Let’s talk negative emission technologies



Negative emissions technologies or NETs have become a hot topic in climate policy discussions.

Afforestation, direct air capture (DAC), land management, ocean fertilization, enhanced weathering and bio-energy with CCS (BECCS) are some of the NETs deemed necessary to reach our ambitious climate targets.  In fact, it is thought that to curb greenhouse emissions to the levels we require to meet the Paris climate targets, NETs need to be deployed at a very large-scale by the end of the century.

However, there are concerns and challenges around the large-scale deployment and sustainable implementation of these technologies.

To gain a better understanding of the potential, but also the challenges facing some of these technologies, the Global CCS Institute interviewed Dr Niall Mac Dowell, Senior Lecturer of Energy and Environment Technology and Policy at Imperial College in London. Niall leads the Clean Fossil and Bioenergy Research Group at Imperial College and is the Principal Investigator on the UK’s Natural Environment Research Council funded international investigation – ‘Comparative assessment and region-specific optimisation of Greenhouse Gas Removal (GGR)’.

In this interview we discuss the implications of NETs (in particular BECCS and DAC), the opportunities and challenges offered by the technologies, and their role in meeting climate targets.



Terminology is an important part of understanding and framing the discussion around climate change. Could you explain NETs and greenhouse gas removal technologies? How do you define these technologies? Why do they play an important role in climate mitigation pathways?


Dr Niall Mac Dowell: There are several terms used to define what is ostensibly the same suite of technologies. Greenhouse gas removal, negative emissions, or carbon dioxide removal technologies, all describe technologies or processes which can remove CO2 from our atmosphere.

The CO2 removal can be done through a human engineered capture process such as direct air capture, or the enhancement of a biological process such as afforestation. The end result is the net removal of CO2 from the atmosphere to be permanently locked away in the geosphere.

We are currently emitting about 35 gigatons of carbon dioxide every year.  By the end of the century, in about 82 years, some of the climate mitigation scenarios have us removing as much as 22 gigatons of CO2 from the atmosphere.  Given that the global carbon dioxide production industry is currently emitting about 35 gigatons of CO2 per year, this means the industry that we need to build to remove this CO2 will need to be nearly as big. This is a huge ask!

There is no scenario accepted by the Intergovernmental Panel on Climate Change (IPCC) that can deliver a 1.5 to 2 degree scenario at the end of the century without extensive use of NETs.

It is not a case of just improving or helping our current situation, the reality is that without some form of NET we will not be able to meet the Paris commitments.


It appears that NETs have a significant role in meeting our future emission reduction commitments, yet several experts have recently questioned the future use these technologies raising concerns about the limits of their deployment. They say NETs are no ‘silver bullet solution’ to tackling climate change. Would you agree that NETs should not be seen as an alternative to current mitigation actions?


Dr Niall Mac Dowell: Absolutely.  It seems there is a quite a bit of confusion around this point.

NETs are not an ‘instead of’ or a trade-off. We can’t continue to burn unabated fossil fuels on one hand, and then on the other, create an industry to suck CO2 from the atmosphere. That is simply not a sustainable solution.

Right now, we need to be using everything in our climate mitigation arsenal to reduce the amount of CO2 that we are putting into the atmosphere.  This means drastically improving energy efficiency, switching to renewable sources of energy, ending fossil fuel use without CCS technology, nuclear energy – all the currently available CO2 mitigation options. By reducing the CO2 emissions that are put into the atmosphere now, we reduce our future dependence on technologies and processes that can remove that CO2.

However, since countries around the world signed up to keep global temperatures to “well below” 2 degrees above pre-industrial levels in the Paris Agreement, a huge amount of work has been undertaken to model our energy systems and carbon mitigation pathways to understand what it will take to limit warming to an aspirational 1.5 degrees. All of the integrated assessment models (IAMs) that get you close to a 1.5 degree limit rely on the use of NETs in addition to all of the current, more familiar CO2 mitigation activities.[1] 

BECCS is a good example. It is highly prevalent in most of these mitigation scenarios – having an important role as a relatively mature NET.  However, successfully creating a large-scale BECCS industry, first requires large-scale CCS infrastructure.

I think of this as the ‘AND AND’ response to climate change – we need to greatly increase our current CO2 mitigation efforts AND be actively working to improve the economic and environmental sustainability of many of the promising NETs.


What are the common challenges and concerns around BECCS? How would you characterise these challenges?


Dr Niall Mac Dowell:  BECCS is a very commonly cited technology on most climate mitigation scenarios largely because it is a relatively well understood and quantifiable technology. The common concerns around BECCS are not whether we can turn biomass into energy, or capture and store the resulting CO2 - the challenge with bioenergy lies in the sustainability of the supply chain. 

In our recent research, we analysed whether we can sustainability produce a sufficient supply of biomass and bioenergy crops and deliver them in a low-carbon way to the conversion site.  

Conventional wisdom would dictate that BECCS would remain small-scale because it will need to use local bioenergy crops – that is not going to deliver the gigaton level of carbon dioxide removal we need to achieve global climate targets.

I believe we are a heading to a world with a truly global bioenergy or negative emissions supply chain. To me, this is entirely conceivable but it would require a concerted global effort.

Imagine a future, where we could have Brazilian or North American biomass cultivated and harvested to be transported into the UK. There it would be transformed into bioenergy for UK consumption. The CO2 could be stored in the North Sea, a region with a great storage capacity.

This would be a complex global supply chain, with the potential to grow to be a billion or trillion-dollar industry. This would be very interesting to potential investors. However, for the supply chain to be successful, it is absolutely critical that it is not contaminated with unsustainable biomass. If this happens, we will be emitting more CO2 than we are removing.


What would you define as unsustainable biomass?             


Dr Niall Mac Dowell:  Unsustainable biomass is derived from the act of cutting down mature forests and engendering huge amounts of indirect or direct land changes and associated emissions.  All of that can result in emitting more CO2 emissions in the atmosphere than you would have removed by converting the biomass into energy and then sequestering the CO2.

It is also possible to grow the wrong biomass in the wrong location. An important part of our research is to consider the total mass balance of carbon, or the carbon efficiency of a BECCS facility.

For BECCS or any of these other technologies to be useful, they must net sequester carbon, and they must also factor in delays in the carbon payback depending on the timing of the project. We have explored this in much more detail in a recent paper published in Energy and Environmental Science.


BECCS is not the only NET that has issues with sustainability though is it?

Dr Niall Mac Dowell: No, just like some countries and regions are better suited to certain sources of renewable energy, NETs are a portfolio of different technologies that countries and regions will be to a greater or lesser degree, naturally endowed with the ability to deliver different kinds of negative emissions.  In some places, it might be more beneficial to do BECCS, in others afforestation, some areas may not be well suited for any type of NETs.

The work we are doing on Comparative assessment and region-specific optimisation of Greenhouse Gas Removal (GGR) is helping us to understand the portfolio of technologies available in different parts of the world.  We are also trying to find out the way this portfolio of technologies will be deployed. This in the same way we are deploying portfolios of low-carbon energy technologies to provide low-carbon heat power and transport. Finally, we want to try and understand how collaboration and cooperation within and between regions to deliver this negative emission target in a cost-optimal and sustainable manner.


In the NETs portfolio, where do you see DAC fitting in?


Dr Niall Mac Dowell: There is disagreement in the academic community on the cost involved in deploying this technology.  Some say direct air capture can be done for tens of dollars per tonne, going right the way up to thousands of dollars per tonne.

What seems to be true is that Climeworks in Switzerland is doing direct air capture. Like BECCS, we know it is technically feasible.

Although Climeworks is not involved in producing negative emissions today, they are doing good work in avoiding the emission of some CO2. They have publicly quoted a figure of 600 dollars a tonne. Importantly they are able to do this at that cost because they have built their facility on top of an incinerator so they can get the energy they need to recover the CO2 and regenerate their solvent. They can effectively get that for free.  If Climeworks had to go in a green field location where this energy wasn’t available, obviously that infrastructure would cost a lot more.  

Similar to BECCS, DAC also relies on the existence of some form of CO2 storage infrastructure in order to safely and successfully sequester carbon dioxide.


What is the role of CO2 utilisation in delivering negative emissions?


Dr Niall Mac Dowell: In my opinion, CO2 utilisation does not have a major role to play in terms of climate change mitigation, and I believe it has effectively zero role to play in terms of negative emissions. That does not dismiss the value of CO2 utilisation.

One example is the chemical industry where CO2 is used in chemical processes because it allows to displace other compounds that are bad from an environmental and human perspective. However, in terms of climate mitigation, the scale of carbon capture and utilisation (CCU) is usually too small, and the duration of the CO2 storage too short to make an impact. 

The exception to this rule may be in the creation of products like building materials enriched with CO2. In order to deliver negative emissions, we need products that sequester the COpermanently.  The carbon would be permanently bound into the product – mimicking the permanent storage of CO2 in the geosphere.


How do you see NETs interacting with other established technologies to meet the Paris global climate targets? How can they be effectively deployed? Should we be investing in such technologies?


Dr Niall Mac Dowell: My preference would be to see everything, meaning NETs, CCS and other important climate mitigation actions, happen today. This is the only way to meet the agreed climate targets on time. We need to move ahead as rapidly as possible with the wide scale deployment of CCS. 

CCS infrastructure not only helps to mitigate the use of fossil fuels and energy intensive industries, it is also a prerequisite for two of the best established forms of NET.

In that context, the 45Q tax credit in the United States could be quite helpful. Hopefully, we will start seeing some progress primarily in industrial carbon capture as a result. Beyond that, the next obvious things are biogenic emission sources.  The capture and storage of biogenic CO2 emissions are an important opportunity to achieve net zero. 

The Illinois Decatur plant already captures and stores around 1Mtpa of CO2 from a corn-based ethanol production plant. This good example of an early-mover facility capturing and storing biogenic CO2, thus delivering negative emissions.

We are right to be concerned about the risks surrounding the use of NETs. It is critical that as we develop these technologies and industries we are rigorous in assessing their carbon efficiency and the wider social and environmental impacts of these technologies/ processes.

Moving forward, I would like to see an emphasis on large-scale BECCS projects. Obviously, there will commercial challenges. However, there are no insurmountable technical or engineering challenges for BECCS or conventional CCS.  Pushing these projects forward as soon as possible is key.



[1] For more information on the current modelling being done to understand the potential pathways to 1.5degrees see this recently published paper in Nature:  and the Carbon Brief write up of the issues surrounding the paper: