Sulfate geoengineering impact on methane transport and lifetime: results from the Geoengineering Model Intercomparison Project (GeoMIP)
Sulfate geoengineering (SG), made by sustained injection of SO<sub>2</sub> in the tropical lower stratosphere, may impact the CH<sub>4</sub> abundance through several photochemical mechanisms affecting tropospheric OH and hence the methane lifetime. (a) The reflection of i...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/11209/2017/acp-17-11209-2017.pdf |
| Summary: | Sulfate geoengineering (SG), made by sustained injection of SO<sub>2</sub> in the
tropical lower stratosphere, may impact the CH<sub>4</sub> abundance through several
photochemical mechanisms affecting tropospheric OH and hence the methane
lifetime. (a) The reflection of incoming solar radiation increases the
planetary albedo and cools the surface, with a tropospheric H<sub>2</sub>O decrease.
(b) The tropospheric UV budget is upset by the additional aerosol scattering
and stratospheric ozone changes: the net effect is meridionally not uniform,
with a net decrease in the tropics, thus producing less tropospheric
O(<sup>1</sup>D). (c) The extratropical downwelling motion from the lower
stratosphere tends to increase the sulfate aerosol surface area density
available for heterogeneous chemical reactions in the mid-to-upper troposphere,
thus reducing the amount of NO<sub><i>x</i></sub> and O<sub>3</sub> production. (d) The tropical
lower stratosphere is warmed by solar and planetary radiation absorption by
the aerosols. The heating rate perturbation is highly latitude dependent,
producing a stronger meridional component of the Brewer–Dobson circulation.
The net effect on tropospheric OH due to the enhanced
stratosphere–troposphere exchange may be positive or negative depending on
the net result of different superimposed species perturbations (CH<sub>4</sub>,
NO<sub><i>y</i></sub>, O<sub>3</sub>, SO<sub>4</sub>) in the extratropical upper troposphere and lower
stratosphere (UTLS). In addition, the atmospheric stabilization resulting
from the tropospheric cooling and lower stratospheric warming favors an
additional decrease of the UTLS extratropical CH<sub>4</sub> by lowering the
horizontal eddy mixing. Two climate–chemistry coupled models are used to
explore the above radiative, chemical and dynamical mechanisms affecting
CH<sub>4</sub> transport and lifetime (ULAQ-CCM and GEOSCCM). The CH<sub>4</sub> lifetime may
become significantly longer (by approximately 16 %) with a sustained
injection of 8 Tg-SO<sub>2</sub> yr<sup>−1</sup> starting in the year 2020, which implies an increase of tropospheric
CH<sub>4</sub> (200 ppbv) and a positive indirect radiative forcing of sulfate
geoengineering due to CH<sub>4</sub> changes (+0.10 W m<sup>−2</sup> in the 2040–2049
decade and +0.15 W m<sup>−2</sup> in the 2060–2069 decade). |
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| ISSN: | 1680-7316 1680-7324 |