Appendix A. Comments on Appendix 5: Frequently Asked Questions

The Committee found the Frequently Asked Questions (FAQs) Appendix to be a useful resource and complement to the main text of the draft NCA4. The FAQs generally provide relevant information that is easily accessible to a non-technical audience.

For the FAQs that relate to specific draft NCA4 content, the provided information is generally consistent with the main text and it is useful when the main text is referenced. However, the emphasis and specific information sometimes differs between the FAQs and the main text. The FAQ information also seems out of date in some cases and should be updated to reflect the current state of knowledge.

The challenge of including a FAQs section is the inevitable feedback that there are other important and appropriate questions that could be addressed. For instance, there could be additional questions on islands and whether climate change impacts them differently, some of the other health-related challenges associated with a changing environment, and other topics. Draft NCA4 topics that do not get much treatment in the FAQs include water, agriculture, energy, ecosystems, and the interconnections between ecological and social systems. Additionally, there are no region-specific FAQs. There could also be an FAQ on scientific consensus. The included FAQs, “Why are scientists so certain that human activities are the primary cause of the recent global warming?” and “How reliable are the computer models of Earth’s climate?” relate to important issues of scientific consensus, but do not address it directly. The lack of scientific consensus is a myth that could be addressed directly.

While a wide range of FAQs are relevant to the draft NCA4 and communicating climate science, the draft NCA4 authors could give further consideration to how FAQs are selected for inclusion. Are they intended to largely address climate science and the types of topics included in the CSSR report, or are they intended to be aligned with the content of the NCA report itself? Or both? Most of the FAQs focus on climate change science rather than impacts, risks, and responses that are the primary focus of the draft NCA4. Both types of information have value, but it may be useful to be more focused in what is included in Appendix 5 of the draft NCA4.

Technical review of each FAQ was provided by a committee member or a consultant with relevant expertise. Only FAQs where changes are recommended are listed in this section. Editorial comments for some FAQs are included in the line comments (see Appendix B). Regarding the inclusion of citations in the FAQs, it is recommended that they either be omitted or listed at the end of the FAQ, so as not to break up the text for the intended audience. As such, it is broadly suggested that all references cited in the FAQ answers be removed.

COMMENTS ON INDIVIDUAL FREQUENTLY ASKED QUESTIONS

How do we know Earth is warming?

Improved accuracy of this FAQ is needed. While some suggestions are provided here, it is recommended that a subject matter expert be consulted.

Early in the answer to this FAQ, it would be more accurate to note that earth is generally warming. The current text that there is warming could be misinterpreted and obscure the changes in extremes.

The statement, “Decades of temperature readings from thermometers and other scientific instruments around the world” (page 1444, lines 3-4) understates the robustness of the understanding of warming since the language arguably at least excludes data from satellites, boreholes, geological data, and other non-instrumental data sources. A more comprehensive statement is needed. Something similar to “multiple independent lines of evidence point conclusively to a warming planet. These include thermometer measurements…etc.” would better reflect current understanding.

The phrase “volunteers or automated instruments” (page 1444, line 11) is a strange choice of language in that it excludes non-volunteer (i.e., professional) human observers, of which there must be some; if not now, then in the earlier years of the thermometer record. Also, mentioning volunteers may tend to devalue the reliability of the measurements. It would be better simply to say “At thousands of ground-based weather- and climate-stations around the world, instruments record.”

Page 1444, lines 12-16 states, “Observations from these stations agree with readings from satellite instruments that measure land temperatures from space.” Measuring land surface temperatures from satellites is extremely challenging, and reviewers are unaware of any datasets showing multidecadal global trends based on this approach. Would it be more appropriate to say “lower atmosphere” instead of “land”? The statement also refers to temperatures, not temperature trends, and thus may be strictly speaking true. However, readers could infer it to refer to temperature trends, and thus be misled. This should be clarified.

In the discussion of land ice in this FAQ, it is suggested that the Antarctic and Greenland ice sheets be specifically mentioned. It is recognized that these are sometimes included in the term “glaciers” but they merit specific, independent mention.

For Figure A5.2, the concept of “mass balance” in panel (f) may be too technical for the target audience without explanation. For panel (i) wildfire, the length of record shown is too short to establish a meaningful trend, and in fact there does not appear to be much of one. Lengthier comments on this figure are provided in response to the main text of the draft NCA4, where the graphic appears at Figure 1.1.

How is recent global warming different than warming in the past?

The supporting text for this FAQ implies that only warming epochs have been linked with orbital cycles, while attributing cooling epochs specifically to volcanic eruptions. It is the understanding of the reviewer that both warming and cooling epochs are forced by orbital cycles,

with either a lack of volcanic eruptions or many volcanic eruptions playing primarily contributory roles to warming or cooling epochs. Orbital variations do not change the total amount of sunlight reaching the earth over the course of a year, but they affect its distribution by season and latitude, which then drive the growth or decay of high-latitude ice sheets and the strong positive climate feedback loops that go with them. The literature on this topic should be revisited by the draft FAQ authors to ensure the state of knowledge is accurately conveyed and the text should be revised to clarify the relationship between orbital cycles and both warming and cooling periods. Rewording of the supporting text (page 1447, lines 13-15) could state: “Warming and cooling epochs were driven by natural variations of the earth’s orbit that altered the amount of sunlight that reached the Earth’s Arctic and Antarctic regions, driving the retreat and advance of massive ice sheets. Additionally, quiescent or active periods of volcanic eruptions also could contribute to warming or cooling epochs, respectively.”

Page 1447, line 8, the phrase “at least” should be deleted. Long-term paleoclimatic time series from ocean sediments clearly show that the 100,000-year cycle extends back to roughly a million years ago; before that, a 40,000-year periodicity was dominant.

What’s the difference between global warming and climate change?

The supporting text for this FAQ begins stating that “global warming” means a period when Earth’s annual average surface temperature is increasing (page 1449, lines 12-13). Since global warming actually refers to the phenomenon of warming and not a time period, this statement is not technically accurate and the language should be revised.

The explanation of the difference between “climate change” and “global warming” currently states, “The entire globe isn’t warming uniformly though” (page 1449, line 17). While this is true, this is not why the term “climate change” has a different meaning from “global warming.” The main reason is, as explained elsewhere, the former term encompasses a wide range of phenomena in addition to an increase in global temperature. The sentence that follows (page 1449, lines 18-21) furthers this misinterpretation and it is recommended that it be deleted from the FAQ. The phrase “side effects” (page 1499, line 22) may seem trivializing. It is suggested something more specific, such as “associated consequences” be used. It is also unclear why this FAQ states that “climate change is less precise” (page 1450, line 6) and it is suggested that this statement be deleted.

Were there predictions of global cooling in the 1970s?

The primary point made in the italicized answer to this FAQ—that there is a preponderance of scientific literature noting warming occurred during this time period—seems to get lost, or even contradicted, in the supporting text. To approve readability and accuracy, it is suggested that the supporting text (beginning page 1450, line 25) be revised to something like “ending of ice ages led a few scientists in the 1970s to contemplate that the current warm interglacial period might be ending soon leading to a new ice age over the next few centuries. These few speculations were picked up and amplified by the media. But at that time there were far more scientific articles describing how warming would occur from the increase of greenhouse gases from the burning of fossil fuels (Figure A5.5). Scientists continue to study.”

Also, for clarification, it is suggested that the last sentence on page 1450 (line 30) be revised to “the composition of the atmosphere in such a short period of time relative to natural orbital processes that the next ice age has now likely been delayed.” The NCA4 authors could also consider noting that this delay could be tens of thousands of years (Clark et al., 2016; full citation in “References” section of this review report).

What are greenhouse gases?

Rather than (or perhaps in addition to) listing greenhouse gases on page 1451, lines 12-13, it is suggested that the text note that the buildup of greenhouse gases in Earth’s atmosphere is the primary driver of recent warming and other climatic changes. It would also be useful to indicate that the anthropogenic greenhouse effect is an incremental increase in the existing (natural) greenhouse effect.

The supporting text could indicate that the molecular structure of greenhouse gases causes them to absorb terrestrial infrared radiation (unlike N2 and O2, the most abundant gases in the atmosphere).

Why are scientists so certain that human activities are the primary cause of recent global warming?

Important information is missing from this FAQ and it is suggested that additional experts be consulted to ensure the accuracy and completeness of the included materials. Additional topics that should be included are: (1) satellite measurements show no trend in energy output of the sun, and (2) paleo data show that warming observed since around 1900 is extremely unusual in the contact of the last 500-1,500 years.

On page 1453, lines 2-5, the provided argument suggests incorrectly that attribution of warming to human activities relies strongly on models. Paleo data show that there was actually a slow cooling trend in the centuries before the start of fossil fuel use, so this should be mentioned, with modeling studies discussed as a way to confirm that this observed cooling was due to orbital forcings.

The argument about the human origin of increased atmospheric greenhouse gases given on page 1453, lines 9-11, logically should be presented first, not last.

Citations that could be reviewed to support this FAQ include Marcott et al. (2013) and Marsicek et al. (2018).

How do we know that carbon dioxide is the main driver behind global warming?

The answer provided in response to this FAQ does not directly address the question. Focus on the recent rate of increase in atmospheric carbon dioxide does not in itself demonstrate that humans are the cause of the rise in temperature. Important arguments that should be noted here include the paleo record showing a very strong correlation between atmospheric carbon dioxide concentration and temperature going back hundreds of thousands of years. This would be a much more effective figure than what is currently illustrated in Figure A5.8.

Alternatively, the FAQ itself could be modified. Examples could be, “How do we know humans are changing the amount of carbon dioxide in the atmosphere? Or “How are humans changing the amount of carbon dioxide in the atmosphere?”

What role does water vapor play in global warming?

The explanation for this FAQ is difficult to understand and inaccurate in some places. On page 1456, lines 9-10 the FAQ response states, “water vapor cannot be the driver of global warming.” One might argue that water vapor actually is a driver of global warming, since it is the dominant greenhouse gas. The point the FAQ authors may be trying to convey is that humans have no direct control over how much water vapor is in the atmosphere, although there is a strong indirect influence since evaporation and water vapor holding capacity increases with temperature. This should be stated more directly.

The language included on page 1456, lines 11-19 is likely too technical for the intended audience and it is recommended that it be deleted. In this text, line 12: “does accumulate” should probably be “does not accumulate” and line 12-13 “water vapor that would otherwise evaporate from the surface” is confusing because water vapor is already evaporated and direct versus indirect changes in water vapor will be difficult to understand for most readers. It might be better to say that water vapor is indeed the strongest greenhouse gas, with respect to current warming of the Earth’s surface, but by itself it is not a strong driver of current and expected changes. Water vapor is not produced by combustion of fossil fuels, but it tends to be an amplifier of the warming effects caused by rising concentrations of carbon dioxide, methane, and nitrous oxide because a warmer atmosphere can hold more water vapor, thereby strengthening the associated greenhouse effect. See Myre et al. (2013) for quantification for the strength and sign of the water vapor feedback.

Figure A5.9. does not effectively communicate the main messages of this FAQ response and the inset graph is incomprehensible. It is recommended that this figure be deleted.

The response to this FAQ would be improved by simplifying it to begin with the second paragraph (page 1456, line 20) and also noting that humans cannot directly control the amount of water vapor in the atmosphere, although humans indirectly affect water vapor as global temperature rise from increasing greenhouse gas concentrations.

Have the sun or other natural factors contributed to the observed global warming of the past 50 years?

For page 1457, lines 10-11, it may be simpler to say directly that there is a cooling tendency from volcanic activity. According to the IPCC Fifth Assessment Report (IPCC, 2014), the estimated net effect of solar variability and volcanoes over this time period is essentially zero (but if anything a tiny warming tendency; IPCC Synthesis Report Figure 1.4, see also Chapter 2 of the draft NCA4). There may be a slight cooling tendency from orbital forcing, but over 50 years it would be minuscule.

For the discussion of cosmic ray effects on page 1458, lines 9-14, is it known whether this cosmic ray effect would theoretically produce a warming or cooling tendency in the most

recent 50-60 years? If the effect is known to be a cooling tendency, then one does not need to argue that the magnitude is small.

For page 1458, lines 10-16, the information should be rephrased to help the reader. The aerosol effects of volcanic eruptions cannot be responsible for a 50+ -year warming trend, because that would require a trend of gradually diminishing aerosol loading from a decreasing sequence of eruptions, which has not been observed. On line 14, “eventually” should be replaced with “after a few years.”

How are El Niño and climate variability related to global warming?

In the phrase “Are not caused by humans” (page 1458, line 25) it is recommended that “but their frequency and/or intensity might be affected by human greenhouse gas emissions” be added.

The statement “However, there is much uncertainty as to how climate change will affect ENSO events.” on page 1459, lines 26-27, is a weak formulation that focuses on what is not known. It would be more effective to restate as, “However, these events might be affected by climate change.” Similarly, page 1459, lines 30-32 provide an unnecessarily weak formulation. It would be more informative to say something to the effect that new research is shedding light on the many factors influencing how climate change affects the ENSO cycle.

The discussion about the spring transition in this FAQ does not seem particularly useful and could be omitted.

Finally, stating that “ENSO is a complex system that is controlled by hundreds of factors” (page 1459, lines 30-32) does not provide useful information to the reader interested in understanding the relationship to climate change. It would be more effective to describe studies that consider how ENSO may change under global climate change.

It is also important to note that ENSO has varied in the past, according to proxy evidence.

Is the global surface temperature record good enough to determine whether climate is changing?

This FAQ response is generally accurate, but the authors should consider that 300-year records have their own issues and are not by themselves adequate to measure global surface temperature warming. Even though this FAQ does not explicitly say that these long records have more power than they actually do, it could be inferred from the text. Instead, it might be more appropriate to emphasize records beginning in the latter part of the 19th century, from which global surface temperature estimates have been derived.

It is recommended that this FAQ answer indicate that the amount of warming varies with location, but the vast majority of Earth’s surface has warmed since 1901. Since the 1950s, for the approximately 70% of Earth’s surface that is ocean, all latitudes of each ocean basin have exhibited warming. The FAQ answer could also note that since 1980, the rate of warming has increased, and since 1980 each decade’s global surface temperature has been successively warmer. All of the ten warmest years have occurred since 1997.

How is climate projected to change in the future?

This FAQ response emphasizes that warming will continue, but it depends on the emission scenarios, and precipitation will also increase, with wet areas getting wetter and dry areas getting drier, and heavy precipitation will be more frequent. These are all key points to convey and easily understood by the intended audience.

In the primary response to the FAQ (page 1462, lines 18-23 and page 1463, line 1) as well as in the caption of Figure A5.14, it would be clearer to add “emissions” in front of scenarios so it would not be mistaken as scenarios that are made up with no scientific basis. It is also important to point out that the U.S. temperature and precipitation changes will have large regional variation.

A robust change under warming is that more precipitation will fall as rain rather than snow, which reduces snowpack. This simple concept could be introduced in this FAQ question.

How reliable are the computer models of Earth’s climate?

This FAQ is scientifically accurate, but it would benefit from further clarification of some key points.

In the text provided as the primary response to this FAQ (page 1464, lines 11-17), it may not be clear to the audience what “broad features” means. Giving an example such as jet streams, continental temperature, and precipitation patterns could help the audience to visualize what “broad features” can now be accurately reproduced by models.

On page 1464, lines 19-20, it is recommended that additional text be added: “By dividing the atmosphere, land, and ocean into smaller spatial units to solve the equations, climate models capture.” Breaking up the atmosphere, land, and ocean into smaller units does not make sense unless it is noted that this is how the equations are solved.

For page 1464, lines 20-21, it is suggested that examples be added other than atmospheric-related variables, such as ocean currents and soil moisture. These are also relevant to many people and it is important to convey that climate is not all about the atmosphere.

The FAQ might also point out the many paleodata-model comparisons in which earth system models have been shown to successfully simulate major features of past climates (e.g., simulating a cold last glacial maximum, in part caused by low carbon dioxide levels at the last glacial maximum) and used to constrain estimates of climate sensitivity.

Figure A5.15 is not the right figure to use in correspondence to the sentence that cites the figure. Page 1464, lines 21-23 focuses on how well models can simulate large-scale climate features, but Figure A5.15 is about how well models can reproduce the observed trends in warming over the United States. If the main idea is to convey that climate models can now reproduce observed trends, then Figure A5.15 is not the best example. To a general audience, the disagreements between the observed and simulated temperature change in the Canadian Prairie and Mexico are enough to disqualify the statement in line 13 (page 1464) that “today’s climate models can accurately reproduce broad features of past and present climate.” After all, what qualifies as “accurate” is in the eyes of the beholders. Furthermore, the title of Figure A5.15, “Climate Models and Temperature Change,” does not make sense. If this figure is to remain, a better title would be “Observed and model simulated temperature change.” It is recommended

that Figure A5.15 be replaced with a figure comparing observed and simulated large-scale features such as annual mean temperature and precipitation patterns rather than observed and simulated trends.

Can scientists project the effects of climate change for local communities?

In the sentence “(1) A statistical approach where strong, local observations are used” (page 1465, lines 21-22), it is unclear what “strong” means. Common statistical methods employ historical observations of whatever amplitude the anomalies are.

This FAQ would also benefit from some additional context. As a backdrop, it seems worthwhile to note that usually local scale conditions are strongly related to larger scale climate anomalies, whether it be calculated using statistical methods relating local to larger scale, or using a fine scale dynamical model that is driven by larger scale model guidance.

It should also be noted that regional dynamical models have biases and errors, just as do statistical methods. One can make local/regional projections, but uncertainties from global model simulations remain (climate forcing [emissions, etc.], climate sensitivity and other forms of model uncertainty, natural variability) on top of uncertainties introduced in the downscaling process.

What are key uncertainties when projecting climate change?

This FAQ is generally written at an appropriate technical level and is scientifically accurate, but some clarification is needed.

On page 1467, line 20, “climate sensitivity” should be defined for the audience or replaced by a less technical explanation such as “differences in how climate responds to doubling of greenhouse gas concentrations.”

Line 13 on page 1467 could be misleading. The phrase, “different models produce slightly different projections of change,” is only accurate when referring to global mean change, but audiences tend to think about climate change in terms of regional changes, so they may be misled to think that different models produce slightly different projections of regional change. This could be clarified by adding “global mean” in front of “change” as a qualifier. Also, it is suggested that “slightly different projections” be revised to “small differences in projections,” since slightly is used to describe a very small difference, but certainly global mean warming between 1.5-4^oC is a rather large range.

Figure A5.17 does not correspond to the description on page 1467, lines 13-14 where the figure is cited. The sentence in lines 13-14 emphasizes the range of projected outcomes due to variability, but Figure A5.17 shows the fraction of variance due to three sources of uncertainty, not just variability, and not projections but rather fraction of variance. Also, it is unclear whether Figure A5.17 is showing the fraction of variance derived for global mean temperature or some specific variable such as U.S. mean temperature. This information should be provided in the figure caption because the fraction changes with spatial scales and regions.

Is it getting warmer at the same rate everywhere? Will the warming continue?

The text provided is accurate; a few additional points would strengthen this FAQ.

It could be noted that high latitudes should warm more than other latitudes given current understanding. Also, coastal and island regions should warm less than interior continent regions.

Regarding the part of the FAQ on “Will the warming continue?,” it might be useful to point out that because Earth’s system still has more energy entering than leaving (oceans still warming over a very deep strata from surface to depth), global warming has not yet equilibrated to the load of increased greenhouse gases that have already accumulated in the atmosphere. Some greenhouse gases have long lifetimes (e.g., carbon dioxide resides in the atmosphere for a century or more). Thus, even if the emissions of greenhouse gases were to be sharply curtailed so as to bring them back to natural levels, Earth is committed to continued warming of more than 1^oF by 2100.

Some plausible scenarios of future greenhouse gas emissions would have global surface temperature increasing by 4-9^oF by 2100. Scientists think that warming in excess of 3.6^oF (2^oC) would have particularly dangerous consequences, placing some of these plausible scenarios into this very serious category.

What do scientists mean by the “warmest year on record”?

It is suggested that the first sentence (page 1470, lines 8-9 and 12-13) be specific and refer to the global surface temperature, which includes land and ocean surface.

The global surface temperature is affected by natural variability in addition to climate change; e.g., El Niño years are generally unusually warm and La Niña years are generally cool. Thus, it is unrealistic to expect each year to set a new record and this should be noted.

How can scientists project climate changes decades in the future when they cannot predict weather more than 2 weeks in advance?

Weather is an individual storm and climate is the average of many storms over many years. In the definition of climate provided, the word “average” is the key ingredient and “30 years or more” may be more appropriately explained as “over multiple years or decades.” It is suggested that the text on pages 1471 be edited to read, “The climate—the average weather over multiple years to decades—varies far less.”

Was there a “hiatus” in Global Warming?

Page 1473, lines 23-24, the statement is not quite right, and could by corrected by revising to, “Temporary speedups have also occurred, most notably from the early 1900s to the 1940s, and from the 1970s to late 1990s.”

On page 1473, line 31 the text should distinguish an uninitialized multi-model average from initialized decadal climate predictions; the latter do show skill in simulating the slowdown. It is suggested that “less than what was expected by the models,” be replaced with: “less than the average of models run with the traditional long-term increases of greenhouse gases. However,

models started from specific observed conditions and run for 10 year periods successfully simulated earlier speed-ups and the more recent slowdown in the rate of surface warming.”

Once again on page 1473, line 32, the different methodologies of simulating climate (uninitialized versus initialized) need to be clarified. Wording could be revised to “are more consistent with the traditional long-term model simulations with increasing human-produced greenhouse gases and have been attributed to human influence.”

What is an extreme event?

The introductory sentence (page 1475, lines 6-7) is not needed given that the definition is provided in the next sentence and stated much more succinctly there.

On page 1475, line 15 the term “compounding events” is jargon and is not clear. Revised wording that could be used is, “Conversely, it is also possible for several types of extremes to occur close to the same time (e.g., a sequence of hot days that occur during dry conditions that make both worse; or several rainfall events occurring one after another that, taken together, produce flooding) that may not be considered extreme individually, but may cause.”

Does global warming affect extreme weather?

The first supporting paragraph for this FAQ (page 1475, lines 26-29) would benefit from inclusion of greater context about the role of increasing greenhouse gases in the noted observation of changes in extremes. Rewording could be: “As average temperatures have warmed due to increasing human-produced greenhouse gases, extreme high temperatures have become more frequent and extreme cold temperatures less frequent. In the United States, more than twice as many daily high temperature records, as compared to low temperature records, were broken over the 2001-2012 period. With ongoing increases of greenhouse gases, the chances for extreme high temperatures will continue to increase, with the occurrence of extreme low temperatures becoming less common. However, even with much warmer average temperatures later in the century, there will still be occasional record cold snaps, though occurrences of record heat will predominate.”

Including physical reasons for increasing extremes makes statements more robust. On page 1475, line 30 it is suggested that the text be modified to “Also, because warmer air can hold more moisture, in many areas heavy rainfall events have become.”

The Knutson et al. (2017) reference cited with this FAQ is not an appropriate reference for the result it refers to.

How is climate change affecting society?

Figure A5.26 could be improved by including oceans, which would make it more consistent with the main text of the draft NCA4.

Figure A5.27 is hard to understand at first glance and thus not very effective in supporting the FAQ. The FAQ authors should consider replacing it.

It may also be worthwhile to refer readers to the “Does global warming affect extreme weather?” FAQ, particularly the discussion of extreme cold temperatures. Explaining that extreme cold weather is not proof that global warming is not happening could also be added. If an effective illustration to convey this message could be found or developed, that could also be useful.

The authors could reconsider how this FAQ is framed. The answer to “are there any benefits to climate change” is rather negative and does not frame mitigation and adaptation in the more solution-orientated nature recommended for the draft NCA4 as a whole. In some cases, mitigation and adaptation efforts may be business and innovation opportunities.

What is the social cost of carbon?

The social cost of carbon is most appropriately used to offer estimates of the economic value of changes in emissions generated by other policy interventions. It is not intended to frame mitigation development. This message should be better conveyed in the FAQ. A reference that could be used to inform this FAQ language is the “Valuing Climate Damages: Updating Estimates of the Social Cost of Carbon Dioxide” (NASEM, 2017). The social cost of carbon depends on many social values such as pure rate of time preference, relative risk aversion, etc. So, appropriate application includes a range of social cost of carbon estimates. An example of the use of the social cost of carbon would be in determining the value of increased vehicle gas mileage standards. Another type of example that could be included in the FAQ (perhaps in a more simplified form) would be siting a windmill farm that will replace fossil fuel energy. Since the effect of a single wind farm would be marginal relative to global carbon emissions, the economic value of that action can be estimated as the product of an estimate of the social cost of carbon times the reduction in carbon emissions from fossil fuel sources.

What is the difference between climate change mitigation, adaptation, and resilience?

On page 1481, lines 19-21, an example of mitigation benefits from natural systems would benefit from a longer list (“tropical forests” is not going to resonate with many U.S. residents, and thus this approach sounds like something that U.S. residents cannot participate in). A sentence pointing out that both protection and restoration of marshes, forests, and wetlands can increase carbon sequestration and storage would be more inclusive.

Is timing important when combating global warming?

It is suggested that this FAQ response lead with explaining that waiting to reduce greenhouse gas emissions will require a more rapid response later that will also be more expensive. A key point that should be added to this FAQ answer is that transient and equilibrium temperature change is driven by cumulative emissions. This could be conveyed by explaining that year-to-year emissions do not matter if the sum over a determined future is satisfied.

Are there benefits to climate change?

The answer to this FAQ should emphasize that benefits are short term and depreciate significantly in a warming world.

Are some people more vulnerable than others?

This FAQ should note that some subpopulations are more affected by environmental exposures, such as air pollution or extreme heat, in the present day. Such disparities are not limited to the future under a changing climate.

How will climate change impact economic productivity?

It is unclear what is meant by point two in the FAQ answer, “private physical capital that firms rely on to produce goods and services, such as equipment and property, will be impaired as a result of climate change” (page 1484, lines 27-28). Revision for clarity is suggested.

Can we slow or even reverse global warming?

It is suggested that the answer to this FAQ provide clearer statements first about slowing warming, and then about reversing it. Reducing the rate of emission of greenhouse gases would slow warming. That is certainly possible. To reverse warming, the amount of greenhouse gases in the atmosphere must decrease, but it is not necessary to reduce to 1750 levels, as stated on page 1485, lines 21-22. If humans stop emitting greenhouse gases, natural processes will remove carbon dioxide from the atmosphere too slowly to have any significant near-term effects on warming. Natural and technological means might be used to remove carbon dioxide (see also the FAQ related to geoengineering), but application at the necessary scale is difficult. The discussion of adaptation is off-point and should not be included in response to this FAQ.

Can geoengineering be used to remove carbon dioxide from the atmosphere or otherwise reverse global warming?

On page 1487, lines 11-16, “planting forests” is only one natural pathway for removing carbon dioxide from the atmosphere. A more general formulation would be to say that removal of carbon dioxide from the atmosphere could be undertaken by applying land management methods that increase carbon storage in forests, soils, wetlands, and other terrestrial or aquatic reservoirs.

On page 1487, lines 22-29, it is important to emphasize that solar radiation management does not reverse global warming caused by the presence of carbon dioxide and other greenhouse gases in the atmosphere (which is what the question asks). Instead, it introduces another forcing which partially cancels some of the effects of increased greenhouse gases. This is an important distinction that should be made explicitly.

Is Antarctica losing ice? What about Greenland?

This FAQ response should discuss the new ideas related to the effects of different ice sheets on different parts of the United States.

The statement: “The West Antarctic Ice Sheet, which contains enough ice to raise global sea level by 10 feet, is likely to lose ice much more quickly if its ice shelves disintegrate” (page 1491, lines 4-6), is oversimplified. In fact, much of the focus is on warming oceans eating away at the place the ice sheets go afloat in West Antarctica. A balanced statement is needed on how both warming air temperature that will make more meltwater and warming ocean could make this ice sheet collapse rapidly.

How fast are glaciers melting in Glacier National Park?

This FAQ answer is confusing and misses an important educational opportunity. Glacier recession is considered one of the important lines of evidence for climate warming and that should be emphasized. The answer should include a broader message about glaciers in general, using those in Glacier National Park as an example and less specific detail.

Some text that draws on recent findings of researchers from the U.S. Geological Survey working in Glacier National Park (GNP) as well as information from the National Snow and Ice Data Center is provided here and could be reviewed and included in the FAQ response.

• Glaciers around the world are retreating at unprecedented rates. Several ice caps, glaciers, and ice shelves have disappeared altogether this century, and many more will vanish within a matter of decades.¹⁹ The cause is increasing temperatures and decreasing precipitation falling as snow. Glaciers retreat when melting and evaporation outpace the accumulation of new snow. In recent decades, the mountains of GNP have experienced an increase in summer temperatures and a reduction in the winter snowpack that forms and maintains glaciers. Since 1900, the mean annual temperature of GNP has increased by 1.33^oC, spring and summer minimum temperatures have risen, and increases in annual precipitation have come in the form of more rain rather than snow (Pederson et al., 2010; 2011a; 2013). Mountain snowpacks now hold less water than they used to and have begun to melt at least two weeks earlier in the spring. This earlier melting alters glacier stability as well as downstream water supplies, wildlife, agriculture, and fire management.
• In a recent study, scientists looked at 39 glaciers in and around GNP and compared aerial photos and digital maps from 1966 to 2016.²⁰ Currently, only 26 glaciers are bigger than 25 acres, the minimum size used for defining a glacier. When GNP was established early in the last century there were an estimated 150 glaciers that were larger than 25 acres. Long term studies of glacier size have shown that the rate of melting has fluctuated in response to decade-long climate cycles and that the melting rate has risen steeply since about 1980 (Pederson et al., 2004, 2011b). Over the next 30 years, glaciologists project that most glaciers in GNP will shrink to a point where they are too small to be active glaciers, and some will disappear completely. All glaciers in the park have a severe threat of completely melting by the end of the century.

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¹⁹ See https://nsidc.org/cryosphere/glaciers/questions/climate.html.

²⁰ See https://www.usgs.gov/news/glaciers-rapidly-shrinking-and-disappearing-50-years-glacier-change-montana.

Supporting references to review when revising this FAQ response are listed in Appendix B.

How are the oceans affected by climate change?

The information in the answer to this FAQ, is accurate and in keeping with the latest available science. In general, the narrative is technical and oriented to an individual with a relatively strong science background, as it appears to make assumptions about the level of understanding the reader will have for scientific jargon (e.g., “reducing ecosystem structure and complexity” and “Atlantic Ocean’s overturning circulation, known as the “Ocean Conveyor Belt”). The examples are a very good feature of the answer and additional examples could be used. Referrals back to the draft NCA4 Chapters 9, “Oceans and Marine Resources,” and 24, “Northwest,” are helpful. Reference to the draft NCA4 Chapter 8, “Coastal Effects,” discussion of coastal ecosystems and pathways from these ecosystems to the open ocean (and to land) may also be of interest to readers and could be added.

The answer to the FAQ might be clearer to the reader if the general impacts and examples alluded to are more apparent. For instance:

• On pages 1492-1493, lines 37-38 and 1, respectively, “Dissolved CO2 reacts with seawater and makes it more acidic. This acidification impacts marine life like shellfish and corals (see Ch. 24: Northwest).” In what ways are the examples of marine life impacted?
• On page 1493, line 3, “A warmer ocean holds less oxygen and changes the physical mixing (for example, upwelling and circulation) of oxygen in the oceans, which affects marine life.” What are the examples of, or indications that, marine life has been affected?

Figure 33 could benefit from additional explanation in the caption that would link the discussion on changes in ranges of species because of climate driven ocean changes to the economic activity of fisheries (which is not really mentioned in the narrative). It should also be “A5.33” to be consistent with the numbering of other figures in the FAQ section.

What is ocean acidification and how does it affect marine life?

The pteropods example in this FAQ answer is a good one, and it especially grabs peoples’ attention because they are an important food source for Pacific salmon. It would be worthwhile to add another sentence that states the link to salmon specifically. This would then provide a specific example relevant to the point in the following paragraph about ripple effects of ocean acidification in food webs.

On page 1494, lines 19-20, it would be useful to highlight corals as an especially vulnerable habitat-forming species.

How do higher carbon dioxide concentrations affect plant communities and crops?

The text is factually correct but could do more to tease out the main takeaways because as a FAQ, this text is for a busy reader that may not dig deeper. The answer could be more readable using a logical flow similar to the following:

1. Along with water, nutrients, and sunlight, carbon dioxide is one of four resources necessary for plants to grow.
2. At the level of a single plant, an increase in carbon dioxide will tend to increase or accelerate growth because of accelerated photosynthesis. Exactly how much growth stimulation will occur varies significantly from species to species.
3. However, the interaction between plants and their surrounding environment complicates the relationship. For example, [give an example, e.g., for stressed plants].
4. At the ecosystem level, the response is further complicated by the competition between species. For example, [can use the pine + poison ivy example here].
5. The expected effects of increased carbon dioxide in agricultural plants are in line with these same patterns. Crops that are not experiencing stresses from nutrients, water, or biotic stresses such as pests and disease will be expected to benefit from carbon dioxide increases. The magnitude of the effect varies greatly from crop to crop. For many crops in most U.S. regions, the benefits will likely be mostly or completely offset by increased stresses. [examples here].

This FAQ might also mention that plants often become less water stressed as the carbon dioxide concentration rises because they experience strong water-carbon tradeoffs at the leaf scale (leaf stomata let in carbon dioxide to the plant tissue [good for plant] but allow water to escape [bad for plant]). Higher atmospheric carbon dioxide concentrations allow plants to photosynthesize more with lower water losses and higher water use efficiencies.

It is suggested that the comment about nutritional quality be removed because this is still emerging science.

Comments about downstream impacts could also be a good fit here. Pollinators are mentioned, but this list could be expanded to other topics that the draft NCA4 Chapter 10, “Agriculture and Rural Communities,” talks about. A topic sentence to clearly delineate for the reader that the discussion is pivoting from the strict subject of the FAQ question to closely related downstream impacts will be needed.

Is climate change affecting U.S. wildfires?

This FAQ contains some irrelevant information and omits new information that is more germane to the question and addresses recent extreme fire seasons. It is suggested that some new data/research results be reviewed to update this topic, and some simplification be made to clarify the major points.

The FAQ answer could be restructured to emphasize recent information on climate-wildfire linkages, including the attribution of recent fires to human-driven climate change. The second paragraph provides more regional detail than is necessary and should be reduced. It is also noted that much of the information is not a major topic of the draft NCA4 Chapter 6, “Forests,” and the FAQ authors should consider whether better alignment is necessary.

A suggested revision to the italicized FAQ response (page 1496, lines 15-19) is: “Yes, wildfire activity occurs during periods when dry weather, adequate fuels and ignition sources co-occur. Weather determines how much area is burned, and wildfire intensity and rate of spread are closely tied to temperature, relative humidity, precipitation and wind speed. Rising temperatures

and more drought have increased the frequency of wildfires as well as their size in the U.S. in recent decades (see Ch. 6: Forests, Figure A5.35).”

Long records of fire provided by tree-ring and charcoal records show that climate is the primary driver of fire on time scales ranging from years to millennia. Globally, the length of the fire season (the time of year when climate and weather conditions are conducive for fire) has increased by 19% from 1979 to 2013, and it has become significantly longer over this period in most of the United States (Jolly et al., 2015). Recent increases in the number of wildfires and area burned in most U.S. forests are a result of rising temperatures, increased drought, longer fire seasons and earlier snowmelt. Since 1985, more than 50% of the increase in area burned by wildfire in the western U.S. is attributed directly to human-caused climate change (Abatzoglou and Williams, 2016).

The frequency of large forest fires has increased since the 1970s most dramatically in the Northwest (1000%) and Northern Rocky Mountains (889%), followed by forests in the Southwest (462%), Southern Rockies (274%), and Sierra Nevada (256%) (Westerling, 2016). Dry forests in these regions account for about half of the total forest burned since 1984. The high levels of fire in these forests results from a combination of extreme weather events as well as the lasting effects of fire suppression, past logging, grazing, and invasive species in building up fuel loads. Large, high-severity fires convert unnaturally dense and structurally homogeneous dry forests to non-forest ecosystems in many places, with attendant loss of ecosystem services (Schoennagel et al., 2017).

References to review when editing this FAQ are included in Appendix B.