Hurricanes

Rising sea surface temperatures could conceivably fuel an increase in the number or the intensity of tropical cyclones (also called typhoons or hurricanes), because warm

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Fig. 12.8 The frequency of different intensities of hurricanes, from Webster et al. (2005).

surface waters are the source of energy for these storms. Hurricanes do not form if the sea surface temperature is cooler than 26°C. As the sea surface temperature increases, the maximum possible intensity goes up. Cyclone intensity also depends on the temperature at the top of the atmosphere, however, which goes up more or less in parallel with sea surface temperature, counteracting some of the effect of the sea surface temperature rise. Cyclone intensity depends on the way that winds are blowing with altitude, whether they tear a tropical storm apart or allow it to grow into a full cyclone. The evolution of a cyclone depends on the subsurface temperatures of the ocean, also, because the wind tends to mix warm surface waters with the cooler waters of the subsurface, slowing things down. The bottom line is, there are reasons to worry that cyclone might get more intense with rising CO2, but the connection is not as simple as, say, the link between CO2 and global mean temperature.

Every year there are about 90 tropical storms, 40 of which tend to become cyclones. A cyclone requires a jump-start, some chance combination of winds and pressures that enables the tropical storm to get going. One might guess that the number of tropical storms would depend on climatic conditions; el Niño or warming or something like that. However, the frequency of tropical storms is observed to be fairly constant through time (Fig. 12.8). No one knows why.

The intensity of hurricanes, on the other hand, seems to be climate sensitive. Model simulated hurricanes are more intense in a doubled-CO2 world. Ordinary climate models do not have enough resolution, meaning that their grid points are not close enough together, to simulate hurricanes very well. Hurricanes can be simulated in global models, however, by a technique called adaptive grid refinement, essentially inserting grid points into a model when and where they are needed, or you can think of it as running a second, high resolution model in the area where a hurricane is, coupled to and exchanging information with a low-resolution global model. Using a wide range of models and for a wide range in parameter space, the study of Knutson and Tuleya (2004) found that the cyclones in their model shift to higher intensity in

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Fig. 12.9 The distribution of wind speeds in model hurricanes for a control world (natural climate) and doubled CO2.

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Fig. 12.9 The distribution of wind speeds in model hurricanes for a control world (natural climate) and doubled CO2.

a high-CO2 world (Fig. 12.9). Wind speeds increase by 10% or so. The destructive power of a hurricane increases, not linearly with the wind speed, but as the wind speed raised to the third power. So an increase in wind speed of 10% leads to an increase in damage of about 33%. The cyclones to be feared the most are not the average ones, but the highest intensity ones. The frequency of the highest power, Category 5, cyclones is predicted by the model to double or worse in a high-CO2 world.

The frequency of the most powerful hurricanes, called Category 5 storms, has doubled in the past 30 years (Fig. 12.8). This by itself does not look to me like iron-clad proof that a global warming future would have more intense cyclones because there is natural variability in these things. A more compelling piece of evidence is that the total destructive power of cyclones in the past 50 years has correlated extremely tightly with variations in sea surface temperature (Fig. 12.10). The cyclones are getting stronger even than the models or theory say they should, given the amount of warming that the sea surface has undergone. It is dangerous to extrapolate this trend to the future until we understand the observation better. However, the 2-3°C of sea surface temperature warming we might expect in the future is much larger than the 0.5°C of warming we've had already, and cyclones have doubled their power already. This gives reason for pause, even if we don't know for sure what's going on.

The really provocative observation is a massive increase in the cost of weather-related damages in the past 50 years (Fig. 12.11). The number ofweather-related events has increased by a factor of five since the 1950s. The number of nonweather related disasters has gone up also, but only about half as quickly as weather-related claims. Insurance payments for weather-related property destruction have increased by a factor of about 13 in the past 50 years. A trend of rising affluence and coastal construction can

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Fig. 12.10 The destructive power of hurricanes correlates with variations in sea surface temperature, in (a) the North Pacific, (b) the North Atlantic, and (c) both basins combined, from Emanuel, (2005), (d) is a possible future sea surface temperature rise. Disclaimer. There is no hurricane power prediction on the right-hand part of the plot because we do not know what the hurricane response will be. The simplest assumption would be that the response in the future will be proportional to the response in the past, which would be huge given the huge future temperature changes. But it might not work like that. The point is that the future temperature changes could be huge compared to what we have seen so far.

explain at least part of the increase in payouts, perhaps all of it. Some of the damage is due to floods, which are probably caused by land-use changes, cutting forests, draining wetlands, and building in flood plains, as much as by climate variability. A number of sources, including Working Group II of the IPCC (the human impacts group), blame the increase in insurance payouts on climate change, but the observational support for that conclusion is thin.

Fig. 12.11 Economic losses (insured + uninsured) due to weather-related disasters, from IPCC (2001).

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