This study is aimed to examine the changes which extratropical cyclones might experience in a warmer world caused by increasing CO2. The CSIRO atmosphere-ocean coupled global circulation model is used for the simulation according to IPCC IS92a scenario. The Melbourne University cyclone finding and tracking numerical scheme is used to identify and track lows in the NH and the SH.

As CO2 concentration becomes doubled, mean frequency of mean sea level pressure cyclones decreases, but mean Laplacian of the central pressure, scale, depth and velocity of cyclone movement appear to increase. However, in detail these changes are shown to be different according to regions. With tripled CO2 concentration, mean frequency, Laplacian and depth show broad regions of both increase and decrease, but the significance level for the decreases is somewhat greater. At Z500 the pattern of changes is similar to that at mean sea level, but more of the changes at Z500 are significant.

We also investigate the vertical structure of cyclones by 'tracking' low pressure systems from the surface to Z500. As the level of CO2 increases, cyclones tend to have a more vertically organised structure, which seems related to greater intensity, larger scale and greater depth of cyclones.