As a person who has spent years on carbon dioxide (CO₂) capture technologies innovation and development, I am often asked why is it that CO₂, this tiny molecule in the air, can cause global warming? And this is a fair question.

And my response always starts with: “it is not only CO₂, but a group of gases called greenhouse gases (GHGs)”, and then follows with this simple explanation: “sunlight reaches the earth, some energy is reflected back to the space, some is absorbed and re-radiated as heat, CO₂ and other GHGs have the ability to absorb the heat emitted from the earth surface and keep it from escaping out to space, and then reflect it in all directions, warming the earth”.

CO₂ is the most important driver of global warming, but it is not the only one. Certain group of gases in atmosphere that are long-lived and remain semi-permanently there and do not respond physically or chemically to changes in temperature are considered as “forcing” climate change, e.g. Carbon Dioxide (CO₂), methane (CH₄), Nitrous oxide (N₂O), Chlorofluorocarbons CFCs. Moreover, there are other gases, e.g. water vapour, which respond physically and chemically to changes in temperature, these gases are defined as “feedback”.

CO₂ is the most long-lived and abundant forcing gas in atmosphere. It releases through natural processes and through human activities such as deforestation, land use changes, and burning fossil fuels. Human activities caused increase in atmospheric CO₂ concentration by more than a third since the Industrial Revolution. Before the Industrial Revolution in the 19th century, global average CO₂ concentration in atmosphere was about 280 ppm (part per million), and in 2019 this value reached to nearly 415 ppm with an incremental growth of 2.3 ppm per year (average of the last decade), considering 1 ppm of atmospheric CO₂ is equivalent of 2.13bn tonnes of CO₂, each year nearly 5bn tonnes of CO₂ is added to atmosphere. Most credible climate scientists believe that the world needs to reduce its atmospheric CO₂ concentration to the “safe” level at 350 ppm at the most. This means we need to reduce the CO₂ concentration of around 65 ppm to hold the line against irreversible climate change. The awful truth is if all decarbonisation methods are effectively used from today and the whole world achieves Net-Zero emissions today (e.g. full electrification, 100% renewables, all electric/hydrogen transportation, etc.), we are only eliminating the growing rate (i.e. 2.3 ppm per year). And the 65 ppm which is the cumulative atmospheric CO₂ concentration since industrial revolution needs solutions other than those mentioned above for removal.

This means we need to counterbalance the legacy CO₂ by taking it back out of the atmosphere, which is also necessary to achieve Net-Zero Emissions by 2060 according to the IEA Technology Perspective Scenario to keep global warming below 2°C. This makes Negative Emissions solutions as part of the “Plan A” to combat climate change contrary to previous notion that negative emissions solutions are down the road far off solutions.

Negative emissions solutions refer to any means or technology that capture atmospheric CO₂ and store it permanently in a safe place. There have been several solutions proposed that could contribute to reduce atmospheric CO₂. Here I am reviewing and evaluating the top three promises:

Reforestation and afforestation

Reforestation and afforestation are great natural strategies to combat climate change via negative emissions as trees are fundamentally designed to absorb CO₂ from atmosphere. Reforestation and afforestation are universally desirable solutions to tackle climate change, and they can represent up to 30% of share for climate change by 2050. Many countries are already practicing reforestation, such as Brazil, which has a target to restore 12m hectares of forest by 2030. Estimations suggest a capture rate of nearly 3.7 tonnes of CO₂ per hectare per year is achievable with associated costs of $20 to $100 per tonne.

Reforestation means restoring areas where the trees have been cut down or degraded. Afforestation means planting trees where there were previously none. In recent decades there has been a movement toward people moving into cities which have freed up some lands and plus better farming methods could help to turn those lands to forest.

There are some considerations for this solution: e.g. land availability and suitability, plus forests cannot be planted indefinitely. Planting new forests could conflict with other requirements for land such as growing food. Also, planting vast areas of forests could cause complex changes in cloud cover, reflectivity, and the soil-water balance. Moreover, rising competition from bioenergy (will be discussed in the following section) is creating a conflict.

Bioenergy power generation with Carbon Capture & Storage (BECCS)

BECCS is another negative emission technology which is the most-popular and the most-talked about technology. BECCS achieves net negative emissions through capturing and storing CO₂ emitted from burning the biomass in a power station, where all the CO₂ absorbed by the plant (energy crops) during its lifetime is not re-released to the atmosphere but removed permanently from the atmosphere resulting in net negative emissions. BECCS has been considered in almost all climate management scenarios. For instance, BECCS accounts for 20% share in the IEA Energy Technology Perspective for limiting the global warming below 2°C.

What makes BECCS attractive is its negative emissions being able to compensate for the remaining emissions from other sectors such as transport or industry that are technically and economically very difficult to abate (e.g. the case for aviation or cement industry). Deployment of BECCS at scale is crucial to ensure the energy industry achieves net-zero emissions by mid-century with current carbon budgets.

Despite BECCS being considered to remove an average of 3.3bn tonnes of CO₂ per year by 2100 (according to the IPCC scenarios to limit global warming below 2°C), it faces one significant limiting factor, which is not the technology, but the supply of biomass. One study in 2018 estimated to achieve negative emissions using BECCS, every billion tonnes of CO₂ stored per year requires approximately 30 to 40 million hectares of BECCS feedstock. This is equivalent of 300 to 700 million hectares of land devoted to bioenergy crops depending on the efficiency of production. The cost of BECCS deployments varies widely depending on the source of biomass too. A cost range of $20-400 per tonnes of CO₂ captured are estimated depending on biomass sources and the sector being used.

Deployment of BECCS would also be limited by availability of land. Growing biomass (energy crops) enough for negative emissions of 10bn tonnes of CO₂ would take up more than one-fifth of the area currently used for growing food. BECCS benefits could be reversed (contributing to positive emissions) if existing forests should be removed to make land for energy crops.

Direct air capture

Direct air capture, sometimes referred to as DAC, is a relatively new and innovative technology that collects and removes CO₂ directly from the atmosphere, and when combined with permanent storage of CO₂, it creates negative emissions. DAC technologies are sometimes called artificial trees whilst unlike a real forest, they would need little land.

DAC is not an alternative to conventional, point-source CCS, but is can be used to complement compensating CO₂ emission from extremely hard-to-abate sources as mentioned earlier. However, finding a safe site with enough capacity to store CO₂ permanently is still an open question for all CCS applications.

Estimations suggest direct air capture could remove all the CO₂ currently emitted each year, but we need to ensure whether Direct Air capture can live up to its promise? Currently, the barrier to its commercial realization are its financial and practicality. The atmospheric concentration of CO₂ is nearly 0.04% whilst the CO₂ concentration in the flue gas of a gas or coal power plant is 100 and 300 times higher, respectively, this would make CO₂ capture using DAC expensive. Currently, the cost of CO₂ capture by DAC is around $200-1000 per tonne of CO₂. This technology needs to scale to bring its cost down, by reaching the economy of scale the cost of CO₂ captured by DAC is expected to reach below $80 by 2050.

As mentioned earlier, today the level of CO₂ is higher than any time in human history mainly due to human-caused reasons. Climate scientists widely agree the earth’s surface average temperature has already increased by 1°C since the Industrial Revolution, and without proper actions to control and limit emissions, the atmospheric CO₂ emission could reach above 500 ppm by 2050, leading to an increase in the average temperature to more than 3°C, a level that climate scientists say would cause extreme weather and sea level rise that would endanger global food supplies, cause disruptive mass migrations, and even destroy the Amazon rainforest through drought and fire.

Lots of grand promises have been made about capturing and removing CO₂ and now it is the right time to sort through reality from promise. The goal is using the right mix of carbon removal solutions as a part of humanity’s response to climate crisis. Now the big question is what form of carbon capture at what scale is going to make the best and the most socially just contribution? There is no single solution, but we need to bring everything we can to the table to combat climate crisis before it leaves behind irreversible and catastrophic scars.