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?
How active was the 2007 hurricane season?
How active was the 2008 hurricane season?
How does El Niño influence hurricane activity?
Is the frequency of hurricanes increasing?
Is the intensity of hurricanes increasing?
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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How active was the 2007 hurricane season?

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

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

In all, the North Atlantic experienced the following activity:

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

This activity fits the description of being much above normal. The total number of named storms was predicted accurately, whereas the number of hurricanes was overestimated. Even so, two major hurricanes made landfall at category 5 strength for the first time during the same season. In 2005, hurricanes Katrina, Rita, and Wilma all reached category 5 strength at sea, but weakened to category 3 at landfall. There were also eight named storms in September 2007, tying the largest number of storms to form in a single month.

Hurricane Dean struck the Yucatan Peninsula of Mexico with category 5 force on August 21. 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 $2 billion. Thousands of buildings were destroyed and thousands of families were left homeless.

Hurricane Felix struck Nicaragua at category 5 strength on September 4. 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.

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How active was the 2008 hurricane season?

The Climate Prediction Center predicted much above normal hurricane activity for 2008:

• 14-18 named tropical storms
• 7-10 hurricanes
• 3-6 major hurricanes (category 3 or higher)

December 5 update: There were 18 named tropical storms so far this year. With the traditional hurricane season now officially over (see below), the number of storms to date has met or exceeded the predictions listed above. 2008 storms to date:

• 18 named storms
• 8 hurricanes       
• 5 major hurricanes   (Bertha-category 3, Gustav-cat. 4, Ike-cat. 4; Omar-cat.3; Paloma-cat.4)

There have also been a large number of landfalling tropical storms in the Atlantic this year, with ten storms making landfall overall. According to NOAA's Climate Prediction Center: "For the first time on record, six consecutive tropical cyclones made landfall on the U.S. mainland and a record three major hurricanes struck Cuba.  This is also the first Atlantic season to have a major hurricane (Category 3) form in five consecutive months [July-November]."  This season was also the third most costly on the record, behind 2004 and 2005. It is the 10th season with above normal activity in the past 14 years. Although the season officially ended on November 30, the North Atlantic is still warm and more storms could still form before the end of the year. We will not know the final storm count for 2008 until after this year has ended.

Landfalling tropical storms of 2008:
* = U.S. landfall

• Tropical Storm Arthur struck Belize on May 31, where it caused severe flooding, killed five people, and caused $78 million of damage.
• * Hurricane Dolly struck the Yucatan Peninsula on July 21 and then struck South Padre Island, Texas, on July 31. It caused several deaths in Guatemala and more than $1 billion of damage in Texas. 
• * Tropical Storm Edouard made landfall near Port Arthur, Texas, on August 3. It dropped heavy rain of up to 6.5 inches in areas near Houston.  
• * Tropical Storm Fay formed in the Caribbean Sea and struck the island of Hispaniola and Cuba, then made the record books as the first recorded storm to make landfall four times in the state of Florida. Fay caused record levels of flooding in many parts of northern Florida, then moved westward into other Gulf Coast states. Fay became a tropical depression centered over southern Mississippi, and the National Weather Service issued flood watches and warnings for parts of Louisiana, Mississippi, Alabama, and Florida. Fay dropped more than 27 inches of rain in some locations and set many new records for daily rainfall amounts. This demonstrates that hurricane-strength winds are not necessary for a tropical storm to become hugely disruptive. 
• * Hurricane Gustav struck Haiti, the Dominican Republic, and Jamaica on August 27, killing at least 85 people. It later intensified to a category 4 hurricane just before striking Cuba. Gustav later made landfall in Louisiana, southwest of New Orleans, as a category 2 hurricane. Gustav avoided directly striking major population centers in the U.S. but caused severe flooding and wind damage in more rural areas. 
• * Hurricane Hanna briefly attained category 1 strength but spent most of its life as a strong tropical storm.  It struck Haiti on September 1, dropping heavy rains and causing severe flooding and landslides that killed more than 500 people. Hanna was the deadliest tropical storm in the Atlantic so far this year. On September 6, Hanna made landfall in the U.S. near the South Carolina-North Carolina border and caused severe flooding in several states as it moved northward along the Atlantic Coast. 
• * Hurricane Ike made landfall in the Bahamas and Turks and Caicos islands as a category 4 hurricane, causing devastation on some of the islands. Later, Ike made landfall as a category 3 hurricane in Haiti and Cuba, where it killed at least 82 people (mostly in Haiti). On September 13, Ike made landfall in Galveston, Texas, as a strong category 2 hurricane. Galveston and other coastal towns were devastated by wind and storm-surge damage. Wind damage was also severe in nearby Houston, where windows were blown out of downtown skyscrapers. The remnants of Ike remained intense as the system moved from Texas into the Midwest, causing severe flooding and wind damage in Arkansas, Kentucky, and Ohio, with large-scale power outages, damaged buildings, and severe crop damage. An estimated 4,000 cattle were killed in Texas, and rice and corn crops were severely damaged in several states. At least 63 deaths in the U.S. are blamed on Ike. 
• Hurricane Kyle made landfall in Nova Scotia as a category 1 hurricane. Kyle was remarkable for maintaining hurricane strength so far north. Although Kyle did not make landfall in the U.S., it prompted the state of Maine to issue its first hurricane watch in 17 years. 
• Tropical Storm Marco made landfall near Vera Cruz, Mexico, on October 7. Marco was a small storm and caused flooding in smaller coastal towns but did not cause damage in the major city of Vera Cruz.

The severe destruction, economic losses, and deaths caused by Fay, Hanna, and Ike in the U.S. and elsewhere demonstrate that tropical storms do not have to make landfall as major hurricanes to cause severe and widespread damage. So far in 2008, landfalling tropical storms have caused more than 1,000 confirmed fatalities throughout the North Atlantic region.

Check back for more updates in the coming weeks.

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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. According to the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), globally "[t]here is no clear trend in the annual numbers [i.e. frequency] of tropical cyclones."

However, in the North Atlantic there has been a clear increase in the frequency of tropical storms and major hurricanes. From 1850-1990, the long-term average number of tropical storms was about 10, including about 5 hurricanes. For the period of 1998-2007, the average is about 15 tropical storms per year, 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 the intensity of hurricanes increasing?

Several peer-reviewed studies show a clear global trend toward increased intensity of the strongest hurricanes over the past two or three decades. The strongest trends are in the North Atlantic Ocean and the Indian Ocean. 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].”

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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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