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Effects of Atmospheric CO2 on Hurricanes
Walsh, K. and Pittock, A.B.  1998.  Potential changes in tropical storms, hurricanes, and extreme rainfall events as a result of climate change.  Climatic Change 39: 199-213.

What was done
This authors review the current ability of climate models to predict the effects of CO2-induced global warming on tropical cyclones and extreme rainfall events, as well as certain real-world observations related to these subjects.

What was learned
The following statements are exact quotations of some of the authors' findings:
1. "The effect of global warming on the number of tropical cyclones is presently unknown."
2. "There is little relationship between SST (sea surface temperature) and tropical cyclone numbers in several regions of the globe.  Thus there is little evidence that changes in SST, by themselves, could cause change in tropical cyclone numbers."
3. "Tropical cyclone numbers in some regions are highly correlated with the complex interannual variability of the ocean-atmosphere system associated with ENSO."  But: "Since there is insufficient confidence in the predictions of current models regarding any changes in ENSO, regional changes in tropical cyclone numbers caused by possible changes in the characteristics of ENSO in a warmer world are likewise unknown."
4. "Tropical SSTs are likely to slowly increase in enhanced greenhouse conditions.  Experiments with limited-area models have shown some increase in intensity with SST."  But: "It is also possible that evaporative feedbacks may lower SSTs in the central region of a tropical storm, thus tending to minimize any increase in intensity."  Also: "Since the models suggest that the tropical atmosphere will become more stable, this may tend to limit intensities."
5. "Under enhanced greenhouse conditions, the number of (model-predicted) storms is substantially reduced."  And: "This result is largely related to increases in tropical tropospheric stability."
6. "There is some evidence from climate model studies that, in a warmer climate, rainfall events will be more intense."  And: "There is considerable evidence that the frequency of extreme rainfall events may increase in the tropics."

What it means
When all is said and done, with respect to the possible impacts of potential global warming on "the climatology of tropical cyclones and extreme rainfall events," the authors conclude that "because of the insufficient resolution of climate models and their generally crude representation of sub-gridscale and convective processes, little confidence can be placed in any definite predictions of such effects."

Reviewed 1 October 1998