Southern Hemisphere climate change in an era of ozone recovery

Over the past thirty years, the westerly winds blowing around the Southern Ocean have grown stronger, and this change in the circulation of the atmosphere has led to a cooling over most of Antarctica, and a warming of the Antarctic Peninsula and southern South America. Recent research suggests that this trend has also influenced the ocean circulation and temperature, and its absorption of the greenhouse gas carbon dioxide. Climate models indicate that this change in atmospheric circulation has been caused partly by the increase in greenhouse gases, and partly by the ozone hole. Greenhouse gas levels are very likely to continue to increase over the next fifty years, but the ozone hole is expected to start to recover, due to international regulation of emissions of the chemicals which destroy ozone. However, it is very uncertain when and how quickly the ozone hole will start to recover, and different computer simulations produce very different results. Previous climate model predictions of future climate change have generally either assumed no recovery of the ozone hole, or they have used a single prediction of future ozone change. We plan to use a state-of-the-art climate model, developed at the Met Office, to derive a range of predictions of future climate change which take account of our uncertainty in future ozone change. We will start by comparing simulated changes in atmospheric circulation over the past 25 years with climate observations to help us understand the causes of past trends. We will also examine the effects of these trends on the climate of the Southern Hemisphere, and on the circulation of the Southern Ocean. We will then test a range of predictions of ozone change. These predictions are made using computer simulations of changes in atmospheric chemistry, and we will test them by comparing simulations of past ozone changes with observations made by satellite and balloon. Once we have identified the four most realistic simulations of past ozone change, we will use the latest Met Office climate model to simulate the climate response to each of the four corresponding scenarios of future ozone change over the next 45 years. Future greenhouse gas changes will also be specified in the climate model. We will use these climate model simulations to predict the likely range of possible future circulation changes in the Southern Hemisphere, and the impacts of these changes on the atmosphere and ocean. If historical circulation trends are reversed by ozone recovery, such impacts could include enhanced warming over Antarctica, and hence faster ice melt and sea level rise; enhanced warming and drying of Australia; and changes in the absorption of carbon dioxide by the ocean.