|I find the manuscript improved in the second round of submission. However, there are still major revisions necessary. But the only new analysis that I suggest is a supplementary table or figure showing changes in major ozone precursor emissions throughout the time period analyzed. The other major revisions I suggest involve word-choice, organization, and clarity of the manuscript, which I think need substantial improvement before publication. I would like to see more discussion of the calibration of the ozone damage parameterization discussed and model evaluation of the high vs. low ozone sensitivity simulations in the main text (especially in regards to the implications of the model evaluation for the investigation here), as they are central to the novelty of the study and thus its findings. I also think the authors need to describe supplemental material and findings in the main text; not only refer the readers to them without context.|
To help readers, I encourage the authors to name their sensitivity simulations and refer to their sensitivity simulations by these names, as opposed to saying things like “the CO2 only run”, or “O3 and CO2 simulations”, “varying CO2 and O3 together”, as these phrases are rather ambiguous.
Line by line comments
Line 31: I disagree that the “impact of the gas on European vegetation and the land carbon sink is largely unknown” - the authors show in their discussion of the literature that there is a substantial amount of work on this. I urge the authors to motivate their work in a way that complements the previous work.
Line 35: I don’t think the authors can call their new stomatal conductance parameterization “an improved” one. I would suggest finding another way to describe it.
Line 41: Where is this discussed in the paper?
Line 82: Please cite papers showing that ozone damage is “key” take into account
Line 98-101: Please clarify the relevance of this text to the analysis
Lines 107-108: The authors should revise their phrasing here.
Line 118: A paper that examines the ozone budget and source/sink terms would be more appropriate to reference here. Fowler et al. (2009) can be the citation for “primarily uptake by … vegetation”, but these papers don’t show that it’s an important sink. Admittedly it’s hard to find papers with the surface ozone budget, but there are many papers that examine the parts of the tropospheric ozone budget.
Line 131-132: Instead of saying “future concentrations of ozone predicted for 2050”, the authors should give the concentration, as these numbers are highly dependent on emission scenario and not necessarily generalizable to a given year.
Line 150: What are the “new measurements”? The g1 parameter? g1 can’t be measured, only inferred. Please revise.
Line 151: “conduct a dedicated analysis” has little meaning. Please revise.
Line 157-158: I would cut everything in this sentence starting with “such that” because I think that it implies independent responses.
Line 169-170: Here the phrase about not including stomatal sluggishness is a bit awkward.
Line 167-176: This is a rather technical paragraph for the introduction. I wonder if the authors could illustrate the novelty of the study without as much jargon.
Line 183: “look at the interaction between O3 and CO2” is ambiguous. Same thing for Lines 196-197, and other points in the text. Please revise.
Line 183-185: As discussed below, I don’t think this is a reason for why this study is novel, and urge the authors to cut this from the introduction.
Line 189: I’m not certain how the high and low ozone sensitivity simulations represent the large variation within and between species specifically, rather than just the large uncertainty in the ozone response generally. Please clarify that for both the high and low sensitivity simulations, there is a distinction between the sensitivities of crops vs. grasslands. What about forests?
Line 193-6: Please clarify here that the authors are forcing with daily ozone concentrations that are scaled to a diurnal cycle. The authors’ phrasing implies that hourly concentrations are archived and used to force the model.
Line 199-201: The authors should also note here that not using coupled chemistry and climate also creates additional uncertainty.
Line 216: The order of the supplemental figures should reflect the order that they are mentioned in the main text
Line 226: Lombardozzi and colleagues’s work shows that there are separate impacts of ozone on stomatal vs. photosynthesis. This merits mention, as this is how ozone damage is configured in one of the only other land surface models with ozone damage. Ozone damage is also a function of cumulative ozone exposure, rather than an instantaneous effect. Please comment on this.
Line 247-249: Nonstomatal conductances are highly variable and substantially larger than this single prescribed value on average across sites. Including this term as 0.04 cm/s should only decrease stomatal uptake by a very small amount. I disagree that this adds any value to the authors’ study.
Line 253: How are leaf dimensions defined?
Line 262: What about the resistance to turbulence in the canopy? This is highly uncertain, but can be substantial. Could it be that too much ozone is getting deep into the canopy?
Line 270-273: I’m finding this hard to follow, especially because how the authors refer to the sensitivity simulations is “high/low plant ozone sensitivity”. I understand that within each sensitivity simulation there are variations among land cover types in terms of the degree of the sensitivity to ozone applied. Further, ozone “dose-response functions” is never defined. Again, it seems like this calibration is a fundamental part of the authors’ analysis. I would suggest that some supplemental material is moved to the main text and cleaned up so the methods are very clear.
Line 286-9: What observations? I am certain FO3crit cannot be measured, only inferred. Cumulative ozone uptake over what time period?
Line 317-324: This discussion seems out-of-place here. It might be more appropriate in the conclusions w.r.t. the “next steps”, please revise.
Line 324-325. Please include more details in the main text as to why the author would go to the supplemental for this analysis.
Line 326: Please briefly state the results of the FLUXNET model evaluation in the main text. Also, the text in the supplemental says that there are large improvements in the seasonal cycle. Large seems like a stretch - instead I would quantify the changes in the RMSE.
Line 335: I would urge the authors to stay away from suggesting that their analysis will allow a “full understanding”
Line 345: By “no form of land management”, do the authors mean that there is no harvesting of crops or grazing of grasses? If so, how “big” do crops and grasses get, and what does this mean for their results?
Line 347: Does the change follow Hurtt et al. 2011? I would refrain from using the term “little” as this gives the reader little understanding of what is going on. Showing only 2050 is not too helpful.
Lines 356-8: Whether emissions or meteorology matters more is going to depend on the emissions and climate variability. Langner et al. (2012b) only examine 1990 onwards and so I don’t think the authors can use this work to comment on emissions vs. meteorology in 1900-1959.
Lines 358-361: I don’t really know what this means. The authors should show the trend in emissions of NOx, methane, and isoprene from 1900-2050 over Europe, which is standard practice in atmospheric chemistry papers, so that readers can fully understand the emission scenarios used, as this is central to the findings.
Line 362-3: This is confusing. I would cut this everything after “however”
Line 367-369: Please clarify how the authors map the ozone concentrations from the land cover categories to the model. What do the differences in ozone concentrations over the different land cover types represent? Differences in dry deposition, BVOC emissions, or just turbulent mixing? Instead of saying “more accurate”, the authors should just say something like ozone concentrations peak during the day so it’s important to take the diurnal cycle into account.
Line 377: Typo
Lines 381-394: Some of this is incorrect and the discussion is lengthy. I simply wanted the authors to note changes in the seasonal cycle of ozone depend strongly on anthropogenic NOx (not because the timing of emissions during the year, rather nonlinear ozone chemistry), the emissions scenario matters for the results regarding uptake of ozone to vegetation, which it seems like they are getting at eventually. I would cut most of this.
Lines 394-397: Jumping from surface ozone seasonality to plant phenology seems erratic. I would suggest moving this discussion elsewhere.
Line 420: I see that the authors examine ozone impacts on stomatal conductance, which could be referred to as “plant physiology”, but it doesn’t seem to me like GPP and C sink are “plant physiology” entities.
Line 421-423: The authors should tell the reader why we should go to the supplemental. “for calculation of the effects due to” is vague.
Line 435: What is a wet site? Specify in the main text.
Line 441: Same for dry site.
Line 442-5: Why should one wet and one dry site represent the entire domain?
Line 445-447: It’s not clear what the authors’ point here is. Since the authors’ simulations are uncoupled, it’s an added uncertainty that changing stomatal conductance is going to impact energy partitioning and thus meteorology. Is that all they are trying to get at here?
Line 455-457: Why do the authors show the bottom row? Is it giving more information then the top row? I would understand if the stomatal uptake and ozone damage fed back onto the ozone concentrations the authors would need the bottom row. As this is not the case, this bottom row should be cut; but I agree that the authors should make this point in the text, which they do. Please clarify in the text what further details are in the supplemental.
Line 473: What do the authors conclude about the comparison between the simulations with high and low ozone sensitivity vs the MTE-GPP product?
Line 484-5: Please re-phrase so that it is clear that the GPP simulated by the low vs. high ozone sensitivity is significantly different
Lines 483-503: It’s confusing in the text whether the authors are discussing changes in the trend from 1901-2001, or changes in the average, due to ozone. Please revise the text accordingly.
Line 516-7: Again, suggesting that the O3 impact on the land carbon sink is a source of carbon is not really appropriate; re-phrasing would allow for the same take-away
Line 523: Please quantify the “large” spatial variability
Line 527-529: With “therefore”, are the authors suggesting that the decreases in GPP are from springtime increases in temperate/Mediterranean regions are because springtime ozone is increases? Please clarify in the text. What is going on in the boreal region?
Line 529: Ok, so the previous sentences are analyzing the simulations without CO2 fertilization? It would be best to make this clear before this point.
Line 533-534: What are the implications of this?
Line 567: “Over the Anthropocene” is ambiguous
Line 634: The authors’ use of “leaf-level” stomatal conductance in this paragraph is confusing; earlier they define leaf-level stomatal conductance as non-canopy integrated stomatal conductance; is this what they are examining here?
Lines 633-648: I would like to see some discussion of the model evaluation of the stomatal conductance models (e.g., FLUXNET). Regarding the last sentence of this paragraph, I would make this statement specific to the uncoupled approach. Higher deposition would reduce ozone concentrations in a coupled chemistry-land study.
Lines 649-661: Do the authors have any hypotheses for why their study shows lower impact on GPP, or do the authors think their results are reasonable in comparison to the other work? On that note, I do not see any support for the last sentence of the paragraph. I would encourage the authors to change the phrasing to be more speculative (instead of saying that this is “likely” the result of).
Line 687-691: Using a stomatal conductance parameterization that simulates higher gs will certainly lead to higher uptake. The higher uptake may decrease ozone concentrations, but the stronger ozone damage may increase ozone concentrations. It’s hard to say which will dominate in the authors’ uncoupled simulations, especially because ozone is fairly well-buffered in models (one sink reduces, another sink kicks in), how the high vs. low ozone sensitivity simulations will be different, and if this high sensitivity study is indeed an “upper bound”.
Line 711: Typo
Line 718: Here it is relevant to discuss the findings of Lombardozzi that there are separate impacts of ozone on photosynthesis and stomatal conductance
Line 724: By gmax are the authors referring to input, or output of the model? If input, I don’t think the authors’ reasoning makes sense. The parameterizations are different and act to scale stomatal conductance by very different entities.
Line 728-730: Ozone deposition can have a substantial impact on surface ozone concentrations (Val Martin et al., 2014). I would not argue this.
Line 730-732: Where is there evidence that stomatal conductance does not influence ozone concentrations above the canopy? I would not argue this. Further, strong vertical mixing above trees means that this is not the limiting factor for deposition - rather stomatal deposition and nonstomatal deposition are.
Line 732-733: Again, where in the literature is there evidence of this?
Line 733-736: I’m not sure what the authors are getting at here.
Line 774: Is there no evidence because studies have not been done? These are rather old papers - no work has been done since?