This chapter provides a comprehensive overview of research focused on carbon budget associated with urban areas. It summarizes active research aimed to quantify spatial and temporal variability in carbon emissions at fine scales needed to understand the drivers of those emissions and document efficacy of management strategies. It provides a very good discussion of governance and management at the urban scale that influence carbon emissions, and it distinguishes what is controllable from what is uncontrollable due to decisions being made at larger state to national scale or the long turnover time of built infrastructure.
This chapter, which follows the Energy chapter that focuses on energy production, would be strengthened by including a brief discussion of energy consumption patterns in North America and by sharpening the introductory section so that the rationale for singling out urban areas is highlighted. Somewhere in SOCCR2, there needs to be a discussion about carbon management choices at state and national levels that mirrors the excellent section in the urban chapter. Instead of leaving off at stating there are factors that can’t be controlled at urban level, provide some discussion elsewhere on what the options are and what is being done to better understand them.
The urban chapter stresses the observation that urban emissions contribute disproportionately to anthropogenic emissions of carbon relative to their land area, which is true but not as illuminating as starting from the observation that urban areas concentrate population and economic activity that are responsible for carbon emissions. This point isn’t made until p.175, Line26. It would make an effective starting point for the introduction. Follow up by noting how urban emissions diverge from a constant per capita value. It is precisely the divergence from constant per capita emissions that make urban emissions especially interesting and provides the reason to study them as a separate entity. As the text points out (at the end of the 3rd paragraph in the introduction) there is a need “to explore how urban infrastructure and urban morphology will influence current and future energy consumption and development.” A figure could be added here to show the range of per capita carbon emissions for different cities or as a function of population density. Because there are emission differences, there are opportunities to influence them and a research need for understanding what causes those differences.
The chapter is right to point out that some of the differences in emission strength between urban areas decrease when the indirect emissions (energy, goods, and services consumed in the urban area that were produced and counted as emissions elsewhere) are considered. Discussions about emission intensity correlating with various factors need to caution against basing mechanistic understanding on correlations and trends alone.
The chapter makes in important point that urban structure and infrastructure investments influence carbon emissions and that governance structures that operate at the urban level are either not present or very different at state and national levels. This point needs to be noted early in the text and given a strong emphasis.
Wherever possible the discussion and estimates of what has been and might be accomplished in the future in terms of managing the carbon cycle at the urban level should be more quantitative.
Also of interest but not discussed quantitatively are the time constants for changes to be effected. The notion of turnover times for infrastructure is alluded to by the phrase “infrastructure lock-in”, but this could be expanded in a more quantitative way by noting typical lifetimes for different classes of
infrastructure. Infrastructure isn’t necessarily locked, but it’s difficult to replace before the end of its designed life. Past experience provides some guidance on issues like how quickly the vehicle fleet is updated compared to time constant for appliances, housing stock, transportation networks, and energy delivery. This is not to suggest that the concept of infrastructure lifetime needs to be exhaustively reviewed in the chapter. It needs to be highlighted as critical factor with a rich history to guide our understanding of how quickly emission changes can be accomplished.
The urban chapter has a very thorough section documenting Societal Drivers, which seems to provide the specific examples of carbon being embedded in societal activities that is the key finding for Chapter 6. Is there sufficient cross reference between these chapters?
Statement of Task Questions
- Are the goals, objectives and intended audience of the product clearly described in the document? Does the report meet its stated goals?
Although the chapter does meet several goals that can be identified by reading through the text, the goals and objectives are not articulated clearly enough and the intended audience is not specified. Having a summary of the objectives and audience in the introduction would guide the reader and provide a focus to sharpen the remainder of chapter.
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
The chapter covers scientific literature up to the onset of writing and chapter review. However, there are a few recent papers that could be incorporated to illustrate some key new results. Notably, the emerging field of using satellite observations to quantify emissions from large urban regions is not given enough attention. Hakkarainen et al. (2016) demonstrate the ability to quantify CO2 emission hotspots from satellite observations and provide a very good illustration of how anthropogenic carbon emissions are concentrated into urban areas.
- Are the findings documented in a consistent, transparent and credible way?
Yes, the chapter provides excellent documentation of its key findings.
- Are the report’s key messages and graphics clear and appropriate? Specifically, do they reflect supporting evidence, include an assessment of likelihood, and communicate effectively?
The graphics have room for improvement. Neither of the two figures that are in the chapter are quantitative, and they don’t effectively provide information beyond what is in the text. At the very least, it would help to have a graphic artist review them with an eye toward making them more effective in the web-published version of the report.
It would help to add a figure in the introduction illustrating the sharply focused carbon emissions from urban areas, based on left-most panel of Figure 1 in Hakkarainen et al. (2016) that shows CO2 hotspots together with NO2 concentrations and emission inventory. The point that urban emissions do not follow a constant per capita ratio could be effectively presented by figures adapted from recent reports
that show emission per density, or that have aggregated different urban areas and ranked them by emission.
Figure 4.2 doesn’t clearly show the relationships described in the text that refers to this figure. Need to clarify the point that figure is supposed to illustrate and revise accordingly. If all the icons represent GHG emissions it is confusing to include a wind turbine in the electricity generation. How does water fit into GHG emission?
Comments on specific Key Findings:
Key Finding 1. The confidence in this finding seems to be understated and could be “high confidence and very likely”, rather than medium. The first sentence in of the introduction section clearly states that carbon fluxes resulting from urban activities account for 80% of the total North American anthropogenic CO2 flux to the atmosphere. The key finding statement would be improved by making it more quantitative (e.g., replace “large proportion” with a number). Consider rephrasing the statement so the result and its reason are given together. Urban areas are a primary source of anthropogenic carbon emissions because humans and human activity are concentrated there. In addition, they are an indirect source of emissions embedded in goods and services consumed by urban dwellers.
Key Finding 2. This would be a logical point to frame the issue in context of infrastructure turnover time. Urban infrastructure is built to last decades if not longer. Major changes are difficult if not impossible and expensive as well as an additional carbon emission associated with demolition and new construction. Infrastructure improvements could be evaluated in terms of payback time. The support for this finding presents a long list of citations, but it would help the reader to present a brief summary of the overall results that are common to those studies.
Key Finding 5. The statement could be phrased positively to focus on what has been learned instead of what we don’t know. 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 emission rates and highlighting infrastructure characteristics that affect CH4 emission.
- Are the research needs identified in the report appropriate?
The research needs to have improved information on fluxes and their drivers, and improved understanding of successful mitigation (as identified in Section 4.7) are appropriate, but more details should be provided. For example, how is the urban carbon flux projection capability expected to improve in the future? How should the various approaches for estimating urban carbon fluxes be integrated and reconciled? Emerging technologies—for example, connected and automated vehicles—potentially will have a large impact on urban emissions and ought to be identified as a topic for future research. Future research needs is another place where the use of satellite observations should be noted as an emerging approach.
- Are the data and analyses handled in a competent manner? Are statistical methods applied appropriately?
Yes, though the chapter lacks a summary figure or table that presents a quantitative bottom line for carbon stocks, budgets, and transfers.
- What other significant improvements, if any, might be made in the document?
Suggestions on ways to sharpen the introduction are noted earlier. Overall the chapter would be more useful if it included specific quantitative statistics on the magnitudes of carbon fluxes/stocks and their trends at the national and continental scale. Some additional figures would be very helpful. The first additional figure would show a map of emission fluxes. A second would rank total carbon flux or per capita flux for specific urban areas. The table of urban carbon budget studies might be accompanied by a table that summarized some quantitative results from those studies.
P175, Line 4
One should not aim to *improve* urban heat islands.
P175, Line 18
Better explanation of the respiration component is needed. In part, for top-down budgeting from atmospheric measurements, respiration needs to be estimated and separated from total urban CO2 emission to get the fossil carbon component. In a full C budget if respiration is included in the emissions the carbon uptake from growing the food that is respired needs to be counted as well
It is not clear from the text whether respiration is being treated consistently with the agricultural products that are being consumed. Notably, the executive summary mentions that the biomass in agricultural systems isn’t included in the budget (presumably because it has short lifetime, though this is not explicitly stated).
P174, Line 28-32
An observation pertaining to the structure of the report overall: Energy efficiency is clearly a major driver of urban carbon emissions. However, as the chapter points out in several places, urban governments have some leverage over energy efficiency (and the carbon intensity of energy), but it is limited. Probably the majority of the capacity to change energy efficiency and carbon intensity in urban areas belongs to federal and state governments. But the report does not have a logical place to discuss these carbon management capabilities and policies, unless it is Chapter 3. As a consequence, energy efficiency improvement and transition to low-carbon energy sources are generally underrepresented in this report.
P175, Line 26-27
Punctuation, Change “;” to “,”.
P179, Line 7-8
Has any estimate been reported for Mexico?
P180, Line 34-41
Why so many examples for UK? It is better to use examples of North America.
P180, Line 21
Extra “,” before the citation should be removed.
P180, Line 29 – P181, Line 6
In the behavior section it could be noted that residents in different cities or geographical regions also have different lawn management practices (e.g., fertilization, watering).
P182, Line 23 – 30
This section could just be labeled Climate. The examples in the text cover large-scale climate as much as local climate moderated by heat island.
P182, Line 35-36
A source is needed for the national growth rate numbers.
P182, Line 31-36
This section is mainly qualitative, and more quantitative trend analyses are needed.
P184, Line 19
Presumably this sentence means positively correlated; that should be stated explicitly, or phrased as consumption increases with area per person, or is it more informative to state that consumption is inversely proportional to urban density?
P186, Line 3- P188, Line 26
The ability to discuss energy efficiency improvement and low-C energy is limited by constraining the scope to the governance capacities of urban areas. As noted on P186, lines 34-37, mitigation is strongly affected by vehicle energy efficiency, but this has been addressed at the national level almost exclusively (efforts of the CARB, and of cities, states or provinces to enact feebates would be an exception). Vehicles with lower emissions due to energy efficiency improvement can accomplish as much CO2 mitigation as transition to low carbon energy (see, e.g., NRC, 2013).
P190, Line 30-32
This statement shows that the level of confidence for Key Finding 1 (Lines 3-6 on this page) should be high confidence and very likely rather than medium confidence and likely.
P190, Line 27
TBD is not really acceptable at this stage.
P190, Line 3-6
Give quantitative estimates.
P190, Line 27
Says TBD. Was there something to fill in for the likelihood of impact, or is that component not appropriate for this finding?
P191, Line 27-30
The challenge could be expanded to include the actual data collection. It is not just analysis and uncertainty quantification of multiple carbon flux approaches that is challenging. Designing and executing urban flux studies is far from routine.
P212, Figure 4.2.
In this chapter it wasn’t exactly clear how agricultural products are treated. Agriculture is noted explicitly as a city process, but doesn’t clearly show up as an upstream process. The accounting could all be correct, it is just that the text is not always clearly indicating that agricultural biomass is being treated consistently across all sectors.