Barry McMullin, Paul Price, Mike Jones and Alwynne McGeever explore the background motivation and concepts underpinning the recent interest in NETs, and the opportunities - and risks - associated with their deployment here


The idea of ‘negative emissions technologies’ (NETs) – technologies which could help address climate change by actively removing greenhouse gas pollution from the atmosphere – has attracted rapidly growing academic attention in recent years. In Ireland, the EPA has recently funded a small preliminary study of the realistic potential for NETs to contribute to our national ‘fair share’ of climate change mitigation (based on the commitments in the Paris Agreement).

The project is called IE-NETs and is a collaboration between Dublin City University (DCU) and Trinity College Dublin (TCD). Professor Barry McMullin of DCU, along with other members of the project team, presented some initial findings at Engineers Ireland in December. In this follow-up article, the project team explore the background motivation and concepts underpinning the recent interest in NETs, and the prima facie opportunities – and risks – associated with NETs deployment in Ireland.

Climate change: real, immediate and potentially overwhelming

The parties to the UNFCCC Paris Agreement, including Ireland, have committed to the goals of limiting the global average temperature increase, caused by human activities (particularly the uncontrolled release of greenhouse gases into the atmosphere) to ‘well below 2℃’ over pre-industrial levels, and taking actions to attempt to respect a lower limit of +1.5℃ over pre-industrial levels.

These goals are based on the best available scientific assessment of likely severe risks of dangerous climate change on a very widespread global basis as these temperature limits are approached or exceeded. Indeed, significant impacts are already clearly visible in many parts of the world, including Ireland.

Failing to hold to these temperature goals would raise very real potential for climate change impacts on a scale that could overwhelm any possibility of effective adaptation. These impacts would be initially manifested on a localised basis in the most climate vulnerable regions, but would expand relatively quickly to have devastating global effects. This analysis accurately reflects the trajectory of current climate policy responses around the world, and it represents the outcome of the most rigorously honed scientific understanding ever achieved in human history.

This is no longer an issue of an uncertain, distant future: climate change profoundly threatens the security and welfare of younger generations already living today, in Ireland and globally.
Of course, climate change is a global problem and needs a global response: but some of us contribute much more than others to the problem, and also have a much greater capacity (material wealth, infrastructure and so on) to act.

Accordingly, some will need to play a much bigger role in the response than others. Given that, on a per capita basis, Ireland’s total annual emissions are currently among the highest in the world, fairness and justice suggest that we have a particular obligation to work harder (and earlier) at reducing them.

The role of ‘negative emissions’ (greenhouse gas removal)

There is no doubt that deep reductions in greenhouse gas emissions are now unavoidable if we are to have an effective, managed, response to global climate change over the next small number of decades. But this is very challenging precisely because our current ‘industrialised world’ patterns of production and consumption are so deeply entwined with processes that intrinsically generate these emissions.

For that reason, it seems that even if we reduce ongoing emissions very rapidly (which we must do), this may not be sufficient on its own to align Ireland’s transition to a low-carbon society with the Paris Agreement goals. In that case, it is now being argued that we will also have to try to proactively remove some, or a lot, of the greenhouse gases that are already accumulating to dangerous levels in the atmosphere: that is, through achieving some level of ‘negative emissions’. This is true on a global basis, but is especially true for countries like Ireland that have very high current (per capita) emissions.

It is well understood that the recent Irish National Mitigation Plan (NMP) does not (yet) lay out a sufficient reduction of emissions to stay within our ‘fair share emissions budget’ over the immediate years (and decades) ahead: in fact, there is a very big and widening gap between our current policies and our stated aspirations. One possible interpretation of this is that the plan is tacitly assuming that negative emissions will be achieved at sufficient scale to close this gap. But what are these supposed technologies and how feasible or practical are they to deploy?

Bioenergy with carbon capture and storage (BECCS)

One of the most commonly proposed approaches to negative emissions is based on the use of ‘bioenergy’ crops, at large scale. Plants absorb carbon dioxide and produce oxygen via photosynthesis, automatically removing carbon dioxide from the atmosphere and storing the carbon in their biomass tissue. Usually the plants then die and decompose, or are harvested and eaten (by people or as animal feed), or are otherwise ‘consumed’ (e.g. burned for heating or cooking). In all cases, the captured carbon in their biomass is released back to the atmosphere as CO2.

However, if we intervene in this cycle, and instead process the plant biomass in a controlled way, we can still exploit the energy embodied in it, but, at the same time, (re-) capture the CO2 that is released, and put it into some kind of stable, long term, storage. Such an arrangement is called ‘BioEnergy with Carbon Capture and Storage’ or BECCS.

But while the various elements of BECCS are certainly possible in principle, it remains very unclear whether such arrangements can be effectively scaled up, or at what cost. In particular, it potentially involves a progressively larger allocation of land area for cultivation of bioenergy crops, and would therefore potentially conflict with other land uses, especially food production.

An issue with any proposed expansion of bioenergy is the choice of plant, and especially the duration of the growth and harvest cycle. For example, forests may take decades to grow, but timber might be harvested and burned in a matter of a few weeks; whereas short rotation bioenergy crops (such as miscanthus or willow) are grown and harvested on a cycle much more closely matched to the time in which they are consumed. Plant choice is constrained by local soil and climate conditions and competing land uses, and varies significantly even within Ireland.

Certainly, in the case of current, ‘unabated’, bioenergy use (without carbon capture and storage), there is a clear advantage to focusing on short rotation energy crops.

Separately, consideration would have to be given to whether bioenergy cultivation can or should displace existing farming systems. However, current Irish agriculture is dominated by beef and dairy production, which are intrinsically greenhouse gas intensive (relative to nett nutritional output).

Accordingly, particularly if enabled by a change in farm subsidies, a significant shift of land use from beef or dairy to bioenergy (and, more especially, to BECCS) might offer a ‘win-win-win’ of reducing emissions of methane and nitrous oxide (assuming both the number of ruminant animals and total fertiliser use are reduced) while simultaneously achieving net removal of CO2 and significantly enhancing national energy security.

In any case, it is clear that the bioenergy resource (nationally and globally), will be finite and constrained. Therefore, in order to get the maximum climate benefit from that resource, it would make sense to ensure that it is developed in a way that is compatible with BECCS deployment as soon as is practical. This is an important immediate policy consideration because BECCS is generally only feasible where the combustion (direct or indirect) of the biomass happens in large scale plants, i.e., where it is practical to install carbon capture and storage technology.

Normally that means use in electricity generation or other large-scale industrial settings – which would specifically argue against allocation to small-scale or mobile combustion (small-scale heating or transport use in the form of ‘biofuels’). But at least some current Irish policy appears to run directly contrary to this. For example, it seems that the recently announced Renewable Heat Incentive scheme (which will support new equipment and infrastructure which may remain in operation for decades into the future) will specifically favour the use of bioenergy in numerous smaller plants, unsuitable for CCS.

‘Direct’ CO2 removals?

A second possible approach to negative emissions is to echo the role of plant photosynthesis in BECCS with machinery that is capable of filtering and concentrating CO2 from air directly. This is called ‘Direct Air Capture [of CO2]’ or DAC. Of course, this must also be combined with long-term storage, i.e., ‘Direct Air Carbon Capture and Storage’ or DACCS.

In principle, DACCS could operate without the use of large land areas (air might be drawn into a relatively small, fixed area using large fans). However, unlike BECCS, instead of yielding usable energy output, DACCS would require very significant net energy input: so unless this energy is available from extremely low carbon sources (renewables or, perhaps, nuclear) then it will not result in net CO2 removal. Even if it can be supplied with suitable low carbon energy, the energy requirement alone will tend to make it relatively high cost: so a business model to support it would have to be created.

Nonetheless, for a country like Ireland with a large low-carbon renewable energy resource (particularly offshore wind), DACCS might offer an excellent way of dynamically balancing the intermittency of such resources if it could be deployed as a large-scale ‘dispatchable’ load (where the energy being consumed can be quickly ramped up or down to compensate for variability in renewable generation, so that the marginal cost of the low-carbon energy used for DACCS could still be relatively low, with co-benefits in facilitating very high penetration of renewable energy sources into overall electricity generation).

Ireland may thus have a particular, strategic, national interest in promoting and leading DACCS deployment.

Other negative emissions possibilities

There are a variety of other candidate ideas for achieving negative emissions: increasing natural accumulation of carbon in soils (soil carbon sequestration, SCS), increasing the total carbon storage in standing forest (but which must, therefore, be in the form of new, additional, forestry plantations, and left unharvested, again with significant potential for land use conflict), using biomass to produce a form of charcoal called biochar that can then be added back into soils, or ‘enhanced weathering’ (EW) where crushed silicate minerals might be spread on land surface and would naturally absorb CO2 (however, as with DACCS, this would be highly energy intensive).

In the short term, Ireland does have some potential to increase carbon sequestration in standing forest and in soils, and indeed there is evidence that grassland soils are doing so already; but this effect still needs much further research to assess its potential capacity (before saturation), and its deployment would likely require very extensive monitoring and verification of the carbon stock to ensure significant additional carbon storage. Further, we have to carefully acknowledge that soil and forest carbon are both inherently impermanent and vulnerable to loss, particularly in the context of further global warming and resulting climate change stress.

Therefore, while all of these merit study for their potential in Ireland, they are generally more speculative, less permanent or have more limited capacity for scaling up for reliable, geologically stable, climate change mitigation compared with BECCS or DACCS. The table, right, gives a qualitative summary of all the NETs techniques considered here.

Conclusion: the opportunity to lead

It is clear that there is indeed some potential for development and deployment of negative emissions technologies in Ireland; nonetheless, it is also clear that they are currently at very early stages of investigation, with large uncertainty about scalability, costs, and potential interactions or conflicts with other critical activities (especially food production).

While the government should make clear what the current assumptions and plans are for NETs in Irish policy, it would be unwise to rely at this point on essentially speculative prospects for large-scale NETs deployment at some indefinite future time. At this point, the working policy assumption should be that adequate mitigation (consistent with the Paris Agreement temperature targets) must be achieved without significant contributions from NETs.

Thus, for Ireland to play anything approaching a ‘fair share’ part in this global effort, we now likely require a sustained reduction in gross greenhouse gas emissions (especially, but not exclusively CO2) of at least seven per cent year on year, every year, until they have fallen effectively to zero (i.e. well below natural, background, processes of CO2 removal).

Compared to current policy, this would represent, in effect, something more akin to a ‘national emergency’ scale response – but an emergency response that would have to be sustained, and indeed likely intensified, for decades into the future. However, given the rapidly closing window for effective action (i.e., consistent with reliably meeting the Paris Agreement temperature goals), it seems clear that nothing short of such a scale of mitigation commitment can now represent international leadership that is commensurate with the scale of the climate change challenge.

Secondarily (and in some ways complementary to this), there is a clear opportunity for Ireland to show climate action leadership by early deployment, at significant scale, of carbon capture and storage (CCS), including the development of a first national CO2 storage facility. CCS is a critical underpinning technology for immediate abatement of conventional fossil fuel combustion emissions, industrial emissions, and for any future deployment of either BECCS or DACCS.

Negative emission technology deployment, as with energy decarbonisation is general, is likely strongly facilitated by early electrification, at large scale, of both heating and transport.

In conclusion, while we would recommend the explicit, open and transparent consideration of all applicable NETs into Ireland’s national climate mitigation planning, they present no panacea or alternative to the hard choices now needed to achieve early, deep and permanent reductions in gross emissions, beginning without delay:

“Negative-emission technologies are not an insurance policy, but rather an unjust and high-stakes gamble. There is a real risk they will be unable to deliver on the scale of their promise. If the emphasis on equity and risk aversion embodied in the Paris Agreement are to have traction, negative-emission technologies should not form the basis of the mitigation agenda. This is not to say that they should be abandoned. They could very reasonably be the subject of research, development, and potentially deployment, but the mitigation agenda should proceed on the premise that they will not work at scale. The implications of failing to do otherwise are a moral hazard par excellence.” Anderson and Peters, 2016, ‘The Trouble with Negative Emissions’ Science 354 (6309): 182–83.

Authors: Barry McMullin (DCU), Paul Price (DCU), Mike Jones (TCD), Alwynne McGeever (TCD) O'RiordanElecclimate change,DCU,TCD
The idea of 'negative emissions technologies' (NETs) - technologies which could help address climate change by actively removing greenhouse gas pollution from the atmosphere - has attracted rapidly growing academic attention in recent years. In Ireland, the EPA has recently funded a small preliminary study of the realistic potential...