The increase in global hurricane intensity since 1970 has been associated directly with a global increase in tropical sea surface temperature (Emanuel, 2005; Webster et al, 2005; Hoyos et al, 2006). Figure 2.3 shows the variation of tropical sea surface temperature (SST) in each of the ocean regions where tropical cyclone storms form. In each of these regions, SST has increased by approximately 0.5°C (or 1°F) since 1970. The causal link between SST and hurricane intensity was established over 50 years ago, when it was observed that tropical cyclones do not form unless the underlying SST exceeds 26.5°C and that warm SSTs are needed to supply the energy to support development of hurricane winds. The role of SST in determining hurricane intensity is now generally understood and is supported by case studies of individual storms and by the theory of potential intensity.
While globally the number of tropical cyclones has not increased (Figure 2.1), there has been an increase in the number of tropical cyclones in the North
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