The impact of greenhouse gases on past changes in tropospheric ozone

[1] The impact of changes in the abundance of greenhouse gases (GHGs) on the evolution of tropospheric ozone (O₃) between 1960 and 2005 is examined using a version of the Goddard Earth Observing System chemistry-climate model (GEOS CCM) with a combined troposphere-stratosphere chemical mechanism. Simulations are performed to isolate the relative role of increases in methane (CH₄) and stratospheric ozone depleting substances (ODSs) on tropospheric O₃. The 1960 to 2005 increases in GHGs (CO₂, N₂O, CH₄, and ODSs) cause increases of around 1–8% in zonal-mean tropospheric O₃ in the tropics and northern extratropics, but decreases of 2–4% in most of the southern extratropics. These O₃ changes are due primarily to increases in CH₄ and ODSs, which cause changes of comparable magnitude but opposite sign. The CH₄-related increases in O₃are similar in each hemisphere (∼6%), but the ODS-related decreases in the southern extratropics are much larger than in northern extratropics (10% compared to 2%). This results in an interhemispheric difference in the sign of past O₃ change. Increases in the other GHGs (CO₂ and N₂O) and SSTs have only a small impact on the total burden over this period, but do cause zonal variations in the sign of changes in tropical O₃ that are coupled to changes in vertical velocities and water vapor.