If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is because a white roof will offset 10 tons of carbon dioxide over its 20-year lifetime. In a real-world 2008 case study of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.