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

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.