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Is there a relationship between increasing hurricane intensity and frequency and climate change?

Author: Jacqui Yeates

Last updated: August 2007

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Headline messages

The key scientific questions on the effects of global warming on tropical cyclones are related to whether the frequency, geographic distribution and intensity of tropical cyclones will change in the future. Current understanding as presented in the IPCC Fourth Assessment Report indicates that it is likely that future tropical cyclones will become more intense, with larger peak wind speeds and more heavy precipitation as a result of ongoing increases in tropical sea surface temperatures.

Recent research based on analyses of observational records have concluded that the overall trend of increases in sea surface temperatures and in the number of tropical cyclones in the Atlantic have been influenced by greenhouse warming.

Summary of research findings 

• The IPCC’s Fourth Assessment Report (4AR) concludes that there is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures.  The assessment also found suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 were found to complicate the detections of long-term trends in tropical cyclone activity. On this basis, the IPCC concluded that there is no clear trend in the annual numbers of tropical cyclones.

• It is generally believed that increasing sea surface temperatures (SSTs) will lead to more intense tropical cyclones. This conclusion is based on observations that show that tropical cyclones cannot form unless SST reach a particular threshold.  SST, however, is not the only determining factor for tropical cyclone formation. Wind shear (variations in the wind at different heights in the atmosphere) with stronger winds aloft has been observed to prevent the formation of the vertical structure necessary for tropical cyclone formation and intensification. Some models have projected an increase in the incidence of atmospheric circulation patterns that would normally be accompanied by strong wind shear. This has lead some to conclude that climate change could result in fewer tropical cyclones.

• In terms of projections, the IPCC 4AR concluded that based on a range of models, it is likely that future tropical cyclones will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical sea surface temperatures. The assessment found less confidence in projections of a global decrease in numbers of tropical cyclones. It is also worth noting that the IPCC indicated that the apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated for that period by current models.

• A recent study by Holland and Webster (Philosophical Transaction of the Royal Society in London, 2007) found that long-period variations in tropical cyclone frequency over the North Atlantic have occurred as three relatively stable regimes separated by sharp transitions. Each regime has seen 50% more cyclones that the previous one and is associated with a distinct range of SSTs in the eastern Atlantic. Overall their analysis concludes that there appears to have been a substantial 100-year trend leading to related increases of over 0.7˚C in SST and over 100% in tropical cyclone numbers. They also concluded that these overall trends have been substantially influenced by greenhouse warming.