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ENERGY-EFFICIENT Urban Form
Reducing urban sprawl could play an important role in addressing climate change. JULIAN D. MARSHALL UNIVERSITY OF MINNESOTA
I
mproving city layouts and transportation networks could reduce carbon emissions more than replacing all gasoline with corn ethanol (1). Although much attention on mitigating climate change has focused on alternative f uels, vehicles, and electricity generation, better urban design represents an important yet undervalued opportunity. Fortunately, such decisions are well within the reach of local governments and leaders and can reduce long-term carbon emissions. © 2008 American Chemical Society
The impact of cities—and urban design—on the global climate is becoming increasingly important. In 2008, urbanites will outnumber rural dwellers globally for the first time in human history (2). China’s population doubled between 1952 and 2003, but its urban population increased 7-fold; today, 170 Chinese cities have at least 1 million residents (3). The U.S. has 39 such cities (4). In coming decades, urban populations are expected to double while rural populations level off or decline. May 1, 2008 / Environmental Science & Technology ■ 3133
Vehicle use is rising rapidly. From 1970 to 2005, U.S. total vehicle-kilometers increased 3× faster than the population (annual increases: 3.0% vs 1.0%) (5). Similar trends occurred in China (8.3% vs 1.7%, a 5‑fold difference) and the world (4.3% vs 1.8%) during 1970–1990 (6). If current trends in total vehiclek ilometers continue, vehicle CO2 emissions may increase even if emissions per mile decline (7). FIGURE 1
Where do Americans drive the most? Vehicle usage records indicate an inverse relationship between population density and vehicle-kilometers traveled. Data are for passenger vehicles in the 47 U.S. urban areas with populations >750,000, for year 2000. (Adapted from Ref. 4.)
Vehicle-kilometers traveled (km person–1 d–1)
60
Indianapolis Dallas San Antonio Orlando
Houston Atlanta Kansas City
40 Pittsburgh Norfolk Providence Buffalo
20
0
0
Boston Chicago Philadelphia
San Jose Ft. Lauderdale Los Angeles Miami
New Orleans New York Las Vegas San Juan
1000
y = 338x –0.32 R 2 = 0.36 2000
3000
Urban population density (people km–2)
In an influential paper in Science, Socolow and Pacala (8) argue that climate stabilization during the next half century means reducing CO2 emissions by 175 GtC (33%) relative to a business-as-usual (BAU) scenario. They propose seven strategies, with each “stabilization wedge” representing emission reductions of 25 GtC during 2005–2054 (each wedge grows from no reduction in 2005 to 1 GtC per year [yr] reduction in 2054). The race is now on to figure out ways to design and implement these wedges. Often neglected in the debate is the role of urban form (e.g., land-use patterns and the layout of transportation infrastructure) in meeting climate objectives. My estimates suggest that reducing urban sprawl in the U.S. alone could represent half or more of a stabilization wedge.
Impacts of urban form on transportation CO2
Compact urban form can cut on-road gasoline emissions, the largest segment (62%) of transportation CO2 in the U.S. The transportation sector is the largest emitter (33%) of CO2, outpacing the residential, industrial, and commercial sectors. (Electricity generation, when totaled for all sectors, accounts for 41% of CO2 emissions.) Records of automobile usage (Figure 1) show an inverse relationship between population density and per capita daily vehicle-kilometers traveled (VKT) (4, 9). Evidence suggests that VKT is causally related to population density and other urban form attributes, and therefore, sprawl reduction 3134 ■ Environmental Science & Technology / May 1, 2008
policies may curtail VKT (10–14). In denser urban areas, trip origins and destinations (e.g., home, work, shopping) are closer; driving disincentives (e.g., congestion, parking costs) are greater; and alternative modes of travel (e.g., walking, bicycling, mass transit) are more common (15). Sprawl encompasses many aspects of land use, including leapfrog development, segregated land use, and automobile dependence (16, 17). Calculations presented next focus on one aspect of sprawl— declining urban average population density. To estimate the potential carbon benefits of reducing urban sprawl, I considered five urban growth scenarios for the U.S., 2005–2054: high sprawl, BAU, reduced sprawl, no sprawl, and infill. Changes per decade in average urban population density are –47, –39, –13, 0, and +11%, respectively, for the five scenarios. In comparison, changes per decade in average urban population density were –13% during 1960–1990 and –34% during 1990–2000 (18). For the no-sprawl scenario, average density is constant, so urban area increases at the population growth rate (1% annually). The infill scenario approximates a strict urban growth boundary, with total urban area roughly constant (unrealistic, but a useful bounding estimate). For each urban growth scenario, I considered no technology innovation and innovation (1% annual reduction in fleet-average gasoline CO2 emissions per kilometer). As a sensitivity analysis, I evaluated the impact of rapid innovation (3% annually). Innovation could include more-efficient vehicles or reduced-carbon fuels. Historically, annual improvements >1% have been achieved, but not maintained, for 50 yr. Annual changes in U.S. passenger vehicle fleet-average fuel economy were –0.4% during 1936– 1973, +2.0% during 1973–1995 (23 yr), and near-zero (+0.03%) during 1995–2005 (19–21). Year 2005 passenger-vehicle average fuel economy was 20 miles per gallon (mpg; 21). The Energy Independence and Security Act, signed by President Bush in December 2007, will increase fuel economy by ~1.5% annually for the next 23 years (assumptions: 10-yr vehicle turnover rate; continuation of the ~20% gap between actual passengervehicle fuel consumption [20 mpg; 21] and Corporate Average Fuel Economy [CAFE]-rated consumption [25 mpg; 22, 23]). The act will increase CAFE standards (currently 27.5 mpg for automobiles and 22.2 mpg for light-duty trucks) to 35 mpg for both vehicle classes in 2020; this is below today’s CAFE-type standards in China (37 mpg) and the EU (44 mpg) (23). For each urban growth and technology scenario, I calculated U.S. total VKT and the resulting on-road gasoline CO2 emissions. See the Supporting Information (SI) for details. The results indicate that sprawl reduction could play an important role in addressing climate change. Reduced sprawl, without technology innovation, decreases emissions by 10 GtC during 2005–2054 (by 0.5 GtC/yr in 2055) compared with BAU. These savings represent 41% of a wedge. The no-sprawl and infill scenarios offer 53% and 60% of a wedge, respectively, compared with BAU, with no innovation. Improvements in emissions per kilometer would
reduce emissions for all urban growth scenarios and also would reduce the differences between them. For BAU growth, innovation reduces emissions by 38% of a wedge compared with no innovation—almost the same reduction as converting, without innovation, from BAU to reduced sprawl. Converting from BAU without innovation to reduced- or no-sprawl growth with innovation would offer 66% or 74% of a wedge, respectively. These improvements are impressive, especially given that they involve action by the U.S. only (5% of the global population). Disruptive technologies such as low-carbon fuels or rechargeable hybridelectric vehicles that use low-carbon electricity could deliver environmental innovation rates significantly higher than historic levels. Increasing innovation from 0% to 3% annually would reduce U.S. emissions by 83% of a wedge under BAU growth. Analyses above use the extant (Figure 1) relationship between population density and VKT (specifically, a density-VKT elasticity of –30%; 9, 24). Greater attention and commitment to energy-efficient urban form (see below for example policies) could increase the carbon benefits from sprawl reduction. As an illustration, I repeated the analyses above, but assumed aggressive support for energy-efficient urban form (represented by doubling the density-VKT elasticity to –60%). I found that, without innovation, reduced- and no-sprawl scenarios offer 125% and 149% of a wedge, respectively, compared with BAU. Sprawl reduction would have a greater CO2 impact if aimed at high-elasticity situations (9, 12) rather than low-elasticity ones. Overall, these findings suggest that long-term climate impacts from shifts in urban form could be comparable to those from technological innovation, and that climate-mitigation strategies would have greater impact by addressing urban form and technological innovation, rather than only one of those two. Reducing sprawl could help minimize U.S. dependence on foreign oil and could ease economic impacts of price shocks while providing financial savings to help offset the costs of low-carbon energy technologies. Conversely, if sprawl accelerates, it becomes another source of emissions to reduce or offset. In that case, additional mitigation wedges will be required. For example, compared with BAU, the high-sprawl scenario increases emissions by 14% of a wedge with innovation or by 21% without. The values presented here are rough estimates only, involving several simplifications (see SI; even a highly detailed analysis would involve significant uncertainty). Still, the results highlight the potential importance of urban design for reducing CO2 emissions and suggest that further investigation is warranted. One source of uncertainty is the influence of urban form on vehicle choice. The perceived convenience of a small vehicle is greater at high than at low density. The evidence suggests that the relationship is causal: lower densities and longer commutes increase preferences for light-duty trucks instead of automobiles (11). Including those interactions would increase the estimated benefit of sprawl reduction on CO2 levels. Perhaps the largest source of uncertainty is the density-VKT elasticity. If elasticity were
between –4% and –15% (9), the impact of reduced sprawl compared with BAU would shrink from 41% of a wedge to 4–17%. (If elasticity were zero, the CO2 impact of urban form shifts would be zero.) Using the elasticity of –30% to –50% reported by modeling (12) and empirical (24) studies would increase the estimated CO2 benefits of sprawl reduction. Urban form is important outside of the U.S., too. Globally, the number of megacities (population >10 million) increased from 3 in 1975 to 20 in 2005 (25). However, most urbanites live in cities with 55% (Berlin, Madrid, Vienna) to
