What are CO2 removal technologies?
CO2 removal technologies aim to remove carbon dioxide from the atmosphere.1. They can also be referred to as NETs (negative emissions technologies)2.
The IPCC (Intergovernmental Panel on Climate Change) identified NETs as requisite for limiting global temperature rise to 1.5C without overshooting targets3. The European Academies’ Science Advisory Council have also taken a similar stance. They recognise that CO2 removal technology will be vital for reducing global emissions in the atmosphere4.
Several proposed CO2 removal technologies have been the subject of intense research and development. These include the likes of afforestation, biochar, bioenergy, enhanced weathering, (DAC) direct air capture, and ocean fertilisation1.
Afforestation, Reforestation and Habitat Restoration
Firstly, afforestation refers to the planting of new trees where there were previously none. You also have reforestation where trees that have been cut down or degraded are restored 1.
As natural solutions, trees act by removing CO2 from the atmosphere and storing it in the biomass and soils of ecosystems5. On a large scale, it is easy to see the effectiveness with U.S. forests estimated to absorb 13 percent of the nation’s carbon emissions6. With space available for trees greater than previously thought, both afforestation and reforestation are potentially large-scale methods for CO2 removal7.
Moreover, the cheap cost of this solution means tree planting could remove more CO2 from the atmosphere at between $0 to $20 per ton of carbon6. As opposed to other solutions, this makes afforestation and reforestation a viable solution in a wider range of countries8 when compared to other CO2 removal technologies. However, land suitability remains a key challenge1.
Aside from trees, coastal and marine ecosystems can also capture and store CO2 from the atmosphere. This solution, referred to as blue carbon habitat restoration, has the potential to be highly effective as these ecosystems can absorb CO2 even faster than terrestrial forests1.
However, uncertainties over blue carbon projects include substantial variation in the amount of CO2 removed by coastal ecosystems in different locations9. These unknowns raise a question mark over effectiveness on a large scale.
Biochar and CO2 Removal
Biochar is a form of charcoal produced by heating biomass without any oxygen present. This process is known as pyrolysis1. Biochar production consumes more energy than it produces, unlike typical burning processes which would be a major source of CO210. Studies have found that biochar has the potential to sequester up to 4.8bn tonnes of CO2 per year11.
Yet it has drawbacks. Biochar, often used as a soil additive, can lower the reflectivity of the soil surface. This result can potentially exacerbate climate change12.
Bioenergy and CO2 Removal
BECCS (Bioenergy with carbon capture and storage) involves burning biomass, capturing the emissions and locking it away deep underground13. BECCS can draw significant quantities of CO2 out of the atmosphere1. This, along with low costs compared to other solutions, make it favourable.
The criticism levelled at BECS’s suitability by the scientific community includes questions over scalability, potential side effects, and whether it is truly able to deliver any negative emissions at all13.
The weathering of huge amounts of rock is another way of potentially removing CO2 from the atmosphere14. The process sees rocks break down by reacting with CO2 in the air creating bicarbonate, a carbon sink. The bicarbonate eventually runs off into the ocean where it stores the CO2 that derives from the process of weathering6.
This normally slow process absorbs around 3% of global fossil-fuel emissions1, and it produces beneficial by-products too. As part of the process, alkaline bicarbonate runoff washes into the ocean and partially helps neutralize ocean acidification6.
Yet the potential of these enhanced weather CO2 removal technologies is limited14, according to studies14. It would likely work best as a small additional contribution to support climate change mitigation.
DAC (Direct Air Capture)
DAC uses machines to suck CO2 out of the atmosphere and either bury it underground or convert it into something useful15.
There are a number of ways to do this. Firstly, industrial-scale facilities use a solution of hydroxide to capture CO2. There is also the possibility of using amine adsorbents in small, modular reactors16.
One study projected that direct air capture could sequester 0.5 to 5 gigatonnes of CO2 a year by 205017. This gives it significant CO2 removal potential compared to other solutions.
However, it is costly and largely inefficient. For example, research suggests DAC could use a quarter of global energy’ in 210016. DAC solutions often increase local air pollution from the energy required to run them, exacerbating public health issues.
Ocean fertilisation works by injecting nutrients into the ocean to trigger a ‘bloom’ of phytoplankton1. Phytoplankton need iron to be able to photosynthesise, and if it is the only limiting element, it can stimulate huge ‘blooms’. During this process of photosynthesis, phytoplankton also need inorganic carbon. By absorbing CO2 from the atmosphere and helping dissolve it in the sea, the blooms help remove atmospheric CO2 levels18.
However, whilst ocean fertilisation would help decrease atmospheric CO2 the impact would largely be minimal. It also carries risks to the ecosystem with possible side effects including changes to phytoplankton species which will have an effect on the food web18.
Finding the Best Solution for Climate Change
There is no single CO2 removal technology that is the ultimate solution to climate change. One proposed way forward involves using a NETs portfolio19 where solutions can be deployed at a more modest scale to help manage risk.
Of course, each technology is feasible at some level but uncertainties about cost, scalability, technology, implementation, or environmental risks remain. No single location and no technology in isolation will be sufficient to solve this huge problem by itself19. A range of solutions will seemingly have to work together where possible in order to remove atmospheric CO2.
- Carbon Brief. 2016. Explainer: 10 ways ‘negative emissions’ could slow climate change.
- The Conversation. 2018. Why we can’t reverse climate change with ‘negative emissions’ technologies.
- Carbon Brief. 2018. In-depth Q&A: The IPCC’s special report on climate change at 1.5C.
- European Academies’ Science Advisory Council. Forest bioenergy, carbon capture and storage, and carbon dioxide removal: an update.
- Johan Busch et al. 2019. Potential for low-cost carbon dioxide removal through tropical reforestation. Nature Climate Change. 9, pp.463-466.
- Columbia University. 2018. Can Removing Carbon From the Atmosphere Save Us From Climate Catastrophe?
- BBC News. 2019. Climate change: Trees ‘most effective solution’ for warming.
- Cosmos. 2019. Rebuilding forests is a cost-effective way to cut carbon.
- Climate Analytics. 2017. The dangers of Blue Carbon offsets: from hot air to hot water?
- Guardian. 2017. Negative emissions tech: can more trees, carbon capture or biochar solve our CO2 problem?
- Pete Smith. 2016. Soil carbon sequestration and biochar as negative emission technologies. Global Change Biology. 3, pp.1315-324
- Sebastian Mayer et al. 2012. Albedo Impact on the Suitability of Biochar Systems to Mitigate Global Warming. Environmental Science and Technology. 46, pp.12726-12734.
- Grantham Institute. 2019. The ups and downs of BECCS – where do we stand today?
- Science Daily. 2018. Enhanced weathering of rocks can help to pull CO2 out of the air — a little
- Quartz. 2019. A tiny tweak in California law is creating a strange thing: carbon-negative oil
- Carbon Brief. 2019. Direct CO2 capture machines could use ‘a quarter of global energy’ in 2100
- Sabine Fuss et al. 2018. Negative emissions—Part 2: Costs, potentials and side effects. Environmental Research Letters. 13
- University of Southampton. 2014. Ocean fertilization – A viable geoengineering option or a pipe dream?
- Carbon Brief. 2018. Guest post: Seven key things to know about ‘negative emissions’