Cities, traffic, and CO2: A multidecadal assessment of trends, drivers, and scaling relationships

Abstract

Emissions of CO2 from road vehicles were 1.57 billion metric tons in 2012, accounting for 28% of US fossil fuel CO2 emissions, but the spatial distributions of these emissions are highly uncertain. We develop a new emissions inventory, the Database of Road Transportation Emissions (DARTE), which estimates CO2 emitted by US road transport at a resolution of 1 km annually for 1980-2012. DARTE reveals that urban areas are responsible for 80% of on-road emissions growth since 1980 and for 63% of total 2012 emissions. We observe nonlinearities between CO2 emissions and population density at broad spatial/temporal scales, with total on-road CO2 increasing nonlinearly with population density, rapidly up to 1,650 persons per square kilometer and slowly thereafter. Per capita emissions decline as density rises, but at markedly varying rates depending on existing densities. We make use of DARTE's bottom-up construction to highlight the biases associated with the common practice of using population as a linear proxy for disaggregating national- or state-scale emissions. Comparing DARTE with existing downscaled inventories, we find biases of 100% or more in the spatial distribution of urban and rural emissions, largely driven by mismatches between inventory downscaling proxies and the actual spatial patterns of vehicle activity at urban scales. Given cities' dual importance as sources of CO2 and an emerging nexus of climate mitigation initiatives, high-resolution estimates such as DARTE are critical both for accurately quantifying surface carbon fluxes and for verifying the effectiveness of emissions mitigation efforts at urban scales.

Keywords: carbon dioxide; emissions; on-road; transportation; urban.

Fig. 1.

Map of 2012 on-road CO₂ emissions for the coterminous United States and selected urban areas at a resolution of 1 km. (Insets) Maps show details of metro areas surrounding Seattle (A), Los Angeles (B), Houston (C), Atlanta (D), and Boston (E).

Fig. 2.

Time series of US on-road CO₂ emissions. Urban roads accounted for 80% of total emissions growth since 1980. Rural road emissions have been declining since 2002.

Fig. 3.

Time series of US per-capita on-road CO₂ emissions by county, using a Census 2000 Metropolitan Statistical Area (MSA) classification. Per capita emissions increased from 1980, both in urban and nonurban counties, with brief declines during the 1981–1982, 1990–1991, and 2007–2009 economic recessions. Since 2009 per capita emissions in non-MSA (rural) and outlying MSA (suburban) counties have grown rapidly, whereas central MSA (urban) per capita emissions have continued to decrease.

Fig. 4.

Plots of on-road emissions at multiple scales. (A1 and A2) The fitted spline Ψ₁ for the partial prediction of total on-road CO₂ in teragrams (Tg) and per capita CO₂ in megagrams (Mg) (A1 and A2, respectively), is plotted against county population density. The rug plots show the distribution of US counties pooled across all years. The values of Ψ₁ are the model-estimated emissions relative to the conditional mean of each county. (B) Decadal per capita emissions vs. density for 14 US cities. Movement in time is denoted by point size and arrows. (C) Per capita on-road CO₂ plotted vs. the share of residents who commute using public transit. (D) Same cities as in B, overlaid on all US census-designated places (gray points), for the year 2010. The dashed blue line in A1, A2, B, and D identifies the first local maxima of Ψ₁ at 1,650 persons per square kilometer.