Hurricanes and Global Warming FAQs

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Below are some frequently asked questions about hurricanes, El Niño events, and the scientific link between increased North Atlantic hurricane activity and global warming.

What is a hurricane?
When is the typical hurricane season?
What do we mean by hurricane activity?
Was the 2004 hurricane season more active than normal?
Was the 2005 hurricane season more active than normal?
Why was the 2006 hurricane season so quiet?
What is in store for the 2007 hurricane season?
How does El Niño influence hurricane activity?
Is the frequency of hurricanes increasing?
Is global warming changing the intensity or frequency of hurricanes?
Is global warming generating other types of severe weather?
Will the damages from severe weather become worse in coastal regions?
What can we do to reduce the consequences and costs of severe weather events?

IN DEPTH: Was Hurricane Katrina a product of global warming?



What is a hurricane?

A hurricane is an intense tropical storm. Tropical storms form over warm tropical oceans during periods when local sea surface temperatures are above 26.5 °C (80 °F). Under these conditions, evaporation from the ocean surface generates very high humidity in the atmosphere, which in turn generates thunderstorms. A tropical storm forms when a system of powerful thunderstorms converges and begins to rotate in the atmosphere, forming a vortex known as a tropical depression. Heat from the ocean surface is drawn up through the center of the vortex and released to the atmosphere as water vapor condenses to form rain around the perimeter of the vortex. Energy from ocean heat also generates high winds. The more heat available in the surface water, the more potential there is to generate heavy rain and high wind. If wind speeds exceed 35 mph, the National Hurricane Center deems the system a tropical storm and assigns it a name (e.g., Tropical Storm Arlene, which formed as a depression in the tropical North Atlantic on June 8, 2005, and subsequently reached maximum wind speeds of 70 mph).

A hurricane is simply an intense tropical storm with maximum sustained wind speeds of 74 mph or greater. The National Hurricane Center categorizes hurricanes according to their maximum wind speed based on the Saffir-Simpson scale:

High winds associated with hurricanes push ocean water onto the shore during land fall, causing a storm surge (see table). The potential for damage from storm surge, heavy rain, and high wind increases dramatically as wind speeds increase, and hurricane categories indicate the potential damage from a given storm. The National Hurricane Center defines major hurricanes, those with the most destructive potential, as category 3 or higher. Note, however, that a tropical storm does not have to become a hurricane to be destructive. For instance, Tropical Storm Arlene made landfall in Florida with winds of 60 mph, dropped 6 inches or more of rain in locations from Florida to New York, spawned tornadoes in Indiana, caused power outages in New York, and caused monetary damages of more than $10 million.

The terms “tropical storm” and “hurricane” are standard in North America, but not in other parts of the world. The more general term for “tropical storm” is “tropical cyclone.” In some regions of the world, hurricanes are called typhoons or simply cyclones.

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When is the typical hurricane season?

The North Atlantic and Eastern Pacific hurricane seasons, which directly affect the U.S., run from June 1-November 30, and the peak season is from mid-August through October. However, it has become more common in the past decade for hurricanes to occur outside of this window. During 2003-2005, for instance, the last named tropical storm occurred in December.

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What do we mean by hurricane activity?

There are three terms generally used to describe a hurricane season: frequency, intensity and activity. Hurricane frequency refers to the number of hurricanes that occur. Hurricane intensity is a measure of the strength or maximum wind speed of a hurricane (the category). Hurricane activity is the term used by the National Hurricane Center that encompasses both the frequency and intensity of hurricanes in a season.

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Was the 2004 hurricane season more active than normal?

The Climate Prediction Center of the National Weather Service predicted that the 2004 Atlantic hurricane season would see above normal tropical storm activity, with 12-15 named storms, 7 reaching hurricane strength. This prediction was close but conservative, as there were 15 named storms and 9 hurricanes. The National Hurricane Center described this number of events as "well above-normal activity."

The number of hurricanes making landfall in the U.S. (four: Charley, Frances, Ivan, and Jeanne) was also above normal. Monetary damages from these events exceeded $40 billion and were concentrated in Florida, which had never before experienced more than three land-falling hurricanes in a single season.

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Was the 2005 hurricane season more active than normal?

The Climate Prediction Center expected significantly greater activity for 2005 than occurred in 2004, predicting 18-21 tropical storms and 9-11 hurricanes, 5-7 of which were expected to reach category 3 or greater. The actual activity significantly exceeded these expectations, as well as all previously recorded activity for a single season. The following activity occurred during the 2005 season:

• 28 tropical storms for the first time since systematic record keeping began about 150 years ago. The previous record was 21 storms, set in 1933.
• 15 hurricanes
• 7 major hurricanes
• The earliest date on record by which four named tropical storms formed (Arlene, Bret, Cindy, Dennis formed before July 5)
• The earliest date on record by which two category 4 hurricanes occurred (Dennis formed July 4-7; Emily formed July 10-16)
• The most powerful hurricane ever recorded in the Atlantic basin (Wilma, central barometric pressure of 882 mBar)
• Three of the six most powerful hurricanes ever recorded in the Atlantic basin (Katrina, Rita, Wilma)
• The first time three category 5 hurricanes have ever been recorded in the same year in the Atlantic basin
• The most destructive hurricane in U.S. history (Katrina)
• Tied the record for latest date of storm formation set in 1954 (tropical storm Zeta formed on Dec. 30, 2005)

In terms of overall hurricane activity (number and intensity of storms), the 2005 Atlantic hurricane season was the most active season ever recorded.

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Why was the 2006 hurricane season so quiet?

Actually, the 2006 North Atlantic hurricane season had a normal level of activity. It was only quiet relative to expectations generated by its hyperactive predecessors in 2004 and 2005. The Climate Prediction Center forecast a very active season, but the number of storms fell far short of initial predictions. With 10 tropical storms, 5 hurricanes, and 2 major hurricanes, the North Atlantic 2006 season was near normal.

One likely reason for the reduced Atlantic hurricane activity relative to initial predictions is that an unanticipated El Niño event formed during the summer of 2006. El Niño events typically suppress hurricane activity in the North Atlantic, but enhance it in the North Pacific. Indeed, while the Atlantic was experiencing a near-normal season, hurricane activity was far above normal in the Pacific.

Ordinarily, hurricane activity would have been reduced below normal activity during an El Niño event, so why did the activity reach near-normal levels? According to the National Weather Service, "The near-normal season mainly reflected the competing influence of El Niño and the ongoing conditions associated with the current active hurricane era that began in 1995." In other words, while El Niño acted to suppress activity, other climate conditions were more favorable to hurricanes than usual, boosting activity above the below-normal activity expected for El Niño events.

Another factor calming the Atlantic in 2006 may have been extremely large dust storms from the African continent that covered the tropical Atlantic Ocean in the early summer of 2006. This dust shaded the sea surface from sunlight, lowering the sea surface temperatures in the main region where hurricanes develop. The dust also absorbed moisture from the atmosphere, thus lowering humidity. Such events are unpredictable and add uncertainty to tropical storm projections. Improved understanding of these massive dust storms may improve the ability of scientists to forecast hurricane activity in the future.

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What is in store for the 2007 hurricane season?

The Climate Prediction Center forecasts much above normal hurricane activity for 2007:

• 13-16 named tropical storms
• 7-9 hurricanes
• 3-5 major hurricanes (category 3 or higher)

As of November 2, the North Atlantic has seen the following activity:

• 14 named tropical storms
• 5 total hurricanes
• 2 major hurricanes (both category 5)

Hurricane Dean struck the Yucatan Peninsula of Mexico with category 5 force on August 21 and hurricane Felix struck Nicaragua also at category 5 strength on September 4. This is the first time in recorded history that more than one North Atlantic hurricane has made landfall with category 5 intensity during the same season. In 2005, Katrina, Rita, and Wilma all reached category 5 strength at sea, but all three weakened to category 3 at landfall. There were also eight named storms in September, tying the largest number of storms to form in one month.

Dean directly affected Jamaica, Haiti, Martinique, Dominica, St. Lucia, Belize, and Mexico. Jamaica was the most severely affected; an initial, conservative estimate placed economic losses in one Jamaican parish (St. Mary) alone at two billion dollars. Thousands of buildings were destroyed and thousands of families were left homeless.

Felix affected several Caribbean islands and Honduras, but its main impact was in Nicaragua, where early reports indicated that 9,000 houses were destroyed and 130 people were killed. The area most affected by the storm was relatively sparsely populated.

Check back periodically for updates on the 2007 North Atlantic hurricane season.

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How does El Niño influence hurricane activity?

The El Niño Southern Oscillation is a pattern of short-term climate variability in the tropical Pacific. Warm phases are known as El Niño events and cold phases are known as La Niña events. El Niño events in the Pacific, which occur every 4-7 years, tend to suppress hurricane activity in the Atlantic, particularly inhibiting the formation of major hurricanes (category 3 or higher). For example, every year since 1995 has seen above-average hurricane activity, with two exceptions: 2006 and 1997—both El Niño years. La Niña events create conditions more favorable for Atlantic hurricanes.

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Is the frequency of hurricanes increasing?

Globally (not just in the North Atlantic), there is an average of about 90 tropical storms every year. This average changes very little over time and there has been no detectable change over the 20^th century overall, although we do not have good historical data for all regions where hurricanes occur.

In the North Atlantic, for which we have the best records, there has been a clear increase in the number and intensity of tropical storms and major hurricanes. From 1850-1990, the overall average number of tropical storms was about 10, including about 5 hurricanes. Since 1995, the 10-year average has risen dramatically, with the 1997-2006 average at about 14 tropical storms, including about 8 hurricanes. This increase in frequency correlates strongly with the rise in North Atlantic sea surface temperature, and recent peer-reviewed scientific studies link this temperature increase to global warming.

There is an ongoing scientific debate about the link between increased North Atlantic hurricane activity and global warming. The 2007 report of the Intergovernmental Panel on Climate Change rates the probability of such a link as “more likely than not.” View a figure of the frequency of tropical storms in the North Atlantic.

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Is global warming changing the intensity or frequency of hurricanes?

Intensity: According to the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), it is “more likely than not” (better than even odds) that there is a human contribution to the observed trend of hurricane intensification since the 1970s. In the future, “it is likely [better than 2 to 1 odds] that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical [sea surface temperatures].”

Frequency: According to the IPCC-AR4, on a global scale, “[t]here is no clear trend in the annual numbers [i.e. frequency] of tropical cyclones.” As discussed above, however, the frequency of tropical storms has increased dramatically in the North Atlantic. Reasons for this increase are currently subject to intense debate among climate scientists. At least two recent peer-reviewed scientific studies indicate a significant statistical link between the increased frequency and global warming, but research to identify a mechanism explaining this link is ongoing.

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Is global warming generating other types of severe weather?

Global temperature has increased and precipitation patterns have changed over the 20th century as a result of human-induced global warming, resulting in some increases in extremes of temperature and precipitation. According to the IPCC-AR4, “[i]ncreases in the amount of precipitation are very likely [better than 9 to 1 odds] in high-latitudes, while decreases are likely [better than 2 to 1 odds] in most subtropical land regions,” and “[i]t is very likely [at least 9 to 1 odds] that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent.”

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Will the damages from severe weather become worse in coastal regions?

The biggest reason for increased loss of life and property in coastal regions is population growth and increasing development in coastal areas. As growth and development continue, the damages caused by severe weather will increase regardless of global warming. It stands to reason that climate change, namely sea level rise and increases in tropical storm activity, would exacerbate the damage as global warming continues.

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What can we do to reduce the consequences and costs of severe weather events?

It is clear that the United States is not prepared to handle multiple catastrophic events in a short time period, and many other countries are even less capable of bearing the economic and logistical burdens of catastrophic weather events. We probably will never be completely prepared, but we could do better. We can plan our communities better, we can design our buildings and infrastructure better, and we can manage resources better. Steps taken today to reduce future greenhouse gas emissions will limit the effort necessary to adapt to climate change that will inevitably result from global warming.

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