Despite progress since the Paris Agreement, a peak in greenhouse gas emissions is only just within sight – and time is fast running out to stay below 1.5C of human-caused global warming since the preindustrial era.
As a result, almost all pathways that keep 1.5C within reach now involve a temporary “overshoot”.
This term refers to a period where the best estimate of warming exceeds 1.5C, until temperatures are brought back below the limit by removing carbon dioxide (CO2) from the atmosphere.
While this idea is growing in prominence, there have only been limited efforts to understand what it would mean to breach the 1.5C limit, even if this is only during a temporary period of overshoot.
In a new Nature paper, we present the findings of a three-year Horizon Europe-funded project, looking at what overshoot means for emissions, temperatures, climate impacts and adaptation.
Our results show that overshooting 1.5C comes with significant uncertainty in terms of warming outcomes, climate impacts and associated risks. For example, climate uncertainty means that what is referred to as a 1.5C “pathway” carries a notable risk of much greater levels of warming.
In order to hedge against the risk of higher-than-expected warming, the world would need to develop substantial capacity for “net-negative” CO2 emissions. This could be used to reverse a temporary overshoot and reduce long-term risks, if warming is no more extreme than expected.
Even so, overshoot would come with irreversible consequences for humans and ecosystems, our research finds, such as rising sea levels and ecosystem loss.
Overshoot overconfidence
The Intergovernmental Panel on Climate Change (IPCC) has been key to shaping our understanding of overshoot scenarios. In its latest sixth assessment report (AR6), the IPCC considered a range of pathways that limit median warming in 2100 to below 1.5C.
The report categorised the pathways according to their probability of breaching 1.5C, but also offered information on the amount of any expected overshoot.
Specifically, the C1 “no or limited overshoot” pathways allow an overshoot of “up to about 0.1C”. The C2 pathways return warming to 1.5C “after a high overshoot” of between “0.1C-0.3C”.
These categorisations give the impression that overshoot can be neatly and confidently constrained – to within a few tenths of a degree – and that in choosing a particular pathway, the countries of the world would have full control over the planetary thermostat.
Crucially, however, the numbers refer only to median warming outcomes. Considering the uncertainties in Earth system feedbacks, it is not possible to rule out much higher peak warming. For example, this could be up to 2.5C under C2 scenarios (at the 95th percentile of all model runs).
If the increase in temperatures is indeed much higher than expected under median warming, or if warming continues even when CO2 emissions reach net-zero, then returning to below 1.5C after an overshoot would require much more CO2 removal than thought.
Even with stringent emissions reductions, we therefore cannot rule out the possibility that reversing a 1.5C overshoot would require the removal of hundreds of billions of tonnes CO2 by 2100.
Indeed, based on the simple climate model FaIR, our findings show that 400GtCO2 of additional removals could be needed to return temperatures to 1.5C by 2100, if warming reaches the 75th percentile of expected levels rather than the median (about 1.7C instead of 1.5C, an outcome with a likelihood of one-in-four).
(This is based on generating more than 2,000 physically plausible climate outcomes for an emission pathway that limits median warming to around 1.5C and achieves net-zero CO2 by around mid-century, without the need for net-negative emissions thereafter.)
To reach 400GtCO2 of removals by 2100 would mean taking nearly 10GtCO2 out of the atmosphere every year after global CO2 emissions reach net-zero. For comparison, current removals amount to around 2GtCO2 per year, from all sources.
The 400GtCO2 of removals that could be needed to deal with higher-than-expected warming is similar to the amount of removals that is typically being relied on in 1.5C pathways, assuming median levels of warming in response to a given level of emissions.
This is shown in the figure below, where the first row illustrates the range of cumulative CO2 removal needed to return temperatures to below 1.5C by 2100, depending on how sensitive the climate is to a given level of emissions. The bottom two rows show removals in C1 “no or limited overshoot” and C2 “high overshoot” 1.5C pathways, assuming a median warming response.
Our findings imply the world may therefore need a “preventive” capacity to remove hundreds of billions of tonnes of CO2 by 2100, to hedge against the risk of higher-than-expected warming.
Moreover, given the political, economic, sustainability and other constraints on the speed and scale at which CO2 removal can be scaled up, it therefore may not be possible to rely on removals to compensate for a failure to reduce emissions in other parts of the economy.
Irreversible impacts
If warming is no more severe than expected under median outcomes, then preventive CO2 removal capacity could be used to steadily reduce temperatures after overshoot.
This could be an important way to minimise long-term climate risks following overshoot.
For example, for every 100 years of overshoot above 1.5C, our findings show that there would be an additional 40cm of sea-level rise by 2300. There would be similarly irreversible consequences for the world’s frozen ecosystems, such as permafrost and peatlands.
In addition, overshoot increases the risk of crossing irreversible climate “tipping points”.
These findings show that even if a global temperature overshoot is reversed, the temporary breach of the 1.5C limit would still come with some irreversible consequences.
Peak and decline
Our study offers a framework for minimising the risks associated with higher-than-expected warming and potentially irreversible climate impacts after temperature overshoot.
Instead of the current categories of mitigation pathway, which focus on peak warming and end-of-century temperatures – apparently with a high level of precision – our paper suggests “peak and decline” (PD) scenarios that allow us to consider a wide range of plausible climate outcomes.
These scenarios aim to achieve a peak in warming, followed by sustained temperature reductions during a period of at least several decades. Global greenhouse gas (GHG) emissions would need to decline towards net-zero CO2 to achieve temperature peaking, followed by net-negative CO2 emissions to enter a long-term decline.
The “peak” is determined by how fast emissions are reduced in the near term, towards reaching net-zero CO2 emissions. This determines the maximum cumulative CO2 emissions of a pathway and therefore the level and timing of peak warming. Importantly, the stringency of non-CO2 GHG emission reductions will also strongly affect peak warming.
The pace of global temperature “decline” after the peak – and therefore the ability to reverse a temporary exceedance of a target limit – depends on the level of net-negative CO2 emissions that can be achieved.
In PD “overshoot” pathways (PD-OS), warming exceeds 1.5C before returning to that level and staying there into the future. These are similar to PD pathways, but the carbon budget, timing of net-zero CO2 and amount of CO2 removal depends on the length, level and timing of overshoot.
In PD “enhanced protection” pathways (PD-EP), warming is kept as low as possible and gradually reversed over time, to minimise climate risks. They entail stringent, rapid cuts in GHG emissions, achieving net-zero CO2 as soon as possible and using sustainable levels of CO2 removal to reduce warming over time, potentially reaching net-zero or even net-negative GHGs.
These pathways are illustrated in the figure below, where the 1.5C limit is shown as a horizontal dotted line, and the different peak and decline pathways are contrasted with a scenario in which temperatures continue to increase, despite reaching net-zero CO2.
Our findings suggest that a peak and decline “enhanced protection” pathway would offer the best way to hedge against the uncertainties and minimise the risks around overshoot and the response of the climate system. This would entail two actions from countries worldwide.
First, it would mean reducing emissions as fast as possible to slow down temperature increase, reduce peak warming, and reduce the dependency of needing large amounts of CO2 removals to even achieve net-zero CO2 emissions.
Second, it would mean rapidly scaling up global capacity for CO2 removal to hedge against high-risk outcomes from stronger than expected climate feedback.
The scale of preventive removal capacity that we estimate could be needed, is only just achievable within sustainable limits. If some removal capacity is used to compensate for a failure to rapidly reduce emissions, then it would not be available to manage higher-than-expected warming.
Overall, our paper reinforces the idea that earlier emissions reductions are the best way to minimise far-reaching climate risks in the 21st century and beyond.
The post Guest post: How to minimise the risks from overshooting the 1.5C limit appeared first on Carbon Brief.
Guest post: How to minimise the risks from overshooting the 1.5C limit
When José Antonio Kast took office as Chile’s new president in March, one of his first moves was to put the brakes on plans to expand two protected marine parks – raising doubts about the country’s high-profile bid to host the headquarters of the High Seas Treaty.
But during the UN General Assembly last month, the right-wing leader reaffirmed his leftist predecessor’s ambition for Chile to host the landmark global pact, which came into effect in January and provides a legal framework to protect the waters of the high seas beyond national jurisdiction that cover about two-thirds of the world’s oceans.
Hailing the country’s “maritime vocation”, Kast’s government said the bid to host the treaty’s secretariat in the port city of Valparaíso was state policy and testament to its commitment to multilateralism.
“For the government of Chile, it is of high interest to achieve this recognition, and we will carry out all the efforts to obtain the necessary support,” Foreign Minister Francisco Pérez Mackenna was quoted as saying by local media.
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Tough competition from Chinese, Belgian bids
But to garner the votes it needs, Chile must fend off competing bids by Belgium – which has proposed its well-connected capital, Brussels – and China, whose well-funded bid includes the provision of free premises in the coastal city of Xiamen and five years of free utility costs.
Parties to the treaty, formally called the Agreement on Marine Biological Diversity of Areas Beyond National Jurisdiction (BBNJ), will choose the headquarters at their first summit (COP1), in New York from January 11 to January 22, 2027. They will seek consensus, falling back on a two-thirds majority in successive secret ballots.
Chile has offered to provide a restored waterside warehouse “at its own expense” as office space, pitching itself to developing countries as a Global South alternative to Europe-centred Brussels. Among richer countries, it is promoting its democratic credentials and greater transparency as an alternative to China.
Rolling back environmental safeguards at home?
While the South American country remains the top pick among environmental campaigners and other civil society groups, critics of Kast’s government say recent policy moves may make it harder for the country to garner the support it needs.
On March 12, the day after Kast took office, the Environment Ministry withdrew 43 decrees awaiting legal approval, including a push to expand the Mar de Juan Fernández and Nazca-Desventuradas marine parks – vast protected areas in Chile’s Pacific Ocean waters. Both remain under review, the ministry told Climate Home News.
The decrees also included an emission standard for coal power plants and regulations for Chile’s new biodiversity and protected areas service.
A month after holding up the decrees, Kast questioned aspects of Chile’s urban wetlands protection law, suggesting it was sometimes an unjustified obstacle to much-needed housing development, and his administration has also sought to ease the rules on salmon farming in the world’s second-largest producer.
In August, the Constitutional Court struck down the salmon farming reform after a challenge by opposition lawmakers and warnings from green groups including Greenpeace, which said it could help farms relocate into protected areas such as the Kawésqar National Reserve.
Valparaíso mayor Camila Nieto, from former leftist President Gabriel Boric’s party, backs the bid to host the High Seas Treaty, but said she hoped “the government’s support also translates into public policies consistent with the goals of protecting and conserving the oceans”.
“The decisions a country makes on environmental matters can influence international perceptions of its commitment to protecting the oceans. That is something we cannot ignore,” Nieto told Climate Home News.
China’s bid stirs concerns over data access, fishing
Chile’s past record on marine protection – it has protected 43% of its jurisdictional waters – could yet give it an edge.
“Chile deserves it for its conservation record as a state. For more than 15 years Chile has been following this path,” Liesbeth van der Meer, executive director at conservation group Oceana in Chile, told Climate Home News.
Concerns in some quarters about the rival bid by China – for example, over a potential conflict of interest due to the vast Chinese fishing industry – may also weigh in Chile’s favour, experts say.
Chinese-flagged vessels did about 30% of detected high seas fishing between 2022 and 2024, according to an Oceana analysis of Global Fishing Watch data. China also sponsors five of the International Seabed Authority’s 31 deep-sea mining exploration contracts, more than any other country.
The secretariat would host sensitive marine data, making transparency paramount, experts say.
China says hosting the secretariat in Xiamen would help make the treaty more globally representative, fostering cooperation between rich and developing countries and aiding equitable access to marine science and technology.
Wang Yi, China’s minister of foreign affairs, said in a statement that the country has “all along championed true multilateralism” and “firmly defended” UN institutions. Given Xiamen’s $38-billion ocean industry, the city lives up to the treaty’s “significance and promising future”, he added.
Meanwhile, Chile is proposing the treaty’s first high seas marine protected area for the Salas y Gómez and Nazca ridges, a chain of more than 110 seamounts stretching about 4,000 km from off Peru to Rapa Nui (Easter Island) that is rich in endemic fish and other marine species.
The ridges are “a true oasis of productivity”, said Carlos Gaymer, director of the Centre for Ecology and Sustainable Management of Oceanic Islands at Universidad Católica del Norte and co-author of a 2021 scientific review of the area.
The water is so clear that phytoplankton thrive around 200 metres down, feeding zooplankton, small fish and, up the chain, sharks and seabirds. Whales, turtles and sharks use the ridges as “a real highway” across the Pacific, Gaymer said. Parts of the Nazca ridge are likely nursery grounds for jack mackerel and swordfish.
Chile has already asked the body that regulates the industry in the area – the South Pacific Regional Fisheries Management Organisation (SPRFMO) – to close the ridges to all fishing vessels. According to Chilean data, more than 80% of fishing in the area involves Chinese vessels.
Meeting on the Chilean request in early September, SPRFMO’s scientific committee agreed only to recommend closing the area to bottom fishing. SPRFMO’s commission will take the final decision at its next meeting, in early 2027.
For Gaymer, Chile’s push to close the area to fishing should be seen as a “starting point” for the treaty proposal.
“Every time there’s a new expedition, species new to science appear,” Gaymer said. “If you don’t protect areas like these … those species simply disappear from the planet.”
The post Race to host High Seas Treaty HQ heats up as Chile reaffirms bid appeared first on Climate Home News.
Race to host High Seas Treaty HQ heats up as Chile reaffirms bid
Mohamed Adow is the founder and director of Power Shift Africa.
What does it mean to ask a continent to adapt to a crisis it did little to create? For Africa, that question has stopped being philosophical as it is answered every day in flooded communities, failed harvests, disappearing livelihoods and public budgets stretched by disasters they cannot afford.
Africa produces a negligible share of global emissions, but between 1970 and 2021 it accounted for around 35% of climate-related deaths worldwide. The continent is therefore confronting a brutal contradiction that is exemplified by the fact that those who have contributed least to the climate crisis are carrying some of its heaviest costs.
And that explains why, for us, it is worrying that much of the climate debate treats adaptation as a technical exercise. Build better infrastructure, they say. Improve early-warning systems, they urge. Develop drought-resistant crops, they preach. Strengthen disaster preparedness, they yell. All of these things matter, but they do not answer the harder question of why some people and countries are so much more vulnerable in the first place.
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That is the question we have spent the last couple of years trying to answer, and it culminates in our recently launched thought-provoking report that shows how Africa’s vulnerability is not an accident of geography, but has instead been shaped by history, economics and power.
Colonial extraction, unequal development, debt dependency, global economic asymmetries and exclusion from international decision-making have all constrained the resources and choices available to African countries, and climate change is intensifying those existing inequalities, shows the report, titled “A Just Transition for Adaptation: A Framework and Vision for Africa”.
Widening adaptation finance gap
I say it is ‘thought-provoking’ because it asks us to rethink adaptation itself – not as a collection of projects designed to help people cope with climate impacts, but as a question of justice, power and structural change.
That distinction might not sit well with some people, but for Africa it matters, because there is a danger in asking vulnerable countries simply to become more “resilient”. Resilient against what? How can you ask people to toughen up while letting them be strangled by a system that continues to reproduce the very inequalities that have made resilience so difficult?
If you want to know where the problem is, see where the money is coming from and where it is going. That becomes quite stark when you consider the fact that Africa’s annual adaptation needs are close to $70 billion but its adaptation finance flows are only around $14 billion.
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At the same time, a growing proportion of climate finance is being delivered through commercial loans rather than grants. That means countries already under severe fiscal pressure are being asked to borrow to protect themselves from climate impacts they did little to cause.
There is something deeply wrong with a system in which climate-vulnerable countries are spending more of their own public money on adaptation than they receive in international support, because that system turns climate injustice into a financial transaction, with those least responsible carrying more of the cost.
From extractivism to African ownership
And, as the new report shows, the problem goes beyond finance. Africa’s land, minerals, forests and other resources have become central to the global transition towards lower-carbon economies. Markets and green industrial projects may offer genuine opportunities, but without African ownership, participation and accountability, there is a risk that old patterns of extraction will simply be repackaged in the language of the green economy, and that is why we argue that a transition can be low-carbon and still be unjust.
It is precisely because of this risk that the report’s idea of a just transition for adaptation becomes important as it expands the idea beyond the familiar focus on emissions and energy systems. It asks what justice looks like when the goal is not simply to reduce carbon, but to enable societies to withstand a changing climate without reproducing the inequalities that made them vulnerable.
The report places four dimensions of justice at the centre: recognition, participation, distribution and rest