Perfect Storm: Climate Change and Hurricanes

Marlene Cimons of Climate Nexus contributed this article to LiveScience’s Expert Voices: Op-Ed & Insights.

Six months later, residents of the New York area are still struggling to recover from a climate-fueled monster storm that surged up and over retaining walls last fall, and destroyed countless homes. So, it’s worth remembering a simple fact: Global warming is increasing the intensity of hurricanes, and Sandy is a prime example of that.

Most of the damage and fatalities from hurricanes come from high-intensity storms ― those labeled category 3, 4 or 5. Historically, those storms have represented a small fraction of the hurricane total. That balance, however, apparently is shifting in ominous ways. We may have fewer storms, but more of them could be catastrophic. Sandy, clearly, was one of those extreme storms.

Perhaps mindful of this, and of the grim lessons learned from Sandy, the National Oceanic and Atmospheric Administration (NOAA) announced Thursday (April 4) it would keep hurricane and tropical-storm watches and warnings in effect — even after such a storm loses its tropical characteristics — if it poses a significant danger to life and property. In addition, NOAA said that the National Hurricane Center (NHC) would continue to issue advisories during the post-tropical phase of such storms to ensure effective communication of ongoing threats. Forecasters had predicted Sandy would become a post-tropical cyclone prior to reaching the coast, possibly prompting residents to underestimate its destructive nature. [As Weather Changes, Forecasts Lag]

Sandy drove a disastrous storm surge into the New Jersey and New York coastlines last fall, with preliminary U.S. damage estimates near $50 billion, making it the second-costliest cyclone to hit the United States since 1900, according to the NHC. At least 147 people died across the Atlantic basin, 72 of them in the mid-Atlantic and northeastern United States ― the greatest number of U.S. direct fatalities related to a tropical cyclone outside of the Southern states since Hurricane Agnes struck in 1972.

"It’s not a pretty picture," said Michael Oppenheimer, professor of geosciences and international affairs at Princeton University, and a longtime participant in the Intergovernmental Panel on Climate Change (IPCC). "Hurricanes could become more intense as the Earth warms. They are frightening, destructive and extremely costly, and we expect future hurricanes to leave an even greater trail of damage in their wake."

The increasing dangers from hurricanes

A hurricane, a type of cyclone, is a low-pressure system that typically develops in the tropics, including areas in the Atlantic, Pacific and Indian oceans. It often is accompanied by thunderstorms and, in the Northern Hemisphere, winds that churn counterclockwise near the Earth’s surface, sometimes as high as 155 mph (249 km/h).

Fierce hurricanes can destroy coastlines and cause damage several hundred miles inland. They also can spawn tornadoes and microbursts, create storm surges along the coast, and produce serious flooding from heavy rainfall. 

The big killers in hurricanes are wind, storm surge and rain. Storm surge, for example, caused most of the flooding during Hurricane Katrina in 2005 in New Orleans, resulting in the breaching of the levees.  As storm intensity increases, so do those dangers.

"We should not be worried about the frequency of hurricanes; we should be worried about the frequency of intense hurricanes," said Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology. "Climate change is causing a greater number of intense storms. The total number of storms has remained constant, but the proportion of high-intensity events has gone steadily upward in most parts of the world. Scientific models and real-world observations both suggest that the frequency of intense storms is going up."

The atmosphere has been heating up, largely due to the burning of fossil fuels like coal and oil, which lead to dramatic increases of greenhouse gases, primarily carbon dioxide. Those greenhouse gases absorb and radiate heat, and are reconfiguring the Earth’s climate. 

The world has always experienced hurricanes, floods, droughts and heat waves, and scientists cannot point to climate change as the direct cause of any single extreme-weather event. Nonetheless, a vast majority of scientists agrees that the continuing patterns of global warming are making those events more frequent and more intense, a situation likely to worsen in the near future.  

Hurricanes and a warming Earth

How does climate change affect the intensity of hurricanes? The evaporation of seawater fuels hurricanes. The power of a hurricane depends on how rapidly water can evaporate from the ocean. Evaporation transfers heat from the ocean to the atmosphere, which converts the transfer of heat into wind energy. Thus, water evaporation ultimately drives the winds in a hurricane. As the temperature of the water rises, the rate of evaporation increases, driving stronger winds.

Climate change is heating up the oceans: Scientists expect a 2-degree Celsius (3.6-degree Fahrenheit) rise in ocean temperature in the next century ― which "translates into a very large increase in the destructive potential of hurricanes," Emanuel said. "It is a whopping big increase." In short, the warmer the ocean, the faster water can evaporate from it, prompting the formation of powerful hurricanes.  [Surprising Depth to Global Warming's Effects]

Heavy rainfall from hurricanes is another likely byproduct of the warming climate. Since warm air holds more water than cold air, moisture-laden air potentially can produce significant rain during a hurricane, even in low-intensity storms. Heavy rainfall raises the probability of dangerous flooding, as seen with Hurricane Irene in 2011. In 1998, flooding from Hurricane Mitch killed more than 11,000 people in Central America.

"The consensus throughout my field is that global warming will lead to appreciable increases in rain falling from all categories of hurricanes," Emanuel said.

Kevin Trenberth, a distinguished senior scientist in the climate-analysis section at the National Center for Atmospheric Research (NCAR), agrees. "In 30 years, the oceans will be warmer, and there will be more water vapor and heavier rains," he said.

Trenberth, who was among the first to attribute seasonal U.S. weather changes to El Niño (a warming water pattern in the equatorial Pacific) and La Niña (a cooling water pattern in the equatorial Pacific), estimated that between 5 percent and 10 percent of Katrina’s precipitation was due to climate change. "That’s probably conservative,” he said. 

"It has been estimated that heavy rains in tropical storms, including Katrina, have increased by 6 percent to 8 percent as a result of higher sea-surface temperatures and more water vapor in the atmosphere," Trenberth said, adding that, at the time of Katrina, the ocean temperatures were the highest ever recorded in the Atlantic.

Similarly, in August 2011, Hurricane Irene barreled up the East Coast, dumping a record rainfall that caused more than $15 billion in damages due to flooding. 

"Irene formed over unusually warm waters and picked up a lot of moisture," said Michael Mann, professor and director of the Earth System Science Center at Penn State. "Because of those record ocean temperatures, Irene was working with a much higher amount of water vapor in the atmosphere than we normally would have. That’s why we saw the record flooding in New England, Vermont and Massachusetts."

The reduction of sulfate aerosols in the atmosphere in recent years also likely has contributed to ocean warming. The sulfate aerosols have a cooling effect on the water but are not uniformly distributed over the globe, and do not accumulate in the atmosphere. They are concentrated in and downwind of industrial regions, and remain in the atmosphere for only about two weeks. Sulfate aerosols began leveling off during the 1980s, due, in part, to clean air regulations in many nations and the collapse of the Soviet Union — when it fell, much of its heavy industry shut down. 

When the sulfate aerosol particles are gone, the air and water grow warm, but only temporarily. "It does not last long," Emanuel said. "On the other hand, excess carbon dioxide will take thousands of years to disappear once emissions cease, so the warming it produces will be with us for a very long time."

With the start of hurricane season less than two months away, experts worry that the congestion of homes and businesses in vulnerable, low-lying coastal zones, often subsidized by states and the federal government, has further increased the danger. Such policies "are encouraging Americans to build in risky places," Emanuel said.

James Fleming, professor of science, technology and society at Colby College in Maine, agrees. "We’ve gone from almost no infrastructure or government support…to an incredibly built environment," he said, adding, "I wouldn’t be too sanguine about the future. Miami, for example, could be absolutely wiped out. That certainly would be a conversation changer."

Read Cimons' latest Op-Ed: Worst Allergy Season Ever?

The views expressed are those of the author and do not necessarily reflect the views of the publisher.