Reflective crops could soften climate change blow

20 January 2009 by Tom Marshall

Shinier crops could help lessen the impact of climate change, according to new research.


Planting varieties of crops like maize with higher albedo could buy time to cut CO2 emissions.

Scientists say planting arable land with varieties whose leaves reflect more light back into the atmosphere could cut regional temperatures by as much as 1°C in summertime.

Many politicians have signed up to a goal of limiting warming to 2°C over pre-industrial levels, so this could be a very significant reduction in some regions and could buy societies time to cut CO2 emissions and find other ways of dealing with the problem.

All plants reflect a certain proportion of the sunlight hitting them away from the Earth. But some reflect more than others. Those with a waxy outer coating on their leaves, or with variegated or hairy leaves, tend to be more reflective, and are said to have a higher 'albedo'. The arrangement of the leaves on the plant is also important. The researchers suggest replacing existing varieties of maize, for example, with higher-albedo alternatives.

The paper appears in Current Biology. Its authors suggest that replacing arable plants with alternatives that were around 20% more reflective would lower summer temperatures by around 1°C throughout much of central north America and Eurasia - equivalent to offsetting around a fifth of the seasonal warming in these regions that is expected to arise towards the end of the century from doubling the current level of atmospheric carbon dioxide.

The question is whether varieties are available that offer this improvement in albedo without compromising the other qualities farmers want, such as high yield, hardiness and resistance to disease. There's plenty of variation in most farmed species, though, so selective breeding or genetic modification could well produce such varieties even if they don't exist right now.

"Persuading farmers to do this would be critical. But if suitable varieties were available with a climate-friendly trait alongside all the other characteristics farmers are looking for, I see no reason why farmers shouldn't adopt them," says Dr Andy Ridgwell, a Royal Society funded Earth scientist at Bristol University and one of the paper's authors.

He notes that such a shift could take place gradually and would not require huge initial investments, since crops are replanted every year as a matter of course. He suggests that existing agricultural subsidies could be changed to encourage farmers to shift to new varieties.

Breeding for albedo

In order to pursue this and fully evaluate the potential of this approach, both conventional breeding and GM approaches may be needed to produce strains that more reflective while still showing all the other good qualities of the varieties used at present. This might take a decade or more.

Ridgwell also cautions, "We have very little information on the existing variation in albedo between different varieties of commercially important crop species at the moment. We need lots more basic research first - it's surprising how little work has been done on this."

The scientists hope the idea, which they call 'bio-geoengineering' could offer a less drastic and cheaper alternative to other recent suggestions to mitigate climate change, which have ranged from seeding the oceans with iron to stimulate plankton growth to building a giant sunshade in space.

"We don't want to suggest anything that will disrupt the agricultural system and threaten food supplies," says Ridgwell. "We're looking for low-cost, undisruptive ways to slow climate change; this is why our proposal is different from previous ideas for geoengineering," he adds.

The researchers used the climate model produced by the Hadley Centre for Climate Prediction and Research to reach their conclusion.

Unintended consequences?

Proposals for large-scale geoengineering to mitigate climate change's effects have caused some observers to worry about unintended side-effects.

Dr Chris Huntingford, a climate modeller at the Centre for Ecology & Hydrology, says that to avoid dangerous levels of global warming "we need massive emissions reductions and soon. However, it is also prudent to plan in the event that this does not occur, and this raises issues of geo-engineering to help mitigate high emissions levels. There is a genuine concern that some geo-engineering proposals could lead to unwelcome and unpredicted consequences."

He acknowledges, though, that this proposal seems less risky than some other others. "Hopefully this would not be the case for the suggestion of this paper, whereby large-scale planting of crops with high reflectivity could reduce ground level warming," he says.

"This study is comprehensive, having used a fully coupled land-atmosphere global climate model to check on feedbacks and thus consequences of revised crop growth. There is also the obvious benefit whereby such a proposal places an emphasis on crops, thus contributing to the debate of feeding an ever-growing global population," Huntingford adds.

One criticism of the proposal is that plants with higher reflectivity may also photosynthesise less and therefore absorb less carbon dioxide, nullifying any potential gains.

Ridgwell says the evidence doesn't suggest this need be a problem. Previous research has suggested that more reflective plants don't necessarily photosynthesise less or produce less biomass. For example, in one study, plants were sprayed with a chalky spray that coated their leaves in a white dusty layer, increasing their reflectivity. This did not significantly reduce their yield.

This could be because more reflectivity helps them by reducing water loss and letting them use water more efficiently. Alternatively, greater leaf reflectivity could cause light to be distributed differently within a plant's canopy, causing its overall productivity to remain the same.

Another potential problem that needs attention is the changes in distribution of soil moisture around the globe that the climate model suggests would follow from shifting to more reflective crops. For instance, the model suggests that American soil moisture could increase but that Australian soils could become drier.

Ridgwell notes that in the long term it is desirable to introduce rules to encourage ways to value mitigating climate change, such as changes to plant albedo, in addition to those that directly affect atmospheric CO2. His back-of-the-envelope calculations suggest that given current prices for emissions in the carbon credit market, the effective benefit of shifting to high-albedo crops globally could be worth some €40bn a year over the current century.

Ridgwell has put together a multi-disciplinary group at Bristol University, called the Bristol Bio-geoengineering Initiative, to investigate the question in more detail. The group involves climate modellers, biologists and biochemists.

He is now seeking funding for trials in glasshouses. He wants to investigate the natural range of variation in the reflectivity of various crop species to assess whether the material is there for selective breeding to create more 'climate-friendly' strains without sacrificing other desirable characteristics like yield or resistance to pests.

To do this the team will investigate both wild and domesticated strains of major crop species. They will also do further work on the effects of greater albedo on photosynthesis, water use, biomass production and other variables together with higher resolution, regional-scale climate modelling.