Published online 15 January 2009 | Nature | doi:10.1038/news.2009.33


Crops that cool

Could shinier farmland help combat global warming?

Wheat fieldsMaking crops shinier could help to cool the planet, a new study suggests.Punchstock

A team of researchers at the University of Bristol, UK, is suggesting that changing the crops farmers grow might help to cool the planet by reflecting more sunlight.

Plants with waxy coatings on their leaves reflect more sunlight than their plainer-surfaced relatives. If more-reflective variants were planted in place of less-reflective ones, the albedo – the proportion of sunlight reflected back into space – of cropland would increase. Other things being equal, that would cool Earth's surface.

Andy Ridgwell and his colleagues suggest that manipulating the waxiness of crops through traditional breeding techniques or genetic modification should raise their albedo by about 20%, from 0.2 to 0.24. On the basis of climate modelling they calculate that the planet would cool by a modest 0.11 ºC. "It's very small on the global average," says Ridgwell. But "what is more important is the summertime effect in specific regions". The mid-latitudes of North America and Eurasia could cool by as much as 1 °C in June, July and August, according to the models. Ridgwell and his colleagues report their results in Current Biology1.

The models also show pronounced cooling in the North Atlantic Ocean and the Barents Sea in the wintertime — which might have a positive effect on sea ice — but a drying out of the soil in some parts of the subtropics. Ridgwell points out that climate models do not predict future precipitation well on a regional basis and treats the latter results more as evidence that there might be effects far from the fields being changed than as a clear indication that there would be damaging consequences.

Shining solution

Various schemes to increase Earth's albedo have been suggested, including injecting aerosol particles into the stratosphere and increasing the reflectivity of clouds over the ocean. Although Ridgwell's approach is less powerful than some previous suggestions, he says that the leaf-albedo idea is probably much easier to implement. "Agriculture is already a globally coordinated undertaking," he points out. Subsidies and other economic considerations can change the types of crop that farmers plant quite quickly.

It is, however, hard to get farmers to plant crops that have lower yields than they are used to, and some think this could be a fatal flaw in the scheme. "The first thing you think about is that it will detrimental to photosynthesis," says Tim Lenton, an Earth-systems scientist at the University of East Anglia, UK.

"Increasing shininess will reduce production, full stop," says Colin Prentice, an Earth-systems scientist at Bristol who was not involved in the work. "We really do not need anything that reduces primary production."

But Ridgwell argues that reports in the scientific literature show that differences in waxiness do not necessarily mean differences in productivity. More and less waxy varieties of barley, for instance, produce much the same yields. And experiments have shown that spraying kaolin on leaf canopies to increase their albedo does not reduce the yield.

Ridgwell and his colleagues would now like to do some practical research on the albedos of various varieties of crop plant, perhaps including fuel crops, and how they might be altered. They are also thinking of trying much higher resolution climate modelling to see whether more-reflective crops could have specific regional benefits by reducing the likelihood of extreme events, such as the 2003 European heatwave.

Cool reflection

The idea that what goes on on farms can alter climate is not new, and by the standards of what has gone before the Bristol idea is quite modest. Calculations by Richard Betts of the UK Met Office and his colleagues have suggested that the clearance of forests (which are darker) to make way for croplands (which are lighter) has cooled agricultural regions in the northern mid-latitudes by up to 2 °C over the past 250 years2.

And in 2007, Robert Hamwey of the Centre for Economic and Ecological Studies in Geneva, Switzerland, looked at changing the albedo of Earth's grasslands by encouraging plants with particular leaf geometries3.

Such plans are in no way a solution, cautions Ken Caldeira of the Carnegie Institution in Stanford, California. "This kind of approach will be at best a minor niche player in our overall response to the climate-carbon problem. That said, if there are cheap and easy things to do, they might be worth doing if they help a little bit and doing them does not create some kind of greater harm."

Lenton thinks that more such ideas, and all sorts of other geoengineering schemes, will be suggested in the years to come, and wants to put in place new systems for judging their potential benefits and costs. Despite some resistance to such research, "it isn't going to go away", he says, and "nor should it". Lenton and his colleagues at East Anglia are now developing a centre for such evaluation likely to be known as the Geo-Engineering Assessment Research (GEAR) unit.

Ridgwell hopes to give them more to evaluate. A former Greenpeace activist, he has no qualms about looking at ways to minimize the effects of climate change, even while being adamant that they cannot replace the imperative need to cut carbon emissions. "If it is low cost but it has some impact," he says, "why not look at it?" 

  • References

    1. Ridgwell, A., Singarayer, J. S., Hetherington, A. M. & Valdes, P. J. Curr. Biol. 19, 1–5 (2009). | Article | ChemPort |
    2. Betts, R. A., Falloon, P. D., Goldewijk, K. K. & Ramankutty, N. Agric. For. Meteorol. 142, 216–233 (2007). | Article |
    3. Hamwey, R. M. Mitig. Adapt. Strat. Glob. Change 12, 419–439 (2007). | Article |
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