Many pathways to staying below 1.5C delay deep cuts in carbon dioxide (CO2) emissions and rely instead on huge amounts of CO2 removal (CDR) later this century.
Land-based CDR is used extensively in the 1.5C pathways presented by the Intergovernmental Panel on Climate Change (IPCC) and also features heavily in the climate plans of many governments and businesses.
Yet the large-scale deployment of land-based CDR could come with major challenges. These include significant ecological and societal risks – particularly to biodiversity loss, food security, freshwater use and human rights, among others – which have not been comprehensively assessed.
In our new paper, published in Science, we assess the level of sustainability risks that could be triggered by the use of various different land-based CDR techniques, such as bioenergy with carbon capture and storage (BECCS) and afforestation and reforestation (A/R).
We show that risks are triggered at much lower levels of deployment than previously thought. Moreover, many of the “Paris aligned” 1.5C pathways presented by the IPCC would exceed the CDR sustainability limits defined by our evaluation.
CDR deployment in mitigation pathways
Many mitigation pathways assessed by the IPCC envisage large deployments of CDR throughout the 21st century.
This is significant because, although the IPCC is not “policy prescriptive”, these pathways – and the policy options within them – strongly shape the “solution space” as seen by policymakers when considering how to meet the goals of the Paris Agreement.
The use of CDR is particularly widespread in the pathways labelled by the IPCC as “1.5C with high overshoot”. In these pathways, emissions cuts are not fast enough to avoid breaching the carbon budget for 1.5C and global temperatures temporarily overshoot the 1.5C limit, before extensive use of CDR brings temperatures down later this century.
Within these pathways, CDR is deployed up to 2050 to help compensate for a slower transition away from fossil fuels, to reduce net emissions. When emissions reach net-zero, CDR is being used to counterbalance large remaining residual emissions. Beyond this point, it is used to draw-down global temperature after exceeding 1.5C.
This type of pathway is typified by the IPCC’s “Neg” illustrative mitigation pathway. Here, – some 5.1bn tonnes of CO2 (GtCO2) is taken out of the atmosphere using CDR in 2050 and 15.1GtCO2 in 2100.
In this pathway, one of five outlined in the IPPC’s sixth assessment report (AR6), primary energy from fossil fuel drops only 36% below 2020 levels by 2050 and 73% by 2100, relative to 2020.
This CDR and emissions profile is in sharp contrast to the IPCC’s “Ren” pathway – which relies on rapid scale-up of renewable energy – primary energy from fossil fuel falls 85% by 2050 and 95% by 2100, relative to 2020. (IMP-Ren)
This means that this renewable energy pathway has much lower reliance on CDR, which is only used to take 2.6GtCO2 out of the atmosphere in 2050 and 3GtCO2 in 2100.
Sustainability limits
The large amounts of land-based CDR in many of the pathways assessed by the IPCC come with significant implications in terms of sustainability, with the potential for serious impacts on human livelihoods and food security.
Yet the IPCC report does not comprehensively assess the environmental feasibility of the scenarios, nor their associated sustainability risks. Nor does it put a figure on the scale of CDR that could be deployed without triggering major impacts.
To address this gap, we quantified the sustainability limits to the widespread deployment of BECCS, A/R and “nature-based” CDR, which includes limited reforestation, forest restoration, reduced forest harvest and agroforestry.
To do so, we draw from recent studies that give greater attention to the ecological, biological and societal impacts of land-based CDR.
Based on these studies, we calculated the levels of CDR deployment that would trigger “low”, “medium”, “high” and “very high” risks for sustainability. These risk levels are colour-coded from green through to dark red, for each type of land-based CDR in the figure below.
Reading from left to right, the figure shows increasing levels of CDR deployment in terms of GtCO2 removed per year. The grey bar shows the range of “technical mitigation potential” for each technique, as currently assessed by the IPCC. The upper end of this is the largest amount that could theoretically be deployed, if barriers to rapid scale-up, constraints on feasibility and sustainability risks are not taken into account.
The figure shows that sustainability risks start well below the technical mitigation potential.
For BECCS, the IPCC reports an average technical potential of 5.9GtCO2 per year. Yet we find that deploying more than 1.2GtCO2 of BECCS per year would tip over from “low risk” into “medium” or higher risk levels.
(This figure is based on BECCS plants capturing a “medium” share of their associated CO2 emissions, below 70%. For a “low” capture rate below 50%, the low-risk threshold drops to just 0.7GtCO2 per year.)
Correspondingly, BECCS would cross the high sustainability risk threshold (shown in red) if used to remove 1.3GtCO2 with a low capture rate – or 2.8GtCO2 with a medium rate.
Even these limited levels of BECCS assume significant bioenergy policy reforms that governments have not yet addressed. These include addressing gaps in emissions accounting and ensuring bioenergy is not causing deforestation, either directly or indirectly.
For A/R, the IPCC average technical potential is 3.9GtCO2 a year. Our research shows that associated sustainability risks remain low or medium below 3.8GtCO2 per year, with high risks beyond that point.
We find that nature-based CDR (which includes limited reforestation) carries the lowest sustainability risks. Deployment would trigger high risks beyond 5.1GtCO2 a year (including 3.8GtCO2 per year of non-monoculture reforestation).
Having defined risk levels for each type of CDR, we then mapped those indicators onto the amount of CDR deployed in each of the IPCC’s five “illustrative mitigation pathways” (IMPs).
(These pathways are: gradual strengthening of climate policy, GS; widespread use of CDR, Neg; low energy demand, LD; shifting pathways towards sustainable development, SP; and heavy use of renewables, Ren.)
Our results, illustrated in the table below, show that the three pathways that limit warming to 1.5C with limited to no overshoot are able to do so without greatly overstepping our sustainability risk thresholds.
In contrast, Neg limits warming in 2100 to 1.5C with high temperature overshoot, but exceeds high and even very high sustainability risk thresholds. The GS pathway only limits warming to 2C and still carries significant levels of sustainability risks.
Reading the table from top to bottom, the first set of rows list the change in CO2 emissions, energy demand and fossil fuel use in 2050 and 2100.
The second set of rows show the amount of each type of CDR deployed in 2050 and 2100, colour-coded according to our sustainability risk levels.
The third set of rows show the amount of land needed for CDR deployment – the land footprint. Again, these are colour-coded according to our sustainability risk levels.
Notably, our findings show that the amount of land needed for CDR in the Neg pathway could reach 7.2m square kilometres in 2050 and 13.3m square kilometres in 2100. For comparison, the land area of the US is just 9.1m square kilometres.
Risk assessment
Our findings suggest there is an urgent need to consider sustainability risks when choosing between different mitigation pathways.
One way to do this would be to define a “sustainable CDR budget”, as the amount of CDR that could be deployed sustainably across all CDR methods.
While our research only considered land-based CDR, alternative CDR options are also likely to come with sustainability and deployment risks, which could limit their potential. These include direct air carbon capture and storage (DACCS) or ocean-based CDR.
Another option would be for scientists to identify Paris-aligned scenarios that do not overstep sustainability limits. Our research suggests that this could be a key priority for the IPCC’s seventh assessment cycle, as well as integrating environmental risks and feasibility throughout the IPCC’s work.
Moreover, our findings suggest that delaying fossil fuel cuts, in the hope that emissions can be drawn down later this century using CDR, would come with high sustainability risks.
If, on the other hand, countries wish to account for biodiversity considerations alongside climate goals, while still limiting temperatures to 1.5C, then they would need to follow a mitigation pathway with more rapid cuts in fossil fuel use, our research suggests.
Many of these pathways include behaviour changes and reductions in energy demand.
Countries could take up our findings in their next nationally-determined contributions (NDCs) under the Paris Agreement, due in 2025. For example, they could address sustainability risks by setting separate, transparent targets for CDR, in addition to headline emissions goals.
They could also aim to limit their reliance on CDR – and its corresponding land footprint – in order to avoid climate actions that have negative consequences for their national biodiversity plans under the global biodiversity framework (NBSAPs).
The post Guest post: Heavy use of CO2 removal would trigger high sustainability risks appeared first on Carbon Brief.
Guest post: Heavy use of CO2 removal would trigger high sustainability risks
N.C. Gov. Josh Stein wants state lawmakers to rethink tax breaks for data centers. The industry’s opacity makes it difficult to evaluate costs and benefits.
Tax breaks for data centers in North Carolina keep as much as $57 million each year into from state and local government coffers, state figures show, an amount that could balloon to billions of dollars if all the proposed projects are built.
The Global Environment Facility (GEF), a multilateral fund that provides climate and nature finance to developing countries, has raised $3.9 billion from donor governments in its last pledging session ahead of a key fundraising deadline at the end of May.
The amount, which is meant to cover the fund’s activities for the next four years (July 2026-June 2030), falls significantly short of the previous four-year cycle for which the GEF managed to raise $5.3bn from governments. Since then, military and other political priorities have squeezed rich nations’ budgets for climate and development aid.
The facility said in a statement that it expects more pledges ahead of the final replenishment package, which is set for approval at the next GEF Council meeting from May 31 to June 3.
Claude Gascon, interim CEO of the GEF, said that “donor countries have risen to the challenge and made bold commitments towards a more positive future for the planet”. He added that the pledges send a message that “the world is not giving up on nature even in a time of competing priorities”.
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Donors under pressure
But Brian O’Donnell, director of the environmental non-profit Campaign for Nature, said the announcement shows “an alarming trend” of donor governments cutting public finance for climate and nature.
“Wealthy nations pledged to increase international nature finance, and yet we are seeing cuts and lower contributions. Investing in nature prevents extinctions and supports livelihoods, security, health, food, clean water and climate,” he said. “Failing to safeguard nature now will result in much larger costs later.”
At COP29 in Baku, developed countries pledged to mobilise $300bn a year in public climate finance by 2035, while at UN biodiversity talks they have also pledged to raise $30bn per year by 2030. Yet several wealthy governments have announced cuts to green finance to increase defense spending, among them most recently the UK.
As for the US, despite Trump’s cuts to international climate finance, Congress approved a $150 million increase in its contribution to the GEF after what was described as the organisation’s “refocus on non-climate priorities like biodiversity, plastics and ocean ecosystems, per US Treasury guidance”.
The facility will only reveal how much each country has pledged when its assembly of 186 member countries meets in early June. The last period’s largest donors were Germany ($575 million), Japan ($451 million), and the US ($425 million).
The GEF has also gone through a change in leadership halfway through its fundraising cycle. Last December, the GEF Council asked former CEO Carlos Manuel Rodriguez to step down effective immediately and appointed Gascon as interim CEO.
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New guidelines
As part of the upcoming funding cycle, the GEF has approved a set of guidelines for spending the $3.9bn raised so far, which include allocating 35% of resources for least developed countries and small island states, as well as 20% of the money going to Indigenous people and communities.
Its programs will help countries shift five key systems – nature, food, urban, energy and health – from models that drive degradation to alternatives that protect the planet and support human well-being by integrating the value of nature into production and consumption systems.
The new priorities also include a target to allocate 25% of the GEF’s budget for mobilising private funds through blended finance. This aligns with efforts by wealthy countries to increase contributions from the private sector to international climate finance.
Niels Annen, Germany’s State Secretary for Economic Cooperation and Development, said in a statement that the country’s priorities are “very well reflected” in the GEF’s new spending guidelines, including on “innovative finance for nature and people, better cooperation with the private sector, and stable resources for the most vulnerable countries”.
Aliou Mustafa, of the GEF Indigenous Peoples Advisory Group (IPAG), also welcomed the announcement, adding that “the GEF is strengthening trust and meaningful partnerships with Indigenous Peoples and local communities” by placing them at the “centre of decision-making”.
The post GEF raises $3.9bn ahead of funding deadline, $1bn below previous budget appeared first on Climate Home News.
GEF raises $3.9bn ahead of funding deadline, $1bn below previous budget
Tropical cyclones that rapidly intensify when passing over marine heatwaves can become “supercharged”, increasing the likelihood of high economic losses, a new study finds.
Such storms also have higher rates of rainfall and higher maximum windspeeds, according to the research.
The study, published in Science Advances, looks at the economic damages caused by nearly 800 tropical cyclones that occurred around the world between 1981 and 2023.
It finds that rapidly intensifying tropical cyclones that pass near abnormally warm parts of the ocean produce nearly double – 93% – the economic damages as storms that do not, even when levels of coastal development are taken into account.
One researcher, who was not involved in the study, tells Carbon Brief that the new analysis is a “step forward in understanding how we can better refine our predictions of what might happen in the future” in an increasingly warm world.
As marine heatwaves are projected to become more frequent under future climate change, the authors say that the interactions between storms and these heatwaves “should be given greater consideration in future strategies for climate adaptation and climate preparedness”.
‘Rapid intensification’
Tropical cyclones are rapidly rotating storm systems that form over warm ocean waters, characterised by low pressure at their cores and sustained winds that can reach more than 120 kilometres per hour.
The term “tropical cyclones” encompasses hurricanes, cyclones and typhoons, which are named as such depending on which ocean basin they occur in.
When they make landfall, these storms can cause major damage. They accounted for six of the top 10 disasters between 1900 and 2024 in terms of economic loss, according to the insurance company Aon’s 2025 climate catastrophe insight report.
These economic losses are largely caused by high wind speeds, large amounts of rainfall and damaging storm surges.
Storms can become particularly dangerous through a process called “rapid intensification”.
Rapid intensification is when a storm strengthens considerably in a short period of time. It is defined as an increase in sustained wind speed of at least 30 knots (around 55 kilometres per hour) in a 24-hour period.
There are several factors that can lead to rapid intensification, including warm ocean temperatures, high humidity and low vertical “wind shear” – meaning that the wind speeds higher up in the atmosphere are very similar to the wind speeds near the surface.
Rapid intensification has become more common since the 1980s and is projected to become even more frequent in the future with continued warming. (Although there is uncertainty as to how climate change will impact the frequency of tropical cyclones, the increase in strength and intensification is more clear.)
Marine heatwaves are another type of extreme event that are becoming more frequent due to recent warming. Like their atmospheric counterparts, marine heatwaves are periods of abnormally high ocean temperatures.
Previous research has shown that these marine heatwaves can contribute to a cyclone undergoing rapid intensification. This is because the warm ocean water acts as a “fuel” for a storm, says Dr Hamed Moftakhari, an associate professor of civil engineering at the University of Alabama who was one of the authors of the new study. He explains:
“The entire strength of the tropical cyclone [depends on] how hot the [ocean] surface is. Marine heatwave means we have an abundance of hot water that is like a gas [petrol] station. As you move over that, it’s going to supercharge you.”
However, the authors say, there is no global assessment of how rapid intensification and marine heatwaves interact – or how they contribute to economic damages.
Using the International Best Track Archive for Climate Stewardship (IBTrACS) – a database of tropical cyclone paths and intensities – the researchers identify 1,600 storms that made landfall during the 1981-2023 period, out of a total of 3,464 events.
Of these 1,600 storms, they were able to match 789 individual, land-falling cyclones with economic loss data from the Emergency Events Database (EM-DAT) and other official sources.
Then, using the IBTrACS storm data and ocean-temperature data from the European Centre for Medium-Range Weather Forecasts, the researchers classify each cyclone by whether or not it underwent rapid intensification and if it passed near a recent marine heatwave event before making landfall.
The researchers find that there is a “modest” rise in the number of marine heatwave-influenced tropical cyclones globally since 1981, but with significant regional variations. In particular, they say, there are “clear” upward trends in the north Atlantic Ocean, the north Indian Ocean and the northern hemisphere basin of the eastern Pacific Ocean.
‘Storm characteristics’
The researchers find substantial differences in the characteristics of tropical cyclones that experience rapid intensification and those that do not, as well as between rapidly intensifying storms that occur with marine heatwaves and those that occur without them.
For example, tropical cyclones that do not experience rapid intensification have, on average, maximum wind speeds of around 40 knots (74km/hr), whereas storms that rapidly intensify have an average maximum wind speed of nearly 80 knots (148km/hr).
Of the rapidly intensifying storms, those that are influenced by marine heatwaves maintain higher wind speeds during the days leading up to landfall.
Although the wind speeds are very similar between the two groups once the storms make landfall, the pre-landfall difference still has an impact on a storm’s destructiveness, says Dr Soheil Radfar, a hurricane-hazard modeller at Princeton University. Radfar, who is the lead author of the new study, tells Carbon Brief:
“Hurricane damage starts days before the landfall…Four or five days before a hurricane making landfall, we expect to have high wind speeds and, because of that high wind speed, we expect to have storm surges that impact coastal communities.”
They also find that rapidly intensifying storms have higher peak rainfall than non-rapidly intensifying storms, with marine heatwave-influenced, rapidly intensifying storms exhibiting the highest average rainfall at landfall.
The charts below show the mean sustained wind speed in knots (top) and the mean rainfall in millimetres per hour (bottom) for the tropical cyclones analysed in the study in the five days leading up to and two days following a storm making landfall.
The four lines show storms that: rapidly intensified with the influence of marine heatwaves (red); those that rapidly intensified without marine heatwaves (purple); those that experienced marine heatwaves, but did not rapidly intensify (orange); and those that neither rapidly intensified nor experienced a marine heatwave (blue).
Dr Daneeja Mawren, an ocean and climate consultant at the Mauritius-based Mascarene Environmental Consulting who was not involved in the study, tells Carbon Brief that the new study “helps clarify how marine heatwaves amplify storm characteristics”, such as stronger winds and heavier rainfall. She notes that this “has not been done on a global scale before”.
However, Mawren adds that other factors not considered in the analysis can “make a huge difference” in the rapid intensification of tropical cyclones, including subsurface marine heatwaves and eddies – circular, spinning ocean currents that can trap warm water.
Dr Jonathan Lin, an atmospheric scientist at Cornell University who was also not involved in the study, tells Carbon Brief that, while the intensification found by the study “makes physical sense”, it is inherently limited by the relatively small number of storms that occur. He adds:
“There’s not that many storms, to tease out the physical mechanisms and observational data. So being able to reproduce this kind of work in a physical model would be really important.”
Economic costs
Storm intensity is not the only factor that determines how destructive a given cyclone can be – the economic damages also depend strongly on the population density and the amount of infrastructure development where a storm hits. The study explains:
“A high storm surge in a sparsely populated area may cause less economic damage than a smaller surge in a densely populated, economically important region.”
To account for the differences in development, the researchers use a type of data called “built-up volume”, from the Global Human Settlement Layer. Built-up volume is a quantity derived from satellite data and other high-resolution imagery that combines measurements of building area and average building height in a given area. This can be used as a proxy for the level of development, the authors explain.
By comparing different cyclones that impacted areas with similar built-up volumes, the researchers can analyse how rapid intensification and marine heatwaves contribute to the overall economic damages of a storm.
They find that, even when controlling for levels of coastal development, storms that pass through a marine heatwave during their rapid intensification cause 93% higher economic damages than storms that do not.
They identify 71 marine heatwave-influenced storms that cause more than $1bn (inflation-adjusted across the dataset) in damages, compared to 45 storms that cause those levels of damage without the influence of marine heatwaves.
This quantification of the cyclones’ economic impact is one of the study’s most “important contributions”, says Mawren.
The authors also note that the continued development in coastal regions may increase the likelihood of tropical cyclone damages over time.
Towards forecasting
The study notes that the increased damages caused by marine heatwave-influenced tropical cyclones, along with the projected increases in marine heatwaves, means such storms “should be given greater consideration” in planning for future climate change.
For Radfar and Moftakhari, the new study emphasises the importance of understanding the interactions between extreme events, such as tropical cyclones and marine heatwaves.
Moftakhari notes that extreme events in the future are expected to become both more intense and more complex. This becomes a problem for climate resilience because “we basically design in the future based on what we’ve observed in the past”, he says. This may lead to underestimating potential hazards, he adds.
Mawren agrees, telling Carbon Brief that, in order to “fully capture the intensification potential”, future forecasts and risk assessments must account for marine heatwaves and other ocean phenomena, such as subsurface heat.
Lin adds that the actions needed to reduce storm damages “take on the order of decades to do right”. He tells Carbon Brief:
“All these [planning] decisions have to come by understanding the future uncertainty and so this research is a step forward in understanding how we can better refine our predictions of what might happen in the future.”
The post Marine heatwaves ‘nearly double’ the economic damage caused by tropical cyclones appeared first on Carbon Brief.
Marine heatwaves ‘nearly double’ the economic damage caused by tropical cyclones
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