Understanding Urban Carbon Fluxes

Key Finding 2

Many societal factors drive urban carbon emissions, but the urban built environment and the regulations and policies shaping urban form (e.g., land use) and technology (e.g., modes of transportation) play crucial roles. Such societal drivers can lock in dependence on fossil fuels in the absence of major technological, institutional, and behavioral change. Some fossil fuel–related infrastructure can have lifetimes of up to 50 years (high confidence).

Description of evidence base

Key Finding 2 involves societal factors that drive urban carbon emissions, including consumption and supply chains (Baiocchi and Minx 2010; Peters et al., 2011), wealth (Creutzig et al., 2015a), fuel prices (Creutzig 2014), lifestyle and norms (Patarasuk et al., 2016; Porse et al., 2016), urban form and density (Baiocchi et al., 2015; Creutzig et al., 2015a; Karathodorou et al., 2010; Mindali et al., 2004; Newman and Kenworthy 1989, 1999), technology (Kennedy et al., 2009, 2014, 2015), and climate (Baiocchi et al., 2015; Creutzig et al., 2015a; Glaeser and Kahn 2010; Kennedy et al., 2015). Research continues to establish the relative permanence of large technological and infrastructural systems in urban areas. For example, fossil fuel–burning infrastructures have lifetimes up to 50 years, leading to systemic dependence (i.e., “lock-in”) on fossil fuel–based technology (Unruh 2000; Seto et al., 2016; Erickson et al., 2015).

Major uncertainties

Increasing numbers of studies examine relationships between urban density and 1) atmospheric emissions and 2) building energy use. Uncertainty exists relative to the ability of cities to change their infrastructure because of cost considerations and municipal regulations, as well as state and national regulations that affect city form and infrastructure. Relationships among the core elements of carbon lock-in (i.e., technological, institutional, and behavioral) are poorly understood and involve interactions among scales of governance larger than urban areas. All these aspects vary widely across cities and North American countries.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

Studies are emerging that investigate these relationships, but more research is needed to understand the processes.

Summary sentence or paragraph that integrates the above information

For Key Finding 2, cities are complex systems with a mix of societal factors driving carbon emissions. Uncertainties remain regarding a complete typology of driving factors and the extent to which these factors lead to path dependencies and the ability of urban areas to alter infrastructure and technological trajectories.

Key Finding 3

Key challenges for urban carbon flux studies are observational design, integration, uncertainty quantification, and reconciliation of the multiple carbon flux approaches to detect trends and inform emissions mitigation efforts (medium confidence, likely).

Description of evidence base

Key Finding 3 is supported by recent research that begins to integrate and reconcile carbon flux information from intensive urban study sites in North America. Key supporting references include Gurney et al. (2017), Lamb et al. (2016), Lauvaux et al. (2016), and McKain et al. (2012, 2015).

Major uncertainties

The major uncertainties related to integrating and reconciling urban carbon budget studies are those intrinsic to the different methodologies used. For trend detection and mitigation guidance, major uncertainties arise from the differences in scientific goals versus policy application.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

There is broad agreement that integration and reconciliation remain challenging. However, the various disciplines that pursue different methodological approaches to urban carbon flux assessment have different 1) definitions of uncertainty, 2) needs for attribution, and 3) criteria for successful mitigation guidance. Hence, some disagreement exists over specific policy application and utility.

Estimated likelihood of impact or consequence, including short description of basis of estimate

Continued integration and reconciliation of urban carbon fluxes are likely to achieve methodologically consistent and agreed-on approaches, results of which will be useful for trend detection and mitigation guidance. Assessment of enacted policy has received limited study, and thus the ability to independently assess atmospheric trends and use that information to inform mitigation progress and potential is highly important and relevant to urban carbon mitigation and climate policy.

Summary sentence or paragraph that integrates the above information

For Key Finding 3, the research community recently has begun to integrate and reconcile multiple approaches to urban carbon flux assessments for intensive study sites of urban carbon in North America. These efforts are ongoing but remain challenging due to methodological differences, methodological uncertainties, and differing disciplinary perspectives and criteria. The relevance and importance of these efforts are high because there remains limited independent assessment of urban carbon mitigation efforts or progress.

Key Finding 4

Improvements in air quality and human health and the reduction of the urban heat island are important co-benefits of urban carbon emissions mitigation (high confidence, very likely).

Description of evidence base

Numerous studies contribute to Key Finding 4, including research on the impacts of carbon emissions reductions on local air pollution, related human health benefits, and reduction of waste heat discharge (Harlan and Ruddell 2011; Huang et al., 2013; Jacobson and Ten Hoeve 2012; Milner et al., 2012; Ostro et al., 2011; Sarofim et al., 2016; Viguié and Hallegatte 2012).

Major uncertainties

Uncertainties include the precise magnitude of health and environmental benefits associated with reductions of carbon emissions. Benefits will vary with a number of factors such as urban population sociodemographics, urban meteorology, composition of emissions sources, and energy fuel mix. Tradeoffs require further research and remain uncertain.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

There is broad agreement about the benefits of reducing carbon emissions. Major uncertainties are related to assessing quantitatively the impacts and precise relationships between carbon emissions reductions and urban health and environmental benefits.

Estimated likelihood of impact or consequence, including short description of basis of estimate

Key Finding 4 is of high impact. The quantitative relationship between carbon emissions reductions and urban health and environmental impacts has direct and important implications for stakeholder decision making associated with urban air quality, urban climate policy, and general urban planning.

Summary sentence or paragraph that integrates the above information

For Key Finding 4, fossil fuel energy systems emit carbon dioxide (CO₂) and methane (CH₄). These systems also result in emissions of local air pollution and heat discharge in urban environments. Hence, reducing fossil fuel dependence can provide co-benefits to human health and environmental impacts associated with urban heat. The net benefit of these related outcomes remains uncertain because of potential tradeoffs and unforeseen outcomes.

Key Finding 5

Urban methane (CH₄) emissions have been poorly characterized, but the combination of improved instrumentation, modeling tools, and heightened interest in the problem is defining the range of emissions rates and source composition as well as highlighting infrastructure characteristics that affect CH₄ emissions (high confidence).

Description of evidence base

For Key Finding 5, consistent and persistent evidence of under-reported CH₄ emissions was found in Los Angeles, Boston, and Indianapolis (Lamb et al., 2016; McKain et al., 2015; Wong et al., 2016). Other studies report inverted distributions of CH₄ emissions in Los Angeles (75% thermogenic, 20% biogenic; Hopkins et al., 2016) compared with San Francisco (17% thermogenic, 82% biogenic; Jeong et al., 2017). Intensive field surveys of urban natural gas systems in seven cities indicate large variations in CH₄ leakage rates from urban gas distribution infrastructure attributed to differences in pipeline material and age (Hopkins et al., 2016; Jackson et al., 2014; Phillips et al., 2013; von Fischer et al., 2017).

Major uncertainties

The uncertainties in urban-scale CH₄ emissions estimates are not well established because the number of cities where these emissions have been studied is small and the temporal duration of the studies is very limited. While Key Finding 5 is of high confidence for the limited times and numbers of cities represented in the literature, this finding cannot yet be generalized across other North American cities.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

The assessment of confidence is based on a small number of cities where emissions have been studied over a short period of time. The confidence level is based on the results of these studies, which are robust and agreed upon, but this confidence does not necessarily apply across the continent due to the limited number of studies conducted to date.

Summary sentence or paragraph that integrates the above information

For Key Finding 5, urban CH₄ emissions estimates exist for several North American cities. Yet there are discrepancies between these estimates and governmental inventories. As such, further research is needed to gain a complete understanding of uncertainties and assess the representativeness of these studies.

Key Finding 6

Urban areas are important sites for policymaking and decision making that shape carbon fluxes and mitigation. However, cities also are constrained by other levels of government, variations in their sources of authority and autonomy, capacity, competing local priorities, and available fiscal resources (high confidence).

Description of evidence base

Thousands of North American cities have joined municipal networks to pursue co-benefits from climate mitigation measures, including benchmarking initiatives. However, many cities do not have authority to dictate fuel sources for their energy supply or for vehicles, nor they do control carbon inputs into products that come into cities. Evidence for Key Finding 6 indicates that municipal carbon emissions mitigation initiatives in the United States vary significantly among states. This variation suggests that state-level policies and characteristics may influence the propensity of cities in their borders (Krause 2011). Jurisdictional barriers that restrict decision making by municipalities may impede change because of a lack of authority over decision making (Tozer 2013).

Major uncertainties

Cities vary in extent and type of innovation, though the precise motivation lacks sufficient evidence to provide a clear understanding of the factors involved. In addition, each country has different governmental arrangements that affect city autonomy; even within states in the same country, these arrangements may vary.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

Evidence of the importance of cities is supported by the large proportion of North American anthropogenic carbon emissions (see Key Finding 1). The evidence for the moderated influence over carbon emissions is supported by the mixture of political, economic, and social authority of cities over direct and indirect emissions sources.

Summary sentence or paragraph that integrates the above information

For Key Finding 6, cities are making policies to reduce their carbon emissions, but they also are constrained by many factors that can limit their authority. Moreover, cities vary widely among themselves. An understanding of the limitations in the ability of cities to mitigate their carbon emissions and why certain cities are more proactive than others is still to be developed.