Worsening with Warming?

On August 28, 2005, as it crossed the Gulf of Mexico, Hurricane Katrina strengthened to a category 5 hurricane. As of early August 2006, the death toll exceeded 1,800, and total damages were estimated to be around $125 billion.

Hurricane Rita, at its maximum strength on September 21, 2005, had sustained winds of 180 miles per hour. Rita was the fourth most intense Atlantic hurricane ever recorded, the most intense for the Gulf of Mexico. Image courtesy of NOAA Environmental Visualization Laboratory.

On September 21, 2005, Hurricane Rita strengthened to a category 5 hurricane. It was the first time on record that two category 5 storms struck the Gulf of Mexico in the same hurricane season. On October 19, 2005, Hurricane Wilma became not only the third category 5 storm for the season, but also the most intense hurricane on record for the entire Atlantic Basin.

With a growing sense in the science community that climate change is real—the National Academies of Sciences in the United States, and the Royal Society in the United Kingdom were just two among many scientific organizations issuing statements about human-caused warming—the devastating 2005 hurricane season hardly seemed coincidental. Warming climate was worsening tropical cyclones, right?

Christopher Landsea, science and operations officer at the National Hurricane Center, wasn’t so sure.

Puzzling Periods of Quiet

Christopher Landsea, on a hurricane surveillance flight. Photo courtesy of Sim Aberson, NOAA.

Landsea’s duties at the National Hurricane Center include issuing forecasts for seasonal activity, predicting daily storm activity, and testing new techniques and models that can save lives by giving advance warning of severe storms. He traces his fascination with tropical cyclones to an early age.

“Most kids love storms,” says Landsea. Growing up in Miami in the 1970s, he was no exception. Like many kids, he hoped to see a hurricane up close. He did school projects on hurricanes, watched TV shows about tropical cyclones, and read articles about monster storms. As a high school senior in the early 1980s, Landsea discovered that he could earn class credit while working at the NOAA Hurricane Research Division, gaining hands-on experience while still in his teens.

There was just one thing missing from his youthful hurricane education: an actual hurricane.

For Landsea, the dearth of hurricanes wasn’t just disappointing, it was a little perplexing. Locals who had lived in Miami a long time remembered plenty of big storms. The Atlantic Basin experienced 39 major hurricanes in the 1950s, and 28 major hurricanes in the 1960s. But in the 1970s, only 16 major hurricanes formed in the Basin. In 1979, when Landsea was 14, Hurricane David traveled up the East Coast. Landsea diligently plotted David’s course based on radio bulletins from NOAA, but the storm only brought a little wind and rain to Miami. Landsea’s hometown wouldn’t experience a hurricane until he was 27 years old.

What he now realizes is that hurricanes occur in cycles. “The late nineteenth century was a very busy period,” he explains. “Then from the 1900s until about 1925, it was very quiet. The late 20s to the 60s were very busy. The 1970s to the mid-90s were quiet again, and then from the late 90s onward, it’s been generally very busy.”

This graph shows annual tropical storm counts for the North Atlantic Ocean. Counts have been adjusted to omit short-duration storms (those that lasted less than 2 days) and to account for storms that were likely missed before satellite observations were available. Data from Landsea et al. 2010.

For all the damage and death Katrina caused in 2005, Landsea points out, that storm was not the most destructive hurricane in U.S. history. Records show that the hurricane that struck Galveston, Texas, in 1900 was far worse. Landsea says, “The Galveston Hurricane was a horrific event. Essentially with no warning, the town of Galveston was completely inundated and destroyed by the storm surge from that hurricane. We’re not sure how many people perished, but it was on the order of 8,000 to 10,000 people drowned in just a couple hours.”

On September 8, 1900, a hurricane and accompanying storm surge inundated Galveston, Texas without warning. The left image shows surface pressures deduced after the storm, courtesy of NOAA. The images on the right show some of the storm’s aftermath, courtesy of Wikimedia Commons.

Natural variability apparently plays a significant role in hurricane frequency and intensity, and currently appears to play a bigger role than greenhouse warming. One example of climate variability Landsea cites is the Atlantic Multidecadal Oscillation, or AMO. The AMO is a pattern of sea surface temperature changes between warm and cold phases in the North Atlantic Ocean. The pattern tends to vary on multidecadal
time scales, but the limited length of the Atlantic sea surface temperature record prevents scientists from making more definitive statements about the precise nature of the AMO. “When it’s in a warm phase, you tend to have a very busy hurricane period. You tend to get the opposite in one of the cool phases,” Landsea says.

The mechanisms governing the AMO are not well understood, but several factors have been hypothesized to contribute to it. These include internal variability of the ocean-atmosphere system, as well as radiative forcing changes caused by changing greenhouse gases, and variations in atmospheric dust and aerosol particles from volcanic eruptions and industrial activity.

The Atlantic Multidecadal Oscillation Index and five-year average counts of tropical cyclones. Before averaging, counts of tropical storms were adjusted to omit short-duration storms and to account for storms that were likely missed before satellite technology was available. Data courtesy of NOAA and Landsea et al. 2010.

A Skewed View of the Past
Although Landsea doesn’t dispute that humans are causing climate change, he does doubt the assertion that global warming has significantly increased hurricane activity. To explain his doubts, he points to the imperfections in hurricane records.

The North Atlantic hurricane database, known as HURDAT, is the official hurricane and tropical storm record for the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea. Taken at face value, HURDAT suggests an increase in hurricane activity that could be linked with warming climate. “But the hurricane database reflects just the knowledge of what was available at the time,” Landsea explains. A direct comparison of today’s storm activity with activity recorded a century ago can be misleading. “One hundred years ago, you didn’t have satellites, you didn’t have buoys, all you had were ships and people trying to stay out of the way.”

In other words, hurricanes might seem bigger, stronger, and more numerous now because we’re much better at finding and measuring them.