Effects of Ozone Levels on Climate Through Earth History
Abstract. Molecular oxygen in our atmosphere has increased from less than a part per million in the Archean Eon, to a fraction of a percent in the Proterozoic, and finally to modern levels during the Phanerozoic. While oxygen itself has only minor radiative and climatic effects, the accompanying ozone has important consequences for Earth climate. Using the Community Earth System Model (CESM), a 3-D general circulation model, we test the effects of various levels of ozone on Earth's climate. When CO2 is held constant, the global mean surface temperature decreases with decreasing ozone, with a maximum drop of ~3.5 K at near total ozone removal. By supplementing our GCM results with 1-D radiative flux calculations, we are able to test which changes to the atmosphere are responsible for this temperature change. We find that the surface temperature change is caused mostly by the stratosphere being much colder when ozone is absent; this makes it drier, substantially weakening the greenhouse effect. We also examine the effect of the structure of the upper troposphere and lower stratosphere on the formation of clouds, and on the global circulation. At low ozone, both high and low clouds become more abundant, due to changes in the tropospheric stability. These generate opposing short-wave and long-wave radiative forcings that are nearly equal. The Hadley circulation and tropospheric jet streams are strengthened, while the stratospheric polar jets are weakened, the latter being a direct consequence of the change in stratospheric temperatures. This work identifies the major climatic impacts of ozone, an important piece of the evolution of Earth's atmosphere.
Russell Deitrick and Colin Goldblatt
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1158', Jim Kasting, 15 Nov 2022
- RC2: 'Comment on egusphere-2022-1158', Anonymous Referee #2, 10 Mar 2023
Russell Deitrick and Colin Goldblatt
Russell Deitrick and Colin Goldblatt
Viewed (geographical distribution)
This is a carefully performed study of the effects of variable ozone concentrations on climate of the early Earth using a 3-D model. It’s really O2 that is changing; however, the calculations have been done at a constant surface pressure of 1 bar, eliminating the effect of O2 on surface temperature. To my knowledge, no similar 3-D calculations have been done, although the authors should be aware of (and reference) the recent study by G. Cooke et al. (Royal Soc. Open Sci. 9, 2022). Cooke et al. used a different version of CESM (called WACCM6) to perform a coupled chemistry-climate simulation of the rise of ozone. Their model produces much lower ozone column depths at low pO2 than does our own 1-D model. We have been working with those authors to understand where the discrepancies arise. Suffice it to say that at least some of the discrepancies are caused by incomplete physics and questionable modeling methodology in the 3-D calculation.
The present paper contains no such obvious flaws. I do have a few minor comments that are numbered below. Perhaps the most significant to this reviewer is the one noted in point 4 and reiterated in point 8. The present 3-D calculation of the effect of ozone on global mean surface temperature gives almost the same result as my own 1987 paper, i.e., about 3 degrees of warming (or cooling) from adding (or subtracting) ozone. So, it appears that this result may be relatively robust.
You may reveal my name to the authors.
1. (l. 29) “Therefore, the history of ozone is intimately connected to the history of molecular oxygen. For the first ∼ 2 billion years of Earth’s history (the Hadean and Archean Eons), oxygen and ozone were present only in abundances of â² 10−7 parts-per-volume (Zahnle et al., 2006; Lyons et al., 2014; Catling and Zahnle, 2020).”
--This statement might be true in the troposphere, but it is not true in the stratosphere. In the stratosphere, O2 can reach mixing ratios of 1.-e3 to 1.e-2, depending on the CO2 abundance. See, e.g., J. F. Kasting, Science (1993).
2. (l. 38) “A second rise of oxygen occurred at the end of the Proterozoic Eon and beginning of the Phanerozoic Eon, bringing oxygen up to its modern value of ∼ 21% (Lyons et al., 2014).”
--Well, maybe. Some authors (L.J. Alcott et al., Science, 2019) have argued that O2 did not reach modern levels until around 400-450 Ma in what they term the ‘Paleozoic Oxidation Event’, or POE.
3. (l. 44) “A key take-away from these studies is that ozone abundances are non-linear in oxygen concentrations—for example, ozone reaches near modern levels even at O2 levels of â² 10−3 (Garduño Ruiz et al., 2022).”
--Is this an O2 mixing ratio or is it a value in PAL (times the Present Atmospheric Level)? Also, not all models do this. I would reword it as: “for example, in some models (e.g., Garduño Ruiz et al., 2022) ozone reaches near modern levels even at O2 levels of â² 10−3.” It is a bit of a problem here that the Garduño Ruiz et al. paper is marked as ‘submitted’, so it is not possible to check these results.
4. (l. 50) “Such extremes have been principally studied using 1-D models (Morss and Kuhn, 1978; Levine and Boughner, 1979; Visconti, 1982; Kasting, 1987; Francois and Gerard, 1988). These works found that the global mean surface temperatures decreased by 5-7 K when ozone was removed.”
--I didn’t look at all these papers, but I looked at my own (Kasting, 1987). I got 3 degrees of warming from ozone and another 2 degrees from O2 (mostly through pressure broadening of CO2 and H2O). Surface pressure was allowed to vary with pO2 in that calculation. So, the sentence as written is not quite accurate. Looking forward at Figure 2 of this paper, it appears that the present authors also get about 3 or 4 degrees of warming from ozone. So, our results are not that different.
5. (l. 94) “For ozone, we use vertical profiles from the photo-chemical calculations of Garduño Ruiz et al. (2022).”
--Well, again, this is a bit of a problem. What do these vertical profiles look like? It’s really hard to review this manuscript rigorously without seeing the GR et al. paper. ..Ah, I got down further and saw that these profiles are shown in Fig. 1. Good! You should say that earlier.
6. (l. 324) “great oxidation event” should probably be capitalized.
7. (l. 328) “While Kasting (1987) and Francois and Gerard (1988) found cooler surface temperatures, using 1-D radiative-convective models, the 3-D GCM modelling of Jenkins (1995, 1999) resulted in a warmer global mean surface temperature, using the GENESIS model.”
--There is a known (to some) flaw in the GENESIS model calculations. See Payne, R. C., A. V. Britt, H. Chen, J. F. Kasting, and D. C. Catling (2016), The response of Phanerozoic surface temperature to variations in atmospheric oxygen concentration, J. Geophys. Res. Atmos., 121, 10,089–10,096. This paper was written in response to the following one: Poulsen, C. J., C. R. Tabor, and J. D. White (2015), Long-term climate forcing by atmospheric oxygen concentrations, Science, 348, 1238–1241. Poulsen et al. used the GENESIS model and found that decreasing O2 increased surface temperatures. Payne et al. found the opposite using a 1-D climate model. The Poulsen et al. result is thought to have been caused by an unrealistic cloud feedback in GENESIS (D. Pollard, private communication). Dave Pollard, who worked here at Penn State for many years, was the chief architect of the GENESIS model.
8. (l. 330) “Now, with CESM 1.2.2, our results (for Constant CO2) are more similar to the Kasting (1987) and Francois and Gerard (1988) results, albeit with a smaller global-mean surface temperature difference: a cooling of ∼5 K, compared to their ∼ 5 K and ∼ 7 K, respectively.”
--As noted in point 4 above, the Kasting (1987) model predicted 3 degrees of cooling from removing ozone.