Impact of the 1994–1997 temporary decrease in Northern Hemisphere stratospheric methane on the 1990s methane trend

Abstract

Methane (CH₄) ranks as the second most significant anthropogenic greenhouse gas following carbon dioxide (CO₂). It originates from a wide range of surface sources and subsequently enters the stratosphere through the tropical tropopause. In line with the observed positive trend in tropospheric CH₄, stratospheric CH₄ has shown an overall increase in the long-term trend. However, contrary to the continuous increase in tropospheric CH₄, stratospheric CH₄ exhibits a temporal decrease in the Northern Hemisphere middle and upper stratosphere during short-time periods. This study investigates the causes behind the decreasing trend of stratospheric CH₄ in the Northern Hemisphere from 1991 to 2000. We find that the extreme decrease of stratospheric CH₄ from July 1994 to May 1997 contributes to the overall decreasing trend of CH₄ from 1991 to 2000. This extreme decrease is attributed to the weakened meridional component of the residual circulation. The weakened meridional component attenuates the transport of CH₄-rich air from the low-latitude lower stratosphere to the mid-latitude middle and upper stratosphere, leading to the observed decrease in CH₄. It is further found that the smallest SST gradient in the North Pacific and adjacent regions from 1980 to 2020 is identified as a significant factor contributing to the weakened residual circulation and the decrease in CH₄. Simulations by a chemistry-climate model support the results.