Posted on 22 June 2016 by Guest Author

This is a re-post from Carbon Brief by Roz Pidcock

The Paris Agreement on climate change pledges to keep warming “well below 2C” and “pursue efforts” to limit the increase since preindustrial times to no more than 1.5C.

But what rarely gets discussed is that the modelling by scientists showing how this might be possible typically assumes that the world will deploy “negative emissions” technologies (NETs) later on this century.

In a week-long series of articles, Carbon Brief has been looking at NETs – the options,implications, history and feasibility. In the last part of our series, we turn the spotlight on the UK to see if – and how – it might resort to “sucking” CO2 from the atmosphere, in order to help meet its climate targets in the future.

Does the UK need negative emissions?

A full seven years before the ink dried on the Paris Agreement, the UK was enshrining in law its own national commitment to tackling climate change.

In 2008, the UK’s parliament passed the Climate Change Act, which set a legally binding target for reducing greenhouse gas emissions in 2050 by 80%, relative to 1990 levels.

In light of the Paris Agreement’s tightened temperature limit – it was nudged from the older “below 2C compared to preindustrial levels” commitment to “well below” 2C – the Committee on Climate Change (CCC), the UK government’s independent advisory body, recently determined that the UK’s fifth carbon budget for the 2028-2032 period should remain unchanged. The government will formally respond to the CCC’s advice and set out the policies to meet the target later this year.

Source: The Fifth Carbon Budget – The next step towards a low-carbon economy, Committee on Climate Change, 2015.

But, as far back as 2010, the UK’s Department for Energy and Climate Change (DECC) recognised that “negative emissions” would likely need to feature in the world’s effort to keeping the global temperature rise to below 2C by the end of this century. A DECC-commissioned study into the potential for negative emissions in the UK concluded:

“It seems increasingly likely that CO2 emissions will overshoot the limit on the cumulative total needed to limit a global temperature rise to below 2C above pre-industrial levels. It may therefore become necessary to remove CO2 from the atmosphere.”

Recognising that the UK needed a “robust strategic plan” to uphold its part of the bargain, the study examined potential approaches, concluding that bioenergy with carbon capture and storage (BECCS) had “the most immediate negative emissions potential” in the UK.

Similarly, a 2015 report by the CCC outlining the scientific context for the UK’s fifth carbon budget described BECCS as a “sensible way to maximise emissions reduction”.

How much could BECCS lower the UK’s emissions?

According to the 2010 DECC-commissioned study, carried out as part of the AVOID2project, a middle estimate for the negative emissions potential of BECCS using only domestically-sourced biomass is just under 50m tonnes of carbon dioxide equivalent per year (MtCO2e) by 2030. This is equivalent to about 10% of the UK’s current emissions. The authors concluded:

“[T]his may provide significant flexibility in delivering long-term GHG [greenhouse gas] reduction targets by offsetting emissions that are difficult to capture (e.g. from agriculture and transportation point sources).”

It would take about 11 years to scale-up BECCS to its full potential, the study said. There is also a fair amount of uncertainty around the figures, with estimates of negative emissions in the literature ranging from 18-80 MtCO2e (or 3-16% of the UK’s emissions in 2015). 

The 10% figure assumes that all coal plants in the UK are replaced with BECCS and that 90% of the CO2 released in combustion can be captured and sequestered. It also assumes that biomass plants run at a similar efficiency to coal power plants (around 40%).

Importing the bioenergy from elsewhere in the world would increase the UK’s access to biomass, the study notes. By how much is uncertain, however, since different forecasts of land availability and yields mean estimates of global biomass potential vary by several orders of magnitude. It is an open question how the accounting for these negative emissions would work if the bioenergy is imported.

An alternative estimate for the potential for BECCS to achieve negative emissions in the UK comes from the CCC’s advice to the government for meeting the fifth carbon budget.

Although the figures have not yet been placed in the public domain, a CCC spokesperson has confirmed to Carbon Brief that its central scenario includes negative emissions from BECCS totalling 43 MtCO2 per year in 2050. The CCC tells Carbon Brief:

“Where BECCs is used (in certain CCC scenarios for 2050), this is not because we need a contribution from negative emissions at this stage, but because it makes the most of scarce bioenergy in reducing emissions and therefore provides more room to accommodate difficult sectors in 2050.”

CCS has commenced at an existing biofuel plant in Decatur, Illinois. Credit: ADM

At around 8.5% of current UK emissions, this is a similar contribution to that discussed above, but achieved in 2050 rather than 2030. It also assumes the UK has access to its “pro rata” share of internationally traded biomass, as well as domestically-sourced supplies.

It’s worth noting, however, that to provide this level of abatement by 2050, BECCS begins to come online in the CCC’s central scenario from 2035 onwards. This, in turn, would require a successful programme of CCS deployment during the 2020s.

This is a significant assumption. While some CCS pilot projects are in operation around the world, the technology has yet to be demonstrated at the commercial scale required for this level of BECCS. The UK government has also recently pulled support for its £1bn CCS competition, for which it has been widely criticised.

A February 2015 working paper on negative emissions by the University of Oxford, as part of the Stranded Assets programme, concluded:

“Rollout of the CCS element, and integration of CCS with biomass conversion technologies, appear likely to constrain BECCS more strongly in 2050 than biomass availability.”

DECC’s “Carbon Plan”, published in 2011 to outline how the coalition government intended to meet the UK’s long term carbon commitments, offers a third perspective on how much negative emissions could come from BECCS in the UK.

Of the four illustrative scenarios for what the UK’s energy mix might look like in 2050, DECC’s “Higher CCS; more bioenergy” scenario sees BECCS generating around 50 MtCO2e of negative emissions in 2050. This is equivalent to about 10% of current UK emissions, slightly higher than the CCC’s scenario.

UK primary bioenergy mix in 2050 (top) and sectoral use of bioenergy (bottom) in 2010 and 2050 under the four Carbon Plan pathways. The “Higher CCS, more bioenergy” pathway is second from the right. Source: Konadu et al., (2015)

What are the consequences of large-scale BECCS in the UK?

One implication of using domestically-grown bioenergy is that it would displace other uses of land. A 2013 “horizon-scanning” report by DECC’s Scientific Advisory Group – since disbanded – mentions food insecurity as a potential “tradeoff” of large-scale BECCS deployment.

Talking generally about a global 2C goal, rather than the UK specifically, the report explains:

“BECCS, as currently envisaged, would require a significant increase in land-area used to generate crops, potentially leading to large areas of forest or agricultural land to be replaced by energy crops.”

To better get to grips with these “critically important” issues, the report recommended a programme of research and development. This has been largely achieved through theAVOID2 project, led by the Met Office in partnership with Imperial College London, the Tyndall Centre and the Walker Institute at Reading University, a DECC spokesperson tells Carbon Brief.

In a guest article earlier this week, as part of Carbon Brief’s series on negative emissions, the Met Office’s Dr Jason Lowe, chief scientist of the AVOID2 project, discussed the feasibility of BECCS in meeting the 2C goal.

The availability of negative emissions technologies in the UK is expected to be determined in a research programme later this year, led by the Natural Environment Research Council (NERC), a DECC spokesperson tells Carbon Brief. To kick things off, NERC will get together with a number of other research councils, academics and policymakers at a workshop in London later this month to discuss options for how best to harness “negative emissions” technologies.

In the meantime, what can existing research tell us about the potential impacts of large-scale BECCS in the UK?

A 2015 study by a group of Cambridge academics looked in detail at how much extra land would be needed for bioenergy in each of the scenarios outlined in DECC’s 2011 Carbon Plan.

The authors calculated that the “High-CCS, more bioenergy” pathway, which sees BECCS generate negative emissions equivalent to 10% of the UK’s current emissions (discussed above), requires that 28% of the land currently used to grow food in the UK be given over to growing energy crops by 2050.

Under a more optimistic scenario in which crop yields increase by 30% by 2050, the amount of farmland needing to be repurposed for BECCS drops to 12%.

But this is not very realistic once you consider the potential impact of climate variability on factors that influence yields, such as soil quality, precipitation and temperature, says Dr Dennis Konadu, lead author on the study. He tells Carbon Brief.

“12-28% of UK agricultural land is huge…Appropriating this amount of good quality land for bioenergy cropping is not feasible…Moreover, current top UK land use priority is for food production, hence, this may not get state backing. The antidote to this will be the use of marginal lands, which will result in reduced yield levels (unless irrigation and fertilisers are applied) and, thus, more land will be required to meet feedstock targets.”

Although the Carbon Plan pathways “appear to deliver the 80% GHG reduction target”, this belies a “fundamental mismatch” between energy policy and the physical limits of natural resources that could undermine the UK’s GHG emissions target, say the authors in the paper.

Would the successful deployment of BECCS guarantee lower emissions?

Even assuming access to enough bioenergy, the availability of commercial-scale CCS and a fully functioning agricultural sector, BECCS still isn’t a guaranteed way to achieve negative emissions.

For example, emissions resulting from changing land use need to be factored into estimates of carbon savings and not all bioenergy crops will reduce emissions relative to fossil fuels, which makes the selection of energy crops critical. (See Carbon Brief’s investigation last year into whether burning imported biomass at Drax – the UK’s largest power station -helps to lower emissions. In short, it’s complicated, depending on the type of biomass used and what would have happened if the land had been used for other purposes.)

Soil emissions resulting from repurposing different types of land for growing bioenergy crops. Source: Hillier, Smith et al (2009) from the CCC Bioenergy Review, 2011.

A conversation overdue

Negative emissions technologies, typically BECCS, are now baked into the majority of the scenarios modelled by scientists showing how the world can avoid breaching the 2C limit. These models tend to assume a growing amount of BECCS being deployed globally from the 2040s onwards.

The vision for the UK is no different. The CCC has confirmed to Carbon Brief that its own recommended “central scenario” for the UK’s carbon reduction pathway for the decades ahead also assumes a rising amount of BECCS from 2035 onwards. And yet research and development – let alone the commercial upscaling of a demonstration project – is still at a tentative, early stage.

Given that there are still large uncertainties about the efficacy and scalability of BECCS – for example, the land-use implications; the choice of bioenergy crop; the safe, available storage of sequestered carbon – it seems that a conversation about negative emissions among scientists, policymakers and the public is overdue.