Thousands of reflective white greenhouses in southeastern Spain are having a cooling effect on local air temperatures, according to new research published in the Journal of Geophysical Research – Atmospheres.

Since the 1970s, large areas of semi-arid pastureland in the province of Almeria have been turned over to greenhouse horticulture in an effort to boost vegetable production. Today, they form the world’s largest concentration of greenhouses, covering an area of around 260 square kilometres.

By monitoring temperature trends around the greenhouses using weather stations and comparing the results with those of uncultivated pastureland, Pablo Campra of the University of Almeria and his colleagues found that the greenhouses have caused air temperatures to drop by an average of 0.3°C per decade since 1983. This is in stark contrast to the air temperatures in other areas, which have risen by around 0.5°C.

The cooling is believed to be caused by the greenhouse roofs, which have a much higher albedo – ability to reflect the sun’s rays – than the surrounding farmland.