By Richard Black
Environment correspondent, BBC News website
Some varieties of crop are more reflective than others
Farmers could help curb rising global temperatures by selecting crop varieties that reflect solar energy back into space, researchers say.
Scientists at Bristol University calculate that switching crops in North America and Europe could reduce global temperatures by about 0.1C.
Temperatures have risen by about 0.7C since the dawn of the industrial age.
Other experts say the idea is feasible but could not cool the world enough to combat rising greenhouse gas levels.
The Intergovernmental Panel on Climate Change (IPCC) projects that by the end of the century, the average global temperature will have risen by 1.8-4.0C from 1990 levels.
The scientists behind the new research suggest that compared to other engineering concepts for curbing temperature rise, the farming switch would probably have no downside and would be easy to do.
"There's lots of talk about whether we can build sunscreens in space or pump aerosols into the atmosphere," Bristol's Andy Ridgewell told BBC News.
"But these schemes require trillions of dollars of investment in infrastructure and it seems unlikely we'll get round to it.
"Whereas farming already has a global infrastructure and it's done each year, so it should be relatively simple to utilise it to provide a climate benefit."
The principle, expounded in the scientific journal Current Biology, is certainly simple enough.
Some crop varieties are naturally more reflective than others. A field of more reflective leaves will send more solar energy back into space than a field of a more absorbent variety.
Most of the data on this has come from satellites that monitor albedo - reflectivity - across the planet's surface.
Dr Ridgewell acknowledged that more research needed to be done on which varieties would prove most suitable to the task.
Satellites can give a remarkably detailed view of crop cover
It would be natural to think that a plant that absorbs a greater proportion of the incoming solar energy would produce a higher yield.
But that does not appear to be borne out in practice, according to another of the scientists involved, Bristol University botany professor Alistair Hetherington.
"Having said that, however, we would want to repeat and extend all these experiments," he told BBC News.
"But another possibility for the future would be to produce a [leaf] surface that differs in reflectivity at different wavelengths, so it could selectively absorb wavelengths involved in photosynthesis."
This might be possible either through intensive plant breeding or genetic engineering, he said.
One downside to the idea is that it only works with crops grown in Europe and North America, according to Andy Ridgewell's computer modelling.
In the Asian growing season, he explained, most of the reflection is done by clouds, so changing the ground's albedo would have little impact.
The corollary of that is that most of the temperature reduction would happen over North America, Europe and neighbouring regions such as western Russia, and would be concentrated in the summer, when the net regional cooling could be as much as 1C.
But with the IPCC forecasting a greater number of summer heatwaves over Europe, the continent's politicians might see the concept as desirable, the researchers believe.
Ken Caldeira from the Carnegie Institution for Science at Stanford University, California, has analysed many different potential climate "fixes"; and he suggested the approach seemed feasible, especially given other research indicating that conversion of dark forests to lighter-coloured croplands during the settlement of North America might have lowered temperatures by about 1C.
But, he said: "This kind of approach can never be quantitatively important on a global scale.
"It could help marginally in certain places but it shouldn't cause anybody to think we can slow up our efforts to stop dumping carbon dioxide into the atmosphere."
Another obvious question with the approach is how to persuade farmers to make crop choices that might impact on their income, if they were asked to adopt strains that fetched less at market.
One way would be to allow farmers to gain carbon credits for making a reflective choice, although Professor Caldeira suggested "starting to price albedo could open up a whole can of worms".
But for Andy Ridgewell, rewards such as those the EU offers to stimulate the growing of biofuel crops would be one way to incentivise farmers - and this approach should not generate any of the same problems that biofuels raise, he suggested.
"You certainly wouldn't have to replace food crops, so food production shouldn't be affected by our idea," he said.
Most engineering "fixes" do not address a separate problem posed by the build-up of carbon dioxide in the atmosphere - the acidity change in the oceans, which is likely to impact marine organisms such as coral and molluscs.