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Negative emissions – a key assumption in limiting temperature rise

by Ella Karnik Hinks

The application of “negative emissions” technology has been widely used by the IPCC and incorporated into models which predict the most modest estimates of limiting the global temperature rise to 2 degrees [2]. Bioenergy and Carbon Capture and Storage, commonly known under the acronym BECCS, is one such technology. In principle, it works by using biomass as an energy source, and then capturing and storing the carbon dioxide release associated with this process.

Bioenergy and Carbon Capture and Storage infographic
Bioenergy and Carbon Capture and Storage – infographic by Ella Karnik Hinks

Using bioenergy as a fuel is not a new concept – for example burning wood or waste agricultural products, such as sugar cane. So far so good.

However, “negative” emissions are only achieved if the amount of carbon dioxide subsequently stored is greater than is emitted during the biomass production, transportation and utilisation [1]. Theoretically, CO2 can be stored by pumping it underground into geological formations. Yet there remains, in practice, a big “if”, when looking at the current existing facilities of carbon capture and storage – the two processes (of BE and CCS) currently only operate in one commercial plant in the United States [3].

Fundamentally, the issue with the inclusion of such technologies into IPCC models is two-fold: firstly, what are the consequential social and environmental risks involved? In the case of BECCS, a key limiting factor is the amount of land that is required to grow the biomass, leading to questions of the impact of land use change and the risk of increased competition for land. The second problem is in the feasibility of such ideas – what is the value of including prospective technologies into a scenario? Of course, the purpose of such scenarios is to imagine how a different world might look like, but it may be misleading for policy makers and environmental groups to assume that 2.6 RCP (minimizing global temperature rise to 2 degrees) is a plausible pathway, when a key assumption was the successful inclusion of BECCS to the model.

  • Problem 1: It’s not actually happening anywhere – is it realistic?

What is the problem with modellers using BECCS? Climate modellers conceptualize different pathways to consider how we can reduce the global temperature rise. The most optimistic of these scenarios include BECCS technology into the models, meaning the supposed possibility of limiting warming to a 2-degree limit rests on an assumption that this technology will be used. However, despite the inclusion of BECCS into scenarios by the IPCC, it is more or less a theoretical concept.

So what would the world have to look like for BECCS to work? Modellers optimise pathways, based on financial costs, and hence entail some social and environmental compromises [2]. In the case of BECCS, the requirement is land. A lot of land. An estimation of twice the size of India has been suggested as the amount needed to reach the BECCS target the models suggest [2], clearly raising the question of how feasible this technology is.

  • Problem 2: Is burying a problem a long-term solution?

Ask any therapist. Although it remains a tempting option to merely bury your problems, or dump them in the ocean, there is significant historical evidence that this is a temporary solution which can lead to other problems down the line. The basic concept for BECCS is that by using bioenergy as a fuel, the only carbon dioxide released into the atmosphere is simply that which was originally contained in the biomass – it is a renewable energy source. In order for BECCS to offer an overall removal of carbon dioxide, it must be stored permanently, for example geologically underground. The question remains therefore if and how this can be done in a way that will not lead to future environmental issues, and naturally, in an economically efficient manner. It is possible to pump the CO2 into the ground, where it can be trapped in a porous rock. This porous rock must be at a sufficiently deep level, with a layer of non-porous rock above it to trap the carbon. The risks involved with this are that there are many uncertainties in the geological impacts, for example the effect on seismic activity, or the potential of leakage. Of course one cannot help but think there must exist more sustainable solutions than simply burying waste. Which leads me to the final point:

  • Problem 3: It lessens and distracts from the urgent need to reduce emissions

Inherently, the best way to solve a problem is to deal with the root cause. Global emissions of carbon dioxide must be reduced dramatically. This is at the heart of the international targets set at the Paris Agreement, with countries across the globe aiming for net zero emissions. However, the ambiguity as to whether this “net zero” can be reached from reducing emissions or including negative emission technology leaves climate policy without a clear indicator of how quickly and decidedly action must be taken [4]. The danger here lies in that countries are given a loophole in their commitment to reduce emissions. One recommendation would be that there is a clear separation of targets aiming to reduce emissions, and those to include negative emission technologies [4].


[1] Global CCS Institute 2019

[2] 2020 “Guest Post: Where in the UK might be suitable for BECCS”

[3] Stiftung Wissenschaft und Politik 2020 “Unconventional Mitigation: Carbon Dioxide Removal as a New Approach in EU Climate Policy”

[4] Frontiers in Climate 2019 “Beyond “Net-Zero”: A Case for Separate Targets for Emissions Reduction and Negative Emissions”

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