How Does Tropospheric VOC Chemistry Affect Climate? An Investigation Using the Community Earth System Model Version 2
Abstract. Because of their computational expense, models with comprehensive tropospheric chemistry have typically been run with prescribed sea surface temperatures (SSTs), which greatly limits the model's ability to generate climate responses to atmospheric forcings. In the past few years, however, several fully-coupled models with comprehensive tropospheric chemistry have been developed. For example, the Community Earth System Model version 2 with the Whole Atmosphere Community Climate Model version 6 as its atmospheric component (CESM2-WACCM6) has implemented fully interactive tropospheric chemistry with 231 chemical species as well as a fully coupled ocean. Earlier versions of this model used a "SOAG scheme" that prescribes bulk emission of a single gas-phase precursor to secondary organic aerosols (SOAs). The additional chemistry in CESM2-WACCM6 simulates the chemistry of a comprehensive range of volatile organic compounds (VOCs) responsible for tropospheric aerosol formation. Such a model offers an opportunity to examine the full climate effects of comprehensive tropospheric chemistry. To examine these effects, 141-year preindustrial control simulations were performed using the following two configurations: 1) the standard CESM2-WACCM6 configuration with interactive chemistry over the whole atmosphere (WACtl), and 2) a simplified CESM2-WACCM6 configuration using a SOAG scheme in the troposphere and interactive chemistry in the middle atmosphere (MACtl). The middle atmospheric chemistry is the same in both configurations, and only the tropospheric chemistry differs. Differences between WACtl and MACtl were analyzed for various fields. Regional differences in annual mean surface temperature range between -4 K and 4 K. These surface temperature changes are comparable to those produced over a century in future climate change scenarios, which motivates future research to investigate possible influences of VOC chemistry on anthropogenic climate change. In the zonal average, there is widespread tropospheric cooling in the extratropics. Longwave forcers are shown to be unlikely drivers of this cooling, and possible shortwave forcers are explored. Evidence is presented that the climate response is primarily due to increased organic nitrates in the troposphere, increased sulfate aerosols in the stratosphere and cloud feedbacks. The possible chemical mechanisms responsible for these changes are discussed. As found in earlier studies, enhanced internal mixing with SOAs in WACtl causes reduced black carbon (BC) and reduced primary organic matter (POM), which are not directly influenced by VOC chemistry. These BC and POM reductions might also contribute to cooling in the Northern Hemisphere. The extratropical tropospheric cooling results in dynamical changes, such as equatorward shifts of the midlatitude jets, which in turn drive extratropical changes in clouds and precipitation. In the tropical upper troposphere, cloud-driven increases in shortwave heating appear to weaken and expand the Hadley circulation, which in turn drives changes in tropical and subtropical precipitation.
Noah A. Stanton and Neil F. Tandon
Status: open (until 05 Apr 2023)
- RC1: 'Comment on acp-2023-17', Anonymous Referee #1, 10 Mar 2023 reply
- RC2: 'Comment on acp-2023-17', Alexander Archibald, 28 Mar 2023 reply
Noah A. Stanton and Neil F. Tandon
Noah A. Stanton and Neil F. Tandon
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This manuscript presents original and valuable earth system model results based on sensitivity experiments to investigate the tropospheric VOC chemistry effect on climate. It is well structured, written and presented. I suggest acceptance of the manuscript for publication but I have a few minor comments to be considered.
Something that I missed in the structure of the manuscript was a discussion of the induced radiative forcing due to the perturbation with explicit VOC chemistry (WACtl) versus the case with a simplified SOAG scheme and no explicit VOC chemistry (MACtl). Would you think that an estimate of effective radiative forcing from these two experiments would be feasible following the regression method of Gregory et al. (2004). For this method, it is the time development before the steady state is reached which is of interest and in your simulations the first 39 years of output (which were discarded because they showed significant trends in global mean surface temperature, indicating that the model had not fully equilibrated) may offer this opportunity.
line 130: I would suggest to add a few references for SOAG scheme implemented?
lines 198-199: The fact that the two cases were not initialized from the same ocean and sea ice states is a limitation. Although you mention that in the future, you plan to examine possible sensitivity to different initialization approaches, could you speculate based on other studies on the degree that this could influence your results?
lines 297-299: The authors mention that " This makes sense because, if low cloud cover is fixed (which is approximately true around Antarctica) and SIC decreases (increases), then the radiative effect of low clouds gets stronger (weaker), implying a negative (positive) CRE change." Maybe the authors can clarify that this is related to albedo changes and not cloud fractions changes.
Figure 5: I am puzzling by the fact that the statistical significant near surface warming over the North polar regions seen in Figure 1 is not shown in Figure 5. Do you have any explanation for this inconsistency among the two figures?
line 325: There is a weakening of the NH stratospheric polar jet but it is not clear the equatorward shift of the tropospheric mid-latitude jet.
line 328: This is confusing as in the next sentence you clarify that the responses are opposite to the poleward shift due to global warming. Are you referring to stratosphere warming (as shown in Figure 5) or the thermal forcings that qualitatively mimic three key aspects of anthropogenic climate change: (warming in the tropical troposphere, cooling in the polar stratosphere, and warming at the polar surface) discussed in Butler et al (2010) referenced here?
lines 330-332: The authors mention that "based on such past work, we would expect that, aside from any regional temperature changes, widespread tropospheric cooling would also shift the midlatitude jets equatorward". Do you refer to past work related to the impact of aerosol cooling? Please prove some references.
line 342: The use of arrows on Fig.6b could help the reader to identify the counterclockwise anomaly extending poleward of the SH HC edge and a clockwise anomaly extending poleward of the NH HC edge. This is only a suggestion.
lines 360-361: Pacific. The statement that "in addition to these HC changes, Fig. 6b also shows weakening of the Ferrel cells in both hemispheres and weakening of the polar cell in SH" needs more elaboration as it is not clear.
line 383: Please define the acronym SPS.
Line 405: The authors mention here that the warming in the Antarctica is likely explained by shortwave heating, which does increase. A link to Figure 8 that shows clearly this would be helpful for the reader.
line 448 : Are PANs included in NOy as part of organic nitrates?
Line 547: The authors mention that in the midlatitudes of both hemispheres, the downward shifts of the clouds result in negative CRE as expected. This is clear for the SH but it is not clearly evident in the case for NH midlatitudes according to Figure 13.