Boosted by the sun Fuse/Getty

The US corn belt is having its moment in the sun. Corn yields have been steadily rising since the first hybrid plants were introduced in the mid-1930s, but it turns out that better agricultural technologies like fertilisers, pesticides and equipment aren’t the only reason, as had long been thought.

Instead, 27 per cent of the increase is due to a phenomenon called solar brightening over the northern states that make up the corn belt. This happens when the air is clear and more light can reach the ground.

Places like China and India have seen solar dimming over the past few decades, but since the mid-1980s, the US corn belt has brightened significantly. The exact causes of brightening aren’t yet certain.


“There’s a question to what extent it’s pollution driven or to what extent it’s natural variation in cloudiness and cloud properties, which can be related to air pollution but don’t necessarily have to be,” says Martin Wild, a climate scientist at ETH Zurich in Switzerland who wasn’t involved in the study.

Crop physiologists Saratha Kumudini and Matthijs Tollenaar created a computer model looking at corn yields over a 30-year period between 1984 and 2013. They included inputs such as fertiliser use and how much solar radiation reached the ground during the 60 days following plant pollination.

Solar energy

To their surprise, they found that changes in the sun’s radiation accounted for a quarter of the overall increase in corn production.

“Solar radiation is the driving force for photosynthesis. You need to convert light energy into biomass or glucose and therefore this is an important driver for yields,” says Kumudini.

Without solar brightening, we would need to expand the area needed for corn production from 90 million acres to 135 million acres to produce the yields seen today, Tollenaar says.

Solar brightening hasn’t been incorporated into agricultural models before, says Kumudini. “They’re assuming, like we all did, that investment in agricultural technologies would continue and we would continue to have these yield improvements year after year,” she says.

That may not be the case. Tollenaar says we haven’t reached the limit of crop yields yet, but without solar brightening, the increases would be far lower. Starting in the 1980s, fertiliser use decreased but that was offset by solar brightening. There is no telling how long the effect will last, he says.

Strangely, it doesn’t seem to have an effect on temperature in the corn belt.

Warming hole

“Climatologists have been looking at this region and trying to evaluate why it is that there’s been this increase in temperatures in other parts of the world, but not in the US corn belt. They call it a warming hole,” says Kumudini.

Some have suggested that it is due to the corn itself transpiring, or sweating out water that turns into vapour and cools the plant, say Tollenaar and Kumudini.

Corn is the basis of much of the world’s feedstock and contributes to sweeteners, oils, industrial alcohol and fuel ethanol.

“Corn is in almost everything,” Tollenaar says. “It’s the cheapest renewable energy source we have and that’s the reason why it’s so dominant. Corn is so efficient at converting solar energy to usable solar energy.”

As the world’s population increases, we will need to produce more corn to feed livestock as well as humans, says Tollenaar.

And if it is used to increase biofuel production, the corn crop may end up producing more solar brightening. Ethanol-based fuels can help reduce air pollution by cutting out fossil fuel use, which can allow more solar radiation to reach the crops needed to produce them.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate3234

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