|Referee Comment on: Uncertainties in isoprene photochemistry and emissions: implications for the oxidative capacity of past and present atmospheres and for climate forcing agents |
Achakulwisut et al have dramatically improved this manuscript and the scientific contribution is clearly stated. The only major issue that was not satisfactorily addressed in the first round of revisions is the justification of their “cold LGM” scenario. In their response they cite Holden et al (2010) and Gilder et al (2014), however neither of these papers specifically address tropical SSTs. Holden et al (2010) investigated interhemispheric coupling between the West Antarctic Ice Sheet and the AMOC. Gilder et al (2014) examined how sea ice affected the hydrologic cycle. Using the tropical SSTs from these model sensitivity studies is taking their studies out of context. To my knowledge, the current best proxy-based reconstruction of LGM tropical SSTs comes from the MARGO project that found tropical SSTs that are close to their “warm LGM” scenario SSTs. It is also instructive that the Webb et al (1997) paper is not cited in the IPCC AR5 in the chapter about paleoclimate archives. Instead, the IPCC AR5 table 5.2 lists a host of studies for low latitude SSTs that do not exceed 3.2 C (including reported uncertainties). In addition to the MARGO work, they cite: Ballantyne et al. (2005); Lea et al. (2000); de Garidel-Thoron et al. (2007); Leduc et al. (2007); Pahnke et al. (2007); Stott et al. (2007); Koutavas and Sachs (2008); Steinke et al. (2008); Linsley et al. (2010). It appears that the authors are using the “cold LGM” conditions primarily because this was the model framework used in Murray et al (2014). It is my view that this rationale is insufficient for including the “cold LGM” scenario for these reasons: 1.) Just because someone else did it doesn’t mean it should be done again if the justification is lacking. 2.) How many of the conclusions in the present manuscript and to what extent are the magnitudes of the sensitivities dependent on the cold LGM scenario? If future work cites ranges of uncertainties reported in this paper, they will be citing ranges that include a climate scenario that is not consistent with paleoclimate archives. Since one of the central messages of this study is to point out that there are large uncertainties in understanding the oxidative capacity of the atmosphere in the past, this is an important issue.
Pg 4, line 3 – “of this record” is not needed in this sentence.
Pg 4, line 7 – “…past variations in atmospheric methane concentration” -> concentration should be plural: “concentrations”
Pg 4, end of 1st paragraph – There are other studies here that have been left out, principally:
Levine, J. G., E. W. Wolff, A. E. Jones, L. C. Sime, P. J. Valdes, A. T. Archibald, G. D. Carver, N. J. Warwick, and J. A. Pyle. “Reconciling the Changes in Atmospheric Methane Sources and Sinks between the Last Glacial Maximum and the Pre-Industrial Era.” Geophysical Research Letters 38 (December 2011). doi:10.1029/2011gl049545.
In re-reading Levine et al 2011, there should probably be more comparisons with this paper throughout the present manuscript since they cover very similar topics.
In section 2.4 in the paragraph about prescribed methane concentrations, there is an inaccurate statement. The manuscript says: “In GEOS-Chem, atmospheric methane concentrations are prescribed with imposed meridional gradients derived from observations, except for the tropical LGM in which model results are used (Murray et al., 2014, Table 3).” However, in Murray et al. 2014 they state that present day values come from flask measurements, but BOTH preindustrial & LGM values are derived from ice core measurements for the poles while tropical values are assumed to be 4% higher than Antarctic methane concentrations. There are no preindustrial methane observations from the tropics.
Pg 28, 1st paragraph: “Results from this sensitivity studies…” should be “Results from THESE sensitivity studies…”
This is a stylistic note, but on pg 29, last paragraph the part starting with “We are reluctant to offer “best guess” estimates…” seems to undercut or qualify their central contribution before they describe what their central contribution actually is. I would suggest moving this section to the end of the first paragraph on page 31. However, this is up to the authors.