Low latitude mesospheric clouds in a warmer climate

Observations show that mesospheric clouds (MCs) have been increasing in recent decades, presumably due to increased mesospheric water vapor which is mainly caused by greater methane (CH₄) oxidation in the middle atmosphere. Past warm climates such as those of the early Cretaceous and Paleogene periods are thought to have had higher CH₄ concentrations than present day, and future CH₄ concentrations will also likely continue to rise. Here, idealized atmosphere chemistry-climate model experiments forced with strong polar-amplified sea-surface temperatures and elevated carbon dioxide (CO₂) and CH₄ concentrations predict a substantial spreading of MCs to middle and low latitudes, well beyond regions where they are currently found. Sensitivity tests show that increased water vapor from CH₄ oxidation and cooling from increased CO₂ and CH₄ concentrations create favorable conditions for cloud formation, producing MC fractions of 0.02 in the low latitudes and 0.1 in the mid-latitudes in the Northern Hemisphere when CH₄ concentration is 16× higher than pre-industrial. Further increases in CH₄ result in a monotonic increase in low- and mid-latitude MCs. A uniform surface ocean warming, changes in polar amplification, or the solar constant do not significantly affect our results. While the appearance of these clouds is interesting, their ice and liquid water content is not sufficient to cause a significant radiative effect. On the other hand, dehydration of the mesosphere due to these low- and mid-latitude MCs could potentially lead to a reduction in atomic hydrogen, thereby affecting mesospheric ozone concentration, although further study is required to confirm this.