Chris De Freitas: Not simply climate change

Chris De Freitas

The devastation wrought by Katrina and Rita has reignited the debate over a possible link between hurricanes and global warming. How real is it and what are the implications for our region?

Six hurricanes, or what in New Zealand we call tropical cyclones, appeared in the southwest Pacific during the six-week period beginning January 31 this year, five of which hit the Cook Islands.

Many climate scientists believe global warming may increase the odds in favour of more intense and more frequent tropical cyclones. These cyclones require warm water to form and global warming means more warm water.

But even without global warming, there is ample warm water in tropical oceans.

Some of the highest sea surface temperatures in the world are found in the tropical Mid-Atlantic and Caribbean, yet most years there are fewer than 15 tropical cyclones and frequently fewer than five.

Given the great expanse of warm water, the important question is why do tropical cyclones form only some of the time, and why are there so few?

Tropical cyclones need exactly the right conditions to form, and warm water and resulting high humidity are just two of the ingredients. So many factors influence the formation of tropical cyclones that it is difficult to predict how global warming will influence their frequency and intensity.

In the tropical Mid-Atlantic and Caribbean, for example, the number and strength of easterly pressure waves coming across the Atlantic and the change in wind with height through the atmosphere (wind shear) are as important as warm sea surface temperatures to forming tropical cyclone. It is not known how global warming might affect these processes.

There is research that shows tropical cyclones occur in distinct multi-decadal cycles and are linked to higher-than-average sea-surface temperatures.

Warm anomalies are associated with increased major tropical cyclone activity; cold anomalies with suppressed activity.

Sea water globally has warmed up about a half degree celsius over the past 50 years, but in the region where Atlantic hurricanes form - from 5 to 20 degrees latitude north, from Africa over to the United States - it has actually been cooling.

The number of intense hurricanes in the Atlantic declined during the 1970s and 1980s, and the period 1991-1994 experienced the smallest number of hurricanes of any four years over the past half century.

At the same time, it is too early to dismiss the role of these changes, as trends are different in the Pacific region.

Researchers Chu and Clark writing in the journal Bulletin of the American Meteorological Society analysed the frequency and intensity of intense tropical storms that occurred in the central north Pacific region over the 32-year period from 1966-1997.

They found storm activity had risen, amounting to an increase of about 3.2 storms over the 32 years. Accompanying the increase in intense tropical storms was a similar increase in maximum tropical cyclone intensity.

In contrast, we have the results of the recent experiments by Australian researchers Nguyen and Walsh, who simulated the occurrence of tropical cyclones in the Australian region using a global climate model that assumed a tripling of the atmospheric concentration of carbon dioxide. They showed the numbers of tropical cyclones in the region would decline.

As a first approximation of tropical cyclone activity over the next two or three decades, one can simply extrapolate past variations in occurrences.

Tropical cyclone researcher Chris Landsea did just this. He examined the data for the Australian, the northwest Pacific and the Atlantic basins in enough detail to allow some suggestions as to what the first decade of the 21st century may bring.

Landsea reported the Australian basin showed a decline in tropical cyclone frequency since the late 1960s, the northwest Pacific showed an increase after experiencing a decrease in frequency from the late 1950s through to 1980, while the Atlantic has been constant since the mid-1940s.

For mean intensity, he reported there was little or no trend in the Australian basin, the northwest Pacific showed a downward trend during the 1960s and 1970s and an upward trend in intensity of events since. Regarding the future, Landsea concluded there was no convincing evidence for systematic changes in the frequency, mean intensity, maximum intensity, and area of occurrence of tropical cyclones.

Notwithstanding all this, the number and scale of weather-related disasters worldwide has been rising rapidly in recent decades because of social and economic changes, not global warming.

In the case of tropical cyclones, the reasons are a greater concentration of people and high-value property in vulnerable areas, mainly coastal, and the fact that business processes have become more susceptible to damage.

Even if global warming never eventuated, the frequency and size of climate-related disasters would continue to rise as more people inhabited vulnerable locations around the world.

The effects of future Katrinas and Ritas will be determined not by our efforts to control changes in global climate, but by the decisions we make about where and how to build and rebuild in vulnerable locations and how well-prepared we are for the future.

* Chris de Freitas is an Associate Professor in the School of Geography and Environmental Science at the University of Auckland.