Non-Tilled Wheat Fields Mitigate Extreme Heat

Wheat fields that are not tilled after crop harvesting reflect 50 percent more sunlight than cultivated fields, helping to mitigate extreme heat by as much as 2 degrees Celsius (36 degrees Fahrenheit), according to a new study.

Post-harvest tilling is a widely practiced and common management technique in Europe - consisting of removing the light-colored stubble and crop residues from the soil surface and bringing dark bare earth to the top. However, ploughed fields can have a negative effect on the local climate during a heat wave.

Researchers at ETH Zurich found that untilled land was effective at mitigating extreme heat due to the albedo effect - the measure of the reflectance capacity of reflective surfaces.

Unploughed stubble, which is lighter in color and therefore reflects more light than tilled surfaces, can reflect about 30 percent of solar radiation back into space. Comparatively, ploughed fields reflect only about 20 percent of incoming solar radiation. Model simulations have shown that this difference results in a 50 percent higher level of reflection in unploughed fields and that this in turn has a significant effect in extreme heat.

"Cropland albedo management has more effect during heat waves because there is almost no clouds during these events and more radiation can be reflected back into space," first author Edouard Davin said in a statement.

What's more, the hotter it becomes, the greater the albedo effect and the more the temperature cools.

"In other words, if all French farmers were to stop ploughing up their fields in summer, the impact on temperatures in Germany would be negligible," added Sonia Seneviratne, co-author and professor of land-climate dynamics.

Though, researchers add, this effect is only short term and local.

The report also notes that the mitigating effects of untilled land are only part of the equation. Crop residue acts as an insulating layer, dragging out the evaporation process by slowly releasing moisture to deeper soil, which in turn also helps reduce the air temperature during a heat wave.

Overall, says Seneviratne, no-till farming makes more sense in regions where summers are regularly very hot due to high levels of sun exposure - for example, around the Mediterranean.

"It is important that cropland albedo management can dampen heat waves because these events, although rare by definition, have a large impact on humans and ecosystems," Seneviratne explained.

The findings were published in the journal Proceedings of the National Academy of Sciences.