Along with all other planetary surfaces, urban climates are subject to global changes due to radiative forcing. The impact of global change on cities is the subject of this section.
The climate system and global climate change
The elements of the global climate system include the atmosphere, biosphere, hydrosphere, cryosphere, and lithosphere (Figure 3.9). The climate system is coupled, in the sense that the components interact at many spatial and temporal scales.
The Earth’s climate is determined by the long-term balance between incoming solar radiation and outgoing terrestrial radiation. Incoming solar radiation is partly absorbed, partly scattered, and partly reflected by gases in the atmosphere, by aerosols, and by clouds. Under equilibrium conditions, there is an energy balance between the outgoing terrestrial or "longwave" radiation and the incoming solar or "shortwave" radiation. Greenhouse gases are responsible for an approximately 30 °C elevation of global average surface temperature. Since the Industrial Revolution, increasing greenhouse gas concentrations due to fossil fuel combustion, cement-making, and land use changes has increased the mean surface temperature of the Earth by approximately an additional 1 °C.
These and other climate changes and impacts have been documented by an international panel of leading climate scientists, the Intergovernmental Panel on Climate Change (IPCC), formed in 1988 to provide objective and up-to-date information regarding the changing climate. Key findings of the 2007 Fourth Assessment Report (AR4; IPCC, 2007) included the following (as summarized in Horton et al., 2010):
Figure 3.9: The global climate system. Shown are the many interactions among the different components of the climate system, which includes cities.
• there is a greater than 90 percent chance that warming temperatures are primarily due to human activities (IPCC, 2007);
• atmospheric concentrations of carbon dioxide (CO2) are now more than one-third higher than pre-industrial levels;
• concentrations of other important greenhouse gases methane (CH4) and nitrous oxide (N2O) have increased by more than 100 percent and close to 20 percent respectively over the same time period;
• further increases in greenhouse gas concentrations are projected to lead to further temperature increases and associated changes in the climate system;
• over the twenty-first century, global average temperature is expected to increase by between 1.8 and 4.0 °C.
Warming is expected to be largest over land and in the high-latitude North, where some Arctic cities may experience warming exceeding 8 °C by 2100. Outside the tropics and subtropics, the largest warming is generally expected in winter. Generally speaking, precipitation is expected to increase in high-latitude cities and decrease in subtropical cities. Hot extremes and cold extremes in cities are generally expected to increase and decrease respectively.
As CO2 continues to be absorbed by the oceans, ocean acidification will accelerate, with potentially large implications on marine ecosystems. While the implications may be largest for those coastal cities where marine ecosystems are a source of economic livelihood and sustenance, in a global economy all the world’s cities would be indirectly impacted by large-scale ocean acidification.
Drivers of global climate change
The global changes described above can largely be attributed to three drivers, or causes, of climate change: greenhouse gases, aerosols, and land use changes. These three drivers, with emphasis on urban contributions, are described below.
Greenhouse gases
Gases that trap heat in the atmosphere are referred to as greenhouse gases. Three primary greenhouse gases that are directly linked to anthropogenic activities are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). As centers of population, economic activity, and energy use, cities are responsible for a large portion of greenhouse gas emissions. On a per capita basis, however, urban residents in developed nations probably have lower emission rates than ex-urban residents due to the inherent energy efficiencies of mass transit and multiple-resident buildings.
Some greenhouse gases, such as carbon dioxide, occur in the atmosphere through natural processes, while others, such as the hydrofluorocarbons, are created solely from human activities.
Carbon dioxide, which accounts for over 75 percent of all greenhouse gas emissions, is emitted into the atmosphere through the burning of fossil fuels and the clearing of land for agriculture. Methane is emitted from natural gas production, livestock, and agricultural production. Application of nitrogen fertilizer for agricultural production also leads to nitrous oxide being emitted into the atmosphere. Most hydrofluorocarbons, another important greenhouse gas, come from industrial activity.
Aerosols
Aerosols are atmospheric particles of both natural and anthropogenic origin. Natural aerosol sources include volcanoes and sea salt, while key anthropogenic sources include fossil fuel combustion and biomass burning. Aerosol concentrations tend to be higher in urban than rural areas, although during times of extensive biomass burning rural areas can have comparable concentrations. Climatic, human health, and visibility effects of aerosols have been documented. (Shu et al., 2000, 2001; Pawan et al, 2006; Eri et al, 2009). Aerosols modify the earth’s energy budget by scattering and absorbing short- and longwave radiation. As a prime radiative forcer (Charlson et al., 1992), aerosol particles also influence cloud optical properties, cloud water content, and lifetime. That is referred to as the indirect effect of aerosols, or indirect climate forcing (Harshvardhan, 2002). Volcanic aerosols can lead to brief periods of global cooling, and biomass burning of agricultural regions has been shown to affect regional weather. While greenhouse gas effects are primarily global and regional in scale, aerosol effects span from the global to the urban scale.
Land use change
Land use change and urbanization influence the climate through changes in surface albedo, land roughness, hydrological and thermal features. Across the globe, human activities have changed the face of the planet. Deforestation has modified the climate by changing solar absorption and moisture transfer rates, as well as increasing carbon dioxide levels. Agricultural expansion has modified these processes as well. Urbanization and development have led to decreased groundwater absorption by the land and more heat absorption as the built environment expands (Zheng et al, 2002; Gao et al, 2003; Pielke, 2005; Lian and Shu, 2007). Like aerosol effects, the climatic effects of land use changes range from large regional scales to urban scales.
