Potential Increased Hurricane Activity in a Greenhouse Warmed World

Judith A. Curry

The prospect of increased hurricane intensity in a greenhouse warmed world is arguably the greatest short-term risk from greenhouse warming. With observations of increased hurricane activity emerging, the risk appears to be increasing even more rapidly than has been expected based on the initial studies.

That the situation was more serious than had been thought became clear during 2005, both because of the record-breaking North Atlantic hurricane season and because of papers published by Emanuel (2005) and Webster et al (2005) that demonstrated an increase in hurricane intensity that has been associated with an increase in tropical sea surface temperature. Webster et al (2005) examined global hurricane activity since 1970 (the advent of reliable satellite data). The most striking finding from their study was that while the total number of hurricanes has not increased globally, the number and percentage of category 4-5 hurricanes has nearly doubled since 1970 (see Figure 1).

Scientists are debating the quality of the data upon which these analyses are based. The most reliable data are on tropical cyclones in the North Atlantic. The HURDAT data prepared by the National Hurricane Center go back to 1851. Prior to 1944, only surface-based data were available (for example, land-falling storms and ship observations). Since 1944, aircraft reconnaissance flights have been made into nearly all of the North Atlantic tropical cyclones. Since 1970, satellite observations have made observing and monitoring tropical cyclones more accurate.

Figure 2.2 shows the time series in the North Atlantic of the numbers of named storms (tropical cyclones), hurricanes and category 4-5 hurricanes (NCAT45; NCAT45 is not shown prior to 1944 owing to concerns about data accuracy). To highlight the decadal and longer-term variability, the data have been smoothed (11-year running mean) to eliminate the year-to-year variability. A nominal 70-year cycle is evident, with peaks in about 1880 and 1950, and minima in about

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