Conclusions

The original purpose of this study was to test the many statements that had been made, and charts that had been plotted, appearing to show a significant increase in global weather-related catastrophe losses over time attributed to a rise in global temperatures and anthropogenic greenhouse gas emissions. We identified several factors that must be considered in interpreting the results: (i) the variance in the definition of economic loss, (ii) improvements in loss reporting over time, (iii) changing vulnerability over time, (iv) national level statistics to adjust loss amounts that affect only a specific national region, and (v) the large weight of US losses in accounting for "global" normalized losses.

Before normalization, the annual rise in losses was about 8%. After normalization, these normalized losses did show a more modest underlying rising trend of 2% per year (from an average of US$36.4 billion in the 1970s to an average of US$64.5 billion from 1996 through 2005: a rise of almost 80%). Therefore, the large portion of the rising loss trend is explained by increases in values and exposure as well as by an increasing comprehensiveness of reporting global losses through time. For specific regions - in particular, India, Australia, and the Philippines - over this same period, there is evidence for a decline in normalized losses.

In sum, we found limited statistical evidence of an upward trend in normalized losses from 1970 through 2005 and insufficient evidence to claim a firm link between global warming and disaster losses. Our findings are highly sensitive to recent US hurricane losses, large China flood losses, and interregional wealth differences. When these factors are accounted for, evidence for an upward trend and the relationship between losses and temperature weakens or disappears entirely.

Finally, it appears that just as hurricane activity and intensity, correlated with a rise in equatorial Atlantic sea surface temperatures (SSTs), have shown the strongest evidence for an increase since the 1970s (Emanuel, 2005), it is hurricanes in wealthy regions that are today the principal driver of the evidence for an upward trend in global catastrophe losses.

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