Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean
<p>In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH<span class="inline-formula"><sub>4</sub></span>) mixing ratios have been analysed. However, to quantify the influence of the respective slow cl...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/731/2021/acp-21-731-2021.pdf |
| Summary: | <p>In a previous study the quasi-instantaneous chemical impacts
(rapid adjustments) of strongly enhanced methane (CH<span class="inline-formula"><sub>4</sub></span>) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven
temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH<span class="inline-formula"><sub>4</sub></span> mixing ratios with
the chemistry–climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixed-layer ocean model to account for tropospheric warming.</p>
<p>Strong increases in CH<span class="inline-formula"><sub>4</sub></span> lead to a reduction in the hydroxyl
radical in the troposphere, thereby extending the CH<span class="inline-formula"><sub>4</sub></span> lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl
radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH<span class="inline-formula"><sub>4</sub></span> lifetime in comparison with the quasi-instantaneous response.</p>
<p>Changes in the stratospheric circulation evolve clearly with the
warming of the troposphere. The Brewer–Dobson circulation strengthens,
affecting the response of trace gases, such as ozone, water vapour and
CH<span class="inline-formula"><sub>4</sub></span> in the stratosphere, and also causing stratospheric temperature changes.
In the middle and upper stratosphere, the increase in stratospheric
water vapour is reduced with respect to the quasi-instantaneous
response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH<span class="inline-formula"><sub>4</sub></span> oxidation. However, in the lower
stratosphere water vapour increases more
strongly when tropospheric warming is accounted for, enlarging its
overall radiative impact. The response of the stratosphere adjusted
temperatures driven by slow climate feedbacks is dominated by these
increases in stratospheric water vapour as well as strongly decreased
ozone mixing ratios above the tropical tropopause, which result from
enhanced tropical upwelling.</p>
<p>While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH<span class="inline-formula"><sub>4</sub></span> radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH<span class="inline-formula"><sub>4</sub></span> changes in this chemistry–climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH<span class="inline-formula"><sub>4</sub></span> effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes.</p> |
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| ISSN: | 1680-7316 1680-7324 |