Plans to tackle climate change by sucking carbon dioxide from the air and storing it underground are wildly unrealistic, according to a new analysis, calling into question our ability to meet climate goals.
Humanity has now released so much carbon dioxide into the atmosphere that is impossible for the planet to remain below 1.5°C or 2°C of warming above pre-industrial levels simply by halting emissions. Instead, most climate models that stay below these temperatures assume we will have to actively remove CO₂ from the atmosphere in various ways (see “Carbon clean-up”, below) and find somewhere else to put it.
These models, known as integrated assessment models (IAMs), play a key role in climate policy-making, setting out potential pathways for global decarbonisation. Their outcomes inform the scientific assessments made by the Intergovernmental Panel on Climate Change (IPCC) on global temperature goals and support governments in drawing up emissions-cutting strategies. In short, it is vital that they are as accurate and realistic as possible.
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But IAMs seem to have massively overestimated one carbon storage method, known as geological storage, which involves capturing CO₂ and pumping it underground into places such as depleted oil and gas reservoirs. Currently, around 9 million tonnes of CO₂ are stored in this way each year, but to stay below 1.5°C, most IAMs assume this rate of storage will need to increase 1000-fold to around 9 gigatonnes by mid-century. Some go even further, requiring double or triple this expansion.
Such rapid scaling is unfeasible, says at Imperial College London. He and his colleagues used a new approach to assess the likelihood of delivering gigatonne-scale carbon storage by 2050, taking into account geological, economic, technological and geographic constraints on growth.
They conclude that the maximum geological storage possible by 2050 is 16 gigatonnes a year, but such a scenario is unrealistic, relying on huge growth, sustained over decades. It also requires the US to deliver 60 per cent of the total annual storage – 10 gigatonnes per year – by 2050, far beyond the 1 gigatonne it has promised by that date.
“It’s just really hard to envision a situation where the US would incentivise that much,” says Krevor. A more realistic scenario, which takes into account the US’s stated goal, sees the world hit around 6 gigatonnes of annual geological storage by mid-century. “The projections that are above 6 gigatonnes, certainly above 10 gigatonnes… it’s difficult to see how that would work,” he says.
The team’s most conservative scenario assumes a 10 per cent annual growth rate for geological storage from now until 2050, which is still higher than the . Under this scenario, annual storage capacity reaches less than 1 gigatonne per year by 2050, well below what climate models assume will be needed to deliver the 1.5°C temperature goal.
The study, which hasn’t yet been peer-reviewed, considers only the regions that have existing carbon-capture-and-storage projects or ones due to be up and running by 2030 – limiting its focus to 10 regions covering North America, the European Union, the UK, Australia and parts of Asia.
Krevor says IAMs are overestimating the delivery of carbon sequestration partly because they don’t account for difficulties in scaling the nascent industry, particularly in areas of Asia where current deployment is low. Some of the projections made for carbon storage in China and Indonesia by 2050 assume such massive increases in deployment that it “boggles the mind”, he says.
Models can also skirt over some of the practicalities of using geological storage, such as how quickly reservoirs can be filled. “A reservoir is like a cup. You can pour at a fast rate for a long time. But as you approach the limit, you have to slow down. It’s not just about the volume that’s there, it’s about how quickly you can use it,” says Krevor.
“This is something that many of these models need to work on,” says at Imperial College London, who wasn’t involved in the study.
Even before the team’s analysis, the possibility of remaining below 1.5°C of warming was seen as increasingly unlikely, requiring the heavy industry and energy and transport sectors to be decarbonised in just a few decades. Without large-scale carbon storage, that task is even tougher, either demanding faster and more extensive emissions cuts or the use of riskier mitigation strategies, such as geoengineering. Such revisions are likely to be included in the next round of IPCC assessments, due in 2029.
Climate modelers will always work to find a solution that fits a temperature target, says at the Federal University of Rio de Janeiro, Brazil. However, as the remaining carbon budget for the 1.5°C temperature threshold shrinks, the solutions will become ever more drastic.
“The real issue here is that we have been extremely ambitious to set a target of limiting the temperature [rise] to 1.5°C,” says Schaeffer “It’s not that the solutions of the model are unrealistic. Eventually what is unrealistic is the target itself.”
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Carbon clean-up
Before we can store carbon, we must capture it. These are the main technologies for doing so.
Carbon capture: Factories and power stations are equipped with devices that can separate and capture the site’s carbon dioxide emissions before they enter the atmosphere. Current technology – which has been operating commercially since 1972 – can remove around 90 per cent of emissions, but it is expensive.
Bioenergy with carbon capture: This process involves extracting energy from plant material, such as by burning it in a power station and capturing the emissions given off. In theory, the resulting energy is carbon negative, because, along with the captured emissions, the plants have absorbed carbon while growing. But opponents say the idea is unproven at scale and requires huge amounts of land.
Direct air capture: These technologies extract CO₂ directly from the atmosphere, either using sorbent materials or a liquid solvent. They deliver truly negative emissions and can operate in any location untethered from pollution sources. However, current versions are yet to scale up and are extremely expensive and energy intensive.
