Keeping global temperatures below the limit set in the 2015 Paris Agreement requires rapid cuts in greenhouse emissions.
However, scenarios consistent with the Paris limit also assume heavy reliance on CDR, particularly in the second half of the 21st century.
The study, published in Nature Climate Change, quantifies the “CDR gap” – the difference between the amount of CDR included in national climate plans and what would be needed to limit warming to 1.5°C.
CDR currently removes about 3bn tonnes of CO2 from the air every year, of which almost 100 per cent comes from land-based methods, such as afforestation and reforestation, the study says.
The authors estimate that if countries implement their national targets, CDR will increase by up to 1.9bn tonnes of CO2 per year by 2050.
However, assessing a range of scenarios for limiting warming to 1.5°C, the authors find a “CDR gap” in 2050 of 0.4bn-5.5bn tonnes of CDR per year.
One scientist, who was not involved in the study, tells Carbon Brief that framing the lack of additional plans for CDR as a “gap” is an “interesting idea”. However, he says it may not reflect a “definitive need for action” because the future role of CDR is debated.
Some scientists argue that reliance on CDR should be avoided, because land-based CDR can cause significant ecological and societal risks. Others worry that the promise of being able to use CDR in the future might dilute incentives to cut fossil-fuel use today.
The lead author of the study tells Carbon Brief that he recognises these concerns and made an effort to discuss them in the paper.
However, he says that calculating the CDR gap is important for assessing nations’ progress – and will provide a way of knowing whether countries are under- or over-committing to CDR in the future.
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The implications of the CO2 removal (CDR) gap are much more open to debate as CDR is a category of mitigation action, with the size of the gap either a cause for alarm or not depending on one’s view of what role that option will or should play.
Steve Pye, professor, University College London Energy Institute
CO2 removal
CO2 removal (CDR) refers to methods that draw down CO2 from the air and store it indefinitely on land, in the ocean, in geological formations or in products.
The study authors note that the term CDR “includes human enhancement of natural removal processes, but excludes natural uptake not caused directly by human activities”. The latter includes the huge amounts of CO2 absorbed by the land and oceans each year.
The paper groups CDR into two categories:
- Conventional CDR on land: This includes afforestation, in which trees are planted when previously there were none, and reforestation, which means restoring areas where the trees have been cut down or degraded.
- Novel CDR: This includes all CDR methods that are not based on forestry and land-use change, such as biochar, direct air capture and bioenergy with carbon capture and storage (BECCS).
Using data collected over 2011-20, the authors estimate that total human emissions of all greenhouse gases have reached 60bn tonnes per year. Of this, CDR efforts currently remove around 3bn every year, they find. The study calculates global emissions in CO2 equivalent (CO2e).
The plot below shows current global greenhouse gas emissions and removals. The bar on the left shows emissions of CO2 (blue) and non-CO2 (pink) gases, as well as land emissions (brown). CO2 removal is shown in yellow.
The bars on the right show that 99.9 per cent of CDR comes from conventional CDR on land (dark yellow), while “novel” CDR (light yellow) has a negligible contribution.
Present-day annual CO2 emissions (blue), non-CO2 emissions (pink), land emissions (brown), land-based CDR (dark yellow) and novel CDR (light yellow). Source: Lamb et al (2024).
In 2015, countries agreed under the Paris Agreement to keep warming “well below 2°C” above pre-industrial temperatures, with an aspiration of limiting warming to 1.5°C.
Rapid cuts in emissions are crucial to meet this goal. To make progress, countries are required to submit – and regularly update – their plans for reducing emissions. There is currently a sizeable “emissions gap” between the cuts included in these national proposals and those needed to limit warming to 1.5°C.
In many future scenarios that meet the Paris limit, CDR features heavily. For example, in scenarios where global temperatures initially “overshoot” 1.5°C, before falling below the limit by 2100, large-scale CDR would be used to remove carbon from the atmosphere and allow global temperatures to decline.
In its most recent assessment, the Intergovernmental Panel on Climate Change (IPCC) modelled 541 pathways that hold warming to 1.5°C or 2°C. All of these pathways involve CDR implementation between 2020 and 2100, ranging from a total of 450bn to 1.1tn tonnes of CO2, in addition to deep emissions cuts.
However, there are currently no rules requiring governments to clearly report their CDR plans.
To assess the amount of CDR proposed by governments, the authors therefore had to analyse a range of documents submitted to the UN Framework Convention on Climate Change (UNFCCC), such as countries’ nationally determined contributions (NDCs) and their long-term low-emissions development strategies.
The authors find that if countries implement their national targets, CDR could expand by 1.5-1.9bn tonnes of CO2 per year, compared to levels in 2020. The paper notes that many countries plan to expand land-based removals, but none has yet committed to “substantively scaling” novel CDR methods.
Warming threshold
To assess how much CDR is needed to meet the long-term goal of the Paris Agreement, the authors use Integrated Assessment Models (IAMs). These models look at the energy technologies, energy use choices, land-use changes and societal trends that cause, or prevent, greenhouse gas emissions.
The authors select a range of IAM scenarios from the latest IPCC scenario database for its sixth assessment report (AR6). Scenarios that limit warming to 2°C require emissions to fall by 46-75 per cent between 2020 and 2050, but CDR becomes the “main mitigation strategy” in the second half of the century, the study says.
The authors add that in these scenarios, conventional CDR on land “starts from a high baseline, but quickly reaches saturation by the mid-century due to land area constraints for afforestation/restoration”. Meanwhile, novel CDR scales up throughout the 21st century and accounts for more than half of cumulative emissions by the year 2100.
To assess the pathways in more detail, the authors select three scenarios that limit global warming to 1.5°C above pre-industrial levels
In the “demand reduction” scenario, humanity focuses on efficiency and sufficiency measures. This scenario requires an increase in land-based CDR, but no increase in “novel” CDR methods.
The “renewables” scenario sees a supply-side transformation towards renewable energy. This scenario mainly requires land-based CDR, but also includes a small contribution from novel methods.
The “carbon removal” scenario involves a rapid near-term reduction in greenhouse gas emissions, but fossil fuels are never entirely phased out, leading to higher “residual emissions” at net-zero CO2. Near-equal levels of land-based and novel CDR are needed by 2050, meaning that novel CDR needs to scale up more than a thousand times from its current capacity.
The plot below shows annual CDR under these three scenarios. The blue line indicates current CDR and each yellow line shows a different scenario. A lower (more negative) number means more CDR.
CDR under three future pathways, which limit warming to 1.5°C above pre-industrial temperatures. The blue line indicates current CDR and each yellow line shows a different scenario. A lower (more negative) number means more CDR. Source: Lamb et al (2024).
The study shows that current government plans – which would result in an extra 1.5-1.9bn tonnes of CDR per year by 2050 – are not ambitious enough to comply with any of the three 1.5°C scenarios.
The table below shows the changes in different types of CDR required under the different scenarios by 2050, compared to 2020 levels. The column on the right shows the “CDR gap” between current plans and each scenario in 2050.
Graph: Carbon Brief
The analysis shows that countries “lack progress in this domain of mitigation”, the study says. However, the size of the shortfall depends heavily on the scenario.
Under the demand reduction scenario, the CDR gap in 2050 is only 0.4bn tonnes of CDR per year, but this grows more than tenfold to 5.5bn tonnes of CDR per year under the carbon removal scenario.
Mind the gap
The prospect of relying on large-scale CDR to meet global climate goals is one that prompts concern in many experts.
One fear is that the promise of being able to use CDR in the future might dilute incentives to cut fossil fuel use today, a phenomenon known as “mitigation deterrence”.
Dr William Lamb – a researcher at the Mercator Research Institute on Global Commons and Climate Change and lead author on the study – tells Carbon Brief that the paper acknowledges this concern and tries to be clear that CDR is not a replacement for mitigation.
Prof Steve Pye is a professor at University College London’s Energy Institute, who was not involved in the study. He says that framing the lack of CDR as a “gap” is an “interesting idea”, but does not necessarily reflect a “definitive need for action” in the same way as the emissions gap:
“The implications of the CDR gap are much more open to debate as CDR is a category of mitigation action, with the size of the gap either a cause for alarm or not depending on one’s view of what role that option will or should play.”
He adds that the analysis could even be “interpreted as positive”, because it shows that countries are not being distracted by novel CDR.
Alexandra Deprez – a research fellow at the Institute for Sustainable Development and International Relations, who is not involved in the study – tells Carbon Brief that in her opinion, the new study does not do enough to consider the “sustainability limits” of CDR.
She recently co-wrote a Carbon Brief guest post explaining these limits, which said:
“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.”
Deprez and Lamb have “opposite starting points” in their work on CDR and therefore arrive at different conclusions, she explains.
Lamb starts by asking “how much CDR is needed” by looking at 1.5°C and 2°C scenarios, and concludes that it needs to be scaled up “significantly”, she says. Meanwhile, she tells Carbon Brief that her own work starts by asking “how much CDR can be sustainably deployed” and finds that “a large number of ‘Paris compatible’ scenarios overstep high CDR sustainability risk”.
Lamb says the authors were “very careful” in selecting the three focus scenarios for the study. He adds:
“We have a kind of selection criteria that includes thinking about the sustainability constraints, whether they’re using too much biomass, whether they’re scaling up novel methods too quickly. And so we’re quite conservative about the specific scenarios we choose.”
Meanwhile Prof Joeri Roglej – director of research at the Grantham Institute – tells Carbon Brief that the study “puts pathways that aim to keep warming as close to 1.5°C as possible in the same basket as pathways that keep it below 2°C only, therewith suggesting a lower overall ambition than the Paris Agreement”.
He adds:
“The study doesn’t distinguish scenarios with CDR levels that risk undermining sustainability. These presentation choices therefore perpetuate some of the reasons why CDR research is often criticised, including that CDR scholarship often turns a blind eye to the sustainability risks of large-scale CDR deployment.”
Pye adds a note of caution about using IAMs, saying they have “relied heavily on CDR to meet high ambition targets” without accounting for the “political reality” faced by many governments.
CDR reporting
According to the study, only about 40 countries, including the EU, have outlined scenarios in their long-term strategies that depict quantifiable levels of CDR by 2050.
For the other countries – which account for 62 per cent of current conventional CDR on land – the authors assume that overall CDR levels will remain constant.
Lamb tells Carbon Brief that this is a “big assumption”. He notes that while CDR globally has been “quite stable over the past 20 years”, there is a lot of variation between countries. For example, he says that China has been “rapidly increasing” its CDR through large afforestation projects, while many countries in Europe have seen a decrease due to problems in their forestry sector.
The study also assumes that countries without quantifiable scenarios do not currently plan to implement novel CDR methods. “This includes China, Norway and Saudi Arabia, which are all developing technology roadmaps towards novel CDR and could contribute to closing the gap,” the paper says.
Dr Ajay Gambhir is a visiting senior research fellow at Imperial College London’s Grantham Institute for Climate Change and the Environment, and was not involved in the study. He tells Carbon Brief that many land-based carbon sinks, such as forests, have the potential to transition to sources of carbon over the coming years.
He adds:
“The authors are mindful of potential reversibility of forest carbon, but this highlights the risks that we are even further from our CDR, and emissions reduction, needs than might be indicated in this analysis.”
The lack of clear data shows that “we need more clarity” in CDR reporting, Lamb tells Carbon Brief. He argues that increasing transparency would “allow more critical reflection actually on carbon dioxide removal plans and whether they’re ambitious enough – or even too ambitious at the expense of emissions reductions”.
The analysis from this paper will be included in the next State of CDR report, which will be released this summer.
This story was published with permission from Carbon Brief.
