Cities play a multidimensional role in the climate change story. Urban climate effects, in particular the urban heat island effect, comprise some of the oldest observations in climatology, dating from the early nineteenth century work of meteorologist Luke Howard (Howard, 1820). This substantially predates the earliest scientific thought about human fossil fuel combustion and global warming by chemist Svante Arrhenius (Arrhenius, 1896). As areas of high population density and economic activity, cities may be responsible for upwards of 40 percent of total worldwide greenhouse gas emissions (Satterthwaite, 2008), although various sources have claimed percentages as high as 80 percent (reviewed in Satterthwaite, 2008). Figure 3.1 shows a remotely sensed map of nocturnal lighting from urban areas that is visible from space and vividly illustrates one prodigious source of energy use in cities. Megacities, often located on the coasts and often containing vulnerable populations, are also highly susceptible to climate change impacts, in particular sea level rise. At the same time, as centers of economic growth, information, and technological innovation, cities will play a positive role in both climate change adaptation and mitigation strategies.
Urban population recently surpassed non-urban population worldwide and is projected to grow from 50 percent currently to 70 percent by 2050 (UNFPA, 2007). The urban population growth rate will be even more rapid in developing countries. In terms of absolute numbers, urban population will grow from ~3.33 billion today to ~6.4 billion in 2050, about a 90 percent increase.