Impacts of biofuel cultivation on mortality and crop yields

Fighting climate change by producing more biofuels could actually worsen a little-known type of air pollution and cause almost 1,400 premature deaths a year in Europe by 2020, according to a new study.

The report said trees grown to produce biofuel - seen as a cleaner alternative to oil and coal - release a chemical into the air that, when mixed with other pollutants, could also reduce farmers' crop yields.

"Growing biofuels is thought to be a good thing because it reduces the amount of carbon dioxide in the atmosphere," said Nick Hewitt, who worked on the study with colleagues from Lancaster University, UK. "What we're saying is 'yes, that's great, but biofuels could also have a detrimental effect on air quality'."

Poplar, willow or eucalyptus trees, all used as fast-growing sources of renewable wood fuel, emit high levels of the chemical isoprene as they grow, the study said. Isoprene forms toxic ozone when mixed with other air pollutants in sunlight.

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Ground-level ozone is a priority air pollutant, causing ~ 22,000 excess deaths per year in Europe¹, significant reductions in crop yields² and loss of biodiversity³. It is produced in the troposphere through photochemical reactions involving oxides of nitrogen (NO_x) and volatile organic compounds (VOCs). The biosphere is the main source of VOCs, with an estimated 1,150 TgC yr⁻¹ (~ 90% of total VOC emissions) released from vegetation globally⁴. Isoprene (2-methyl-1,3-butadiene) is the most significant biogenic VOC in terms of mass (around 500 TgC yr⁻¹) and chemical reactivity⁴ and plays an important role in the mediation of ground-level ozone concentrations⁵. Concerns about climate change and energy security are driving an aggressive expansion of bioenergy crop production and many of these plant species emit more isoprene than the traditional crops they are replacing. Here we quantify the increases in isoprene emission rates caused by cultivation of 72 Mha of biofuel crops in Europe. We then estimate the resultant changes in ground-level ozone concentrations and the impacts on human mortality and crop yields that these could cause. Our study highlights the need to consider more than simple carbon budgets when considering the cultivation of biofuel feedstock crops for greenhouse-gas mitigation.

by K. Ashworth, O. Wild & C. N. Hewitt