The Importance of the Ozone Layer for the Response of the Climate System to Increased CO2 Concentrations

The important role of the ozone layer for the climate system – notably how the ozone layer responds to natural and anthropogenic forcings, and how that response then feeds back on the climate itself – remains largely unexplored, apart from the effects associated with gases regulated by the Montreal Protocol. This is because, to date, most climate models do not account for the complex interplay between stratospheric ozone chemistry, dynamics and radiation. We here present recent results illustrating the importance of such interplay in the context of climate sensitivity simulations, imposing increased CO2 concentrations.

First, we present an analysis of the ozone layer response to increased CO2 concentrations in four different CMIP5 models. We show that increased CO2 levels lead to a decrease in ozone concentrations in the tropical lower stratosphere, and an increase over the high latitudes and throughout the upper stratosphere. Then, we quantify the radiative and dynamical feedbacks induced by these ozone changes, by imposing the ozone response to increased CO2 levels in climate sensitivity experiments from the Whole Atmosphere Community Climate model. We show that the global mean radiative forcing induced by the ozone responses to CO2 is small. As a consequence, the effects of ozone on global mean surface temperature are negligible. However, stratospheric ozone has a considerable impact on the tropospheric circulation response to increased CO2 in both hemisheres, leading to sizable changes in regional patterns of climate change, such as a negative North Atlantic Oscillation, and an equator-ward shift of the mid-latitude jet in the Southern Hemisphere; these changes oppose the effects of increased CO2 levels.

Our findings demonstrate that stratospheric ozone feedbacks likely play an important role in shaping the projected climate change patterns in both hemispheres. Neglecting ozone feedbacks in climate models results in an overestimate of the climate system response to increased CO2. Efforts are needed in producing CO2 forcing-consistent ozone data-sets for CMIP models lacking interactive ozone.