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. These numbers underscore the fact that urban climate is becoming the dominant environment for most of humanity.
This topic presents information on four interrelated components of urban climate: (i) the urban heat island effect and air pollution, (ii) the current climate and historical climate trends, (iii) the role of natural climate variability, and (iv) climate change projections due to worldwide greenhouse gas increases. Figure 3.2 provides a schematic of key interactions within the urban climate system.
Teasing out the relative influences of these components on urban climate is challenging. Natural variability can occur at multidecadal timescales, comparable to the timescales used for historical analysis and to long-term greenhouse gas forcing. Another challenge is that different climate factors may not be independent. For example, climate change may influence the amplitude and periodicity of natural variability, such as the intensity and frequency of the El Nino-Southern Oscillation (ENSO).
To survey these issues, twelve cities are selected as examples. The twelve focus cities in this topic – Athens (Greece), Dakar (Senegal), Delhi (India), Harare (Zimbabwe), Kingston (Jamaica), London (UK), Melbourne (Australia), New York City (USA), Sao Paulo (Brazil), Shanghai (China), Tokyo (Japan), and Toronto (Canada) (Figure 3.3) – share a range of characteristics but also differ in key respects. They are all large, dynamic, and vibrant urban areas, and act as hubs of social and economic activity.
Figure 3.1: View of Earth at night: Areas of light show densely populated, urban areas.
Figure 3.2: Conceptual framework of this topic showing major components impacting urban climate.
They also feature long-term twentieth-century climate records that allow trend detection, projections, and impact analysis. All the cities selected are likely to experience significant climate change this century, and several illustrate the influence of climate variability systems such as ENSO. Some of the city examples demonstrate unique vulnerabilities to extreme climate events.
With regard to differences, the selection includes a range for geography and economic development levels. The various geographic locations allow examination of climate change impacts in multiple climate zones. Covering multiple climate zones also allows for the examination of the influence of the major natural climate variability systems such as ENSO.
A range of economic development examples is important for vulnerability studies. Adaptation planning for climate change tends to be more difficult in cities with limited resources; typically but not always the case in developing countries. Our focus cities include examples from both the developed and developing world, with some having already taken steps to prepare for climate change and others that have yet to start. Table 3.1 gives some summary statistics on socio-economic and geographic data, and mean climate for the focus cities.
Effects of cities on temperature: urban heat islands
Urban areas are among the most profoundly altered landscapes away from natural ecosystems and processes. Figure 3.4 illustrates a recent reconstruction of the verdant "Mannahatta" island circa 1609 as compared to the current landscape 400 years later (Sanderson and Boyer, 2009). Pondering this visually arresting reconstruction, it is not surprising that cities have altered microclimates with, among other effects, significantly elevated surface and air temperatures.
The elevation in temperatures is most generally explained in terms of the basic surface energy balance processes of shortwave and longwave radiation exchange, latent, sensible, and conductive heat flows (Oke, 1987). With respect to shortwave, or solar, radiation, surface albedo refers to the reflectivity of a surface to visible light and is measured from 0 to 100 percent reflectivity. The regional albedo of cities is significantly lower than natural surfaces due to the preponderance of dark asphalt roadways, rooftops, and urban canyon light trapping. These urban features have typical albedo values below 15 percent.This leads to efficient shortwave radiation absorption. The urban skyline, with deep urban canyons, results in a greatly reduced skyview at street level and this impedes longwave radiative cooling processes. This urban vertical geometry further impacts winds, generally reducing ventilation and sensible heat cooling. The replacement of natural soil and vegetation with impervious surfaces leads to greatly reduced evapotranspiration and latent heat cooling. The dense impervious surfaces with high heat capacity create significant changes in heat storage and release times as compared to natural soil and vegetated surfaces.
Figure 3.3: Map of cities highlighted in this topic. The cities were selected based on socio-economic factors and the availability of long-term climate data.
Table 3.1: City statistics and mean temperature and precipitation for 1971-2000*
|
City |
Latitude |
Longitude |
Population |
Mean annual temperature |
Annual precipitation |
|
Athens |
37.9 N |
23.7 E |
789,166 (2001) |
17.8 °C |
381 mm |
|
Dakar |
14.7 N |
17.5 W |
1,075,582 (2007) |
24.0 °C |
357 mm |
|
Delhi |
28.6 N |
77.2 E |
9,879,172 (2001) |
25.1 °C |
781 mm |
|
Harare |
17.8 S |
31.0 E |
1,435,784 (2002) |
18.1 °C |
830 mm |
|
Kingston |
17.9 N |
76.8 W |
579,137 (2001) |
27.5 °C |
691 mm |
|
London |
51.3 N |
0.4 W |
7,556,900 (2007) |
9.7 °C |
643 mm |
|
Melbourne |
37.8 S |
145.0 E |
3,806,092 (2007) |
15.7 °C |
652 mm |
|
New York City |
40.8 N |
74.0 W |
8,274,527 (2007) |
12.8 °C |
1181 mm |
|
Sao Paulo |
23.5 S |
46.4 W |
11,016,703 (2005) |
19.5 °C |
1566 mm |
|
Shanghai |
31.5 N |
121.4 E |
14,348,535 (2000) |
16.4 °C |
1155 mm |
|
Tokyo |
35.7 N |
139.8 E |
8,489,653 (2005) |
16.2 °C |
1464 mm |
|
Toronto |
43.7 N |
79.0 W |
2,503,281 (2006) |
7.5 °C |
793 mm |
Population data for all cities are from the United Nations Statistics Division Demographic Yearbook, United Kingdom National Statistics Office, and Census of Canada. Annual temperature and precipitation statistics are computed for all cities using data from the National Climatic Data Center Global Historical Climatology Network (NCDC GHCNv2), UK Met Office and Hadley Centre, Australian Bureau of Meteorology, and Environment Canada.
Figure 3.4: Manhattan-Mannahatta: on right is a reconstruction of Manhattan Island circa 1609 (called "Mannahatta" by the Lenape native Americans), as compared to today, based on historical landscape ecology and map data.
There are additional atmospheric and heat source processes in cities that interact with these energy balances. Aerosols tend to reduce the amount of incoming solar radiation reaching the surface (a net cooling effect), while elevated ambient urban carbon dioxide levels may further reduce net radiative cooling.
The high density of population and economic activity in urban areas leads to intense anthropogenic heat releases within small spatial scales. These include building heating and cooling systems, mass transportation systems and vehicular traffic, and commercial and residential energy use. Anthropogenic heat emission has been well documented and researched in developed countries as a major factor causing the heat island phenomenon (Ohashi et al., 2007). As economic development, urbanization, and population growth continue in the developing countries, anthropogenic heat has increased there as well (Ichinose and Bai, 2000). Growth in urbanization increases energy demand in general and electricity demand in particular.
In analogy with the well-established urban heat island, it is tempting to define additional atmospheric urban "islands" such as rainfall islands, and relative humidity islands, which refer to potential urban alterations of precipitation and reductions in urban soil moisture availability due to impervious surfaces. In some cities, there may be good evidence for their existence and effects, such as in Shanghai (Section 3.1.3.1). In general though, the case for additional urban atmospheric islands, such as rainfall islands, is not as straightforward as the heat island and needs further research and characterization.
There can be little question, however, about the broad array of quality-of-life issues that are generally negatively impacted by excess urban heat. These include extreme peak energy demands, heat wave stress and mortality risk, air quality deterioration, seasonal ecological impacts including thermal shocks to waterways following rain events and impacts on urban precipitation.
Effects of cities on local precipitation
There is a longstanding interest in the question of urban impacts on precipitation both locally and regionally. Although there is evidence that urbanization and precipitation are positively correlated, a consensus on the relationship has not yet been reached. Early studies by Horton (1921) and Kratzer (1937, 1956) provided indications that urban centers do play a role in strengthening rainfall activity. Studies by Landsberg (1956), Stout (1962), and Changnon (1968) discussed the extent to which urbanization may induce and strengthen precipitation. The strongest argument used in those studies to substantiate the role of urban centers on rainfall was the enhancement of downwind rainfall. Landsberg (1970) and Huff and Changnon (1972a,b) used observed data to support this hypothesis.
Balling and Brazel (1987), Bornstein and LeRoy (1990), Jau-regui and Romales (1996), Selover (1997), Changnon and Westcott (2002), and Shepherd et al. (2002) have shown evidence that corroborates the earlier findings of enhanced precipitation due to urbanization. However, the hypothesis has been disputed, and even challenged, by other data studies that show no local effect on precipitation (Tayanc and Toros, 1997) or even deficits in precipitation that accompany urbanization (Kaufmann et al., 2007).
Recent studies by Burian and Shepherd (2005) and Shepherd (2006) point specifically to increases in downwind rainfall due to urbanization. Although topological effects may be partially responsible for this finding in the Shepherd (2006) case, Chen et al. (2007) supports the Shepherd (2006) study. Shepherd et al. (2002) and Simpson (2006) have defined an "urban rainfall effect," which is defined as the impact of urban centers on enhancing downwind and peripheral rainfall. However, there is again no consensus for a unifying theory for the urban rainfall effect. Various explanations of why urbanization positively impacts convection have evolved over the years (Shepherd, 2005). The arguments include sensible heat flux enhancement, urban heat island-induced convection, the availability of more cloud condensation nuclei in urban areas, urban canopy alteration or disruption of precipitation systems, and increased surface roughness convergence.
Climate change impacts on the urban heat island processes
Climate change may well modify the urban heat island and rainfall effects, but the quantitative extents are unknown at this time. This is a ripe area for potential future climate research. For example, warmer winter temperatures may decrease energy combustion for heating, but also increase summertime air conditioning needs, thus affecting these anthropogenic heat sources. Higher temperatures in summer are likely to lead to higher levels of pollutants such as ozone. Changes in temperature, precipitation and ambient levels of carbon dioxide will all impact local vegetation and ecosystems with effects on urban parks and vegetation restoration, which is an important adaptation strategy. Climate-induced changes in winds could also impact urban climate. It should also be noted that urbanization often increases the impact of a given climate hazard. For example, a precipitation event is more likely to produce flooding when natural vegetation is replaced with concrete, and temperatures above a certain level will cause greater mortality when air pollution levels are high.
Shanghai: many urban environmental islands
Due to its land use patterns (Figure 3.5) and high densities of population and buildings, Shanghai experiences perhaps five interrelated microclimatic impacts, which we frame as metaphorical "islands" in analogy with the heat island: the heat island, dry island, moisture island, air pollution island, and rain island.
The Shanghai heat island appears from afternoon to midnight under certain weather patterns. According to ground observation records from the past 40 years, there is an apparent mean annual temperature difference of 0.7 °C between downtown and the suburban areas (16.1 °C and 15.4 °C, respectively). The corresponding mean annual extreme maximum temperatures are 38.8 °C and 37.3 °C, respectively. This urban warmth tends to appear from afternoon to midnight in mid autumn and in early winter, summer, and spring under clear conditions with low winds (Figure 3.5).
The second and third Shanghai islands can be referred to as the dry and moisture island effects. The elevated inner city temperatures and greater soil moisture availability in rural areas should result in lower urban relative humidity. Annual average data from Shanghai confirm this effect (Figure 3.5). However, the daily cycle of urban-rural humidity can be more complex due to differences in dewfall, atmospheric stability, and freezing, which can dry out rural air overnight and lead to greater urban humidity at night (Oke, 1987). This can lead to a cycle of relative dry and moisture islands that alternate during the day and night.
Figure 3.5: Shanghai urban islands effects. From top left to bottom right are Shanghai’s land use pattern, temperature, relative humidity, optical depth, and precipitation. For all the climate variables, a distinct pattern emerges in the area of greatest development.
These differences in relative humidity may have implications for human comfort indices during heat waves and cold periods.
The fourth island is an air pollution island, since urban air quality is poorer than that of suburban and rural areas. The inversion layer over the urban heat island holds back the diffusion of atmospheric pollutants, increasing pollution levels locally. This in turn leads to acid rain. In 2003, the average pH value of precipitation in Shanghai was 5.21, with a percentage of acid rain of 16.7 percent. In the downtown area, where industry, commerce, traffic, and residents interact closely, air pollution is severe, as shown in Figure 3.5.
The fifth Shanghai urban island is termed the urban rainfall effect. According to ground observation records in the flooding season (May-September) and non-flooding season (October-April in the following year) over the period 1960-2002, the central city experiences greater precipitation than the outlying regions (urban rainfall effect), with an average precipitation that is 5-9 percent higher than in the surrounding regions. There are a number of hypothetical scenarios that may produce an urban rainfall effect: (1) the urban heat island effect may contribute to the rising of local air currents, which help to develop convective precipitation; (2) reduced urban windspeeds may slow the movement of storm systems over urban areas and could lengthen the duration of rain events; (3) aerosol pollutants may provide potential rainfall "nucleation sites" in low-forming clouds over the city. Thus, the urban heat island, the urban wind regime, and the urban pollutant island may all be partially responsible for the higher precipitation amount in the central city and the leeward area (Figure 3.5).
