Southern Hemisphere climate change in an era of ozone recovery

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences


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.


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Description This project was very successful, both in terms of meeting its stated objectives, and in other accomplishments not originally set out the proposal. More than 12 publications were published resulting fully or in part from work carried out as part of this project. PDRA Alexey Karpechko carried out extensive analysis of Southern Annular Mode trends in the CMIP3 simulations, which found that the SAM response to ozone is sensitive to model resolution in the lower stratosphere, but not to the vertical resolution above this (Karpechko et al, 2008). A detailed study of the abilities of the CMIP3 models to simulate the climate impacts of the SAM demonstrated that the SAM response in some variables, such as sea ice, is very poorly simulated by some models, while other variables such as SST and precipitation are more realistically simulated (Karpechko et al, 2009). Analysis carried out as part of this project also contributed to the first study to attribute changes in Antarctic near surface temperature to anthropogenic influence, which was published in Nature Geoscience and received considerable media attention (Gillett et al, 2008). Work carried out as part of this project also contributed to a careful study of the mechanisms of response of Southern Ocean SST to SAM variability (Screen et al, 2008). Much of the project was focused on an analysis of ozone changes simulated in the CCMVal coupled chemistry climate models (CCMs). A quantitative metric was developed to quantify the realism of CCMs simulated ozone (Karpechko et al, 2009), and analysis of these simulations carried out as part of the project was included in the SPARC CCMVal Report Radiation chapter, on which the PI Nathan Gillett and PDRA Alexey Karpechko were coauthors. These results were also presented in Forster et al. (2011). A figure showing layer temperatures in the CCMVal simulations and observations prepared by PDRA Alexey Karpechko during the project was included in Chapter 3 of the SPARC CCMVal report, and in Chapter 4 of the 2010 WMO Ozone Assessment, of which PI Nathan Gillett was an author. The WMO Ozone Assessment is the key science document informing international policy on ozone depleting substances. This analysis of the CCMVal simulations also fed into the first study to apply formal detection and attribution methods to stratospheric temperature and ozone (Gillett et al, 2011). An analysis of the simulated response to volcanic eruptions in the CMIP3 simulations found a significant tropospheric response in the models for the first time (Karpechko et al, 2010a). Lastly, consistent with the original project objectives, the project examined the sensitivity of future Southern Hemisphere climate to the rate of ozone recovery (Karpechko et al, 2010b), finding different Southern Hemisphere tropospheric climate trends depending on which scenario of future ozone change was used.
Exploitation Route The numerous studies mentioned above are information current and future research in this area
Sectors Environment