Bioenergy with carbon capture and storage (BECCS) involves burning biomass for energy and capturing the resulting emissions. This controversial “negative emissions” technology is currently restricted to a few pilot projects, but features heavily in model pathways that limit global warming to 1.5C or 2C above pre-industrial temperatures.
The new research, published in Nature, assumes that biofuels and residues from food crops are both used for BECCS. The authors find that as global temperatures rise, crop yields will decline, resulting in reduced residues. This will limit the efficiency of BECCS, reducing the ability of the technology to remove emissions from the atmosphere.
The study finds that if the warming is allowed to cross a “threshold” of 2.5C warming, the effectiveness of BECCS could “rapidly decrease”. This could make it impossible to use BECCS to push global warming back below 2C, the authors find.
The research shows that models which rely on BECCS to limit global temperature rise “could be unduly optimistic”, says an accompanying commentary. The analysis “demonstrates convincingly” that “waiting for salvation could well hasten our demise, because delays might restrict the technologies available to us now”, it adds.
The paper also warns of food insecurity as crop yields fall. Developing countries – which often see the most severe impacts of warming – will be among the first to be affected. This reinforces the urgency of early climate mitigation “to avoid irreversible climate change and serious food crises”, the study adds.
Bioenergy
To meet the warming limits set out in the Paris Agreement, most emissions scenarios rely heavily on negative emissions technologies to remove carbon from the atmosphere later in the century, as well as rapid decarbonisation in the near term.
Bioenergy with carbon capture and storage (BECCS) is one such technology. It involves burning biomass – from agricultural waste and/or growing dedicated bioenergy crops – to produce energy, and capturing the resulting emissions for long-term storage.
In the latest Intergovernmental Panel on Climate Change report on climate change mitigation, all pathways that limit global warming to 1.5C or 2C rely on land-based CO2 removal methods – such as tree planting and BECCS – to some extent. The report estimates that large-scale BECCS could remove 5.9bn tonnes of CO2 equivalent (GtCO2e) each year over 2020-50. However, it adds that the technology has sparked “fervent debate” in recent years.
BECCS is currently only operational in a handful of pilot projects and will need to be scaled up to notably reduce atmospheric CO2 levels. It is often touted as a way to produce carbon neutral energy from food waste. But in practice, scaling up the technology could require setting aside vast areas of land to grow bioenergy crops – possibly reducing the amount of land available to grow food for human consumption.
Prof Rong Wang from Fudan University is an author on the new study. He tells Carbon Brief that the paper focuses on “scenarios where food crops are increased globally to ensure food security before using the limited land to grow the dedicated energy crops”.
The study assumes that cropland will expand – converted from forests or marginal lands – to help feed a growing global population. It further assumes that half of this land is used to grow new energy crops and the other half is used for food crops with the residues given over to BECCS.
The study reveals a potential flaw with reliance on BECCS – the impact of global warming on crop yields. The authors explore the optimal growing conditions for wheat, rice and maize – key crops whose residues could be used for BECCS – by looking at crop growth data from 13 countries distributed across the globe.
They find that crops achieve optimal growth when average temperatures during the growing season are around 25C, but that yield drops off quickly at higher temperatures. CO2 fertilisation – in which higher atmospheric CO2 levels aid plant growth – partly counteracts this effect, as does an increase in soil nitrogen levels.
Overall, the authors conclude that temperature is the more significant driver of crop yields, which will drop as the climate warms. This means that the later BECCS is implemented, the less effective it will be, creating a “feedback loop”, the authors warn.
A News & Views article that accompanies the study – penned by Dr Gernot Wagner, a climate economist at Columbia Business School, and Prof Wolfram Schlenker from Columbia University – neatly summarises this finding:
Feedback loop
To determine the impact of this feedback loop on future warming, the authors investigate how global temperatures would evolve between the present day and 2200, if widespread mitigation and BECCS were implemented from 2040, 2050 or 2060.
They use the OSCAR Earth system model – calibrated with results from the fifth and sixth phases of the Coupled Model Intercomparison Project – to determine interactions between climate change and the global carbon cycle. Average growing-season temperatures for maize, rice and wheat were estimated by country based on global crop calendar data.
The paper projects a very high emissions scenario (SSP5-8.5) until mitigation and BECCS are implemented, after which emissions drop to meet current emissions pledges in a moderate warming scenario (SSP2-4.5).
The authors also include two different BECCS responses – one excluding the agricultural “feedback loop” – and the other including it. The figure below, from the News & Views article, shows these results.
The dotted lines show the model runs without the agricultural feedback loop, while the solid lines show model runs including the feedback loop. The red, blue and yellow lines indicate the implementation of mitigation and BECCS from 2040, 2050 and 2060, respectively.
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