Climate change has been found to increase the rainfall and storm surge associated with hurricanes, and there is strong evidence that climate change also increases the intensity and wind speed of hurricanes. While the details are still being explored, the basics are simple. Sea level rise fuels the storm surge driven by hurricanes.  Warmer air holds more moisture, feeding more precipitation from all storms including hurricanes. Hurricanes need warm water, and climate change warms sea surface temperatures, contributing to an increase in hurricane intensity, and with that an exponential increase in damage. Future warming increases the chances of a Katrina-like event two to seven times per degree C of warming.

Cyclones, typhoons or hurricanes—three different names for the same type of storm—are among the world’s most devastating and costly natural disasters. Hurricanes are ranked by the intensity of their sustained wind speed, but extreme winds are only part of what causes a hurricane’s massive devastation. For many hurricanes, storm surges (exacerbated by extreme winds and sea level rise) and flooding (made worse by extreme rainfall) cause the most damage and loss of life.

In the U.S., hurricanes accounted for seven out of ten of the costliest natural disasters from 1980-2014. Hurricane Katrina in 2005 is at the top of the list, having caused 1,322 fatalities and overall losses of $125 billion. Hurricane Sandy in 2012 was the second worst disaster with 127 fatalities and $65 billion in losses. In Asia, there were four major typhoons during 2013-2014 that ranked among the top 10 costliest storms in the region from 1980-2014, with 2013’s Typhoon Haiyan ranked on top.

Hurricanes form over ocean waters near the equator using warm, moist air as fuel. While their formation depends on warm sea surface temperature (SST), the interactions between warm ocean water and regional atmospheric conditions, known as atmospheric stability, are what centrally affect hurricane intensity. In addition to SST, atmospheric stability depends on air moisture and temperatures as well as wind patterns, since too much vertical wind change, or wind shear, can disrupt hurricane formation.

While the impact of climate change on hurricanes is still an active area of research, experts agree that climate change is connected in various ways to more devastating hurricanes in many ocean basins. Climate change can impact hurricane activity in many different ways by: (1) increasing storm intensity, (2) altering storm size (3) decreasing storm frequency, (4) increasing rainfall intensity, (5) causing higher storm surges due to sea level rise, (6) altering El Niño Southern Oscillation and Madden-Julian Oscillation patterns, and (7) shifting the locations where storms develop and travel. Because a combination of these factors can influence a hurricane’s overall impact, experts also consider two metrics—the Power Dissipation Index (PDI) and Accumulated Cyclone Energy (ACE)—that summarize hurricane activity by combining intensity, duration and frequency data.

The following provides an overview of the current scientific understanding of climate change impacts on hurricanes in different regions and what climate models anticipate for the future.

GLOBAL TRENDS

• Intensity: According to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change, the intensity (or maximum wind speed) of the strongest hurricanes has likely increased in some regions since 1970. Global ocean warmth has also been linked to an increase in atmospheric pressure believed to increase hurricane intensity, while inhibiting overall hurricane occurrence. Cooler ocean basins with larger temperature increases in recent years show the greatest upward trend in hurricane intensity. These include the North Atlantic, eastern North Pacific and southern Indian basins. By late this century, models project an increase in the number of the strongest (Category 4 and 5) hurricanes. As hurricanes increase in intensity, the potential for greater damage increases exponentially (by a power of three). For example, a 150 mph wind is 20 percent stronger than a 125 mph wind. However, the destructive power of a 150 mph wind compared to a 125 mph wind is actually 73 percent greater.

• Sea surface temperature: There is a strong link between SST and hurricane intensity, suggesting that increased SST leads to stronger storms. However, the picture isn’t that simple. Research indicates it is the temperature gradient between local SST and average SST in the tropics that fuels stronger storms, and that changing atmospheric temperatures affect the influence of SST.

• Frequency: Each year, about 90 hurricanes occur globally, and this number has remained roughly steady since satellite records began in the mid-1970s, according to the AR5. However, there can be substantial variability in hurricane frequency within individual ocean basins. In the future, models project that the total number of cyclones will decrease by the late 21st century, with one study projecting an overall global frequency decrease ranging from 6 to 34 percent. The Southern Hemisphere ocean basins in particular show a marked decrease. The decrease may be due to increases in vertical wind shear or a weakening of tropical circulation. It is important to keep in mind that model projections predicting fewer but stronger hurricanes in the future do not imply that overall damage will be the same in the future. On the contrary overall damage will be much higher.

• Rainfall intensity: Increases in tropical water vapor and rainfall have been identified and there is evidence for related changes in tropical cyclone-related rainfall. One study quantifying the effects of climate change on typhoon rainfall near Taiwan in the Northwest Pacific finds that modern-day typhoons yield more rainfall by about 5 percent. Another study finds that there is a positive significant trend in total July-November rainfall over the North Atlantic and western North Pacific, and that hurricanes have been feeding increasingly more to rainfall extremes. Projections show a 20 percent increase in hurricane precipitation within 100 kilometers of the storm center by the end of the century. Models are highly consistent in projecting increase in rainfall of between 3 and 37 percent within the area near the hurricane center.

• Storm surge: Sea levels are rising and will continue to rise for many centuries, resulting in increasing baseline elevations for waves and storm surges. Extremes in sea level tend to be caused by large storms, especially when they occur at times of high tide. The damages wrought by heavy rain and storm surge are often much worse than the damage from heavy winds.

• Storm tracks: The literature suggests that storm tracks and the location of maximum intensity have shifted poleward, away from the tropics in recent years as north to south SST gradients change. The shift is particularly clear in the Southern Hemisphere. Similarly, the literature suggests an eastward track shift in several basins including the North Atlantic and Northwest Pacific. Superstorm Sandy provided an example of the result of such a shift.

• El Niño-Southern Oscillation and Madden-Julian Oscillation: Both El Niño-Southern Oscillation (ENSO) and the Madden-Julian oscillation (MJO) have been documented to impact hurricane activity around the globe. El Niño events are associated with warmer SSTs in the eastern and central equatorial Pacific, and changes in the strength and location of a pattern of atmospheric circulation over the Pacific known as the Walker Circulation. These fluctuations have been shown to change hurricane frequency around the globe. For example, there is generally increased vertical wind shear across the tropical Atlantic during El Niño years, reducing the chance for hurricane formation. Climate models provide evidence for a doubling in the occurrences of extreme El Niño events in the future in response to climate change. The MJO is a pattern of tropical variability that propagates around the globe on an approximately 30 to 60 day time scale. As it moves, the MJO alters vertical wind shear and moisture, among others, leading to periods of increased precipitation and dryness. These alterations have been implicated in the tendency of hurricanes cluster in time around the globe.

• Observational challenges: Due to historical hurricane records with different observing technology and reporting protocols, detection of hurricane trends remains a significant challenge. Much of the global hurricane data is confined to the period from the mid-20th century to the present, with the exception of the North Atlantic. As a result, scientists remain cautious when reporting observed long-term (i.e. 40 years or more) increases in hurricane activity.

• Paleoclimate evidence: By looking at proxy records, scientists are able to metaphorically “observe” the record of hurricanes over hundreds to thousands of years. In doing so, they have found that periods of intense hurricane activity occur when sea surface temperatures are abnormally high, as they are currently with climate change. In general, studies of ancient hurricane activity find that active and quiet periods are influenced by climatic forces, particularly El Niño and La Niña, which warm and cool the oceans, respectively.