How Much Human-Caused Global Warming Should We Expect with Business-As-Usual (BAU) Climate Policies? A Semi-Empirical Assessment
Round 1
Reviewer 1 Report
This is a very long paper but not excessive considering the amount of material covered. It is well-written and tightly argued. Still there are sections I would recommend can be shortened or removed:
- The opening paragraphs of Section 2 that review early investigations of atmospheric infrared absorption
- The discussion of Kuhn on page 15; and the "4 paradigms" can be shortened substantially
- Most of the CO2 charts in the BAU section
- The discussion of the Consensus surveys and most of Section 4.1: all that is needed is a decision on how you will define pre-industrial temperature
- The section on climate sensitivity can be made much shorter. The listing of "paradigms" over and over is extremely tedious. Since you are going to rely on the empirical sensitivity estimates explain those and state why you prefer them
In your discussion of emission trends, the tendency for over-estimation of emission paths by the IPCC goes back a long way. McKitrick et al (2013) which refers to the SRES scenario group is useful for supporting your position (https://onlinelibrary.wiley.com/doi/full/10.1002/for.2248)
Figure 13 is confusing because the lines start at different heights. I assume that is because each chosen ECS value implies a different amount of historical warming that can be attributed to greenhouse gases, hence a different starting value. But we aren't interested in the starting values we are interested in the future warming. So the lines need to be positioned to all begin at the same level at a suitable starting year.
The entire analysis basically comes down to the point that the answer depends on ECS. If assumptions about the likely emissions pathway and the airborne fraction are also influential, it would be helpful to select one ECS value (say 2K) and show how the warming path would change under different assumptions about emissions and airborne fraction. Such a presentation would also permit shortening the earlier sections because you won't be trying to argue decisively for one position over another; just defending why you prefer one to the other.
Author Response
We thank Reviewer 1 for their encouraging comments, and constructive review. We have revised our paper in several places accordingly. With some of his/her recommendations, after much consideration, we have kept the manuscript as-is. However, we appreciated all of the suggestions and recommendations, and we thank the reviewer for his/her careful and systematic review of our paper.
This is a very long paper but not excessive considering the amount of material covered. It is well-written and tightly argued. Still there are sections I would recommend can be shortened or removed:
- The opening paragraphs of Section 2 that review early investigations of atmospheric infrared absorption
- The discussion of Kuhn on page 15; and the "4 paradigms" can be shortened substantially
- Most of the CO2 charts in the BAU section
- The discussion of the Consensus surveys and most of Section 4.1: all that is needed is a decision on how you will define pre-industrial temperature
- The section on climate sensitivity can be made much shorter. The listing of "paradigms" over and over is extremely tedious. Since you are going to rely on the empirical sensitivity estimates explain those and state why you prefer them
It is apparent that the reviewer has a lot of expertise on many of the topics covered in this paper. As a result, we appreciate that the reviewer probably finds the discussions highlighted above to be somewhat self-evident, and not as essential as other parts of the paper. However, after much consideration, we have decided to keep these sections as is. This is because we believe the paper should be accessible to as wide an audience as possible, and many readers might not be as familiar with these concepts as the reviewer.
In your discussion of emission trends, the tendency for over-estimation of emission paths by the IPCC goes back a long way. McKitrick et al (2013) which refers to the SRES scenario group is useful for supporting your position (https://onlinelibrary.wiley.com/doi/full/10.1002/for.2248)
We thank the reviewer for pointing out this useful reference. We have added a reference to it, as well as the correspondence arising from the earlier Castles & Henderson (2003) critique of SRES to Section 2.2:
“None of these six “SRES” scenarios match well with our BAU projection. “A1G” and “A2” both imply a growth in emissions that is considerably higher than BAU. This is broadly consistent with the findings of McKitrick et al. (2013)[57] as well as the earlier critique by Castles and Henderson (2003)[58] which led to considerable debate, e.g., [59–62] Meanwhile, the rest of the scenarios imply changes that are considerably less than BAU. “A1” and “A2” match our BAU projection reasonably well up to 2050, but then they diverge in opposite directions (“A1” increasing faster than BAU and “A2” decreasing).”
Figure 13 is confusing because the lines start at different heights. I assume that is because each chosen ECS value implies a different amount of historical warming that can be attributed to greenhouse gases, hence a different starting value. But we aren't interested in the starting values we are interested in the future warming. So the lines need to be positioned to all begin at the same level at a suitable starting year.
We thank the reviewer for pointing out that more clarification is needed here. Another reviewer also made a similar point. As the reviewer assumed, the reason for the divergence between the various projections over the historic period, 1980-2019, even though the magnitude of global warming over this period is already quite well known, is that the projections are of “human-caused global warming”. The actual global warming observed over this period might be either greater than or less than the presumed “human-caused” component. However, when the reviewer says that, “But we aren't interested in the starting values we are interested in the future warming”, this is not necessarily true. For instance, the Paris Agreement limits are explicitly defined in terms of “pre-industrial levels” rather than current levels. Still, we appreciate the reviewer’s suggestion that many readers might be more interested in future warming. With that in mind, we have added to the Supplementary Information an equivalent version of Figure 13 which begins in 2020.
We have also added the following text to the description of Figure 13:
“Figure 13 (a) shows the results of our analysis for a TCR of 0.5°C, 1.0°C, 1.5°C, 2.0°C, 2.5°C and 3.0°C, while Figure 13 (b) shows the results for an ECS of 1°C, 2°C, 3°C, 4°C, 5°C and 6°C. One point which might initially seem surprising is that the results are already different for the historic period, 1980-2019. That is, the magnitude of “human-caused global warming” which is presumed to have already occurred over the historic period increases with the value of the higher climate sensitivity which is assumed. Some readers may wonder at why there should be uncertainty over this given that we have reasonable estimates of the global warming which occurred over this period. However, as discussed in Section 4.1, the magnitude of “global warming” over a given period does not automatically tell us the magnitude of “human-caused global warming” from increasing greenhouse gases. Some (or even all) of the observed global warming may have been due to natural factors and/or other non-greenhouse gas-related factors. On the other hand, there may have been additional “global cooling” factors – either natural (e.g., decreases in solar activity) or human-caused (e.g., increases in aerosols) - that led to a reduction in human-caused global warming. That is, the amount of human-caused global warming which should have already occurred might be less than or greater than the amount of observed global warming. Indeed, this is a major part of the reason why there is still such uncertainty over the actual climate sensitivity to greenhouse gases.
At any rate, for us, probably the most striking result is the sheer range of possible values by the end of our BAU projections in 2100. This is quite problematic given that recently international climate policies have been framed within the context of limiting the magnitude of future human-caused global warming to within a specific value. In particular, the 2015 Paris Agreement involved a voluntary international agreement for, “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C above pre-industrial levels” [19]. More recently, in 2018, the IPCC issued an intermediate Special Report entitled, “Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty.”[266]. For readers who are more interested in future human-caused global warming relative to present, in the Supplementary Information we provide an equivalent figure using 2018 greenhouse gas concentrations as the starting baseline. “
The entire analysis basically comes down to the point that the answer depends on ECS. If assumptions about the likely emissions pathway and the airborne fraction are also influential, it would be helpful to select one ECS value (say 2K) and show how the warming path would change under different assumptions about emissions and airborne fraction. Such a presentation would also permit shortening the earlier sections because you won't be trying to argue decisively for one position over another; just defending why you prefer one to the other.
These are interesting suggestions, and we have considered them carefully. However, one of the main goals of this study was to present a single “Business-As-Usual” projection of future greenhouse gases (with uncertainty bars). Therefore, especially given that the paper is already long, we believe an equivalent analysis to Figure 13 for all the many emissions scenarios used by the IPCC reports would be beyond the scope of this paper. It might make for an interesting follow-on study.
Nonetheless, the reviewers comments have prompted us to include some extra analysis on the role of the airborne fraction in the Supplementary Information. Specifically, we have added a figure to the Supplementary Information in which we show the effects of using the projected airborne fractions implied by the RCP scenarios on future GHG concentrations as opposed to assuming the empirically-observed airborne fractions remain constant under BAU. We also added the following reference to this in Section 3.5:
“For interested readers, in the Supplementary Information, we compare the total projected greenhouse gas concentrations under BAU (in CO2 equivalent concentrations) to alternative projections that used the projected airborne fractions implied by the RCP scenarios.”
Again, we would like to thank Reviewer #1 for his/her encouraging and constructive comments. We hope that these revisions (along with the others to address the comments of the editor and other reviewers) have satisfactorily addressed his/her concerns.
Reviewer 2 Report
The paper seems to be submitted as a review paper, therefore please clearly state this in the template provided by MDPI. In my pinion, the paper is extremely long: 53 pages among which almost 13 pages with references. The structure provided in the Contents section is rather similar to the one a book has - not sure it is required by a scientific paper in MDPI format - please check. Also, please improve the abstract to give more insight over the need for such a research. I think that the information related to the steps needed to be taken is not important for a reader wanting to read more and should be eliminated from the abstract (should be only discussed in the paper in the methodological section). Regarding the content of the paper, I think it should be re-thinked as the paper is very descriptive and very personal (please see rows 868-876). A pro-con analysis related to the achievements made by the papers would have been necessarily in this case.
Author Response
The paper seems to be submitted as a review paper, therefore please clearly state this in the template provided by MDPI.
We apologise, as we were unfamiliar with the MDPI template, and did not notice that the “Article” word in the header section needed to be changed to “Review” if the paper was to be submitted as a review. We have now fixed this, and we thank the reviewer for pointing this out.
“the paper is extremely long: 53 pages among which almost 13 pages with references.”
Yes, as we discussed in the Introduction there is a lot of material and concepts that need to be simultaneously addressed, and a lot of relevant references often presenting conflicting perspectives. Therefore, by necessity, the review is relatively long and includes a rather comprehensive list of references. It was partly for this reason that we chose to submit this review to Energies, since they do not have an explicit page limit.
The structure provided in the Contents section is rather similar to the one a book has - not sure it is required by a scientific paper in MDPI format - please check.
We had included a Table of Contents section after the abstract as we thought it might be helpful for readers given the length of the review (this is a feature that some journals recommend for longer review articles). However, we have now deleted it, since both the reviewer and the editor objected to it.
Also, please improve the abstract to give more insight over the need for such a research. I think that the information related to the steps needed to be taken is not important for a reader wanting to read more and should be eliminated from the abstract (should be only discussed in the paper in the methodological section).
We have now revised the abstract in light of the reviewer’s suggestion.
Regarding the content of the paper, I think it should be re-thinked as the paper is very descriptive and very personal (please see rows 868-876). A pro-con analysis related to the achievements made by the papers would have been necessarily in this case.
In Rows 863-867, we had pointed out that the so-called “hockey stick debate” is a topic which has been remarkably lively and contentious. To give the readers an idea of just how contentious the topic became (and remains), in Rows 868-876 we describe an example of the lively and contentious nature of the debate over a serious of papers which one of us (WS) co-authored. As the reviewer notes, this might make the discussion appear overly personal. We were aware of this when we wrote it, and have strived to be as objective as possible in describing the debate (which is very relevant to Section 4.2, since it is over how variable temperatures were during the pre-industrial period). With that in mind, we presented all sides of the debate, and provided the readers with references to the key relevant literature presenting all sides. Two of us (RC and MC) have had several amicable discussions with both Mann and von Storch (separately) in recent years, and we believe they would agree that we have described the debate objectively.
Nonetheless, we appreciate the reviewer’s point, and so we have slightly changed the introduction to this discussion to stress that this is being provided “as an example of the contentious nature of this topic”.
We hope that these revisions (along with the others to address the comments of the editor and other reviewers) have satisfactorily addressed Reviewer #2’s concerns. We thank the reviewer for their useful feedback, which we believe has helped us to improve our original manuscript.
Reviewer 3 Report
Excellent and clear statements of how the data were obtained, what they look like, and what assumptions are involved. These things are often hidden in such studies. It is noteworthy that they compare their input data to IPCC and can explain how they differ and why. There are only a few small issues:
line 114 drop final "to"
line 624 "it" should be "in"
line 631 drop "actually"
line 864 drop "more"
line 1132 drop "experimental" --you mean empirical
line 1202 "of" should be "on"
Fig 13 need to state what the initiation period was fro the calculations since they already differ in 1980
Author Response
line 114 drop final "to"
line 624 "it" should be "in"
line 631 drop "actually"
line 864 drop "more"
line 1132 drop "experimental" --you mean empirical
line 1202 "of" should be "on"
We thank the reviewer for spotting all of these typos/mistakes. We have changed them all as suggested, although for line 1202, we changed “of” to “in”.
Fig 13 need to state what the initiation period was fro the calculations since they already differ in 1980
We thank the reviewer for pointing out that more clarification is needed here. Another reviewer also made a similar point. The reason for the divergence between the various projections over the historic period, 1980-2019, even though the magnitude of global warming over this period is already quite well known, is that the projections are of “human-caused global warming”. The actual global warming observed over this period might be either greater than or less than the presumed “human-caused” component. Since this initially might be somewhat unintuitive, we feel that it is better addressed in the body of the text rather than in the figure caption. Therefore, we have added the following text to the description of Figure 13:
“Figure 13 (a) shows the results of our analysis for a TCR of 0.5°C, 1.0°C, 1.5°C, 2.0°C, 2.5°C and 3.0°C, while Figure 13 (b) shows the results for an ECS of 1°C, 2°C, 3°C, 4°C, 5°C and 6°C. One point which might initially seem surprising is that the results are already different for the historic period, 1980-2019. That is, the magnitude of “human-caused global warming” which is presumed to have already occurred over the historic period increases with the value of the higher climate sensitivity which is assumed. Some readers may wonder at why there should be uncertainty over this given that we have reasonable estimates of the global warming which occurred over this period. However, as discussed in Section 4.1, the magnitude of “global warming” over a given period does not automatically tell us the magnitude of “human-caused global warming” from increasing greenhouse gases. Some (or even all) of the observed global warming may have been due to natural factors and/or other non-greenhouse gas-related factors. On the other hand, there may have been additional “global cooling” factors – either natural (e.g., decreases in solar activity) or human-caused (e.g., increases in aerosols) - that led to a reduction in human-caused global warming. That is, the amount of human-caused global warming which should have already occurred might be less than or greater than the amount of observed global warming. Indeed, this is a major part of the reason why there is still such uncertainty over the actual climate sensitivity to greenhouse gases.
At any rate, for us, probably the most striking result is the sheer range of possible values by the end of our BAU projections in 2100. This is quite problematic given that recently international climate policies have been framed within the context of limiting the magnitude of future human-caused global warming to within a specific value. In particular, the 2015 Paris Agreement involved a voluntary international agreement for, “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C above pre-industrial levels” [19]. More recently, in 2018, the IPCC issued an intermediate Special Report entitled, “Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty.”[266]. For readers who are more interested in future human-caused global warming relative to present, in the Supplementary Information we provide an equivalent figure using 2018 greenhouse gas concentrations as the starting baseline.“
We think this added discussion should address the Reviewer’s concern.
We thank Reviewer #3 for his/her encouraging and constructive comments. We hope that these revisions (along with the others to address the comments of the editor and other reviewers) have satisfactorily addressed his/her concerns. We thank the reviewer for their useful feedback, which we believe has helped us to improve our original manuscript.
Reviewer 4 Report
GENERAL COMMENTS
The work is very interesting, it has a certain degree of originality, and it is appropriate for publication in the Energies journal, after performing a moderate level revision. Indeed the topics related to climate change effects are very actual and important.
However, there are still some observations and issues that should be clarified and fixed before publication. The first observation is that in my opinion, with more than 250 references and and more than 50 pages the work should be classified as ‘Revue article’ and not as Article. At the same time, I am not sure whether there are not some length limitations for the papers published in Energies journal, because from my perspective this work looks more as a bock chapter rather than a common research paper. Thus, if there is a length limit in this journal, the work should be restructured and reduced to a more succinct form. On the other hand, the work as it is provides indeed relevant information for the topic targeted.
Some of the changes required are:
- some figures and tables should be redesigned and more explanations should be provided in relationship with the results illustrated;
please check carefully the journal template. For example the figures and tables captions are written with italics, and they shouldn’t.
Some specific comments are given next. They are not exhaustive, which means that there are still some other issues to be double checked by the authors before publication.
SPECIFIC COMMENTS
KEYWORDS
I would suggest introducing also the keywords: climate policies, business as usual
ABBREVIATIONS
There are many abbreviations considered in this work. From this perspective, an Index of Notations and Abbreviations would be beneficial for a better understanding of the proposed work.
Furthermore, please, check carefully if all the abbreviations and notations considered in the work are explained for the first time when they are use, even if these are considered trivial by the authors. The paper should be accessible to a wide audience. For example, the abbreviations presented in the legend of Figure 1.
FIGURES & TABLES
Some corrections are required in relationship with the figures and the figures captions, as follows:
Please write ‘Year’ on the x axes of the figures giving time projections of some parameters.
Figure 1b – you should explain better the legend of this figure. Eventually you can associate a table explaining each case considered. Otherwise the results presenting in this figure are confusing.
Figures 2-5 – you should explain better what are the data presented in these figures and what represent the scenarios considered in them (for example A, B, C, D and also IS92- A, B, C, etc.). Eventually, you should clarify better the link of these figures with Table 2.
The writing in Figure 12 is different (larger) than for the others. Please be consistent.
SOME MINOR ASPECTS
Lines 77, 82, 98, 99, 811 (etc) the words [] in brackets are confusing, you should better change to points …. As you used at line 73;
Lines 98, 112, 715, 738, 936, 963, 976, 1152, 1336 (etc) – the words written with bold font are confusing and not necessary to be highlighted in this way.
Line 568, please write approximately 540-568, instead of ~540-568
Author Response
The first observation is that in my opinion, with more than 250 references and and more than 50 pages the work should be classified as ‘Revue article’ and not as Article. At the same time, I am not sure whether there are not some length limitations for the papers published in Energies journal, because from my perspective this work looks more as a bock chapter rather than a common research paper. Thus, if there is a length limit in this journal, the work should be restructured and reduced to a more succinct form. On the other hand, the work as it is provides indeed relevant information for the topic targeted.
We apologise, as we were unfamiliar with the MDPI template, and did not notice that the “Article” word in the header section needed to be changed to “Review” if the paper was to be submitted as a review. This was also pointed out by Reviewer #1. We have now fixed this, and we thank both reviewers for pointing this out.
please check carefully the journal template. For example the figures and tables captions are written with italics, and they shouldn’t.
Again we apologise, as this was another mistake of ours arising from our unfamiliarity with the MDPI template. Although the MDPI template has styles for figure and table captions, these do not appear to be set up to be automatically activated by MS Word’s “insert caption” facility. Instead, the captions need to be manually inserted or else the style of the default captions (italics, etc.) need to be manually changed to the appropriate MDPI styles. We have now done the latter, and we thank the reviewer for bringing our attention to this.
KEYWORDS
I would suggest introducing also the keywords: climate policies, business as usual
Done, thanks!
ABBREVIATIONS
There are many abbreviations considered in this work. From this perspective, an Index of Notations and Abbreviations would be beneficial for a better understanding of the proposed work.
Furthermore, please, check carefully if all the abbreviations and notations considered in the work are explained for the first time when they are use, even if these are considered trivial by the authors. The paper should be accessible to a wide audience. For example, the abbreviations presented in the legend of Figure 1.
We thank the reviewer for this helpful suggestions. We have now added a new Table 3 to Section 2.1, which provides an index of all of the acronyms and abbreviations used in the article.
Please write ‘Year’ on the x axes of the figures giving time projections of some parameters.
We have now added a clarification to each of the relevant figures explaining that, “The x-axes correspond to years.”
Figure 1b – you should explain better the legend of this figure. Eventually you can associate a table explaining each case considered. Otherwise the results presenting in this figure are confusing.
Thanks. We have added the following clarification to the Figure 1 caption to explain the legend,
“Each of the plots for the historical period in (b) represents a different estimate of the CO2 emissions since 1959 as compiled by the Global Carbon Budget project. The acronyms of these estimates are provided in the legend below (b), but for further details on each of these estimates, we refer the reader to Friedlingstein et al. (2019) and references therein. However, for the analysis in this paper we will use the mean ± 2 S.E. of all of these estimates, which is shown with a thick solid red line (with light red envelope).”
Figures 2-5 – you should explain better what are the data presented in these figures and what represent the scenarios considered in them (for example A, B, C, D and also IS92- A, B, C, etc.). Eventually, you should clarify better the link of these figures with Table 2.
Thanks, this is a good point. We have added extra information on the IPCC projection scenarios to Table 2, and we have also added the following, “Details on the projections are provided in Table 2.” to the captions of Figures 2-5 and 9.
The writing in Figure 12 is different (larger) than for the others. Please be consistent.
Fixed, thanks!
Lines 77, 82, 98, 99, 811 (etc) the words [] in brackets are confusing, you should better change to points …. As you used at line 73;
For the quotes, we used open ellipses, i.e., “…”, to indicate that we have omitted some part of the original quotation which included a digression not germane to the point. This is a fairly common practice for some academic writing styles, e.g., APA, etc. Although we note that MLA style recommends including the ellipses in round brackets, i.e., “(…)”, to indicate that the ellipse was not in the original quote, and Chicago recommends using square brackets, i.e., “[…]”.
We used square brackets, i.e., “[]”, to indicate that we have inserted or altered words in the original quotation to clarify meaning and/or provide context. This is a fairly common practice in most of the academic writing styles we are familiar with, e.g., APA, Chicago, MLA. If we did not adopt it and only used open ellipses as the reviewer suggests, there might be a risk that some readers might not realise that some insertion or alteration was carried out to improve clarity of meaning and/or provide context.
See e.g.,
https://owl.purdue.edu/owl/research_and_citation/mla_style/mla_formatting_and_style_guide/mla_formatting_quotations.html
https://www.enago.com/academy/how-to-use-brackets-in-academic-writing-some-common-rules/
https://writingcommons.org/article/inserting-or-altering-words-in-a-direct-quotation/
https://blog.apastyle.org/apastyle/2015/05/punctuation-junction-quotation-marks-and-ellipses.html
http://askus.library.wwu.edu/faq/116674
Therefore, we have decided to keep to this recommended approach of using square brackets whenever the direct quotation has been altered in some way. However, we appreciate the reviewer’s point about how it can be confusing especially when also accompanied by an open ellipse.
With that in mind, as a compromise, we have taken a combined approach whereby we include any ellipses in the square brackets, i.e., the Chicago academic writing style, instead of the open ellipse approach used by e.g., the APA style guide. We note that some APA experts have said that this is also acceptable within APA, e.g., http://askus.library.wwu.edu/faq/213662?m=p
We also have removed the square brackets from places where the alteration was just to convert a capital letter into a lowercase letter, as this is often accepted as not being essential.
Lines 98, 112, 715, 738, 936, 963, 976, 1152, 1336 (etc) – the words written with bold font are confusing and not necessary to be highlighted in this way.
Fixed.
Line 568, please write approximately 540-568, instead of ~540-568
Fixed.
We thank Reviewer #4 for his/her encouraging and constructive comments. We hope that these revisions (along with the others to address the comments of the editor and other reviewers) have satisfactorily addressed his/her concerns. We thank the reviewer for their useful feedback, which we believe has helped us to improve our original manuscript.
Round 2
Reviewer 2 Report
In the light of the authors' comments I think that the paper can be accepted in this current form. Please change the abstract and explain using words the achievement of the paper instead of using so much numbers.
Author Response
We are glad that the reviewer was satisfied with most of our revisions. However, we see that the reviewer would like us to make some further revisions to the abstract itself:
“Please change the abstract and explain using words the achievement of the paper instead of using so much numbers.”
Unfortunately, given the large body of work described in the paper, we are restricted in how much we can modify the abstract while keeping it concise, and simultaneously conveying the key findings to readers. However, we appreciate that there are a lot of numbers reported in the abstract, and this might make the abstract a bit hard to read. With that in mind, we have removed the projected greenhouse gas concentration ranges by 2100 from the abstract. The final revised abstract is as follows,
“In order to assess the merits of national climate change mitigation policies, it is important to have a reasonable benchmark for how much human-caused global warming would occur over the coming century with “Business-As-Usual” (BAU) conditions. However, currently, policymakers are limited to making assessments by comparing Global Climate Model (GCM) projections of future climate change under various different “scenarios”, none of which are explicitly defined as BAU. Moreover, all of these estimates are ab initio computer model projections, and policymakers do not currently have equivalent empirically-derived estimates for comparison. Therefore, estimates of the total future human-caused global warming from the three main greenhouse gases of concern (CO2, CH4 and N2O) up to 2100 are here derived for BAU conditions. A semi-empirical approach is used that allows direct comparisons between GCM-based estimates and empirically-derived estimates. If the climate sensitivity to greenhouse gases implies a Transient Climate Response (TCR) of ≥ 2.5°C or an Equilibrium Climate Sensitivity (ECS) of ≥ 5.0°C then the 2015 Paris Agreement’s target of keeping human-caused global warming below 2.0°C will have been broken by mid-century under BAU. However, for a TCR < 1.5°C or ECS < 2.0°C then the target would not be broken under BAU until the 22nd century or later. Therefore, the current IPCC “likely” range estimates for TCR of 1.0°C to 2.5°C and ECS of 1.5°C to 4.5°C have not yet established if human-caused global warming is a 21st century problem.”
We believe the remaining numbers are important and many readers who are familiar with the literature on climate sensitivities would be interested in the numbers given.
