Doubled CO₂‐induced cooling in the middle atmosphere: Photochemical analysis of the ozone radiative feedback

Abstract

Simple radiative arguments predict that the impact of CO 2 increases on the stratosphere and mesosphere should be a cooling and that the magnitude of the temperature change should be significantly larger than in the troposphere. Considering the temperature dependence of middle atmospheric gas‐phase ozone photochemistry, it is expected that the ozone response will generate a radiative feedback that mitigates the CO 2 ‐induced cooling. The magnitude and vertical structure of this signal need to be characterised in order to distinguish the impact of future CO 2 changes from other processes affecting the temperature evolution, such as changes in chlorine loading and water vapour trends. The Canadian Middle Atmosphere Model (CMAM) has been used in a process oriented study to examine the details of radiative and photochemical feedbacks under current and doubled CO 2 conditions at low and middle latitudes. The model was run both with and without interactive chemistry in order to determine the importance of the radiative feedback through ozone changes on the CO 2 ‐induced cooling signal. Changes in other greenhouse gases, ozone depleting substances, or SST's and sea ice coverage were not considered. The interactive model results show a substantial temperature decrease throughout most of the middle atmosphere with a maximum cooling of 10–12 K at the stratopause. In association with this temperature change, the ozone mixing ratio increases by 15–20% in the upper stratosphere and by 10–15% in the lower mesosphere. Results from the non‐interactive simulations show that the magnitude of the cooling is overestimated by up to ∼4.5 K when the ozone radiative feedback is not considered. In spite of the complexity of the ozone chemistry, the ozone increase at 30–70 km can be understood primarily as a result of the negative temperature dependence of the O + O 2 + M → O 3 + M reaction that controls odd oxygen partitioning. Additional partial contributions to the ozone increase below 60 km are provided by a decrease in the reaction rate coefficient of the Chapman reaction O + O 3 → 2O 2 and by a decrease in the NO 2 abundance.