Hurricanes and Climate Change

Increasingly destructive hurricanes are putting a growing number of people and structures at risk

Sandy. Katrina. Andrew. Ike.

Wilma. Ivan. Charley. Irene.

For coastal communities, the social, economic, and physical scars left behind by major hurricanes can be devastating.

While hurricanes are a natural part of our climate system, recent research suggests that their destructive power, or intensity, has been growing since the 1970s, particularly in the North Atlantic region [1].

A growing number of people and structures are at risk from the increasingly destructive potential of hurricanes, a trend exacerbated by sea level rise and rapid population growth. 

The aftermath of Hurricane Ike in Gilchrist, Texas, in 2008.
Photo: Jocelyn Augusitno/FEMA CC BY (Wikipedia)

Factors that increase the destructive potential of hurricanes

The oceans have taken in nearly all of the excess energy created by global warming, absorbing 93 percent of the increase in the planet’s energy inventory from 1971-2010 [2].

In some ocean basins, hurricane intensification has been linked to rising ocean temperatures [4]. Since 1970, tropical ocean sea surface temperatures worldwide have warmed by about an average of 0.5°C [3]. Warming in the North Atlantic basin has been more rapid—about 0.7°C since the 1980s [5].

Sea levels are also rising in response as the oceans warm and seawater expands. This expansion, combined with the melting of land-based ice, has caused global average sea level to rise by roughly 8 inches  since 1880 [6]—a trend that is expected to accelerate over coming decades.

Higher sea levels give coastal storm surges a higher starting point when major storms approach and pile water up along the shore. The resulting storm surge reaches higher and penetrates further inland in low-lying areas. The risk is even greater if storms make landfall during high tides.

Roads and other crucial infrastructure face growing risks from storm surges.
Photo: USFWS/CC BY (Flickr)

Roughly a third of the US population—more than 100 million people—lives in coastal counties [7]. US coastal county populations are also growing much denser than non-coastal counties. Between 1980 and 2008 coastal counties increased population density by 28 percent (excluding Alaska). In non-coastal counties, population density hardly changed over the same period. 

By concentrating ourselves along the coasts, we have increasingly exposed our communities and homes to powerful storms. As a result of coastal development, storms are exacting rising financial tolls [8].

Observed trends in hurricanes

The number and strength of storms is highly variable from year to year, which makes it challenging to detect trends in the frequency or intensity of hurricanes over time.

Storm counts and strength measurements were also less consistent prior to the 1970s when satellite observations began, further complicating the study of long-term trends [9].

To help address these challenges, scientists run hurricane models calibrated with observations over the historical period to project future trends and understand their major contributing factors [10].

chart of hurricane storm categories

A hurricane is a rotating, organized system of thunderstorms and clouds originating in tropical or subtropical waters. Hurricanes are classified according to wind speed.

Recent research in this area suggests that hurricanes in the North Atlantic region have been intensifying over the past 40 years [11].

Since the mid-1970s, the number of hurricanes that reach Categories 4 and 5 in strength—that is, the two strongest classifications—has roughly doubled [12].

Measures of the potential destructiveness of hurricanes (a measure of the power of a hurricane over its entire lifetime) also show a doubling during this time period. Indices for hurricane activity based on storm surge data from tide gauges further indicate an increase in intensity [13].

Hurricanes in the western North Pacific and the northern Indian oceans—known as typhoons and cyclones, respectively—are also intensifying, though the signal is not as strong as for the North Atlantic [14]. Whether hurricanes are intensifying in other regions is less clear, though other recent evidence suggests that the trend toward more intense hurricanes may extend globally [15].

There has been little change, however, in the frequency of hurricanes globally [16]. Roughly 90 hurricanes occur each year around the world, with by far the greatest number occurring in the largest ocean basin on Earth–the Pacific.

To further address the challenges of detecting long-term trends, scientists also study the core factors that intensify or weaken hurricanes, including the interplay between human-driven climate change and natural factors. 

Percent of Atlantic hurricanes each year from 1970 to 2012 that reached categories 3, 4, and 5. Annual data (light blue) and 5-year running average (dark blue).
Graphic: Union of Concerned Scientists; Data: NOAA Hurricane Research Center (http://1.usa.gov/1YV6SNy and http://1.usa.gov/1QPC22Z).

Rising ocean temperatures fuel stronger North Atlantic hurricanes

Warm ocean temperatures are one of the key factors that strengthen hurricane development when overall conditions are conducive for their formation and growth [17].

Hurricanes require high humidity, relatively constant winds at different altitudes, and can occur when surface ocean temperatures exceed about 79°F (26°C). The rising of warm, moist air from the ocean helps to power the storm.

In order to build up and intensify, hurricanes require warm ocean temperatures, moist air, and low vertical wind shear (i.e. no strong change in wind speed or direction between two different altitudes).
Graphic: physicalgeography.net

Because of this link between warm oceans and hurricane behavior, warming of the surface ocean can increase the intensity of hurricanes, with the stronger ones getting the biggest boost [18]. While hurricanes that make landfall are comparatively rare, they are responsible for vast economic damage in the United States [19].

Two other factors may also be contributing to the rising intensities of hurricanes. First, warm air holds more water vapor than cold air—and the rising air temperatures since the 1970s have caused the atmospheric water vapor content to rise as well [20]. This increased moisture provides additional fuel for hurricanes. Indeed, hurricanes indicate a trend toward producing more torrential downpours, both in the historical record and in climate models that project future conditions [21].

Second, as ocean temperatures rise, there is also less cold, subsurface ocean water to serve as a braking mechanism for hurricanes. When strong storm winds churn up cold subsurface water, the cooler waters can serve to weaken the storm. But if deeper waters become too warm, this natural braking mechanism weakens. Hurricane Katrina, for example, intensified significantly when it hit deep pools of warm water in the Gulf of Mexico [22].

The largest Atlantic hurricane on record, Hurricane Sandy reached over 1000 miles in diameter and made landfall in the U.S. on October 29, 2012.
Photo: NASA

The role of natural cycles in hurricanes

The oceans experience a variety of natural circulation patterns, or oscillations, that influence the distribution of warm and cold water in the upper ocean. These naturally occurring oscillations affect ocean conditions on timescales ranging from just a few years to several decades and are known to affect the intensity of hurricanes.

During the warm, or El Niño, phase of the El Niño Southern Oscillation (ENSO), for example, hurricanes are less likely to make landfall in eastern Australia and Atlantic hurricanes tend to be suppressed [23]. However, El Niño conditions can boost typhoon risks in parts of Asia [24]. 

The presence of these natural oscillations can mask or enhance the potential influence of human-caused warming on hurricane activity.

hurricane sandy damage to mantoloking-new-jersey

The aftermath of Hurricane Sandy in Mantaloking, New Jersey.
Photo: Master Sgt. Mark C. Olsen/US Air Force CC BY (Wikimedia)

What the future holds

As the climate continues to warm, the frequency of intense hurricanes in the North Atlantic is projected to rise while the overall number of hurricanes globally is expected to either decline or remain unchanged [25].

The projected increase in intense hurricanes is substantial—a doubling or more in the frequency of category 4 and 5 storms by the end of the century—with the western North Atlantic experiencing the largest increase [26]. With continued warming, sea level is likely to rise by one to four feet globally by the end of the century, enabling the powerful surge associated with hurricanes to penetrate further inland than today [27]. 

Given the loss of life [28] and the huge costs of rebuilding after hurricanes, it is essential to do whatever we can to avoid dangerous warming and protect coastal communities for ourselves and our children.

References:

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Last revised date: September 1, 2016

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