the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Present-Day Methane Shortwave Absorption Mutes Surface Warming and Wetting Relative to Preindustrial Conditions
Abstract. Recent analyses show the importance of methane shortwave absorption, which many climate models lack. In particular, Allen et al. (2023) used idealized climate model simulations to show that methane shortwave absorption mutes up to 30 % of the surface warming and 60 % of the precipitation increase associated with its longwave radiative effects. Here, we explicitly quantify the radiative and climate impacts due to shortwave absorption of the present-day methane perturbation using the Community Earth System Model version 2. Our results corroborate that present-day methane shortwave absorption mutes the warming and wetting effects of longwave absorption. For example, the global mean cooling in response to the present-day methane shortwave absorption is -0.10 ± 0.04 K, which offsets 29 % of the surface warming associated with present-day methane longwave radiative effects. Similarly, we explicitly estimate 66 % of the precipitation increase associated with the longwave radiative effects of the present-day methane perturbation is offset by shortwave absorption. Unlike other solar absorbers (i.e., black carbon), the decrease in global mean precipitation under methane shortwave absorption is driven by both fast (atmospheric absorption) and slow (surface temperature cooling) responses. Finally, we show that the present-day methane shortwave radiative effects, relative to its longwave radiative effects, are about five times larger as compared to those under idealized carbon dioxide perturbations. The unique responses to methane shortwave absorption are related to its vertical atmospheric solar heating profile. Methane remains a potent greenhouse gas and continued endeavors to decrease methane emissions are necessary to stay below the 1.5 °C global warming threshold.
- Preprint
(1996 KB) - Metadata XML
-
Supplement
(985 KB) - BibTeX
- EndNote
Status: open (until 22 May 2024)
-
RC1: 'Comment on egusphere-2024-872', Anonymous Referee #1, 26 Apr 2024
reply
Allen et al. assess the impact of methane (CH4) shortwave absorption for the increase of CH4 concentration from pre-industrial to present-day conditions.
The study builds on previous work (Allen et al., 2023) which has quantified the impact of CH4 shortwave absorption for idealized CH4 perturbations (2x, 5x, 10x pre-industrial CH4). The present study extends the analysis by explicitly simulating the impact for the present-day CH4 concentration, which corresponds to an increase of 2.5x pre-industrial CH4. Consistent with the 2x, 5x, 10x CH4 experiments, the present study finds that shortwave absorption of methane significantly mutes the effect of its longwave absorption. The study extends the analysis by an assessment of the energy budget and by comparing the effect of methane shortwave absorption to the effect of CO2 shortwave absorption.
The results are presented in a clear and understandable way. In my opinion it is a useful contribution to the understanding of the role of methane shortwave absorption. And - considering methane’s short atmospheric lifetime – the findings are further relevant for the scientific assessment of short-term climate change mitigation options.
Therefore, I recommend publication after some minor revisions detailed below.General comment
In my opinion, the paper is clearly written throughout most of the text. However, there are some formulations that might be misleading, especially if used out of context. At some points, the formulations “negative ERF” or “surface cooling” “under SW absorption” are used. I understand that “SW absorption mutes/offsets the (total) ERF” or “the SW effect/contribution to the ERF is negative” is meant. However, especially the formulation “under SW absorption” might be misleading as it could also mean “total ERF/temperature response if SW absorption is accounted for”. Therefore, I suggest to carefully review the formulations and adapt the text where it might be misleading.
Some examples are:
• l. 114: “For example, the global mean near-surface air temperature (TAS) response under 5xCH4SW and 10xCH4SW (Figure 1a) yielded significant global cooling at -0.23 and -0.39 K.”
• l. 273: “This negative rapid radiative adjustment promotes a negative ERF under methane SW absorption. …”
• l. 297: “.., 2.5xCH4SW yields larger (10-20%) and more negative TOA and surface IRFs, ERFs, and ADJs. The larger negative ERFs (and ADJs) act to promote cooling.”
I think that the SW contribution to TOA IRF (2.5CH4SW) is not even negative, but weakly positive (Fig. 2a)).
• l. 644: The total rapid radiative adjustment for both CO2 perturbations is negative under SW radiative effects at …”
• l. 831: “… leading to a negative ERF.Specific comments
l. 80: The term “rapid adjustments” is used in the introduction without a detailed explanation, which follows in the Methods section. Please shortly explain the term in the introduction or refer to the Methods section.
l. 159: I assume that the simulations are all “time slice simulation” (=cyclic repetition of the boundary conditions every year). This is not explicitly stated.
l. 209: Here an explicit description how the surface temperature driven feedbacks (e.g. Fig. 5) are calculated is missing. I assume that they are also calculated using the kernel method, but with the climate variable from the coupled ocean experiments. The radiative effects of the slow response are then presumably calculated as difference between radiative effects of the fast and total response?Section 3.4 /Fig. 5:
• The second paragraph (l. 443-455) might be moved to section 3.1 as only the rapid adjustments are discussed.
• The radiative effects of the total and slow response are not shown for CH4LW and CH4LW+SW. A figure similar to Fig. 2 b) could be added in the supplement as comparisonSection 3.5.:
I am a bit confused about the sign convention in this section, which makes it difficult to follow the discussion. Could you give more detail on how to calculate LWC and SWC? Do they represent the divergence of LW/SW radiative fluxes in the total atmospheric column (=loss or gain of radiative energy of the total atmospheric column)?
If yes, I would presume that SWC would lead to energy gain (=warming) for reference conditions as the net downward SW flux at TOA is larger than the net downward SW flux at the surface (see e.g. Fig. 7.2 in IPCC-AR6, The Physical Science Basis). The LWC should lead to energy loss (=cooling) for reference conditions as the net downward LW flux at TOA is more strongly negative than the net downward LW flux at the surface, is this correct? The combined effect of LWC and SWC would be cooling (=net energy loss) as the absolute value of LWC is larger than SWC.
Does a positive LWC / SWC represent cooling (= net energy loss) or warming (=net energy gain)?l. 850: It might be worth mentioning here that chemical composition changes of O3 and stratospheric H2O also affect the temperature response and thereby the static stability in the upper troposphere and stratosphere (see e.g. Winterstein et al, 2019, their Fig. 8; https://doi.org/10.5194/acp-19-7151-2019; for the temperature response induced by O3 and H2O changes in the stratosphere). Could this affect the cloud adjustment processes?
Typos / technical corrections:
l. 74: Etminan et al., 2016 (misspelled)
l. 94 : “… isolate the effect of …”
l. 129: estimates (plural)
l. 149: “targeted methane-only equilibrium climate simulations”- This implies simulations perturbed by methane only, but you also conducted CO2 experiments.
l. 217: I assume it is 5% of ERF?
l. 325: Double mentioning of word correlation: “Correlations between … are significant.”
l. 352: Should the unit of static stability be “K/km”?
l. 471: Avoid line breaking between - and corresponding number (-0.31 Wm-2).Captions of Supplementary Fig. 1, 4 and 5: “Total climate responses are estimated using from coupled ocean-atmosphere CESM2 simulations.” – “using data from coupled ocean-atmosphere simulations”
Captions of Supplementary Fig. 2 and 3: “Annual mean global mean spatial fast responses”: The data do not show the global mean, but the spatial distribution.Citation: https://doi.org/10.5194/egusphere-2024-872-RC1
Viewed
HTML | XML | Total | Supplement | BibTeX | EndNote | |
---|---|---|---|---|---|---|
143 | 33 | 11 | 187 | 20 | 6 | 7 |
- HTML: 143
- PDF: 33
- XML: 11
- Total: 187
- Supplement: 20
- BibTeX: 6
- EndNote: 7
Viewed (geographical distribution)
Country | # | Views | % |
---|
Total: | 0 |
HTML: | 0 |
PDF: | 0 |
XML: | 0 |
- 1