Hurricane Intensity Trends Are Natural, Not Manmade

Published December 1, 2007

Florida has the dubious distinction of holding several not-so-enviable records when it comes to hurricane strength, frequency, deaths, and damages.

The Florida Keys Hurricane of 1935 was the most intense Atlantic hurricane ever to strike the United States when it passed over Florida’s Middle Keys on September 2, 1935, killing 408 people.

In 1928, the Lake Okeechobee Hurricane caused a six- to nine-foot lake surge that killed 1,836 people. It ranks second, behind the Galveston Hurricane of 1900, in the list of most deadly U.S. hurricanes.

Scientists estimate a repeat of the Great Miami Hurricane of 1926, the eye of which passed directly over Miami as a category 4 storm, would today cause nearly $160 billion in damages–more than Katrina and Andrew combined.

Much at Stake

Clearly, Floridians have a lot at stake when hurricane season approaches each year. Hence the question of whether human-induced changes in the Earth’s greenhouse effect may impact current or future hurricane patterns is probably nowhere more important than it is in Florida, with its 17.3 million coastal residents (making up half of the vulnerable coastal population from Texas through North Carolina).

Hurricane 1

My October 2007 report published by the Science and Public Policy Institute, “Hurricane Threat to Florida: Climate Change or Demographics?” details the best available scientific evidence concerning anthropogenic climate change and its potential impact on Atlantic tropical cyclones.

The report finds natural variations, on time scales ranging from years to decades, largely dominate any small impacts that a warming climate may have on the frequency and intensity of Atlantic tropical systems. Far and away the most important determinant of future vulnerability is not changes to the hurricanes themselves, but changes to the population, wealth, and structure of Florida’s coastal communities, many of which rank among the fastest growing localities in America.

Observed Trends

Along the Florida coast, according to records from the National Hurricane Center, the number of total hurricane strikes is subject to decadal variations but no real long-term trends.

Hurricane 2

The number of hurricanes affecting Florida during the past 10-year period, 1997-2006, was similar to periods in the first half of the twentieth century. However, since it followed three decades of relative quiet, the recent period has seemed exceptionally active. In fact, it has not been exceptionally active in the greater historical perspective.

Nor has there been any long-term trend towards greater damages from hurricanes–not when changes in inflation and population demographics are correctly considered.

Hurricane 4

New research by a team of scientists led by University of Colorado Professor of Environment Studies Dr. Roger Pielke Jr. examined the historical damage amounts from tropical cyclones in the United States from 1900 to 2005. After adjustments were made for inflation, growth in coastal population, and changes in wealth, they found no long-term change in damage amounts.

They determined the Great Miami hurricane of 1926, not Hurricane Katrina, would have been the most damaging storm in history had it hit in 2005, likely causing $157 billion worth of damage.

After the Great Miami hurricane and Katrina, the remaining top 10 most damaging storms show no obvious predominance toward recent years. In fact, the updated potential damages from 1926-35 storms were nearly 15 percent higher than those in 1996-2005, the last decade Pielke and colleagues studied.

Natural Cycles

Natural cycles dominate the observed record of Atlantic tropical cyclones, which dates back to the eighteenth and nineteenth centuries. Hurricane activity was quiet in the 1910s and 1920s, elevated in the 1950s and 1960s, quiet in the 1970s and 1980s, and has picked up again since 1995.

The timing of these oscillations matches well with the oscillations of a phenomenon known as the Atlantic Multidecadal Oscillation (AMO), which reflects changes in large-scale patterns of sea surface temperatures in the Atlantic Ocean.

While instrumental records of the AMO date back little more than 100 years, recent research has found indirect evidence for oscillations in the frequency and intensity of Atlantic hurricanes dating back thousands of years.

Projected Effects

The question of whether human-induced climate change will affect future patterns of tropical cyclone frequency and/or intensity in the Atlantic basin is currently one of the most active research topics in climate science. There is no general consensus on the matter.

As we head into the future, it is expected that human activity will lead to an overall climate warming. A warming climate is reasonably expected to lead to higher sea surface temperatures in the tropical Atlantic, which is the spawning ground of hurricanes. Sea surface temperatures play an important role in the formation and intensification of tropical cyclones. Therefore, some researchers suggest hurricanes will become stronger in a warmer world.

Natural Fluctuations

However, some of the climate changes that are projected to occur will likely act to hinder tropical cyclone formation and development. This includes projections of increased vertical wind shear and increasing atmospheric stability.

When all of the projected changes are incorporated into climate models, they generally predict only small increases in tropical storm intensity (maximum winds increase by just 6 percent) over the course of the coming century, and decreases in frequency of storm formation.

In addition, the small intensity increases are produced by a climate model driven by scenarios of future carbon dioxide increases that are much greater than current trends suggest will occur. Thus, even the small projected increase in hurricane intensity may well be overstated.

The bottom line is that hurricanes always have and always will set their sights on the Florida coastline. Natural fluctuations in the frequency of such hurricane events will continue to occur. Preparedness and vigilance are the best hurricane defenses–not trying to change the climate.

Robert Ferguson ([email protected]) is president of the Science and Public Policy Institute.

For more information …

Robert Ferguson, “Hurricane Threat to Florida: Climate Change or Demographics?” Science and Public Policy Institute, October 2007: