FIGURE 5.3 Relative variability of US farm output as share of total gross domestic product, 1929-2000. Relative variability is the dollar variability shown in Figure 5.2 as a percent of trend real gross domestic product. Even though the variability in Figure 5.2 has increased over the last seven decades, its size relative to the overall economy has declined because of the declining share of agriculture in total output. (Data from Bureau of Economic Analysis.)

Another human vulnerability to climate change is impacts from severe storms. Total economic losses from hurricanes in the United States over the 1900-1995 period have increased sharply (Figure 5.4). In contrast to agriculture, when these data are normalized to account for inflation, wealth, and population (Figure 5.5), they exhibit an extremely skewed distribution but show no clear trend.² Furthermore, data on floods show no decline in flood damage per unit wealth in the United States (Pielke and Downton, 2000) over an equivalent period.

The relationship between climate and human civilizations has long been a subject of research and speculation among historians and economists.