An example of the greenhouse effect cooling the climate has been seen in Spain, where the roofs of hothouse farms reflect so much sunlight that they may be pushing down local temperatures.
Since the 1970s, semi-arid pasture land in Almeria, south eastern Spain, has been replaced by greenhouse horticulture.
Today, Almeria has the largest expanse of greenhouses in the world, around 26,000 hectares.
According to a report in New Scientist, Pablo Campra of the University of Almeria and colleagues studied temperature trends from weather stations inside the region, and from other areas of Spain.
With the help of satellite data, they compared semi-arid pasture land and greenhouses, looking for differences in surface radiation and albedo, the ability to reflect sunlight.
In the greenhouse region, air temperature has cooled by an average of 0.3 degree Celsius per decade since 1983. In the rest of Spain, it has risen by around 0.5 degree Celsius.
The satellite data revealed that the white greenhouses were much more reflective than farmland.
The team thinks that the white roofs are key to the cooling, demonstrating the potential for placing reflective surfaces in semi-arid regions of the world to offset climate change.