actions.

In the context of analyses related to Overshoot, how significant are greenhouse gas emissions other than CO₂? For example, how important is it to reduce CH₄ emissions and what role does the release of methane resulting from permafrost melting play as a consequence of global warming exceeding the 1.5°C threshold?

CH₄ is the second most important greenhouse gas globally after CO₂. Current CH₄ emissions are contributing a little over 0.5 degrees to the total warming from all human activities of just over 1.3 degrees. The total amount of warming the world experiences at any time will always be the sum of warming from CO₂ and from other gases (as well as aerosols and albedo changes).

Because of the relatively short lifetime of CH₄, the total amount of warming caused by CH₄ depends mostly on the recent rate of emissions, not cumulative emissions as for CO₂. I.e. it matters much less for future warming in the 21^st century how much CH₄ we have already emitted during 20^th and 19^th century – the ‘historical liability’ from past emissions is much less significant than it is for CO₂. This means that significantly reducing future CH₄ emissions would reduce the total amount of warming that the world experiences in future. Significant and sustained further CH₄ reductions after the time when temperature peaks could help deliver a decline in global temperature.

There are two key challenges to make this contribution a reality though.

One challenge is that, in order to limit peak warming to as low a level as possible, most economic models suggest that global CH₄ emissions would (or at least could) have already been reduced by about 50% by the time global temperature peaks. The CH₄  emissions still left at this point are generally considered ‘hard to abate’, mainly from agriculture. Making deep further reductions in those remaining emissions will pose formidable challenges given the large diversity of agricultural production systems and the role that livestock agriculture plays in many developing countries not only in food security but also poverty alleviation, as insurance mechanism etc.

The second is that the amount of temperature reduction that could be achieved by further reducing CH₄ emissions is finite. If the world still emits, say, 200 Mt CH₄ per year in 2050, then an extremely ambitious scenario (that would rely on demand-side changes plus deployment of new technologies such as a vaccine against CH₄ production in ruminant animals) could perhaps reduce emissions by another 100-150 Mt CH₄. That would go beyond what even the most ambitious model studies assume at the moment. Such a reduction would lower global temperature by about 0.2 degrees below the peak over the next 50-100 years. While this reduction in temperature would be non-trivial if we have not shot too far past 1.5 degrees, that is all that a further reduction in CH₄ emissions can achieve. It can therefore complement efforts to reach and sustain net-negative CO₂ emissions but it likely cannot substitute for them.

Other long-lived non-CO₂ gases such as N₂O will also have a cumulative effect on global warming, more like CO₂. Their contribution globally is much smaller than that of CO₂, but they need to be considered in strategies to halt global warming in the first instance and they could be significant at country level. We usually use net-zero CO₂ as the key milestone to halt global warming, but really it is net-zero long-lived GHGs that we need to reach to halt global warming at any level (which is also why the IPCC said that halting global warming at any level requires “at least” net zero CO₂). The more we can reduce emissions of other long-lived GHGs, the lower peak warming and the more feasible a gradual decline in global temperature after the peak will become.

The IPCC is working on the Special Report on Climate Change and Cities. Is there anything specific we can say about the implications of Overshoot for cities, especially regarding the need for mitigation and adaptation options in urban environments?

Cities are obviously exposed to climate extremes, and they will be at increasing risk as warming levels continue to climb past 1.5 degrees. I’m no expert on cities so would not want to speculate, but I’m not aware of a large body of research to look at overshoot (in the sense of peak-and-decline) specifically for cities. It is perhaps worth noting though that cities can play a crucial role in the demand for products and services (energy, food, transport, materials) that will be essential to allow deep reductions in gross emissions, but could also contribute to low-emissions energy supply via solar photovoltaics.

How cities adapt to climate change, how we ensure that institutions and governance systems can cope with the increasing pressures from climate-related extremes and more insidious changes will also be essential to maintain resilience and to be able to benefit from an eventual reduction in the frequency and intensity of extreme events.

What role do you see for nuclear power in the portfolio of solutions that should lead to a reduction in emissions necessary to stay within the 1.5°C threshold or, in the case of overshoot, to bring global temperature back below the limit?

Nuclear is one of many forms of low-emissions energy, but it comes with its own well-known problems and of course relies on an extractive industry for its fuel. It may have a role to play in some contexts, but in others, renewables (which do not need large-scale deployment to be economically viable) will be cheaper, face less public resistance and lesser risk of cost overruns and delays in deployment. Nuclear can only operate on the side of reducing gross emissions, it cannot help on its own with CDR.

Bioenergy is the only energy supply system that in principle could achieve net-negative emissions directly if it is combined with carbon capture and storage (BECCS), which is why many economic models that seek to achieve a decline in temperature deploy BECCS at very large scales.

Are there climate impacts that can be irreversible once the 1.5°C threshold is exceeded? What is their role in Overshoot scenarios?

Many climate impacts are irreversible, for example species extinctions, and of course human deaths. A destruction of social structures and institutions can also be effectively irreversible for generations. However, most of those impacts do not happen precisely on a hard line at 1.5 degrees, but rather on an escalating continuum where 1.5 degrees is a meaningful reference point but not a binary step.

Perhaps a more relevant question though is which risks are irreversible, in the sense of which impacts that have not yet happened would become less likely to happen (or less damaging if they happen) if temperature comes down again. So, while every single death during a heat wave is irreversible, the risk posed by heat waves (i.e. the number of people we expect to die during each event) would likely reduce again if temperature declined.

The risk for many ecosystems though would not necessarily reverse, since the loss of a few keystone species can disrupt food webs and trophic connections that make a transformation of an ecosystem near-permanent even if temperature declines again.

And finally, sea level will continue to rise even if we bring temperature back down again, albeit at a lower rate than if it remains elevated. So there are significant benefits from reversing global temperature, but that does not mean that the associated risks will be reversed.

When we talk about ‘business as usual” for emissions, we are referring to emissions that remain very high. In the current trend, we see that emissions are decreasing, albeit very slowly. If we consider this slow, very slow reduction in emissions, what trajectory does it outline for the increase in the global average temperature?

Global emissions are not yet decreasing – we may be near a plateau, but there is no firm indication that global emissions have indeed peaked. Given that CO₂ is the dominant GHG globally, and given its long lifetime in the atmosphere, temperature will continue to rise even if CO₂ emissions come down significantly. Global CO₂ emissions will need to get to net zero for temperature to stabilize.

Reductions of the emissions of short-lived gases like CH₄ could make a contribution towards a decline in temperature and would limit the level at which global temperature peaks, but unless we reach near-zero global CO₂ emissions, temperature will continue to rise given that CO₂ is globally the dominant driver of global warming. This is a key prerequisite for then taking the next step to turn the temperature trend around and achieve net-negative CO₂ emissions.

While a large number of countries have set targets to reach at least net-zero CO₂ emissions, many do not have clear plans let alone policies to achieve those targets. Under current policies, global GHG emissions might more or less stabilize at or somewhat below current levels, with warming reaching 2.5 degrees or more by 2100 and continuing to rise thereafter.

If all net-zero targets were achieved, this could limit warming to below 2 degrees, but there is a very large gap between many targets and the policies actually in place to achieve them.

In terms of adaptation options, how do we assess how significant the risks we would avoid in an Overshoot scenario are?  

That is a key research question. Most research to date has focused on how risks and impacts increase with rising temperature, not on how much risks and impacts decline if temperature comes back down again. The reason why this does not have a simple answer is that risks are driven not only by climate hazards but also by the exposure and vulnerability to those hazards, and those will in themselves change over time in an overshoot trajectory during the period of temperature exceedance. In addition, how we respond to climate change can also create new risks.

As a result, there will be some risks that are virtually impossible to reduce again even if temperature reduces, while others could be mostly reversed. Some risks might even be smaller than they were at the outset if effective adaptation during the overshoot period causes a sustained and durable reduction in exposure and/or vulnerability (this could include managed retreat, early warning systems, or improved governance and institutions to strengthen preparedness before and recovery after extreme events).

It is difficult to classify which risks belong to which category because how the different drivers of risk change under overshoot will critically depend on local context – not only the system affect but social capacity, finance, and response strategies. But this will be an important research area since it provides the value proposition why it is worthwhile to bring temperature back down again in the first place. In many cases we can be confident that risks would be lower if temperature comes down again than it if remains elevated well above 1.5 degrees, but how much lower, which systems and places would benefit most, and under what conditions and adaptation and mitigation pathways this would be the case, is a critical research question.

Other resources:

• Climate u-turn? What happens if we exceed 1.5°C and then go back – an article from Climate Foresight, the CMCC digital Magazine
• What will a world that exceeds the 1.5°C goal look like? – from the CMCC Linkedin newsletter, an issue dedicated to Overshoot
• Overshoot: what does it mean to exceed and return to 1.5 ºC? A resource to understand overshoot in the context of the science-policy interface – a webpage dedicated to the paper Overshoot: A Conceptual Review of Exceeding and Returning to Global Warming of 1.5°C, Andy Reisinger, Jan S. Fuglestvedt, Anna Pirani, Oliver Geden, Chris D. Jones, Shobha Maharaj, Elvira Poloczanska, Angela Morelli, Tom Gabriel Johansen, Carolina Adler, Richard A. Betts, Sonia I. Seneviratne. Annu. Rev. Environ. Resour. 2025. 50:1.1–1.33. https://doi.org/10.1146/annurev-environ-111523-102029