The southwestern United States have seen some of the fastest-growing cities in the US, and Tucson has experienced a doubling of the population over the past three decades to about three-quarters of a million people. This has also led to dramatic growth in developed urban and suburban land area. The population increase matches well the temperature increase we see for Tucson, with an almost steady temperature in the first part of last century but a marked acceleration in the second part. Research indicates that since 1969 Tucson has seen a 2.6°C increase just from the urban heat island. And since there has also been a temperature increase in the general area, presumably from both global warming and natural variability the average minimum temperature over the past 40 years for Tucson has increased more than 6°C.
Likewise, we have seen an increase in the urban heat island over Athens in the past 50 years. Here maximum temperatures have increased by about 2°C. In downtown Los Angeles, maximum temperatures over the past century have increased by some 2.5°C and minimum temperatures by some 4°C. New York has a similar night-time urban heat island of 4°C.
Recently we have been able to use satellite measurements of the direct temperatures over the entire surface of a city. When researchers looked at Houston, they realized it was a fast-growing city. From 1990 to 2000 it grew by 300,000 residents—a full one-fifth increase. Yet when they looked at the change in temperature as measured from the sky, they found an amazing result. Over a short 12 years, the night-time surface temperature increased by about 0.8°C. Over a hundred-year period that would translate into almost 7°C temperature increase.
And indeed, for huge cities these are the kinds of temperature differences that are being found around the world. Asian cities are today the most rapidly growing regions of the world. Sixteen of the world's 24 megacities (cities with more than 10 million people) will be in Asia by the year 2015. Not surprisingly, this is also where we find some of the largest urban heat islands. The daytime temperature difference between the tropical cities of Bangkok and Manila and their countrysides is 7-8°C. The same temperature differences are found for temperate cities like Seoul and Shanghai. If we go to the mega city of Beijing, researches have found that temperatures diverge some 10°C in the daytime and 5.5°C at night. And Tokyo with its 20 million inhabitants sees some of the most dramatic consequences of the urban heat island. While the daytime temperature of the area surrounding Tokyo in August was 28.5°C, the downtown was measured at 40+°C. And this high temperature is not just affecting a small inner core of the city—the high-temperature area covers some 8,000 km2 or the equivalent of 140 times the area of Manhattan. Nights in August Tokyo only provide a slight solace, as the temperature drops to 9°C in rural areas but remains at 26.5°C in the city.
At the place where the urban heat island was first discovered, London is now also seeing a strong warming. Since the 1950s, the number of nights with intense urban heat has increased by four days each decade. Today temperatures are 4-6°C higher, and during the August 2003 heat wave reached 9°C. These worldwide urban temperature increases tell us at least two things.
First, many of these urban temperature increases over the past half or full century are of the same order or bigger than the 2.6°C that we expect to see over the coming full century. It is likely that for many cities the temperature increases mainly from the urban heat island of the twentieth century arc of a bigger scale than the temperature increases from global warming in the twenty-first century. Yet the increases have not brought the cities tumbling down.
Over the past hundred years, metropolises have had to adapt to temperatures that rose faster and higher than what we will expect in this century. Their inhabitants both were poorer and had less technological ability to adapt. Yet, the higher temperatures did not produce widespread and frequent heat waves killing sizable numbers of inhabitants.
This does not mean that the urban heat island may not have been bad for some or possibly even most cities. Although deaths have in general been declining (as we saw for Philadelphia above), they might have declined even faster without it. But it means that the doomsday predictions are sorely mistaken when they focus solely on ever more heat deaths without taking into account fewer cold deaths, and that adaptation will possibly strongly mitigate the temperature effects. If our forefathers were able to do so, it seems reasonable to assume that, being much richer and having vastly more technical prowess, we will be able to repeat their feat.
This also does not deny that with global warming the impact on cities will be considerably worse, because they will be hit by a double whammy—temperature increases both from CO2, and from still-increasing urban heat islands.
But this leads straight to the second point. Unlike our forefathers, who did very little or nothing about the urban heat island, we are in a good position to tackle many of its effects. Presumably our goal is to prevent part of the problems of increasing temperatures over the coming century. It is curious that we focus so much of our attention on cutting CO2, when it is likely that we could do much more and at much lower cost to cut temperatures by addressing the urban heat island.
Studies show that very simple solutions can make a great difference. One of the two main reasons cities are hotter is that they are drier. Cities lack moist green spaces and have large, impermeable surfaces with drainage, quickly leading any water away. Thus, the sun's energy goes into heating the atmosphere instead of into the cooling evaporation of water. If we plant trees and provide vegetation and water features in the urban environment, this will—apart from making a more beautiful city—dramatically cool the surroundings. For instance, air around the River Thames or within urban parks is on average 0.6°C cooler than neighbouring built-up areas. If we significantly and pervasively increase moisture, models show that at noon on the third day of a fine weather spell temperatures can be decreased by as much as 8°C.
The other main reason that cities are hotter is that they have a lot of black asphalt and dark, heat-absorbing structures. Although it may seem almost comically straight-forward, one of the main solutions is very simple—paint the tarmac and buildings white.