A Large Ozone-Circulation Feedback and Its Implications for Global Warming Assessments

Abstract:

State-of-the-art climate models now include more climate processes simulated at higher spatial resolution than ever. Nevertheless, some processes, such as atmospheric chemical feedbacks, are still computationally expensive and are often ignored in long climate simulations. We use the recently developed atmosphere-ocean chemistry-climate model AO-UMUKCA (HadGEM3) to show that the choice of how to represent stratospheric ozone in climate models lacking interactive chemistry can have a first order impact on estimates of effective climate sensitivity. We find an about 1°C (~20%) larger global mean surface warming after 75 years when ozone is prescribed at pre-industrial levels compared with when it is allowed to evolve self-consistently in response to an abrupt 4xCO₂ forcing. This difference is primarily attributed to changes in longwave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes. This has important implications for global model intercomparison studies in which participating models often use highly simplified treatments of atmospheric composition changes and, in particular, for abrupt 4xCO₂ experiments where this feedback is often ignored.