Dear Bob:

The normal procedure of BG is that you first are invited to submit a revision before uploading a revision, and if needed a detailed rebuttal (additional to public domain reply). Since you already sent me the revised version and accompanying letter, I have read them to speed up matters during the slow summer season.

Science does not always progress with consensus and your paper presents a nice story that shale gases may explain the last decade methane increase and methane isotope depletion trend. Although one might argue on details, the analysis as presented makes sense: shale gases are more negative in d13C than fossil fuels and thus explicit accounting for their contribution allows a substantial input of shale gas while still being consistent with the global methane flux and isotope balance.
Your initial manuscript received quite some critics, but you have adequately resolved the issues identified and I am therefore happy to inform you that your paper may be acceptable for publication pending further minor revision.
Besides a few minor technical corrections, I believe that the presentation of your flux and isotope balance could still be improved. I make a suggestion below.
a) rather trivial, why do you not use Bw for the Worden et al. flux and Bn for your new flux.
b) I believe that you better start with the Bw+FFw= 28.4 Tg of Worden as equation x, then start with Bn+FFn+SG=28.4 Tg y-1 as equation y and then substract x from y to arrive at your equation 1. Your equation 1 drops out of the air for the non-informed reader
c) Next, you might consider rewriting your equation 2 as (Bn-Bw)*Dba + (FFn-FFw)*Dffa + SG*Dsga. Why? It directly links with your equation 1 and you swap signs (or lhs vs. rhs) at the same time as your difference in isotope Dffa vs. Daff. This is confusing.
d) Next you multiple your equation 1 with Dba and to arrive at (Bn-Bw)*Dba + (FFn-FFw)*Dba + SG*Dba (a revised way of presenting your equation 3).
e) Finally, you then substract your equation 2 from 3 to arrive at (FFn-FFw)*(Dba-Daff) + SG*(Dba-Dsga) =0 which can be re-arranged to isolate SG: SG= -(FFn-FFw)*( Dba-Daff)/ (Dba-Dsga). This should be the same as your eq. 5.

A few minor technical remarks:
-p.3, l13-14: the – symbols are different (-- vs. -)
-p.4, l. 7: either 14.0 or 14 Tg per year.


I am looking forward receiving your revision, which after inspection by me, will likely be accepted for publication.

With best regards,

Jack Middelburg, Associate Editor Biogeosciences

Dear Dr. Howarth, dear Bob:

I have read your revised manuscript and identified a number of small issues that need to be resolved before I can accept this paper. I list these corrections below and I am looking forward receiving a revised version.

With best regards, Jack Middelburg, Associate editor

Corrections identified while reading your latest version.

p. 5, line 12: Should it not be FFW rather FFN as written?

p. 6, line 1-2: Schneising et al. 2016 is not in the reference list. Either it is another publication or add the correct reference to the list.

p. 8, line 1-3: I have re-read Schneising et al (2014) and have the impression that these authors are more conservative than your text suggests. They indeed report an increase of methane emissions for two individual production regions in North America, but they do not extrapolate this to the regional, let alone the global, scale. I recommend phrasing this a little more cautious/conditional.

p. 20, Table 1, line 27. Here you mention that uncertainties are shown as in the original publication of Worden et al. (2017). However, I do not see these anymore, they were present in the original submission. Please do include them again.

p. 21, Table 2, line 31. Should it not be .. through Eq. (12) and ….?

Table 1 and 2: presentation wise: it would be most instructive to present coal and oil in the same sequence in both tables.

Figure 3B. Should this figure not be updated? It is not consistent with the numbers presented in Table 1. I believe it is still the old figure with numbers of the original submission.

Dear Dr. Howarth, dear Bob:

Thank you for submitting a revised version. We are almost there, but sorry there are still some issues: three technical issues and one incorporation of literature issue.

Technical issues:
(1) On page 5, line33, you have replaced Schneising et al. (2016) with Schneising et al. (2014) in the latest version, because the former was not in the reference list. But the latter is a remote sensing paper and does not present isotope data. I guess you confused Schneising et al. (2016) with another 2016 paper. Please correct.

(2) The same applies to page 6, line 2: you changed Schneising et al. (2016) to (2014) for a citation to isotope numbers. Please also correct this confusion of references as well.

(3) On page 6, line 5, it should be Nisbet not Nisebet

The proper incorporation of literature issue.

One of attractive features of the Biogeosciences/Copernicus platform is that is open to everyone and your paper is attracting quite some attention. For instance, the nominated referees of your BGD version can already follow our discussion at this stage and one of them alluded me that your citation to Schneising et al. is still not correct and expressed severe concern.
I have re-read that article and agree with this referee. Schneising et al. (2014) do not conclude that the methane increase came from the mid latitudes in the northern hemisphere. They do conclude that “positive methane anomalies associated with the oil and gas industries can be detected…” and they show that methane emissions from two shale gas areas have increased. The data presented in their Fig. 2 show enrichment in methane from sin 0.2 to 0.5 north, i.e. south of 30 degrees north, inconsistent with your idea. Moreover, Schneising et al. (2014) do not claim that the global increase can attributed to this and deliberate do not interpret Fig. 2 data to infer the source of methane increase.

Therefore, I recommend to delete the passages (on page 8) with the two critical citation instances a) "Our finding is also consistent with Schneising et al. (2014), who analyzed data from another satellite and concluded that most of the global increase in methane emissions between 2006-2008 and 2009-2011 came from mid latitudes in the northern hemisphere." and b) "Overall, our results tend to support the conclusions of Turner et al. (2016) and Schneising et al. (2014) on the spatial source of increased methane emissions."
Removal of these lines will not impact the main message of your paper.

However, in the end of the day an author decides who to cite and how to interpret published data, and if you do not agree with my recommendation to delete these references, then I propose to rewrite more conservatively (page 8, line1): e.g. “who analyzed data from another satellite and could link positive methane anomalies to increased gas production in two shale gas areas in North America”.


With best regards, Jack Middelburg, Associate Editor

Dear Bob:

I have read your revision. You have adequately resolved the last issues remaining and I am therefore happy to inform you that your paper is now accepted for publication in Biogeosciences.

With best regards, Jack Middelburg, Associate Editor