Sensitivity of the Eocene climate to CO<sub>2</sub> and orbital variability

Keery, John S.; Holden, Philip B.; Edwards, Neil R.

doi:https://doi.org/10.5194/cp-14-215-2018

Articles | Volume 14, issue 2

https://doi.org/10.5194/cp-14-215-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-14-215-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 2

Research article

|

23 Feb 2018

Research article |

| 23 Feb 2018

Sensitivity of the Eocene climate to CO₂ and orbital variability

John S. Keery, Philip B. Holden, and Neil R. Edwards

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Final revised paper (published on 23 Feb 2018)
Preprint (discussion started on 11 Apr 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review', Michel Crucifix, 07 Jun 2017
RC2: 'Review', David De Vleeschouwer, 08 Jun 2017
AC1: 'Response to M. Crucifix', John Keery, 15 Jul 2017
AC2: 'Response to D. De Vleeschouwer', John Keery, 15 Jul 2017

Peer-review completion

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

ED: Reconsider after major revisions (15 Jul 2017) by Arne Winguth

AR by John Keery on behalf of the Authors (25 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Sep 2017) by Arne Winguth

RR by David De Vleeschouwer (16 Oct 2017)

RR by Michel Crucifix (02 Nov 2017)

Suggestions for revision or reasons for rejection

The authors have provided a revised version of their manuscript. My review has concentrated mainly on the responses to questions. Even though the authors made a case that the climatological results they obtain are robust, there are still some contentious aspects on the experiment design and I will attempt here to make my case more explicitly.

The authors rebutted my comment on the 'a-physical' character of their experiment design as follows:

1. there are no combinations of these parameters which can be excluded for the early Eocene period
2. This approach does not make the assumption that the only effect of eccentricity on the Earth's climate is through its effect on the amplitude of the precession cycle

They argument one is a useful condition to avoid wasting computing time, but not sufficient to guarantee a good design. Condition 2. is perhaps true but in fact comes at the price of wrong assumptions.

I agree that my condemnation of the design as 'a-physical' was not the most explicit, so I will rephrase my argument differently.

Latin Hypercube and related techniques (maxi-min, etc.) are mathematically justified by the hypothesis that distances in the input space (here $e$, $\varpi$, $\varepsilon$) translate _a priori_ into distances in the output space. Of course this is never a posteriori quite true (hence the interest of actually doing the experiment), but this hypothesis is the prior assumption which precisely justifies the interest of maximising distances in the input space (this is, _a priori_, the best way to make experiments maximally informative). See textbooks on this (e.g. Santner, Williams and Notz).

A LHS with maximisation of Euclidian distances in the $\{e,\varpi,\$varepsilon\}$ space, as done here, therefore implies two wrong assumptions: there is a non-zero distance between two experiments with different $\varpi$ but zero eccentricity (while the insolation input is rigorously the same), and there is a twice greater distance between $\varpi$=359 degrees and $\varpi$=0 degrees, than between, say $\varpi$=100 degrees and $\varpi$=280 degrees. Both are wrong, not because of assumptions on model response, but because $\varpi$ is (1) an angle which (2) loses meaning at $e=0$.

On the other hand, as Devleeschouwer or Araya-Melo have shown, an $\{e\sin\varpi$,$e\cos\varpi\}$ input space does not prevent at all to detect effects of eccentricity. However, because of the geometry of insolation, we can never expect the same climate in two experiments with both high eccentricity but two opposite phases of $\varpi$. Net effects of eccentricity tend to come as the result of a non-symmetrical (non-linear) effects of positive and negative climatic precession anomalies (a form of rectification), or possibly more subtle effects in the tropics which could also be detected with a $\{e\sin\varpi$,$e\cos\varpi\}$ experiment design.

Of course I do not want to block a nice paper for that reason alone, but I cannot accept the line of defense that LHS sampling a $\{e,\varpi\}$ space is a good option, out of the fear of spreading bad practice.

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ED: Publish subject to minor revisions (review by editor) (21 Nov 2017) by Arne Winguth

AR by John Keery on behalf of the Authors (01 Jan 2018) Author's response Manuscript

ED: Publish as is (14 Jan 2018) by Arne Winguth

AR by John Keery on behalf of the Authors (17 Jan 2018)

Short summary

In the Eocene (~ 55 million years ago), the Earth had high levels of atmospheric CO₂, so studies of the Eocene can provide insights into the likely effects of present-day fossil fuel burning. We ran a low-resolution but very fast climate model with 50 combinations of CO₂ and orbital parameters, and an Eocene layout of the oceans and continents. Climatic effects of CO₂ are dominant but precession and obliquity strongly influence monsoon rainfall and ocean–land temperature contrasts, respectively.

Sensitivity of the Eocene climate to CO2 and orbital variability

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Sensitivity of the Eocene climate to CO₂ and orbital variability