Intensity of Hurricane Ike Linked to Global Warming?

By the early morning hours of September 4, Hurricane Ike was a Category 4 hurricane, hitting its peak with 145 mph (233 km/h) winds and a pressure of 935 mbar (27.61 inHg) over the open Atlantic Ocean. That made it the most intense storm so far in the 2008 Atlantic hurricane season. At one point the diameter of Ike's tropical storm and hurricane force winds were 450 and 190 miles (720 and 305 km), respectively. The world watched in horrible anticipation as Ike was tracked toward landfall on the Gulf Coast.

After its devastation on Haiti, Ike slammed into Galveston, Texas at 2 am Saturday, Sept. 13 at 110 mph, and moved inland across the Galveston-Houston area The angry storm shredded buildings, flooded streets; it knocked out power for millions of people and damaged water and sewer services, leaving almost 30,000 people in emergency shelters, says Bloomberg.
According to the Environment News Service. “2.2 million people evacuated from Texas and more than 130,000 left Louisiana ahead of the storm. Ike's coastal storm surge caused flooding of up to 20 feet along the Texas coast.” Two million people were without power in Texas, Arkansas and Louisiana and 28 are known dead from the hurricane that struck the Gulf Coast Saturday morning.

Remnants of the hurricane raced northeast at 50 mph and produced heavy rainfall with strong and damaging winds across the southeastern states, spawning flash flooding and tornadoes and wind damage in a swath from southeast Missouri up the Ohio River through southern Indiana, western and northern Kentucky and into Ohio. At 4 pm Sunday the system was racing 40 miles west of Toledo, Ohio, bringing damaging winds and flooding rain to the lower Ohio Valley. Hurricane Category 1 winds were clocked at 75 mph in Louisville, Kentucky that day. The winds toppled trees, wrecked houses and crushed cars and left tens of thousands without electricity in what is being called the worse power outage in 30 years and warranting a state of emergency. Over a million homes and businesses in Ohio and Kentucky were (and still are) without power, including my friend’s. The remnants of Ike produced heavy rainfall over the lower Great Lakes as it then moved into the St. Lawrence River Valley.

I am reminded of the time I was visiting Louisville, KY, last winter when an aseasonal tornado ravaged the city much in the same way. It prompted a blog post on aseasonal tornados in which I investigated the link of increased violent storms with climate change.

So, what’s going on? Are hurricanes increasing in frequency and intensity? And, if so, is climate change responsible?

In the wake of Katrina in 2005 , John Roach of the National Geographic posed this question: Is Global Warming Making Hurricanes Worse? According to hurricane historian, Jay Barnes of Pine Knoll Shores, North Carolina, ocean heat is the key ingredient for hurricane formation and more heat could "generate more storms and more intense hurricanes," said Barnes.

Although previous studies had not shown conclusively that the number of hurricanes and associated storms is increasing because of climate change, Kerry Emanuel, in the journal Nature in 2005, demonstrated that hurricanes and typhoons have become stronger and longer-lasting over the past 30 years and that these upswings may correlate with a rise in sea surface temperatures. Professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge, Emanuel found that the duration and strength of hurricanes has increased by close to 50% over the past three decades.

First off, let’s look at what a hurricane is. A hurricane is simply an intense tropical storm with maximum sustained wind speeds of 74 mph or greater. Tropical storms form over warm tropical oceans when local sea surface temperatures rise above 26.5 °C (80 °F). Evaporation from the ocean surface then generates very high humidity in the atmosphere, promoting 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.

The National Hurricane Center categorizes hurricanes according to their maximum wind speed based on the Saffir-Simpson scale:

Saffir – Simpson Hurricane Scale

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Category ....Wind Speed (mph) ....Storm surge (feet)
1................... 74 – 95....................... 4 – 5
2 ..................96 – 110 .....................6 – 8
3 .................111 – 130 ....................9 – 12
4 .................131 – 155 ..................13 – 18
5 .................> 155 ..........................> 18

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High winds associated with hurricanes push ocean water onto the shore during land fall, causing a storm surge (see table above). The National Hurricane Center defines major hurricanes, those with the most destructive potential, as category 3 or higher.

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.

So, was Hurricane Ike part of a new series of increased climate change-induced violent storms? As with Katrina, no single storm event can be looked at in this way. No scientist will be able to prove or disprove that Hurricane Ike was a result of global warming. However, there is evidence to suggest it; particularly when the event is investigated as part of a “trend”.

Emanuel’s 2005 study in Nature examined a statistical measure of the power dissipation associated with past hurricane activity (i.e., the "Power Dissipation Index" or "PDI"). He found a correlation between increases in this measure of hurricane activity (which is likely a better measure of the destructive potential of the storms than previously used measures) and rising tropical North Atlantic SST, consistent with basic theoretical expectations. As tropical SSTs increased in past decades, so have the destructive potential of hurricanes.

The NOAA National Hurricane Center had suggested that the recent upturn in hurricane activity is due to a natural cycle, e.g. the so-called Atlantic Multidecadal Oscillation ("AMO"). Emanuel’s results argue against this hypothesis as sole explanation, given that the recent increase in SST lie well outside the range of past oscillations. Emanuel concluded in his paper that "the large upswing in the last decade is unprecedented, and probably reflects the effect of global warming."

There are two ways that hurricanes may increase: in intensity and in frequency.

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.” 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.

References:
Emanuel, Kerry. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.
Pielke Jr., Roger. 2005. Meteorology: Are there trends in hurricane destruction? Nature 438, E13
Emanuel, Kerry. 2005. Meteorology: Emanuel replies. Nature 438, E13
Emanuel, K. 2006. Hurricanes: Tempests in a greenhouse. Physics Today 59:74-75.(5)
Trenberth, K. 2005. Uncertainty in hurricanes and global warming. Science 308: 1753-1754.

Pielke, et al. (NOAA). Hurricanes and Global Warming. 2005. http://www.nhc.noaa.gov/pdf/05pielke.pdf

Union of Concerned Scientists. 2006. Hurricanes and Climate Change.http://www.ucsusa.org/global_warming/science_and_impacts/science/hurricanes-and-climate-change.html

Nina Munteanu is an ecologist and internationally published author of novels, short stories and essays. She coaches writers and teaches writing at George Brown College and the University of Toronto. For more about Nina’s coaching & workshops visit www.ninamunteanu.me. Visit www.ninamunteanu.ca for more about her writing.