Understanding Urban Carbon Fluxes

A quantitative understanding of contemporary urban carbon trends continues to face limitations related to data availability across the North American domain. Some understanding can be gleaned from statistics on urban growth in general, along with several case studies of urban carbon fluxes over particular time spans or locations. For example, Mexico’s annual urban population grew at a rate of 1.9% between 1995 and 2015, while both Canada and the United States had urban growth rates of 1.2% (UN DESA 2015). Future projections at the global level and for North America suggest increases in urban land use. For example, there is a greater than 75% probability that global urban land will increase from 652,825 km² in 2000 to 1,863,300 km² in 2030 (Seto et al., 2012). Other studies have projected a near tripling in the percentage of land devoted to urban cover by midcentury (Nowak and Walton 2005).

The future trajectory of urban carbon fluxes is unambiguously tied to increases in aggregate urban energy demand and the proportion met by fossil fuels (Hoornweg et al., 2011; Jones and Kammen 2014; Marcotullio et al., 2013). Theoretically, these increases are the cumulative result of concentrated population and economic activity, which today are predicated on the more energy intensive processes in agriculture, transportation, buildings, industry, and waste management (Liddle 2014). However, despite consensus about the positive correlation between population and energy demand or carbon emissions, there is debate about the magnitude of the effect and the implications of future urbanization. The effect of population size on carbon emissions or energy demand may be contingent on other factors, including, for example, a city’s starting population size (Bettencourt et al., 2007). Some evidence for this scaling relationship suggests that urban areas with larger population sizes have proportionally smaller energy infrastructures than smaller cities (Bettencourt et al., 2007; Fragkias et al., 2013). Other evidence suggests that carbon emissions may increase at a rate greater than population growth rates, so that larger cities exhibit proportionally higher energy demand as they grow than do smaller cities (Marcotullio et al., 2013). Theoretically, such an outcome is possibly due to diminishing returns, threshold effects, negative synergisms, and the disproportionate escalation of cost for maintaining environmental quality with population growth (Ehrlich and Holdren 1971). Finally, the difficulty occurs with predicting not only trends in policymaking, but also the impact of policy change on energy sources (Tuckett et al., 2015). For instance, in some U.S. states, policy is shifting some of the energy generation toward renewables (Lutsey and Sperling 2008). However, cost drivers for energy sources evolve over time and influence the choice of energy supply (Gan et al., 2007).

The generation of waste heat, coincident with carbon emissions from the combustion of fossil fuels, has the potential to initiate feedbacks with the urban carbon cycle through the UHI effect—a phenomenon whereby urban areas are warmer than their unbuilt surroundings (Boehme et al., 2015; Oke 1982). Averaged at the city scale, the magnitude of this waste heat can be up to 100 watts per m² (Sailor et al., 2015), potentially increasing urban warming by 2 to 3^oC in winter and 0.5 to 2^oC in summer (Fan and Sailor 2005). As urban areas warm due to both large-scale changes in climate and localized UHI, the energy consumed for space cooling in summer increases while the energy used for heating in winter decreases, “spilling over” into other seasons (Li et al., 2015; Wang et al., 2010). For example, recent research found that summer electricity demand may increase up to 50% in some U.S. states at the end of this century due to increased cooling needs under climate change alone (Huang and Gurney 2016). In fact, a recent modeling study by Georgescu et al. (2014) found that for U.S. cities, the effects of urban expansion on urban air temperatures by 2100 will be on the same order of magnitude as GHG-induced climate change. The UHI effect, in addition to changes in heatwave event frequency and magnitude, would further exacerbate this feedback (Li and Bou-Zeid 2013).