Brief communication: Increasing shortwave absorption over the Arctic Ocean is not balanced by trends in the Antarctic

Katlein, Christian; Hendricks, Stefan; Key, Jeffrey

doi:https://doi.org/10.5194/tc-11-2111-2017

Articles | Volume 11, issue 5

https://doi.org/10.5194/tc-11-2111-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/tc-11-2111-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 5

Brief communication

|

07 Sep 2017

Brief communication |

| 07 Sep 2017

Brief communication: Increasing shortwave absorption over the Arctic Ocean is not balanced by trends in the Antarctic

Christian Katlein, Stefan Hendricks, and Jeffrey Key

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Final revised paper (published on 07 Sep 2017)
Preprint (discussion started on 02 Jan 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'Total absorption values', Willis Eschenbach, 08 Jan 2017
- AC1: 'Reply to Reviewer comments 1', Christian Katlein, 25 Apr 2017
RC1: 'Referee comment', Sebastian Bathiany, 09 Jan 2017
- AC2: 'Replies to comments from Reviewer 2', Christian Katlein, 25 Apr 2017
RC2: 'Comments on article', Anonymous Referee #2, 08 Feb 2017
- AC3: 'Replies to comments from Reviewer 3 (Anonymous reviewer #2)', Christian Katlein, 25 Apr 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Christian Katlein on behalf of the Authors (10 May 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 May 2017) by Dirk Notz

RR by Sebastian Bathiany (16 May 2017)

RR by Ian Eisenman (06 Jun 2017)

Suggestions for revision or reasons for rejection

This Brief Communication assesses the change in surface shortwave absorption in both polar regions during 1982-2014. The authors find an increase in the Arctic, which is unsurprising given the rapid sea ice retreat, but they also report an increase in the Antarctic despite the observed sea ice expansion in that hemisphere. The manuscript is clearly written, and I find it well suited for publication in this journal, although I think the points raised below should first be addressed.

Major comment:

I am a bit confused regarding the significance of the Southern Hemisphere result. The authors write that in the Southern Hemisphere "energy absorption by the ice-ocean system south of 60S also increased but, due to the large interannual variability, only at a statistically insignificant rate" (page 4 lines 23-25). But despite this, the text says "both hemispheres show a distinct increase in energy deposition" (page 4 line 25) and "positive energy absorption trends in both hemispheres" (page 5 line 1), the title states that shortwave absorption is increasing "at both poles", and the abstract says it is "increasing at both poles". Am I missing something here? If the results do not show a significant increase in shortwave absorption in the Southern Hemisphere, then I think the title, abstract, and main text should be revised to reflect this point.

Minor comments:

page 4 line 22-23: I found the choice of units of the increase in absorbed shortwave to be somewhat unclear. It seems most intuitive to present this in terms of how much the absorbed energy in W/m^2 (averaged over the relevant polar region) increases each year. This is instead presented here in units of J/yr, which I find somewhat confusing. Furthermore, I am confused by the values. The rate of increase reported here for the region 60N-90N is 2.3 x 10^25 J/yr (page 4 line 22-23). But unless I'm mistaken, this is more than the total TOA solar energy incident on the earth, which is 5.5 x 10^24 J/yr (calculated as 1340 W/m^2 * surface area of earth/4).

page 3 line 25: The 1982-2004 sea ice trends reported in this paragraph, averaged between September and March, have a magnitude in the Arctic that is 5.2x larger than in the Antarctic. This is fairly similar to the 1979-2012 annual values reported in the IPCC AR5, which have a magnitude in the Arctic that is 2.9x larger than in the Antarctic. I think it would be more accurate to revise how this is described from Arctic losses being "thus roughly one order of magnitude stronger" than Antarctic increases to "thus five times larger in magnitude" (or something similar).

page 3 line 21: When error bars are given for trends, are these 68% confidence intervals (as are often used), or are they 95% confidence intervals (as implied on page 3 line 19)? It would be helpful if this were specified.

page 1 line 19: The argument being made here may be supported by our own finding that there is a large uncertainty in the 1979-2012 Antarctic sea ice extent trend due to intercalibration across sensor changes (Eisenman et al. 2014, The Cryosphere 8 1289-1296). I don't mean to angle for a self-citation here, and I certainly leave this to the authors' discretions, but I just figured I'd mention this in case the authors find that it strengthens their case.

page 2 lines 21-29: (typo) ice extent trends should be in units of km^2/yr, not km/yr.

page 2 lines 26-27: (typo) "as a climate data record variables" should presumably be "as climate data record variables".

page 3 line 22: (typo) "in the Antarctic in the Southern Hemisphere" should presumably be "in the Southern Hemisphere".

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ED: Reconsider after major revisions (07 Jun 2017) by Dirk Notz

AR by Christian Katlein on behalf of the Authors (29 Jul 2017) Author's response Manuscript

ED: Publish subject to technical corrections (01 Aug 2017) by Dirk Notz

Dear authors,

thank you very much for submitting the revised version of your manuscript. I find that you have very convincingly responded to both my own concerns and to the concerns of both reviewers, and am very happy to accept your paper for publication in The Cryosphere.

In particular, I was impressed when reading this new version of this paper how relatively small changes to the text led to a very clear description of the scientific purpose and the motivation of this study. I also find your argument convincing that for a brief communication this paper contains sufficiently new results and that a deeper analysis of the underlying mechanisms goes well beyond a "Short Communication".

For the final paper, I would like to ask you to address a few rather minor points. To minimize possible delays in the publication process, I mark these as technical corrections, which allows this paper to be published without me having to examine it again. Nevertheless, please carefully address these concerns:

1. Last sentence of abstract: This sentence is not very clear to me, and links in particular only weakly to the first sentence. The first sentence only speaks about the Southern Ocean, but the second sentence introduces some generically retreating sea ice. I find this confusing.
Maybe the following could work, but please feel free to adjust this as needed: "The observations hence show that the small increase in Antarctic sea-ice extent can not compensate for the combined effect of retreating Arctic sea ice and changes in cloud cover, which both result in a total increase in solar shortwave energy deposited into the polar oceans." Note that I also took out the "surface albedo" in this suggestion, as I found this to be too unspecific to be easily understood here.

2. p.3, l.30: Wouldn't it make more sense to turn this around and say "Thus, the slight gains in Antarctic sea-ice area does not compensate for the areal loss of sea ice in the Arctic."

3, p.4, l.27 and l.29: Please double check that the error in the analysis indeed causes a difference of exactly a factor 1e5 in the Arctic, but a factor of 1.35e5 in the Southern Ocean. Depending on the type of error, this factor should possibly be the same.

4. p.4, l.3: I find this sentence not clear. What does it mean that the "Energy flux anomaly shows a relation to interannual variability"? Do you refer to the interannual variability of a specific climate component?

In addition to these comments, I also stand by my impression that the Eisenman et al. ,2014 reference refers to a dataset that is no longer distributed and no longer used by most scientists, and only considers the Antarctic. I am hence still not convinced that it should be used to substantiate the claim that there is no long-term consistent observational dataset for both poles. I leave it to you to decide on the possible inclusion of this reference.

Thanks again for your very helpful and convincing response to all comments from the previous round(s) - and thanks for submitting this paper to The Cryosphere.

Have a really good summer,

Dirk

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AR by Christian Katlein on behalf of the Authors (02 Aug 2017) Author's response Manuscript

Short summary

In the public debate, increasing sea ice extent in the Antarctic is often highlighted as counter-indicative of global warming. Here we show that the slight increases in Antarctic sea ice extent are not able to counter Arctic losses. Using bipolar satellite observations, we demonstrate that even in the Antarctic polar ocean solar shortwave energy absorption is increasing in accordance with strongly increasing shortwave energy absorption in the Arctic Ocean rather than compensating Arctic losses.