Schematic showing a possible mechanism toward an La Niña-like state, reviewed in Lee et al. (2022). The left column shows the relevant processes in the pre-industrial climate and the right column shows how these same processes initially respond to GHG warming (this schematic does not show the final state). Technical Details (left panel): Highlighting the abundance of water vapor in the lower troposphere, and the trapping of infrared radiation (IR) by cirrus outflow from convective towers which otherwise escapes to space. Latent heating warms aloft, and evaporation cools the ocean surface. Right panel: Under GHG warming, the lower troposphere becomes more moist which decreases the gross moist stability and thus strengthens the Walker circulation. In the margins of the convective region, the horizontal moisture gradient increases, and advection from the surrounding drier region diminishes the area of convection. The so-called “iris effect” can also lead to a similar response to increased warming: Precipitation efficiency in convective tower increases, leaving less moisture for cirrus outflow. The resulting contraction of the cirrus cover in the periphery of the warm pool allows for more IR to escape, potentially cooling SSTs and thus enhancing the zonal SST gradient between the warm pool and its surroundings. Schematic by climate.gov.