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6. Observed effects of climate change on tropical cyclones worldwide

6.1 Introduction

Recent international literature concerning observed trends in global tropical cyclone

activity and correlation with increasing sea surface temperature is reviewed in the

following. Several of the papers are reviewed in the following. In Section 6.6

consensus statements by expert groups are reproduced in an endeavour to give a

balanced view of opinions on this somewhat controversial topic.

6.2 Interpretations by Webster et al (2005)

Webster et al (2005) conducted a comprehensive analysis of global tropical cyclone

statistics for the satellite era (1970-2004) in each tropical ocean basin in which they

occur. They found significant increasing trends in sea surface temperature (SST) in

each of the ocean basins except for the South Pacific. Since it is well established that

a SST of 26 0 C is required for tropical cyclone formation in the current climate, it

might be expected that there would be an increase in the number of tropical cyclones.

However there was no significant trend in global cyclones of all strengths.

The North Atlantic region did show a statistically significant increase since 1995 in

tropical cyclones of hurricane strength (defined by Webster et al as having wind

speeds greater than 33 m/s). However an attribution of the increase to increasing

SST is not supported because of the lack of correlation of the number of tropical

cyclones of this strength in other basins.

When Webster et al examined the number of hurricanes by allocated category (using

the Saffir-Simpson scale), they found a significant increasing trend in the numbers of

Category 4-5 storms, and found that the numbers of these strongest storms increased

from about 50 globally per 5-year period in the 1970s to nearly 90 per five-year

period in the decade 1995-2004 (see Figure 6.1). This conclusion has been

controversial and is further discussed in the following. Also as seen in Figure 1

there has been no trend from the 1990-1994 period to the 2000-2004 period (as

pointed out by Klotzbach (2006)).

Figure 6.1 Apparent increase in number of Category 4 and 5 hurricanes world-wide

(from Webster et al (2005))

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