Surface warming and wetting due to methane’s long-wave radiative effects muted by short-wave absorption

• Published in: Nature geoscience 2023-04, Vol.16 (4), p.314-320
• Main Authors: Allen, Robert J., Zhao, Xueying, Randles, Cynthia A., Kramer, Ryan J., Samset, Bjørn H., Smith, Christopher J.
• Format: Article
• Language: English
• Subjects: 704/106/694/1108; 704/106/694/2739; 704/106/694/682; Absorption; Climate; Climate change; Clouds; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Emissions; Gas absorption; Gases; Geochemistry; Geology; Geophysics/Geodesy; Global warming; Greenhouse effect; Greenhouse gases; High level clouds; Long wave radiation; Methane; Methane emissions; Outgoing long-wave radiation; Precipitation; Radiation; Radiation absorption; Radiation-cloud interactions; Radiative forcing; Relative humidity; Short wave radiation; Solar heating; Surface chemistry; Surface temperature; Wave reflection; Wetting
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-023-01144-z

Although greenhouse gases absorb primarily long-wave radiation, they also absorb short-wave radiation. Recent studies have highlighted the importance of methane short-wave absorption, which enhances its stratospherically adjusted radiative forcing by up to ~ 15%. The corresponding climate impacts, however, have been only indirectly evaluated and thus remain largely unquantified. Here we present a systematic, unambiguous analysis using one model and separate simulations with and without methane short-wave absorption. We find that methane short-wave absorption counteracts ~30% of the surface warming associated with its long-wave radiative effects. An even larger impact occurs for precipitation as methane short-wave absorption offsets ~60% of the precipitation increase relative to its long-wave radiative effects. The methane short-wave-induced cooling is due largely to cloud rapid adjustments, including increased low-level clouds, which enhance the reflection of incoming short-wave radiation, and decreased high-level clouds, which enhance outgoing long-wave radiation. The cloud responses, in turn, are related to the profile of atmospheric solar heating and corresponding changes in temperature and relative humidity. Despite our findings, methane remains a potent contributor to global warming, and efforts to reduce methane emissions are vital for keeping global warming well below 2 °C above preindustrial values. Climate simulations suggest that the contribution of methane to climate warming and wetting due to absorption of long-wave radiation is partially counteracted by short-wave absorption.