Robust changes in global subtropical circulation under greenhouse warming.

The lower tropospheric subtropical circulation (SC) is characterized by monsoons and subtropical highs, playing an important role in global teleconnections and climate variability. The SC changes in a warmer climate are influenced by complex and region-specific mechanisms, resulting in uneven projections worldwide. Here, we present a method to quantify the overall intensity change in global SC, revealing a robust weakening across CMIP6 models. The weakening is primarily caused by global-mean surface warming, and partly counteracted by the direct CO 2 effect. The direct CO 2 effect is apparent in the transient response but is eventually dominated by the surface warming effect in a slow response. The distinct response timescales to global-mean warming and direct CO 2 radiative forcing can well explain the time-varying SC changes in other CO 2 emission scenarios. The declined SC implies a contracted monsoon range and drying at its boundary with arid regions under CO 2 -induced global warming.