Assessment of Maximum Possible Urbanization Influences on Land Temperature Data by Comparison of Land and Marine Data around Coasts

Global surface temperature trends, based on land and marine data, show warming of about 0.8 degrees C over the last 100 years. This rate of warming is sometimes questioned because of the existence of Urban Heat Islands (UHIs). In this study we compare the rate of temperature change estimated from measurements of land and marine temperatures for the same grid squares using 5 degrees by 5 degrees latitude/longitude grid-box datasets. For 1951-2009 the 'land' average warmed by 0.02 degrees C decade(-1) relative to the 'sea surface temperature' (SST) average. There were regional contrasts in the trends of land/sea temperature differences: the land warmed at a greater rate compared to the SST for regions north of 20 degrees S, but the opposite occurred further south. Given strong forcing of the climate system, we would expect the land to change more rapidly than the ocean, so the differences represent an upper limit to the urbanization effect.