The Antarctic ozone hole has a major effect on the local climate, and the future of Antarctica will influence global climate and sea level changes.
Modelling the atmospheric chemistry of feedback processes between stratospheric ozone and the Antarctic climate will increase the accuracy of global climate models.
This research programme commenced in 2002. Our research has revealed a number of key findings to date including:
- The year when the ozone layer over Antarctica returns to 1980 conditions is only marginally affected by different future scenarios of greenhouse gas (GHG) emissions. The pathway to that recovery is however affected by GHG emissions.
- The recovery of the Antarctic ozone hole will affect surface climate over southern mid-latitudes including New Zealand.
- There was significant ozone depletion over Antarctica between 1960 and 1980. This depletion is not always apparent in observations since Antarctic stratospheric temperatures were unusually high in the late 1970s and unusually low in the late 1990s obscuring to some extent the halogen induced destruction of ozone.
The ozone hole remains the largest human induced perturbation to Antarctica to date. From the 1940s to the 1990s, industrial and domestic emissions of chlorofluorocarbons, hydrochlorofluorocarbons, halons and methyl bromide significantly increased the concentrations of chlorine and bromine in the stratosphere. These two halogens are released from their source gases through chemical reactions within polar stratospheric clouds, and then destroy large quantities of ozone.
The Montreal Protocol has significantly reduced emissions of halocarbons to the atmosphere and equivalent effective Antarctic stratospheric chlorine (a measure of the combined effect of different chemicals) is now decreasing. In response the ozone layer over Antarctica is expected to return to 1980 levels sometime in the latter half of this century.
However, the timing of the recovery is very uncertain due to considerable natural variability and uncertainties in the models used to project future changes. The use of global climate models to project future Antarctic climate change is limited because there is considerable interaction between changes in stratospheric chemistry, in particular ozone depletion, and climate. Global climate models of future climate in Antarctica use projected data to partially incorporate these interactions, but any feedback between stratospheric change and climate change is lost because the models do not generate their own ozone levels for each year.
To model ozone/climate feedback processes, we will incorporate atmospheric chemistry directly into a GCM so that stratospheric temperature changes go on to:
- change chemical reaction rates
- effect the transport of chemically active trace species, and
- alter atmospheric chemical composition.
Use of chemistry-climate models (CCMs)
Changes in chemical composition, in turn, drive changes in the radiative balance of the atmosphere, thereby changing temperatures, dynamics, and climate. Such a model, in which atmospheric chemistry has been incorporated interactively into a GCM, is called a coupled chemistry-climate model (CCM). At NIWA, we are operating two such models.
CCMs are very complex, often exceeding a million lines of code. The developers of these models try to incorporate all processes known to be important to the problem being addressed. The models divide the atmosphere into hundreds of thousands of boxes and calculate the changes in chemistry and dynamics within each box. Many processes in the atmosphere occur on smaller scales (for example, clouds, convection, thermals, and atmospheric waves) and therefore cannot be explicitly included in the model. Their effects on the atmosphere need to be parameterised. The complex and interdependent nature of models makes it hard to identify where they are deficient, so that they can be improved to better reflect reality.
International Polar Year
From 2007 to 2009, scientists around the globe participated in the International Polar Year, collecting unprecedented data from both polar regions. For atmospheric research, International Polar Year has meant an array of improved stratospheric measurements of ozone and the trace gases affecting ozone.
Within this programme we are using novel diagnostic tools, and the IPY measurements, to probe some of the key chemical processes within our models in order to highlight and address model weaknesses. The improved models will provide more reliable projections of Antarctic ozone recovery.
To achieve the solution described above we have developed diagnostic probes for CCMs in the form of semi-empirical models (SEMs) of key stratospheric processes. These SEMs are fitted to real world observations and to output from CCMs. The SEM fit parameters capture key sensitivities in various stratospheric processes central to Antarctic ozone depletion (e.g. chlorine activation, and denitrification). By comparing these parameters from reality and from the CCMs, we can better identify where the CCMs may be underperforming. These inadequacies can then be addressed, resulting in more robust projections of ozone changes over Antarctica.
Antarctic ozone depletion is known to affect surface climate in Antarctica and over southern mid-latitudes but incorporating the required stratospheric chemistry into a GCM (i.e. to make a CCM) adds a lot of complexity to the GCM making it far more expensive to run. CCM simulations take many months even on high performance supercomputers. As part of this research programme, we are planning to adapt the SEMs used to diagnose the models to develop a new and less expensive scheme to include stratospheric chemistry into simple climate models and into GCMs. This, for example, would improve the climate models used by the IPCC to project future climate change and would also permit these models to make projections of the recovery of the Antarctic ozone hole.
As of 2012, this project forms part of the Chemistry-climate modelling project.