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Climate change
Global-warming projections that rely on bioenergy strategies to offset carbon dioxide emissions could be unduly optimistic, according to a study that accounts for how climate change affects crop yields. See p.299
Climate change is beset with unpleasant surprises1. Yields of maize (corn), wheat, rice and soya beans all fall precipitously when temperatures exceed certain thresholds - for example, 29 °C for maize2. These four staple crops together account for 75% of the calories consumed by humans3, so the non-linear temperature dependence of their yields calls for rapid action to avoid the tipping points, either by limiting the carbon dioxide emissions that are warming the planet4 or by relocating crop fields on a vast scale - probably both. But efforts to curb global warming rely increasingly on the use of plant biomass to reduce emissions, and introduce a feedback loop that endangers attempts to meet essential climate goals, as Xu et al.5 report on page 299.
Plants absorb CO2 from the atmosphere during photosynthesis, and this process can be used to capture and store CO2 when fuels made from plant biomass are burnt without releasing the CO2 back into the atmosphere; this results in a source of energy that has 'negative' emissions. Most models that estimate the costs associated with a changing climate assume that this technology, known as bioenergy with carbon capture and storage (BECCS), will be ramped up substantially over the coming decades6,7. And with good reason - an increase in the deployment of new, improved technologies is typically a safe assumption. However, Xu and colleagues' analysis shows that, as time goes by and the world warms, falling crop productivity rates will reduce the effectiveness of BECCS, highlighting limits of this new technology.
The authors describe the effect as a 'positive' feedback loop. There is, of course, nothing positive about this particular feedback in the conventional sense of the term: increased global average warming leads to reduced crop yields, which, in turn, decreases carbon capture through BECCS, inducing further increases in global average warming. In this scenario, two negative links combine to create one hot mess.
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