• climate change;
  • summer cyclone

[1] The potential impact of increasing greenhouse gases on the frequency of Northern Hemisphere summer cyclones is examined using daily-averaged mean sea level pressure from climate models used in the latest assessment report from the Intergovernmental Panel on Climate Change. Previous studies have found that the models consistently show a strong decrease for winter cyclones. In contrast, there are much smaller changes in the frequency of summer cyclones and little consistency among the models. In particular, there is no consistency among the models as to whether the frequency of hemispheric-averaged summer cyclones will increase or decrease. For some subregions the sign of the trend is consistent across the vast majority of models, but even then there is a large spread in the magnitude of the trends. The general lack of consistency among models indicates that care is required when interpreting projected changes in summer weather systems.