I came across a fascinating news item on Twitter the other day, publicising a paper (Ashworth et al. 2013) about the effects of certain biofuel crops on air quality. This isn’t my area of expertise at all (assuming I have expertise, that is!) but I found it an excellent example of the trade-offs humans make in their interventions in the environment. In densely-populated and highly-industrialised Europe, where this study is focused, there is no such thing as ‘nature’ or the ‘natural environment’. Human and natural systems are intertwined. The study demonstrates how complex this relationship is.
Isoprene emissions from woody biofuels impact human health
In a nutshell, the study showed that replacing crops or grassland with woody biofuel crops like ash, poplar and willow (which they call ‘short rotation coppice’) increases concentrations of a chemical called isoprene in the atmosphere. The vast majority of the isoprene in the atmosphere comes from plants. Isoprene is quite reactive and reacts with other chemicals in the atmosphere. If there is a lot of nitric oxide pollution around, isoprene reacts with it and forms ozone. Ozone is very important for shielding the Earth’s surface from harmful solar ultra-violet radiation, but it is best kept high in the atmosphere because it is also harmful to human health.
Nitric oxide and isoprene, therefore, is a bad mixture to have in the atmosphere. The resulting ozone is linked to asthma, bronchitis and heart attacks. Planting 72 million hectares of biofuel crops across Europe, the study estimated the isoprene-related ozone could cause between 690 and 1,890 additional deaths each year. Ozone is also harmful to crop growth, and they estimated crop losses with a value of $1-2bn (in 2010 dollars).
Other effects of isoprene
Isoprene is also implicated in the formation of secondary organic aerosol. These are tiny particles (‘aerosols’) which both absorb and reflect solar radiation. Most aerosols (except very sooty ones) tend to be more reflective, which means they cool the surface. A cloud of aerosols works much like a cloud of water droplets in this sense, providing a sunshade. These aerosols are ‘organic’ because they come from compounds produced by plants, and ‘secondary’ because these compounds first have to undergo some chemical reactions in the atmosphere before the aerosols are produced.
Secondary organic aerosols are still quite poorly understood and offer plenty of interesting research opportunities. We don’t even understand whether isoprene always increases the amount of secondary organic aerosol or whether it sometimes decreases it.
Choose your biofuel crops carefully
In my research for this post I came across another paper (Crespo et al. 2013) which looked at emissions of isoprene (and other so-called ‘volatile organic compounds’) from different types of biofuel crops. They showed that the problem of isoprene emissions is much bigger for woody crops, such as the ones used in the Ashworth study I mentioned earlier. Most plants emit isoprene but some do so much more than others.
[O]ur data suggest that the use of perennial grasses for extensive growing for biofuel production have lower emissions than woody species, which might be important for regional atmospheric chemistry.
The ‘perennial grasses’ they study are things like ‘elephant grass’, a fast-growing crop which is already being grown in the UK.
I find the interplay between the ‘natural’ and the ‘human’ especially interesting here. Humans may think they are doing something natural (or at least modifying the environment is a sympathetic way) by planting these crops. The ‘natural’ isoprene emissions combine with human pollution (nitric oxide) and produce ozone pollution, which affects human health, crop yields and ‘natural’ plant growth.