A model study of the impact of source gas changes on the stratosphere for 1850–2100

Abstract. The long-term stratospheric impacts due to emissions of CO₂, CH₄, N₂O, and ozone depleting substances (ODSs) are investigated using an updated version of the Goddard two-dimensional (2-D) model. Perturbation simulations with the ODSs, CO₂, CH₄, and N₂O varied individually are performed to isolate the relative roles of these gases in driving stratospheric changes over the 1850–2100 time period. We also show comparisons with observations and the Goddard Earth Observing System chemistry-climate model simulations for the time period 1960–2100 to illustrate that the 2-D model captures the basic processes responsible for long-term stratospheric change.

The ODSs, CO₂, CH₄, and N₂O impact ozone via several mechanisms. ODS and N₂O loading decrease stratospheric ozone via the increases in atmospheric halogen and odd nitrogen species, respectively. CO₂ loading impacts ozone by: (1) cooling the stratosphere which increases ozone via the reduction in the ozone chemical loss rates, and (2) accelerating the Brewer-Dobson circulation (BDC) which redistributes ozone in the lower stratosphere. The net result of CO₂ loading is an increase in global ozone in the total column and upper stratosphere. CH₄ loading impacts ozone by: (1) increasing atmospheric H₂O and the odd hydrogen species which decreases ozone via the enhanced HOx-ozone loss rates; (2) increasing the H₂O cooling of the middle atmosphere which reduces the ozone chemical loss rates, partially offsetting the enhanced HOx-ozone loss; (3) converting active to reservoir chlorine via the reaction CH₄+Cl→HCl+CH₃ which leads to more ozone; and (4) increasing the NO_x-ozone production in the troposphere. The net result of CH₄ loading is an ozone decrease above 40–45 km, and an increase below 40–45 km and in the total column.

The 2-D simulations indicate that prior to 1940, the ozone increases due to CO₂ and CH₄ loading outpace the ozone losses due to increasing N₂O and carbon tetrachloride (CCl₄) emissions, so that total column and upper stratospheric global ozone reach broad maxima during the 1920s–1930s. This precedes the significant ozone depletion during ~1960–2050 driven by the ODS loading. During the latter half of the 21st century as ODS emissions diminish, CO₂, N₂O, and CH₄ loading will all have significant impacts on global total ozone based on the Intergovernmental Panel on Climate Change (IPCC) A1B (medium) scenario, with CO₂ having the largest individual effect. Sensitivity tests illustrate that due to the strong chemical interaction between methane and chlorine, the CH₄ impact on total ozone becomes significantly more positive with larger ODS loading. The model simulations also show that changes in stratospheric temperature, BDC, and age of air during 1850–2100 are controlled mainly by the CO₂ and ODS loading. The simulated acceleration of the BDC causes the global average age of air above 22 km to decrease by ~1 yr from 1860–2100. The photochemical lifetimes of N₂O, CFCl₃, CF₂Cl₂, and CCl₄ decrease by 11–13 % during 1960–2100 due to the acceleration of the BDC, with much smaller lifetime changes (<4 %) caused by changes in the photochemical loss rates.