“Without these technologies, we will hardly achieve our climate targets”

Under the motto “Ecosystem Restoration”, the theme of Word Environment Day on 5 June 2021 is the restoration of the planet’s ecosystem. The only way to achieve this is through climate protection – one of the most hotly debated topics of our time. It is becoming increasingly obvious that this not only requires a drastic reduction in greenhouse gas emissions, but additional measures for reducing the level of carbon dioxide (CO2) in the atmosphere. Unless this happens, it will be impossible to achieve the primary goal of the Paris agreement: to restrict global warming to no more than two degrees.
Tom Kober analyses energy systems and knows that a neutral climate balance can ultimately only be achieved with the help of negative emission technologies. (Photo: Paul Scherrer Institute/Mahir Dzambegovic)

Mr Kober, climate scientists are warning that negative emission technologies (NETs) are needed to achieve our climate targets. What exactly are NETs?

These technologies permanently remove CO2 from the atmosphere by capturing and storing it underground, for example, and preventing it from being released back into the atmosphere. So while it is still important to reduce greenhouse gas (GHG) emissions from fossil sources using conventional climate protection measures such as energy savings and renewables, the purpose of NETs is to remove gases already emitted into the atmosphere. Studies have shown that it will be impossible to achieve our long-term targets unless we do so. Above all, it is important to remember that carbon dioxide stays in the atmosphere for several centuries, and the planet’s climate system only reacts very slowly to its effect. As a result, the CO2 emitted 50 years ago is still affecting our climate today, and the impact of our current incredibly high emissions will continue to be felt over the coming decades – even if we do manage to reduce GHG emissions to zero by mid-century. On top of that, there are some areas of the economy, such as agriculture, where even with the best intentions, it is impossible to eliminate emissions entirely if we are to protect a secure food supply. So to attain our goal of “net zero” emissions by 2050 as planned, we need to deploy negative emission technologies, or NETs for short. They offset emissions that are either unavoidable or can only be avoided at enormous expense.

So is planting trees also a form of NET?

That is actually called a ‘negative emission measure’ but the basic effect is similar: trees capture CO2 from the atmosphere and convert it into carbon, which stays trapped until the wood decomposes or is burnt, ideally only after a very long period. Planting trees, or deliberate afforestation, therefore also benefits the eco-balance. Trials with other non-technological measures are currently ongoing as well: enriching oceans with iron or other nutrients to encourage the growth of algae, for example, which then absorb more CO2. More sustainable soil management created by carbon contained in plants is another alternative: crop residues and carbonised biomass (with the carbon captured in it) can be reincorporated into the soil, acting as a form of fertiliser. This promotes root penetration, improves water retention and stops nutrients leaching out of the soil. At the same time, it encourages carbon sequestration. This not only creates a more reliable food supply, but also benefits our climate. However, my own research group is more interested in technological solutions.

So what are they?

There are basically two types: we already have pilot plants with ventilators that suck air from the atmosphere, pass it over large filter surfaces and capture the CO2, which can then be stored underground, for example. This technique is called carbon dioxide air capture, or direct air capture. Another possibility is not to filter the CO2 directly from the air, but to separate it from the waste gases or process gases emitted when energy is produced from biomass. In a typical waste disposal plant in Switzerland, for example, around half the waste is biogenic. If the plant is connected to a system that captures and stores CO2 (carbon dioxide capture and storage, or CCS for short), it is possible to capture the carbon dioxide extracted from the air by plants through photosynthesis and store it underground, rather than burning it and releasing it back into the atmosphere. At the same time, heat and electricity are generated during this process, along with ethanol and hydrogen which can then be used as carbon-neutral biofuels. This method allows energy to be produced while at the same time removing CO2 from the atmosphere, effectively killing two birds with one stone.

Can the captured carbon be used to produce synthetic fuels as well?

Yes, but then the net emissions would no longer be negative because the burning of synthetic diesel, petrol or aviation fuel would release carbon dioxide back into the atmosphere within a very short time frame. At best, this technological approach would be carbon neutral. The intended gain provided by negative emissions is only possible if we ultimately “lock away” the carbon and permanently remove it from CO2 circulation.

Even so, some critics of CCS warn that the effects of carbon dioxide stored underground have not been properly researched. Perhaps the gas could pollute the groundwater or soil, or even escape back into the atmosphere …

It is fair to say people in Europe have since grown quite sceptical about CSS, and as far as I know, hardly any projects are currently trialling this technology. Previous projects have studied the storage of CO2 in different ways, such as the Ketzin pilot plant in Potsdam. In trials conducted by the German Research Centre for Geosciences, carbon dioxide has been reliably and securely stored in a saline aquifer. In the North Sea, CO2 has been successfully stored in sedimentary rock more than 800 meters depth below the seabed for several years. The Norwegian offshore energy company Equinor has been continuously sequestering large quantities of carbon dioxide since 1996 in the area of the Sleipner natural gas field and since 2008 in the Snøhvit gas field area, in each case carefully monitoring how the CO2 behaves underground. As with natural gas repositories, protective mineral layers – made of clay, for example – act as caps to ensure that the injected CO2 stays sequestered underground. I believe it is essential to conduct more research and trials into CSS, rather than leaving the task to countries outside Europe. Anyone who takes climate protection seriously must keep all options open.

How well advanced is the development of NETs already?

The technology is already working well in pilot and demonstration plants. The Swiss company Climeworks, for example, is running a project in Iceland to extract COdirectly from the atmosphere. A handful of power stations already have CCS systems, but these usually do not use bioenergy and only capture relatively small quantities of CO2. In addition, a number of bioethanol plants with CO2 capture are already in service in North America. But none of this has yet become established on a truly commercial scale, by which I mean specifically power stations with an output in the region of several hundred megawatts. It should happen within the next few years, although the process integration for large-scale plants certainly presents a challenge. Furthermore, NETs still have to overcome several technological challenges, such as converting the biomass in a way that allows pure hydrogen and pure carbon dioxide to be produced continuously. Similar challenges are the transport of large quantities of CO2 over long distances and then storage in underground rock formations. Last but not least, there are economic hurdles for the entire NET chain, as these technologies offer comparatively cost-intensive options for reducing emissions. Our model calculations for Switzerland show that in the long term, waste incineration plants with CSS as well as hydrogen production from biofuels with CO2 capture could be cost-efficient NET solutions that help us achieve our net zero target for emissions, provided their costs and efficiency improve in future. The hydrogen produced in with NETs could then be used directly as an alternative fuel for heavy goods vehicles, and possibly as fuel for aviation or shipping as well. In short, for any application where battery-powered vehicles are not particularly suitable because these modes of transport require high energy densities.

So what quantities of CO2 are these technologies supposed to remove from the atmosphere by mid-century?

According to our analyses, NETs need to extract around ten gigatonnes (ten billion tonnes) of CO2 from the atmosphere every year if we are to reach our global target by 2060. That is almost a third of the 35 gigatonnes of energy-related CO2 currently emitted worldwide every year. In Switzerland, our model outcomes for 2050 show that NETs will need to be able to capture and store around four million tonnes of CO2 for us to reach our ambitious net zero target by 2050.

If they become proven technologies, could NETs reduce the pressure from climate change to give us more time to deal with the problem?

On the contrary! All potential and cost-efficient measures for reducing greenhouse gases must definitely be implemented as quickly as possible. Here negative emission technologies should not be mistaken for a free pass. They are very costly and should only be used to capture the residual emissions that are extremely difficult to avoid.

Does Switzerland even have enough underground storage facilities for CO2?

That is certainly a national challenge. Our models indicate that to achieve net zero by 2050, Switzerland needs to reduce emissions at an annual rate of nine million tonnes of CO2, which can be captured and stored using NETs and other CCS technologies. A study by ETH Zurich has shown, however, that the lack of porous sediment in Switzerland effectively limits national storage capacity to around 50 million tonnes in total – which will not get us very far. As we also need to maintain this rate of storage beyond 2050, it is important to explore potential large reservoirs outside Switzerland that we might be able to share. It could be possible, for example, to transport CO2 via pipelines or ships to the North Sea and take advantage of its potentially enormous CO2 storage capacities.

Will that not be incredibly expensive?

The costs per tonne are likely to be in three figures. But our cost analyses show NETs are still economically sensible and competitive in the long run, provided they are accepted by the general population. This is especially the case considering that alternative ways of reaching net zero can be considerably more expensive for certain applications. In addition, it would be possible to operate the plants in an entirely different location. As far as the planet’s climate balance is concerned, it makes no difference whether the CO2 is captured from the atmosphere in Switzerland or in Asia, for example. We should therefore be on the lookout for suitable locations for NET facilities and prioritise international cooperation. Naturally, this assumes the overall legal framework is in place to support such measures. In other words, emission reduction measures implemented abroad should also count towards national emission targets.

Which options are reasonably priced?

That depends on the technology. Direct capture from the atmosphere is very intensive in terms of physical area and energy, and mainly makes sense where land and energy prices are cheap. Ideally as much sustainably produced biomass as possible needs to come from a nearby source – especially wooden biomass and biogenic waste and crop residues. What’s more, underground CO2 repositories also have to be available. Without having studied these aspects in detail, I could imagine, for example, that suitable biogenic resources and CO2 storage facilities might be available in Eastern Europe or Western Asia. We have just launched a new research project in which we analyse international supply chains for sourcing hydrogen and synthetic fuels. This should provide more insight into the viability of negative emission technologies. Once we have completed that, I will hopefully be able to provide more specific details.


Dr Tom Kober
Head of the Energy Economics Group
Laboratory for Energy Systems Analysis
Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland
Telephone: +41 56 310 26 31, e-mail: tom.kober@psi.ch [German, English]