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
The governor’s office said the city’s two main reservoirs could dry up by May, much sooner than previous timelines. But authorities still offer no plan for curtailment of water use.
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Lena Luig is the head of the International Agricultural Policy Division at the Heinrich Böll Foundation, a member of the Global Alliance for the Future of Food. Anna Lappé is the Executive Director of the Global Alliance for the Future of Food.
As toxic clouds loom over Tehran and Beirut from the US and Israel’s bombardment of oil depots and civilian infrastructure in the region’s ongoing war, the world is once again witnessing the not-so-subtle connections between conflict, hunger, food insecurity and the vulnerability of global food systems dependent on fossil fuels, dominated by a few powerful countries and corporations.
The conflict in Iran is having a huge impact on the world’s fertilizer supply. The Strait of Hormuz is a critical trade route in the region for nearly half of the global supply of urea, the main synthetic fertilizer derived from natural gas through the conversion of ammonia.
With the Strait impacted by Iran’s blockades, prices of urea have shot up by 35% since the war started, just as planting season starts in many parts of the world, putting millions of farmers and consumers at risk of increasing production costs and food price spikes, resulting in food insecurity, particularly for low-income households. The World Food Programme has projected that an extra 45 million people would be pushed into acute hunger because of rises in food, oil and shipping costs, if the war continues until June.
Pesticides and synthetic fertilizer leave system fragile
On the face of it, this looks like a supply chain issue, but at the core of this crisis lies a truth about many of our food systems around the world: the instability and injustice in the very design of systems so reliant on these fossil fuel inputs for our food.
At the Global Alliance, a strategic alliance of philanthropic foundations working to transform food systems, we have been documenting the fossil fuel-food nexus, raising alarm about the fragility of a system propped up by fossil fuels, with 15% of annual fossil fuel use going into food systems, in part because of high-cost, fossil fuel-based inputs like pesticides and synthetic fertilizer. The Heinrich Böll Foundation has also been flagging this threat consistently, most recently in the Pesticide Atlas and Soil Atlas compendia.
We’ve seen this before: Russia’s invasion of Ukraine in 2022 sparked global disruptions in fertilizer supply and food price volatility. As the conflict worsened, fertilizer prices spiked – as much from input companies capitalizing on the crisis for speculation as from real cost increases from production and transport – triggering a food price crisis around the world.
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Since then, fertilizer industry profit margins have continued to soar. In 2022, the largest nine fertilizer producers increased their profit margins by more than 35% compared to the year before—when fertilizer prices were already high. As Lena Bassermann and Dr. Gideon Tups underscore in the Heinrich Böll Foundation’s Soil Atlas, the global dependencies of nitrogen fertilizer impacted economies around the world, especially state budgets in already indebted and import-dependent economies, as well as farmers across Africa.
Learning lessons from the war in Ukraine, many countries invested heavily in renewable energy and/or increased domestic oil production as a way to decrease dependency on foreign fossil fuels. But few took the same approach to reimagining domestic food systems and their food sovereignty.
Agroecology as an alternative
There is another way. Governments can adopt policy frameworks to encourage reductions in synthetic fertilizer and pesticide use, especially in regions that currently massively overuse nitrogen fertilizer. At the African Union fertilizer and Soil Health Summit in 2024, African leaders at least agreed that organic fertilizers should be subsidized as well, not only mineral fertilizers, but we can go farther in actively promoting agricultural pathways that reduce fossil fuel dependency.
In 2024, the Global Alliance organized dozens of philanthropies to call for a tenfold increase in investments to help farmers transition from fossil fuel dependency towards agroecological approaches that prioritize livelihoods, health, climate, and biodiversity.
In our research, we detail the huge opportunity to repurpose harmful subsidies currently supporting inputs like synthetic fertilizer and pesticides towards locally-sourced bio-inputs and biofertilizer production. We know this works: There are powerful stories of hope and change from those who have made this transition, despite only receiving a fraction of the financing that industrial agriculture receives, with evidence of benefits from stable incomes and livelihoods to better health and climate outcomes.
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Inspiring examples abound: G-BIACK in Kenya is training farmers how to produce their own high-quality compost; start-ups like the Evola Company in Cambodia are producing both nutrient-rich organic fertilizer and protein-rich animal feed with black soldier fly farming; Sabon Sake in Ghana is enriching sugarcane bagasse – usually organic waste – with microbial agents and earthworms to turn it into a rich vermicompost.
These efforts, grounded in ecosystems and tapping nature for soil fertility and to manage pest pressures, are just some of the countless examples around the world, tapping the skill and knowledge of millions of farmers. On a national and global policy level, the Agroecology Coalition, with 480+ members, including governments, civil society organizations, academic institutions, and philanthropic foundations, is supporting a transition toward agroecology, working with natural systems to produce abundant food, boost biodiversity, and foster community well-being.
Fertilizer industry spins “clean” products
We must also inoculate ourselves from the fertilizer industry’s public relations spin, which includes promoting the promise that their products can be produced without heavy reliance on fossil fuels. Despite experts debunking the viability of what the industry has dubbed “green hydrogen” or “green or clean ammonia”, the sector still promotes this narrative, arguing that these are produced with resource-intensive renewable energy or Carbon Capture and Storage (CCS), a costly and unreliable technology for reducing emissions.
As we mourn this conflict’s senseless destruction and death, including hundreds of children, we also recognize that peace cannot mean a return to business-as-usual. We need to upend the systems that allow the richest and most powerful to have dominion over so much.
This includes fighting for a food system that is based on genuine sovereignty and justice, free from dependency on fossil fuels, one that honors natural systems and puts power into the hands of communities and food producers themselves.
The post Middle East war is another wake-up call for fossil fuel-reliant food systems appeared first on Climate Home News.
Middle East war is another wake-up call for fossil fuel-reliant food systems
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