Chapter 3 achieves its stated goal to assess the contribution of the North American energy system to the global carbon cycle. It does so by citing a great many statistics from well documented and reliable sources. The chapter is clearly written, comprehensive and well organized. What it does not do is present a clear accounting for the energy sector’s role in the processes, stocks, fluxes, and interactions with the global carbon cycle. As a consequence, it is difficult for the reader to see the forest for the trees. We discuss below some options for addressing this concern.
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?
- Are the document’s presentation, level of technicality, and organization effective? Are the questions outlined in the prospectus addressed and communicated in a manner that is appropriate and accessible for the intended audience?
A great many detailed statistics are presented, but how they fit into the overall concept of the carbon cycle is difficult to discern. One solution might be a diagram (such as a Sankey diagram) that shows the energy system in the context of the overall carbon cycle or a pictorial illustration of energy system stocks and flows by source similar to Figure ES2 (e.g., https://flowcharts.llnl.gov/commodities/carbon) that represents the major components within the context of the global carbon cycle. A summary graphic might help the reader see the big picture and more readily understand how this important chapter fits into the carbon cycle. This might also help to reduce the level of detail necessary by linking the chapter with other chapters that treat the carbon cycle contribution of the energy sector.
Another opportunity is to make greater use of the Kaya identity, which is presented early in the chapter but never used as an analytical tool later on. The Kaya identity categories are actually not “drivers” of emissions change as asserted in the text (e.g., p.123, line 24; p.131, line 13), but rather ex-post accounting categories that are useful for quantitatively decomposing trends. There are several recent decomposition analyses of U.S. and global energy and carbon emission trends that could have been used in conjunction with the Kaya identity to quantitatively analyze recent trends in energy and carbon emissions in North America (e.g., EIA, 2017; Feng et al., 2015; Shahiduzzaman and Layton, 2015; Vinuya et al., 2010).
The chapter is written at an appropriate level for researchers and others with a technical knowledge of the energy sector.
The practice of presenting economic and physical quantities to 4 or 5 significant figures gives an incorrect impression of the accuracy of the estimates and is inconsistent with the discussion of uncertainty.
A minor omission but one that should have been mentioned is the fact that a portion of the nation’s N2O emissions are a result of fossil combustion.
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
- Are the data and analyses handled in a competent manner? Are statistical methods applied appropriately?
The Energy chapter accurately reflects the scientific literature on the subject. The authors know the relevant data sources well, understand them thoroughly, and use the data appropriately. The presentation of data is comprehensive, accurate, unbiased, and well referenced. Two areas that could be improved are (i) the potential for mitigation of the sector’s carbon emissions and (ii) the range of future scenarios presented, each discussed below.
Statements are made about the existence of cost-efficient energy efficiency technologies, followed by selected examples of such technologies. A more effective approach would have been to refer to the peer-reviewed literature on the subject of mitigating carbon emissions from energy use. There is enough that is new since the last report to make this worth considering. Summarizing such studies would better document the potential for mitigation and would provide a context for discussing the extent to which mitigation actions are likely to be cost effective.
The future scenarios presented do not include any that seriously attempt to meet national and international goals for limiting global warming. Carefully constructed scenarios are available from credible sources such as the IEA, EIA, IIASA and IPCC. The Global Energy Assessment is one such comprehensive study that certainly should have been referenced (GEA, 2012). Scenarios that attempt to meet national goals also provide useful information about mitigation potentials and the roles of energy efficiency, low carbon energy sources, prices and behavior in managing carbon flows from the energy system.
The discussion of management of the carbon cycle is accurate but should include a discussion of the time constants for change for different management actions. Section 3.7 on carbon management overlooks the most important effort of all: the UN Framework Convention on Climate Change (UNFCCC), and most recently the Paris Accord. The discussion of the efforts and policies of the three countries should include the Nationally Determined Contributions (NDCs) of Canada and Mexico and the ambiguous position of the U.S., especially now that the U.S. is the only nation not to sign on to the Paris Accord.
The omission of the economic dimension of the energy sector’s role in the carbon cycle is very surprising and a serious oversight. This is especially so given that economics is not included in the chapter on social science perspectives. The economics literature on the energy sector’s carbon emissions and the potentials for mitigation, sequestration and other sinks is vast and deserves attention, especially if assertions are made about cost-effectiveness.
- Are the findings documented in a consistent, transparent and credible way? 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?
All of Chapter 3’s findings, except those pertaining to the potential for cost-effective mitigation, are well-supported by the evidence presented and the evidence has been well and clearly documented. The key messages are the very large role of the energy system as a source of carbon and the large potential for mitigation. The energy system’s major role as a source of carbon is thoroughly documented with data from the most authoritative sources. The relatively non-quantitative treatment of mitigation potential discussed above does not provide adequate support for the existence of a large potential for
cost-effective mitigation. However, as noted above, the literature on this subject is substantial and could have been used to support such a finding. Including evidence from the literature of energy economics and engineering would allow a valuable discussion of the cost-effectiveness of various levels of carbon mitigation and their dependence on future technological advances.
- Are the research needs identified in the report appropriate?
Although there is not a distinct listing of research needs, the research needs identified are appropriate and well supported by evidence. However, once again, the inventory is not complete because the quantification of mitigation potential and its cost-effectiveness has been left out. There are important research needs concerning technological, economic, and behavioral potentials. In addition, the challenge of achieving a large-scale energy transition, which appears to be necessary to meet ambitious carbon mitigation goals, poses many new research questions.
We recommend the following improvements to the chapter:
- In Key Findings 1 and 2, quantitatively connect the energy system to the carbon cycle. For example:
- Since 17XX the North American (or U.S.) energy system has emitted XXX petagrams of fossil C into the atmosphere. Of that amount, YYY petagrams remain increasing the global concentration of CO2 in the atmosphere by ZZ parts per million. This represents W% of the total increase in atmospheric CO2 since 17XX.
- The North American (or U.S.) energy system emitted x petagrams of fossil C in 2016(?). Of that amount, y petagrams will remain in the atmosphere through 2zzz, (account for the fate of the rest).
- A third finding should use the results of recent decomposition analyses (e.g., EIA, 2017; Feng et al., 2015; Shahiduzzaman and Layton, 2015; Vinuya et al., 2010) to quantify the factors responsible for recent trends in carbon emissions from the North American energy system.
- Use the graphical representation of decomposition analysis to effectively illustrate the factors responsible for recent trends in carbon from the U.S. energy system.
- Construct a Sankey diagram showing the sources (e.g., sector, energy type) of carbon emissions from the U.S. energy system and their fates at a very general level of detail (e.g., similar to Figure ES2; for example, see link above).
- If possible, a fourth finding should address the kinds of changes in the energy system that would be necessary to reduce the North American (or U.S.) energy system’s emissions of carbon to levels consistent with international objectives for constraining the increases in global average temperatures. Any reasonable and scientifically supported level could be chosen (e.g., 2°, 2.5°, 3° C). Appropriate research needed to support such a large-scale energy transition should be identified.
- Section 3.7 discusses carbon management decisions at international, national, and state and urban levels but provides no quantification of the impacts of these decisions on the carbon cycle. A quantitative discussion of impacts should be added. Any quantification will be uncertain to some degree, but estimates exist in the literature, and there are ways to describe uncertainty.
P110, Line 152
The chapter contains a wealth of statistics, which makes it difficult to see the forest for the trees. Perhaps a figure illustrating the role of energy systems in sources (mostly) and sinks (to a much lesser degree) would be helpful. It could be accompanied by a discussion of how energy systems fit into the overall framework of the carbon cycle. In a similar vein, the chapter introduces the Kaya Identity as an organizing concept but then doesn’t use it, either as an organizing concept for presenting status and trends or analyzing them. More specific recommendations are made separately.
P110, Line 152
The treatment of mitigation is unsatisfying for several reasons:
- impacts, past and potential future, of management actions are not quantified;
- the critical role of increased energy efficiency in all sectors is given inadequate attention, especially since there have been and are important initiatives in place in North America;
- the challenge of transforming the energy system to a low carbon system (energy transition) is also not adequately discussed and analyzed;
- none of the projections of future energy use come close to achieving climate stabilization goals, a critical issue for the future of the energy system.
If North America seriously attempts to mitigate climate change, the energy system and its role in the carbon cycle will change profoundly. This should be a key topic of the chapter, but it is not.
Add footnote on energy units.
P111, Line 10
What is land-based carbon?
P112, Line 9-29
The historical context discussion reads like it’s all about recessions. Recessions have been important but so have energy prices and energy efficiency regulations. Attributing changes in carbon emissions via the Kaya identity would show this. Specific recommendations on sources of decomposition analyses of trends in carbon emissions from energy are suggested separately.
P113, Line 10 – P114, Line 36
The focus on proved reserves gives a misleading impression of the potential for future carbon emissions from combustion of fossil fuels, especially for petroleum. As the box on the subject acknowledges, proved reserves are a very conservative measure of potential future resources. There is a large literature on this subject that could be summarized as follows: Proved reserves are mainly a stock that energy entities maintain to insure adequate production in the near future. At a global scale, for example, proved oil reserves relative to current production have changed very little over decades. Resources have various definitions, but as a very broad generalization, technological advances have consistently overcome depletion of fossil fuel resources. This is likely to continue. Why is this important? Utilizing resources beyond proved reserves holds enormous potential for increasing the carbon concentrations in the atmosphere.
P114, Line 21-27
Renewable generating capacity is mentioned but not renewable resource estimates. This might be a useful addition with relevance to how the carbon cycle might be changed.
P116, Line 19-22
The residential and commercial emissions of CO2 do not seem to include emissions from electricity generation, or at least they are not consistent with the EIA’s data, which indicate 1.1 Pg for residential and 0.9 Pg for commercial in 2013 [https://www.eia.gov/environment/data.php#summary] .
P118, Line 2
This opening sentence is one of many examples of a “topic sentence” that doesn’t really convey the main point of the section.
P119, Line 7
“As demonstrated,…” is arguable and unneeded.
P119, Line 11-20
Why does this section not mention technologies identified by the EPA/DOT rulemaking for increasing light-duty vehicle fuel economy through 2025, and medium and heavy-duty fuel economy as well. This is all thoroughly documented in the rulemaking and supporting documents. And what about other transportation modes?
P123, Line 16-30
Isn’t the right way to present information on the carbon cycle role of biofuels to quantify the emissions from biofuel combustion as a source and the production as a source and sink?
P124, Line 34-35
“more than an estimated 18.6 million” is confusing unless the intention is that 18.6 is an absolute lower bound.
P124, Line 31 – P126, Line 9
Shouldn’t this section be attempting to quantify the sources, sinks and flows of carbon in the biofuel system, including uncertainty bounds?
P126, Line 11
What is a “feedback mechanism scenario”?
P127, Line 6
Please cite the projection referenced.
P128, Line 1
The Kaya equation can also include sectoral structure (summing over sectors). Why not add that? It is disappointing that the authors do not use the identity or cite the work of others using the identity to decompose trends into components.
P130, Line 31
Total vehicle miles traveled in the U.S. has increased every year since 2011 according to the Federal Highway Administration (FHWA1).2 We are not able to match the other data in this paragraph (e.g., fuel economy/energy intensity) to the FHWA data either.
P131, Line 14
This section deals with carbon intensity and refers to F/E (amount of CO2 emitted per unit of energy produced). The authors do not provide U.S. data for carbon intensity of transportation, however. This will depend on the carbon intensity assigned to ethanol, which is controversial/uncertain.
P132, L 32 - P133, Line 8
Should CCS be considered a carbon sink in the framework of the carbon cycle or a determinant of carbon intensity (as is done here)?
P136, L 36 – P137, Line 1
The rulemaking documents provide reasonable estimates of the carbon source reductions that would be achieved by GHG emission regulations under the Clean Air Act.
P137, L19 – P138, Line 11
California’s comprehensive GHG reduction plan and legislation deserves mention here along with other states that have such plans (less comprehensive and potent, in my opinion). California has a cap and trade system, Zero Emission Vehicle and Low Carbon Fuel standards, among a suite of comprehensive policies.
P138, Line 12 – P140, Line 18
None of the scenarios discussed correspond to a serious attempt to reduce GHG emissions to levels that would stabilize global warming at target levels proposed by climate scientists (e.g., 2°C, 2.5°C, etc.). All are variations on business as usual. In a report of this nature at least one serious mitigation scenario should be included, as such scenarios do exist.
P141, Line 6-12
The list in this paragraph has to include increasing energy efficiency. And in addition to decreasing the use of carbon-intensive fuels it should include a transition to low-carbon energy sources.
P142, Line 1 – P143, Line 5
It is good that this sidebar acknowledges the different definitions of resources and reserves. However, the discussion in the text focuses almost exclusively on proved reserves, which is a less relevant measure than the other discussed in the sidebar.
P144, Line 18-19
The 3% number is relevant for electricity production but because the CO2 reduction for CH4 use in transportation vehicles is only about 15-20%, only about a 1% leakage rate will eliminate GHG benefits.
P145, Line 32-33
See previous comment.
P151, Line 30-33 (also on P111).
It seems odd to rate a finding that net carbon effects may be positive, negative or neutral as having “high confidence”. The authors are perhaps saying they are certain that we don’t know the net effect for biofuels as a whole. Likewise, with respect to CH4 as a fuel (overwhelmingly of fossil origin at the present) we know for certain that fugitive emissions reduce the overall carbon benefits. Considering the report recognizes that biofuels vary in their carbon impacts, we suggest rephrasing this finding or present it in a different way.
This page intentionally left blank.