Alongside measures to reduce the amount of carbon being emitted into the atmosphere, this approach will likely need include another action that scientists see as increasingly, if frustratingly, necessary: carbon dioxide (CO2) removal.
Make no mistake, we need to do carbon removal; but we need to do it responsibly – Rob Bellamy
The term refers to a variety of mechanisms and technologies that would pull CO2 back out of the air and trap it so it cannot contribute to the greenhouse effect that is warming our planet. The idea is to actively reverse the emissions that humans have pumped into the atmosphere. Techniques on the table include everything from direct air capture or bioenergy with carbon capture and storage (CCS) to biochar or enhanced rock weathering.
But how feasible is it to scale the assorted proposed methods for removing CO2 to the levels that would make a meaningful difference to the climate? Are some options better than the others? And which schemes run the risk of simply being too crackpot to ever stand a chance?
Rob Bellamy, a lecturer in climate and society at the University of Manchester, says the IPCC report makes it clear that taking CO2 out of the air is “not just an option – but a necessity”. However he warned carbon removal methods also bring significant risks to both people and the environment.
“We now need a wide-ranging societal conversation about which methods to take forward, how to incentivise them, and ultimately how to govern them. Make no mistake, we need to do carbon removal; but we need to do it responsibly.”
In particular, the new IPCC report notes that CO2 removal will be needed to counterbalance “hard-to-abate residual greenhouse gas emissions” in order to reach net-zero CO2 or greenhouse gas emissions. Sectors with these hard-to-abate emissions include agriculture, aviation, shipping and industrial processes, it notes (these sectors are consider hard to reduce emissions in either due to a lack of technology or the high expense of decarbonisation).
“The IPCC reports show that we can prevent irreversible harm to people and the planet if we scale up proven solutions available now: replacing fossil fuels with renewables, increasing energy efficiency, and reducing energy and resource use are the surest path to limiting global warming to 1.5C,” says Lili Fuhr, deputy director of the climate and energy programme, at the Center for International Environmental Law (Ciel), a non-profit environmental law firm based in Geneva, Switzerland.
“Building our mitigation strategies on models that instead lock in inequitable growth and conveniently assume away the risks of technofixes like carbon capture and storage and CO2 removal ignores that clarion message and increases the likelihood of overshoot,” says Fuhr. (See below for more on climate overshoot)
In its own assessments, the IPCC looks at different pathways to achieving emissions cuts in line with various degrees of ambition. Most of these pathways now require some level of CO2 removal techniques, and virtually all scenarios consistent with the Paris Agreement goals to limit warming to 1.5C or well below 2C include at least some CO2 removal. The world is currently on track for 2.4C rise by 2100 if all UN country climate pledges are fully implemented, or 2.8C by 2100 if only the climate policies already in place are considered.
The exact amount of removal needed to reach this level, however, strongly depends on what cuts to emissions are made – making it hard to put an exact figure on it. Scenarios showing steep and rapid emissions cuts require the lowest amount of CO2 removal.
Some of the IPCC scenarios also allow for “climate overshoot” – a period when the global temperature goals are temporarily exceeded before dropping back to the targeted value. Bringing temperatures down like this would require some kind of CO2 removal, the IPCC says.
What are the options?
There are a huge variety of different ways we could potentially remove CO2 from the air. It’s worth noting, however, that no technological methods have been proven at scale.
The graph below shows the estimated emissions reduction potential of several different techniques from the University of Oxford CO2 removal assessment.
Bioenergy with carbon capture and storage (CCS) is the most prominent technological method in the IPCC report. Here, trees are grown to capture CO2 before being burnt in a plant for energy. The emissions from the plant are captured and stored permanently underground, a practice known as CCS.
Using biochar for CO2 removal, meanwhile, would involve using biomass such as trees or plant matter, which has captured carbon while growing, being pyrolysed (heated in the absence of oxygen) to produce a black, coal-like substance which consists mainly of elemental carbon. This biochar can then be added to soils, in theory locking the carbon away.
Enhanced rock weathering has also been proposed as a way to remove CO2 from the atmosphere. This would consist of spreading large amounts of finely ground silicate rock such as basalt onto land in an effort to mimic natural rock weathering, eventually leading to CO2 being trapped in the ocean as bicarbonate or locked up on the sea floor.
Finally, ocean alkalinisation is another proposed process which would involve adding alkaline materials to the ocean such as silicate or carbonate rocks to increase the amount of CO2 it takes in. Similarly, ocean fertilisation would involve stimulating phytoplankton growth in the ocean to enhance carbon sequestration.
Despite being much hyped, these “novel” methods of CO2 are also in reality still in their infancy. Steve Smith, executive director of Oxford Net Zero and CO2RE at the University of Oxford and lead author of the CO2 removal assessment, notes that even combined all of the above methods remove just two million tonnes of CO2 per year (Mt CO2/yr). That is an amount equal to 0.005% of global fossil fuel and cement emissions in 2022.