Negative Emissions Technologies

The IPCC - the Intergovernmental Panel on Climate Change - published its first assessment report on climate change in 1990. That report barely mentions carbon removal, the intentional human effort to remove CO2 or other greenhouse gases from the atmosphere.

Thirty years later, carbon removal - or Negative Emissions Technologies - is central to any path out of the climate crisis. In the 6th iteration of the report, which came out in August 2021, all scenarios taking us to a world where mean average global temperatures don’t rise by more than 1,5°C assume we will absorb greenhouse gases (refers to article : The case for carbon capture) from our atmosphere. The more realistic scenarios taking us to a 2°C warmer world, also.

But, today, we don’t really know what these Negative Emissions Technologies are.

The case for Negative Emissions Technologies

For a fair chance at keeping our climate - and our world - stable, we need to avoid increasing global temperatures over 2°C by the end of the century. This asks that we reduce our emissions by around 5% to 10%, each year between 2030 and 2050, reaching net-zero emissions soon after mid-century. This is nothing less than radical. As a comparison, the COVID related lockdowns in 2020 lead to about 8% emission reductions ... for three months.

If we manage to drastically reduce our emissions, we will still find ourselves with what some studies call hard-to-avoid emissions. These are emissions that will be extremely difficult to abate. Some of them stem from the need to secure food supplies, others might come from remaining aviation fuel, for example. We’ll need to compensate for the warming effect of these emissions by capturing greenhouse gases from the atmosphere, and storing them safely. Eventually, we might need to capture the equivalent of 10 to 25 years of greenhouse gas emissions.

And if we don’t manage to bring our emissions down drastically, then basic risk management tells us that these technologies become essential.

We need to roll out and scale negative emission technologies in the 2030s already. That’s very soon, considering that we are barely starting to understand and investigate them.

What makes a good Negative Emission Technology?

Capturing CO2 from the atmosphere is not something we’re doing on a large scale today, but ideas and concepts are coming in from many directions. Research labs have been looking into these options since the 1970s and, unsurprisingly, oil majors have been looking into them as well. Today, a fresh wave of startups and research communities want to speed up the research, testing and scaling of these technologies.

Because we can barely afford to waste more time, we need solutions, not well-intentioned ideas with limited reach. (i.e Making emission offsets work)

The most promising Negative Emission Technologies options have a few elements in common:

  • They have potential. They can remove enough CO2 from the atmosphere to make a dent in the real world, not just on paper or in the lab.
  • They are permanent and not reversible. They can store the CO2 - or just C, the carbon atom of the CO2 molecule - for centuries or millennia, with no risk that the captured gases might end up in the atmosphere again.
  • They have acceptable negative side effects - there is no such thing as solutions without side effects - on humans, ecosystems and other elements of the climate system.
  • They maximise positive side effects, including on people and ecosystems.
  • They are cheap. We still live in a material world, after all.

We should also ask ourselves:

  • How quickly we might scale the method. We need practical solutions within 10 years, not 50.
  • If the solution is even legal, today.
  • If the technology is acceptable to most of us or if it might face heavy opposition, NIMBY, or raise ethical concerns.

The options we have

Releasing greenhouse gases in the atmosphere is simple. Just startup a car, or buy anything in a shop, or turn a light on, or eat a steak,… Absorbing them back from thin air is arguably more difficult. Nothing in our daily schedule absorbs CO2 back from the atmosphere.

For all the talk about high - and increasing - CO2 concentrations in the atmospheres, carbon dioxide only accounts for 1 out of 2500 molecules in the air we breathe. Collecting one ton of the gas involves sieving through more than 2500 tons of air, or the volume of about 690 hot-air balloons.

Fortunately, some people see past those numbers and are working on ways to get it done. As a testament to human ingenuity, they’ve come up with a range of interesting - at times intriguing - ideas.

They differ according to:

  • How the CO2 is captured from the air. It might be absorbed through chemical reactions with well-chosen minerals or specially designed solvents. It might also be absorbed by plants, through photosynthesis - the natural process by which plants grow by taking in CO2 through their leaves -.
  • What happens to the carbon atom from the CO2 molecules once they are captured. It could be stored in biomass - plants or other living organisms such as plankton - , soil, rock formations, or old oil and gas reservoirs.

Soil Carbon Sequestration (SCS)

Soils play an important role in regulating our climate (i.e Carbon cycle). They - and the living organisms they house - absorb, store or release carbon back in the atmosphere, depending on environmental conditions. In general, healthy soils, the ones that harbour rich microbial, animal and plant lives, are better at capturing and storing carbon, keeping it out of the atmosphere where it contributes to global warming.

Improving soil management practices by using cover plants between crop rotations or introducing low tilling methods, for example, can tip the balance in the right direction and boost the soil’s natural ability to sequester carbon. These methods usually come with positive side effects, as they can improve yields, help improve water quality, biodiversity and wellbeing for farmers.

They don’t always work though, as some soils might emit other greenhouse gases as they get richer. Soil Carbon Sequestration depends very much on ultra-local conditions: type of soil, climate, local vegetation, or local soil management traditions. And because the methods rely on human behaviour change, they are notably difficult to scale.

They are also quickly reversible: a new change in practice or a new land owner might release all the painstakingly stored carbon back in the air again without even knowing it. That’s a problem, because we want the carbon to stay put for centuries on end.

Afforestation, reforestation and wetland preservation

This is the poster-child of our fight against climate change, or even of environmental activism. Our forests capture a bit less than a third of our emissions today.

We could boost that number by planting trees. The Great Green Wall project aims to plant billions of trees, capture carbon and change the lives of communities across the entire width of Africa, for example.

But the biggest impact we can have is to maintain existing forests, avoid forest fires, stop deforestation or lengthen harvest schedules. That’s because new forests - usually of the boring plantation type - need a precious resource we are running out of: time. Time to grow, time to develop the rich ecosystems that are so good at pumping carbon out of thin air and storing it.

Afforestation and reforestation projects come with loads of side benefits. They usually boost biodiversity, support healthier water cycles and improve livelihoods. Plus, trees are just plain nice to have around.

Here as well, the main issue with this method is us humans. We need to be careful that forest carbon sinks don’t compete with crop land, driving more of the world’s population into poverty. We also need to maintain these carbon sinks intact and functioning for centuries - millennia even -. In a few hours, a mere forest fire releases most of the carbon that might have been sequestered for decades.

Preserving and maintaining wetlands comes with similar advantages and limits. It sits even higher on the list of priorities though, as these natural systems already store over 40% of the planet’s biological carbon although we’ve already lost a third of them.


Biochar is a twist on the Soil Carbon Sequestration technique, aiming for more absorption and longer storage time horizons. Essentially, biochar is barbecue charcoal. It’s a pitch black material, made up almost only of carbon, and manufactured by decomposing biomass - plants - under heat, but without oxygen. This concentrates all the carbon that the plants had absorbed from the air during their growth. By mixing biochar with crop-land or forest soil, we can lock the carbon in place and promote plant growth at the same time.

There is a small market for biochar already. But difficulties arise because different plants create different biochar and different soils react differently under different conditions. There are a lot of uncertainties on how much negative emissions the method could lead to on a global scale. Some studies are even questioning whether Biochar can store carbon in the soil on longer time frames than simple soil sequestration.

Biomass Energy with Carbon Capture and Storage

Biomass Energy with Carbon Capture and Storage - BECCS, pronounced “beeks” - tries to bypass the reversibility issue of methods that primarily rely on biomass as a storage medium - such as SCS, Biochar or afforestation -.

The CO2 is still captured by plants, through photosynthesis, though. The idea is to grow crops, harvest them, use them in a useful production process, capture the carbon they contain before it escapes into the atmosphere, and store it underground.

Ideas for BECCS systems come in all shapes and sizes. The most interesting variation on the concept - to generate negative emissions - seems to be to use the biomass as fuel for a power plant, to capture the exhaust gases - rich in CO2 - and to pump them deep below our feet in rock formations. There, the CO2 gas reacts with minerals and solidifies in carbonate form, sequestered for long enough that we don’t need to worry about it anymore.

Only a few demonstration projects are running today. They’re a good proof of concept but also reveal the limits of BECSS: all of these projects output biofuel, which, like all fuel, ends up generating emissions. That’s very profitable, but not supporting our goal of generating negative emissions. Also, BECSS works best with large mono-culture plantations. As such, it competes against crops for suitable arable land, and can contribute to deforestation, the destruction of ecosystems and local communities.

Direct Air Capture (DAC)

Direct Air Capture stores the carbon using similar technologies as BECCS, but absorbs it from the atmosphere in a very different way. Huge industrial-scale ventilators channel air through a series of pipes and machines containing chemical solvents or specially designed porous materials for the CO2 to bind to.

The first industrial scale plant opened in Island in 2021, making use of abundant and renewable geothermal energy to capture CO2 and pump it deep below the earth’s surface for storage. There, it reacts with volcanic rocks rich in calcium and magnesium, solidifying into carbonate minerals. Once it’s in solid form, it stays there. The facility captures the equivalent emissions of roughly 1000 cars per year. Some say that’s better than nothing.

This shows the difficulty in capturing trace amounts of gases from thin air. The fans, pumps and heat exchangers consume huge amounts of energy, which means any large DAC facility would come with its very own power plant. The entire scheme only generates negative emissions if the power plant itself runs on renewable power.

Side note: capturing CO2 directly at the source, at the exhaust of a coal fired power plant, for example, makes sense. The greenhouse gases are in higher concentration there, and easier to capture. But such a system can only reduce emissions from the plant, not generate the negative emissions we need to balance out our climate system inputs and outputs. (i.e The case for carbon capture)

Terrestrial Enhanced Weathering

While they might look inert to us, rocks react chemically with air and water in natural processes known as weathering. This unfolds over geologic times and captures CO2 from the atmosphere by binding the carbon atom to other minerals.

The idea behind enhanced weathering - as the name suggests - is to facilitate and speed up these natural processes, capturing more CO2, faster. We could mine or recycle rocks from mining waste, crush them and simply spread them on crop land, letting the capture process unfold naturally. The carbon ends up trapped either in the soil itself, or is washed to sea in mineral form.

Enhanced Weathering is barely coming out of the lab, with a few field experiments performed in Canada and Australia, but shows potential. The full climate impact of this low-tech process depends on the type of minerals used, where the minerals are mined, how it is treated before use, the local soil and climate conditions, etc.

Because it increases the soil’s content in useful minerals, the method could help increase soil quality and food production. But things are still blurry, as we’re starting to study the long-term impacts on soil quality, water cycles, ecosystems or even the health of the people involved.

Ocean Enhanced Weathering

Ocean Enhanced Weathering also tries to boost a natural process, the ones through which CO2 dissolved in seawater reacts with other minerals - carbonates and silicates - to form stable bicarbonates, in turn leaving space for more CO2 to dissolve in the ocean.

We could speed up this process by adding carbonate and silicate-rich minerals to the ocean surface waters - in other terms: by dumping rock powder in the ocean -. By increasing the ocean’s PH, this would also help balance out another consequence of global warming: ocean acidification, which is impacting biodiversity already.

Realistically, we still know little about the full ecological effects of Ocean Enhanced Weathering, and the more we look into it, the less attractive it looks.

Coastal Enhanced Weathering

Coastal Enhanced Weathering is an improvement of Ocean Enhanced Weathering. The principles are similar, with crushed rocks spread over coastal areas, making use of the tremendous force of crashing waves to speed up and keep the chemical reactions going. Coastal Enhanced Weathering also skips the complex logistics of moving large quantities of rocks to sea.

The method might help restore coastal environments since it works against ocean acidification and brings minerals useful to plant and marine life growth. But we haven’t looked into the full extent of its impact. The method also raises questions related to public acceptance, with minerals such as Olivine - a widely available mineral known for its green colour - being spread over existing fragile habitats.

Ocean fertilisation

Ocean fertilisation tries to exploit a different pathway to sequester carbon in our oceans. It stimulates plankton and algae growth which is often hindered by low concentration of minerals - iron in particular - in sea water.

Spreading iron or iron-rich minerals in the ocean could, in the right conditions, encourage marine plant and animal growth, capturing carbon from the air. However, the chain of chemical reactions leading to carbon sequestration, either in the ocean or as sediments trickling down to the bottom of the sea, is not clear yet.

Because Ocean fertilisation would rely on complex and expensive logistics and, based on small-scale experiments, seems to have largely negative effects on marine eco-systems, the method has been gathering less and less attention.

Ocean fertilisation could also be achieved through a technique called upwelling, which would use gigantic pumps to push nutrient-rich water from the deep ocean back to the surface.

An overview of Negative Emission Technologies (NETs).

Table data largely taken from Negative emissions—Part 1: Research landscape and synthesis, Jan C Minx et al 2018 Environ. Res. Lett. 13

Negative Emissions Technologies are not ready for deployment

We have plenty of ideas, rough estimates of what these methods would cost, blurry projections of how they might help ease the worst effects of climate change and coarse opinions on their potential side effects.

We’re running out of time though.

While the sheer diversity of potential negative emission technologies - or carbon dioxide reduction technologies - is a demonstration of our species's inventiveness, the speed and extent at which any of these options need to be investigated and eventually scaled is chilling.

While afforestation, reforestation and ecosystem preservation will help us in the short term, we will need to ramp up several other Negative Emissions Technologies in the 2030s. From today’s vantage point, Direct Air Capture, Terrestrial and Coastal Enhanced Weathering seem interesting. But we need systematic, well funded, research programs and field trials framed by courageous policies to clear out the uncertainties and understand the impacts of these technologies.