evidence: A summary of radiative forcings of climate in 2019 resulting from the increases in the principal long-lived and short-lived GHGs from human activities since 1750 is shown in Figure Q17-1. Positive forcings lead to warming and negative forcings lead to cooling of Earth’s surface. Climate forcings also lead to other changes, for example reductions in glacier and sea-ice extent, variations in precipitation patterns, and more extreme weather events. International climate assessments conclude that much of the observed surface warming and changes in other climate parameters over the last several decades are due to increases in the atmospheric abundances of CO2 and other GHGs, which result from a variety of human activities. [...] For example, model projections driven by increases in the atmospheric abundance of GHGs over the coming decades show a strengthening in the global-scale atmospheric circulation that brings air from the troposphere into the stratosphere in the tropics, moves stratospheric air poleward into both hemispheres, and then returns air to the troposphere at middle to high latitudes. These circulation changes will significantly alter the global distribution of ozone and the atmospheric lifetimes of ODSs and other long-lived gases (see Q6). Also, while Earth’s surface is expected to continue to warm in response to positive radiative forcing of climate from GHGs (see Q17), the stratosphere is expected to continue to cool. A colder upper stratosphere leads to increases in ozone because lower temperatures slow down the gas-phase reactions responsible for ozone loss. Conversely, colder conditions at lower altitudes in the polar stratosphere have the potential to exacerbate ozone loss, especially if the presence of polar stratospheric clouds (PSCs) increases.