It is sometimes observed that all explorations of climate change lead back to the Sun. Whether produced from a shift in the surface energy balance or the release of ancient carbon through fuel combustion, the excess thermal energy that now confronts us comes ultimately from the Sun. Of the innumerable pathways through which the Sun's energy warms the environment, human respiration is undoubtedly low on the list of climate-change agents. Yet, in cities at least, the extent to which heat produced through the human metabolism of food is influencing ambient temperatures is now measurable, accounting for almost 5% of the total waste-heat burden in some cities.

Although the dense concentration of human bodies has always influenced micro-climates to some extent, that such a seemingly finite biological process could generate a warming signal at the scale of urbanized regions is wholly a reflection of just how populous these regions have become. Today, urbanized regions are not only home to the majority of the global population, the number of new arrivals to cities each year from births and rural migration is outpacing the number of new arrivals to the planet. As a result of this trend, the next doubling of the urban population will occur in less than 1% of the time required to reach its current size: about 50 years. At that time, around the middle of this century, the total urban population will be equivalent in size to the total global population circa 2004 [1].