Hurricanes and Climate Change
Definition of a Hurricane
A hurricane is a type of tropical cyclone, which is a general term for a low-pressure system with activity like thunder and lightning that develops in the tropics or subtropics, between about 30 degrees north and 30 degrees south latitude. In the Northern Hemisphere, these storms rotate counter-clockwise. In the Southern Hemisphere, they rotate clockwise. Stronger systems are called “hurricanes” or “typhoons,” depending on where they form. Weaker tropical cyclones might be called “tropical depressions” or “tropical storms.”
It’s unclear whether climate change will increase or decrease the number of hurricanes, but warmer ocean surface temperatures and higher sea levels are expected to intensify their impacts.
Hurricanes are subject to various climate change-related influences. Warmer sea surface temperatures could intensify tropical storms wind speeds, potentially delivering more damage if they make landfall. Based on sophisticated computer modeling, scientists expect a 2-11 percent increase in average maximum wind speed, with more occurrences of the most intense storms. Rainfall rates during these storms are also projected to increase by about 20 percent.
In addition, sea level rise is likely to make future coastal storms, including hurricanes, more damaging. Globally averaged, sea level is expected to rise by 1-4 feet during the next century, which will amplify coastal storm surge. For example, sea level rise intensified the impact of Hurricane Sandy, which caused an estimated $65 billion in damages in New York, New Jersey, and Connecticut in 2012, and much of this damage was related to coastal flooding.
The connection between climate change and hurricane frequency is less straightforward.
Globally, the number of tropical storms that form each year ranges between 70 and 110, with about 40 to 60 of these storms reaching hurricane strength. But records show large year-to-year changes in the number and intensity of these storms.
It’s important to note that changes in frequency and intensity vary from basin to basin. In the North Atlantic Basin, the long-term (1966-2009) average number of tropical storms is about 11 annually, with about six becoming hurricanes. More recently (2000-2013), the average is about 16 tropical storms per year, including about eight hurricanes. This increase in frequency is correlated with the rise in North Atlantic sea surface temperatures, which could be partially related to global warming.
One trend analysis published in the journal Nature shows the strongest hurricanes have also increased in intensity over the past two or three decades in the North Atlantic and Indian Oceans. Other areas in the Pacific and Indian Oceans show virtually no significant trends. Other trend analyses that include all hurricanes globally are similarly inconclusive, with upward trends in the North Atlantic and Indian Oceans and no apparent increase in frequency or intensity in other basins.
For the 21st century, some models project no change or a small reduction in the frequency of hurricanes, while others show an increase in frequency. More recent work shows that there is a trade-off between intensity and frequency – that as warmer oceans bolster hurricane intensity, fewer storms actually form. For the continental United States in the Atlantic Basin, models project a 75 percent increase in the frequency of category 4 and 5 hurricanes despite a possible decrease in the total frequency of all storms.
The tracks of tropical storms near the United States. Tropical storms are shown with red lines and major hurricanes (Category 3 and higher) are yellow. Source: National Hurricane Center. To see the legend, click here
Threats Posed by Hurricanes
The National Hurricane Center categorizes Atlantic hurricanes based on wind speed. A storm with winds exceeding 74 mph is a Category 1 hurricane. Storms with winds stronger than 111 mph are considered “major hurricanes” (Category 3 or higher). Many factors contribute to a hurricane’s impact, including its track, size, storm structure, rainfall amount, duration, and the vulnerability of the area it affects.
Eight of the 10 costliest hurricanes on record in the United States have occurred since 2004. Hurricanes Katrina (2005) and Sandy (2012) were by far the most damaging, costing $125 billion and $65 billion respectively. Hurricanes Andrew (1992) and Ike (2008) cost $27 billion each. Sandy ranks as the second most damaging storm or weather disaster since 1980, even though the storm was no longer a hurricane at landfall.
An important driver of the increased cost of hurricanes is increasing development in coastal areas. U.S. coastal populations grew by nearly 35 million people between 1970 and 2010. Coastal counties account for nearly 40 percent of the total U.S. population. As more development occurs in harm’s way — regardless of climate change — the more likely the damage will grow.
How to Build Resilience
Ways communities can bolster their resilience and reduce the impacts of hurricanes include:
- Preserving coastal wetlands and dunes to absorb storm surges.
- Replenishing beaches and improve infrastructure that affords coastal protection, such as seawalls.
- Elevating vulnerable buildings to reduce flood damage.
- Designing structures to be resilient to high winds and flying debris.
- Enacting policies that discourage development in vulnerable areas.
- Preparing prior to a storm’s arrival by boarding windows, clearing property of potential flying debris, and having an evacuation plan.
To Learn More
National Hurricane Center
NOAA’s Geophysical Fluid Dynamics Laboratory
Learn How to Prepare for Hurricanes at Ready.gov