Background
The authors introduce their study by noting that "understanding the characteristics and trends in summer cyclones is important not only for understanding mid-latitude weather systems and extreme events, but it is also important for understanding the Arctic hydrological cycle and radiation budget (e.g., Orsolini and Sortberg, 2009)." In addition, they note that "the surface concentrations of ozone and aerosols, and as a result surface air quality, depend on a range of meteorological factors [that] are closely connected with cyclones (e.g., Jacobs and Winner, 2009)."

What was done
In an attempt to get some feel for the degree of confidence one should place in the projections of state-of-the-art climate models in regard to these subjects, Land and Waugh say they examined "the robustness of trends in Northern Hemisphere (NH) summer cyclones in the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model data set that was used in the Fourth Assessment (AR4) of the Intergovernmental Panel on Climate Change (IPCC, 2007)."

What was learned
The two Johns Hopkins University researchers report that they could find "little consistency" among the sixteen models they studied. In fact, they say "there is no consistency among the models as to whether the frequency of hemispheric-averaged summer cyclones will increase or decrease." For some sub-regions, however, the sign of the trend was consistent among most of the models; but even then, as they report, "there is still a large spread in the magnitude of the trend from individual models, and, hence, a large uncertainty in the trends from a single model."

What it means
In the not-so-comforting words of the two scientists, "the general lack of consistency among models indicates that care is required when interpreting projected changes in summer weather systems."

References
Intergovernmental Panel on Climate Change. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, New York, New York, USA.

Jacob, D.J. and Winner, D.A. 2009. Effect of climate change on air quality. Atmospheric Environment 43: 51-63.

Orsolini, Y.J. and Sorteberg, A. 2009. Projected changes in Eurasian and Arctic summer cyclones under global warming in the Bergen Climate Model. Atmospheric and Oceanic Science Letters 2: 62-67.