An Introduction to Negative Emissions Technologies

A figurative way to visualise the challenge of anthropogenic climate change is to imagine being stuck on a small boat in the middle of the ocean. Ten holes suddenly appear in the bottom of the boat and it rapidly starts filling up with water. You have two options to keep the boat afloat. The first is plugging the holes to reduce the water intake. The second is bailing out the water at a rate greater or equal to the water coming in.

The sudden ingress of water due to the ten holes is a metaphor for the rapid increase in emissions since the industrial revolution. Plugging the holes represents the global effort to reduce emissions through strategies such as renewable energy or the electrification of transport. Unfortunately, it’s increasingly clear that we’re not plugging the holes fast enough and that we also need to focus on bailing out the water, herein referred to as negative emissions technologies (NETs). These can be defined as intentional human efforts to remove CO2 from the atmosphere. Before we look at NETs in more detail, it’s important to first set the scene to understand the boat we’re in and how much water we might need to bail out.

Let’s begin in France…

The Paris Agreement is a legally binding international treaty on climate change that was adopted by 196 parties in 2015. The objective of the treaty was to limit global temperature rise compared to pre-industrial levels to “well below” 2°C and to “pursue” efforts to keep it below 1.5°C. It signified a landmark moment in the multilateral effort to tackle climate change by bringing all nations together under a common cause through a binding agreement. As part of the Paris Agreement, each country had to submit their climate change mitigation strategy to reach the goals of the treaty, known as nationally determined contributions (NDCs).

Why did we choose 1.5°C and 2°C?

Up until the Paris Agreement, a 2°C rise in global temperatures compared to pre-industrial levels had long been the default target for trying to limit the effects of climate change. By 2015, it had becoming increasingly clear that the outcome of a 2°C increase would not be “safe” and hence the need to pursue efforts to keep warming to 1.5°C. In 2018, the International Panel on Climate Change (IPCC) conducted a Special Report on the likely outcome of these two scenarios, which is summarised in the graphic below:

Ok, so how are we tracking?

The carbon budget is defined as the cumulative amount of net CO2 emissions that can be released while still limiting warming with a specific minimum probability to below a given temperature threshold. Returning to our boat metaphor, this is the amount of water that can enter the boat before it starts sinking. Once the carbon budget for 1.5°C is used up, we will need to offset every tonne of CO2 we emit if we want to keep warming to 1.5°C. The following graphic from Carbon Brief, which was adopted from the UNEP Emissions Gap report in 2019, shows how far current policies and NDCs are from achieving either a 1.5°C or 2°C trajectory:

So we’re off track. Technology to the rescue?

In the last few years, NETs have garnered increasing attention in climate change mitigation plans due to the cognitive dissonance between ambitious international policy such as the Paris Agreement and short term emission reduction efforts, as evidenced by the graph above. The increasingly critical role of NETs is evidenced by the IPCC’s Sixth Assessment Report, which modelled five different scenarios to achieve different warming levels. In the extracted figure below, SSP1–1.9 is the only scenario that keeps warming to within the 1.5°C range. It’s important to realise that not only does this scenario require us to reduce global annual emissions from a level of ~40GtCO2 today to 0 by 2055 but thereafter requires us to be removing CO2 from the atmosphere at levels of over 10GtCO2 by the end of the century. It’s clear that the IPCC is betting heavily on rapid NET deployment at scale to achieve 1.5°C, both in the short term to achieve the pathway to net zero but also in the long term, where we’re actually drawing more CO2 from the atmosphere than we’re emitting.

So what are some of these NETs?

Afforestation and Reforestation (AR)

Afforestation is the creation of a new forest by planting trees or seeds in an area where there were no trees before. Reforestation is planting trees in a forest where the number of trees have been declining. These trees capture and store carbon dioxide via the process of photosynthesis. The amount of CO2 captured is estimated based on the above ground dry weight of the tree, of which around 50% is carbon. It’s important to note that this is not a permanent form of storage and once the tree decomposes or is burnt, the CO2 absorbed is then released back into the atmosphere. Therefore, the biomass needs to be managed through activities such as converting it to biochar (more later) or if the forest is well conserved, we can consider the sink “stable”.

Bioenergy with Carbon Capture and Storage (BECCS)

This involves converting biomass into useful thermal or electrical energy and then capturing and storing the carbon dioxide produced. Bioenergy on its own was initially viewed as a net zero energy resource in that the CO2 produced is only equal to the CO2 that was sequestered during the growth of that biomass. However, this does not take into account the production, logistics and transport of producing and burning the biomass. Therefore, if you then implement carbon capture and storage on the CO2 that is produced during the energy conversion process, you’re creating a net carbon sink.

Direct Air Carbon Capture and Storage (DACCS)

This involves capturing carbon dioxide directly from the ambient air and generating a concentrated stream of CO2 for storage. There are two current approaches to drawdown the CO2 from the air. These are either liquid systems, which involve passing air through chemical solutions to remove the CO2, and solid systems, which use sorbent filters that chemically bind with the CO2 and are then subsequently heated to release the concentrated CO2. Currently these processes tend to be quite energy intensive because they are trying to extract a molecule that only represents 0.04% of atmospheric air.

Enhanced Weathering (EW)

Weathering is a process that occurs naturally, whereby exposed rocks absorb carbon dioxide from the atmosphere and transfer it into other compounds in the presence of water. Enhanced weathering accelerates this natural process by spreading finely ground rock on land so as to maximise the reactive surface area. The sequestered CO2 can be stored in several different stages. The first is that it can remain as dissolved inorganic carbon in the soil and groundwater solution. If this gets supersaturated, the carbon minerals can precipitate in the soil and residence times can then be on a geological time scale. In the absence of supersaturation and precipitation, the solution can be transported to the ocean by rivers and help reverse ocean acidification due to its alkalinity.

Biochar

Biochar is the black residue that is obtained from pyrolysis, which is the thermal degradation of biomass in the absence of oxygen. The biochar is then typically spread on soil to ameliorate soil fertility and store the carbon long term. It’s important to note that it is the input biomass that performs the carbon sequestration function when it is alive through photosynthesis whereas the biochar process acts as a means of stabilising the carbon to prevent it returning to the atmosphere.

Soil Carbon Sequestration (SCS)

SCS is when a net removal of CO2 from the atmosphere occurs due to improved land management techniques increasing the soil carbon content. This can be either by increasing the carbon input to the soil from litter, residues, roots, manure etc. or by reducing the carbon output due to soil disturbance, respiration etc.

Other NETs

Ocean fertilisation is another NET that deserves mention and involves adding nutrients such as iron to promote phytoplankton growth. The phytoplankton then draws down atmospheric CO2 and sequesters it when they die and sink to the ocean bed. One of the concerns is that there is a high recycling rate for the stored organic carbon, which limits its ability to sequester CO2 but larger scale trials are needed to understand this better. There is also growing interest in Blue Carbon, which is an umbrella term for the conservation and restoration of sea grasses, mangroves and salt marshes along coasts to expand their ability as a carbon sink. Large scale trials are required to understand its carbon sink potential.

Potentials, costs and side-effects

The following table summarises a research literature assessment of the carbon removal potential of these NETs by 2050, the estimated cost per tonne removed and the positives and negatives associated with each methodology:

So forget renewables, focus on NETs?

There is concern that the widespread integration of NETs in scenario modelling is indicative of technological optimism i.e. the misplaced confidence in the efficacy of technological solutions to solve socially created problems. For example, getting NETs to the deployment level required in many 1.5°C and 2°C scenarios requires adding several hundred BECCs or DACCS plants every year during the period of 2030–2050. However, these technologies are still very much in their nascent phase, with 60% of the scientific literature still dealing with research and development. The resulting moral hazard is that by placing an unrealistic expectation on the future potential of NETs, we lose focus on the effort required for normal emission mitigation strategies such as renewable energy deployment, electrification of transport etc. Therefore, it is imperative that we continue to invest in emission reduction strategies almost under the assumption that NETs will contribute nothing, whilst also pushing these technologies forward so that we develop a diversified portfolio of climate mitigation strategies in isolation.

Ok but how can we progress NETs as quickly as possible?

The deployment pathway of any frontier science technology begins with research and development. This involves assimilating scientific knowledge to model what a possible solution to a problem might look like. Field demonstrations are then required to ensure that this modelled solution actually works when deployed in the natural environment e.g. testing tidal energy turbines in Orkney. Given that such trials are typically capital intensive, it’s vital that there is an appropriate level of both private and public investment at this stage so that promising technologies are progressed.

If the demonstrations are promising, the next stage is to scale up the technology so that it approaches commercial viability. Policies are often required to bridge the gap at this stage so that there is sufficient demand to fund the scale up. For example, the UK implemented generous subsidies for onshore wind turbines which allowed for iterative development of the technology over time until cost parity was achieved and subsidies were no longer needed. The scale up stage can also benefit due to niche markets, where early adopters are willing to pay more for the technology and hence fund its pathway to commercial viability. The final aid is public awareness and acceptance of the benefits of technology, whereby members of society can influence its uptake through democratic pressure encouraging favourable policy.

Shout out to Stripe and Sourceful!

Companies like Stripe and Sourceful have made a conscious effort to create a niche market for NETs to support field demonstrations and accelerate the learning curve towards commercial viability. Stripe purchase negative emissions at any price, with a commitment to spend at least $1 million annually on these purchases. They have a panel of experts who assess which applicants have the most potential of providing a game changing solution and purchase offsets from the winners each year. Sourceful are also pioneering this in the UK by providing a platform whereby businesses can contribute towards a portfolio of NETs and they will match your contribution to help these projects get the capital support they need. If you’re interested in getting involved, learn more here.

To conclude…

The IPCC’s Sixth Assessment Report stated that global warming of 1.5°C and 2°C will be exceeded during the 21st century unless large emission reductions occur in the coming decades. Their two scenarios that keep warming to below 2°C by the end of the century require annual emissions to drop from a current level of ~40GtCO2 to less than -5GtCO2. The negative sign here is not a mistake.Therefore, in tandem with rapidly reducing greenhouse gas emissions, NETs should be supported to ensure that technological optimism is converted into reality. It’s important that a portfolio of techniques are developed given the varying constraints of each methodology and that we avoid the moral hazard of shifting focus away from emission reduction strategies by relying on future NETs.

After all, if you were stranded on a sinking boat in the middle of the ocean, wouldn’t your first reaction be to plug the holes before thinking about bailing out the water?