Natural and human-induced perturbations in the middle atmosphere: A short tutorial

This tutorial paper presents a new two-dimensional model of the atmosphere (0 to 120 km) that describes the interactions between dynamical, radiative and chemical processes. The model is used to assess the response of the atmosphere (e.g., ozone concentration, temperature) to natural perturbations such as volcanic eruptions or solar variability, and to anthropogenic emissions of radiatively or chemically active gases. For example, it is shown that, above 25 km altitude, the depletion of ozone in response to the emissions of man-made chlorofluorocarbons (1990 conditions) reaches a maximum of 10-20% at 45 km altitude, with the highest values in polar regions. A large ozone reduction (typically 30%) is found in the mesosphere near 80-85 km in response to the methane increase over the 1850-1990 period. The predicted ozone reduction has been accompanied by middle atmosphere cooling. These anthropogenic perturbations are predicted to have comparable magnitudes to natural changes (such as those produced by solar variability).