Middle‐atmospheric response to a future increase in humidity arising from increased methane abundance

Abstract

[1] The response of the middle atmosphere to an increase in humidity arising from a possible future increase in CH 4 is examined in a general circulation model with interactive H 2 O and O 3 . A chemical parameterization allows the middle-atmospheric H 2 O change to evolve naturally from an imposed change in tropospheric CH 4 . First, a simulation of the year 2060 using postulated loadings of the radiatively active gases is compared with a control simulation of the present-day atmosphere. Then, the particular contribution of the CH 4 (and hence H 2 O) change to the observed difference is isolated by repeating the 2060 simulation without the projected CH 4 change. Under the Intergovernmental Panel on Climate Change Special Report on Emission Scenarios (SRES) B2 scenario, the middle atmosphere in 2060 cools by up to $5 K relative to 1995, with the CH 4 -derived increase in H 2 O accounting for $10% of the change. The cooling is accompanied by a strengthened general circulation, intensified dynamic heating rates, and a reduction in the mean age of middle-atmospheric air. The component of the circulation change attributable solely to the H 2 O change differs somewhat from the net response: The H 2 O change causes a greater increase in the descent rate in the north than in the south, ages the stratospheric air, and has a distinct effect on age/N 2 O correlations. Around 20% of the increased prevalence of polar stratospheric clouds (PSCs) in 2060 is due to the microphysical effect of the extra H 2 O, with the remainder attributable to the reduced vortex temperatures. Although the PSC increase facilitates release of reactive chlorine, this positive impact on chemical O 3 destruction is outweighed by the negative impact of the reduced total chlorine in 2060. Nonetheless, the H 2 O increase does make the 2060 Arctic O 3 loss $15% greater than it would otherwise be.

Relaxing the well‐mixed greenhouse gas approximation in climate simulations: Consequences for stratospheric climate

“…Their results agreed qualitatively with those of Rind and Lonergan, and also revealed the sensitivity of the results to the exact location and magnitude of the water vapor perturbation [ Oinas et al , 2001; Forster and Shine , 2002]. MacKenzie and Harwood [2004] examined the middle atmosphere response to a humidity increase due to CH 4 oxidation in a future equilibrium scenario. In addition to the expected radiative response, these authors found a modest strengthening of the stratospheric (residual mean) circulation.…”

The role of methane in projections of 21st century stratospheric water vapour

“…Increases in SWV lead to stratospheric ozone depletion both by enhancing odd hydrogen cycles and by increasing the prevalence of polar stratospheric clouds (PSCs), which facilitate polar springtime ozone depletion (Kirk-Davidoff et al, 1999;MacKenzie et al, 2004;Stenke and Grewe, 2005). SWV owes its existence primarily to transport from the troposphere, which occurs predominantly through the tropical cold-point tropopause (CPT) (Brewer, 1949;Holton and Gettelman, 2001) and to methane oxidation via Reaction (R1) (Bates and Nicolet, 1950;Le Texier et al, 1988):…”

Investigating the yield of H<sub>2</sub>O and H<sub>2</sub> from methane oxidation in the stratosphere

“…(1) could be replaced or supplemented by an explicit parameterization of the chemical loss of H 2 O, mostly via photolysis and the reaction with O( 1 D); see MacKenzie and Harwood (2004) and McCormack et al (2008). In the simplified methane chemistry of EMAC, for example, a predefined O( 1 D) is also used for the reaction with CH 4 and could be reused for the reaction with H 2 O.…”