The urban heat island effect is the observed increase in temperature of urbanized areas due to asphalt and cement structures and rising population. The effects of urbanization are believed to be responsible for rising trends in the temperature change record, thus decreasing the reliability of the sea-surface temperature record. Figure 1 shows the observed effects of urbanization on a number of counties throughout the state of California since 1910. Note that the sharpest increases in temperature correspond with the most populated and urbanized areas (top curve). The middle curve corresponds with counties of relatively moderate size (populations ranging from 0.1 million to 1 million). And the bottom trend, indicating those counties with a population less than 0.1 million, shows the least increase in temperature over time.
Figure 1: Warming trends (1910 to 1995) illustrating effect of urbanization on temperature change as recorded by California stations. Top curve represents counties with populations greater than 1 million, the middle curve illustrates counties with populations between 0.1 and 1 million, and the bottom curve represents counties with a population less than 0.1 million people (Goodridge 1996).
However, much debate remains over the extent to which the UHI effect influences sea-surface temperatures. While some say that it has a significant impact on the historical temperature record, thus making sea-surface temperatures inaccurate, others maintain that urbanization has a minimal effect, if any at all. David R. Easterling of the National Climatic Data Center believes that UHI has a minimal impact, contributing approximately 0.1 deg/C to the surface record over the last one-hundred years (McDonald 1999) .