Charlene Bentum, Colleen Cooper-Vanosdell, Hermes Lynn, Robert Petro, Guy Duncan
Spring 2010 ESCI 580- Teaching Middle School Earth System Science using
Problem Based Learning
Assignment: Hurricane Katrina Module- Cycle B Group Discussion/Team Knowledge
Building Document
Date Due: 03/28/2010
Problem Statement
Has the incidence and intensity of Hurricanes increased and if so, is the increase limited
to the North African warm air to Atlantic tropics corridor? And, could damage estimates
be due to an actual increase in hurricane damages, or is it due to inflationary pressures
and affluent properties being built in coastal areas?
Mechanisms Behind Hurricane Formation
There are a number of factors that affect hurricane formation. Size and strength is
primarily dictated by the amount of energy associated with the storm system. The
ultimate source of this energy is from solar heating of the ocean which causes
evaporation. Once the water begins to evaporate as water vapor, the energy is now stored
as latent heat. This latent heat is released when the water vapor condenses to form liquid
water. This happens when the air is cooled to the LCL, meaning that the air is now
saturated and cannot hold the amount of water vapor any longer. In order to get a larger
storm there has to be more convective potential energy which means that the difference
between the surface temperature of the ocean and the air must be great. There has to be
warm ocean water at least 79.7 degrees F down to a depth of at least 50 meters. There
must also be rapid cooling with height in the atmosphere. This phenomenon allows the
water to continue to evaporate feeding the storm. The cool air temps allow the water
vapor to condense and release energy.
Most hurricanes originate 5 degrees or less from the equator where ocean
temperatures are warm enough to satisfy the rapid evaporation rates required. Once the
storm is born, it can travel out of those latitudes and generally follows the prevailing
winds. Another factor which affects hurricane formation is that there must already be a
high humidity, meaning the air is already close to saturation. This allows the evaporation
water vapor to condense rapidly. Wind shear must also be low at the start of hurricane
formation to allow the cell to progress. Lastly there must be some pre existing storm
system which turns into a hurricane when the above conditions are present.
Based on the above information, the single largest contributor to hurricane
formation is warm ocean temperatures. Without this factor, severe storms are not as
likely. Due to global warming/climate change, sea surface temps have been measured to
be increasing, supporting the fact that there has been an increase in the likelihood and
severity of tropical storms.
Is there Evidence that Climate Change Causes More Frequent/Severe Hurricanes?
The data available at first seems overwhelming regarding the evidence for climate change
being the driving force behind an increase. Consider the following:
Figure 1: Frequency of Category 4 and 5 Hurricanes over the past 35 years
Figure 2: Sea Surface Temperature
Since 1970, sea surface temperature (SST) in tropical regions of the world's oceans has been
on the rise. While the average rise, roughly 0.5ºC, may seem slight, this extra heat in trillions
of tons of water amounts to a vast reserve of energy. And it is well known that hurricanes
draw on warm-water energy to gain strength. The graph above charts SST in particular ocean
basins during their tropical cyclone seasons.
http://www.pbs.org/wgbh/nova/sciencenow/3302/07-hott-nf-02.html
Lack of Consensus
Despite the seemingly supportive evidence, there is no consensus that links either sea
temperature rise or climate change to a predicted increase in hurricane numbers or
strength. There is no doubt that cyclonic storms derive their energy from warm tropical
ocean temperatures. Can these temperatures be linked directly or indirectly to Climate
Change? A study conducted by Dr. Michael Mann suggests that the current trend in
cyclonic storm activity rivals that of 1000 years ago. In a New York Daily News article,
tropical storm specialist Julian Heming of the UK Met Office, “I think if there’s one
standout result [from this study], it’s that the high storm counts we've seen in the last 10
to 15 years could have been matched or even exceeded in past periods.”
The following statement from climatecentral.org seems to sum things up: “Beyond that,
while most climate models predict that the strongest tropical cyclones will get stronger as
human-caused global warming continues (that’s why the word likely in the IPCC’s
statement), some models suggest that the number of storms may not increase. Many
factors influence the formation of hurricanes and tropical cyclones, including wind
patterns, ocean currents, and local weather conditions. All of these may change in a
warming world, in ways scientists are not yet certain of. So it is really hard to project
how the numbers will change worldwide. There could be more hurricanes and tropical
storms in the future. Or there could actually be fewer. This is a difficult problem and
more data and research are needed.”
Quite the Opposite
Studies published by Federov et.al., in fact imply that hurricanes and cyclones can
themselves serve as a driving force behind climate change. The reason- they stir up
waters in the upper ocean resulting in a warmer surface ocean temperature change. Their
findings "add support to the idea that tropical cyclones can actively contribute to the
global climate system," says climate scientist Ryan Sriver of Pennsylvania State
University, University Park (Sciencenow).
Earlier studies by Thomas R. Knutson of NOAA’s Geophysical Fluid Dynamics
Laboratory states that cyclonic hurricane activity is not driven so much by changes in
ocean temperatures as they are by oceanic currents, wind speed, and wind direction- not
just surface winds but wind direction variations throughout different layers of the
atmosphere. This differential warming across layers of the atmosphere and ocean currents
likely have more to do with hurricane activity than Global Warming.
Has there been a continued increase in the number and severity of storms like
Hurricane Katrina? Are hurricanes increasing in frequency and intensity in other
parts of the world, not just the Gulf region?
There is research to support the idea that the “strongest tropical cyclones worldwide are
getting stronger” (http://myweb.fsu.edu/jelsner/index.html). This is due to the increase in
the number of warm years versus cold years. The number of storms that are weaker than
Katrina does not seem to be increasing between globally warm and cool years.
(http://myweb.fsu.edu/jelsner/index.html).
The increase in storm intensity seems to be contained to certain regions of the globe. “In
the North Atlantic there has been a clear increase in the frequency of tropical storms and
major hurricanes.” (http://www.pewclimate.org/hurricanes.cfm#freq). From 1850-1990,
the annual average of North Atlantic hurricanes was 5 per season. From 1998-2007, the
annual average had risen to 8 hurricanes per season. The intensity of tropical storms in
the North Atlantic and Indian Oceans has been rising since the 1970s with “future
tropical cyclones (typhoons and hurricanes) [becoming] more intense, with larger peak
wind speeds and more heavy precipitation associated with ongoing increases of tropical
sea surface temperatures” (http://www.pewclimate.org/hurricanes.cfm#freq).
The charts below show data for the total global number of storms from 1970 to 2005.
Some years have higher averages than others, but there doesn’t seem to be a definite
trend towards increasing storm frequency.
Fig. 2. Global time series for 1970–2004 of (A) number of storms and (B) number of
storm days for tropical cyclones (hurricanes plus tropical storms; black curves),
hurricanes (red curves), and tropical storms (blue curves). Contours indicate the year-byyear variability, and the bold curves show the 5-year running average.
(http://www.sciencemag.org/cgi/content/full/309/5742/1844)
Fig. 3. Regional time series for 1970–2004 for the NATL, WPAC, EPAC, NIO, and
Southern Hemisphere (SIO plus SPAC) for (A) total number of hurricanes and (B) total
number of hurricane days. Thin lines indicate the year-by-year statistics. Heavy lines
show the 5-year running averages.
(http://www.sciencemag.org/cgi/content/full/309/5742/1844)
This figure looks at hurricanes occurring solely in the Atlantic Basin, with some
estimation. This again doesn’t show a strong trend towards increasing storm frequency.
Figure 4: Reconstruction of Atlantic tropical storms (1878 to current) with adjustments
during the pre-satellite era (1878-1965) based on weather reporting ship track density in
the Atlantic. Blue curve shows the adjustment for estimated number of missing storms
Does it seem that hurricanes are getting more frequent and intense because we have
better and more equipment to monitor them?
According to some data, it seems to be likely that the frequency of tropical storms
like hurricanes is increasing; however satellite data began in the mid 1960s making an
earlier data uncertain. Therefore, as we go back in time, the data becomes less and less
reliable. (http://www.usgcrp.gov/usgcrp/links/hurricanes.htm). The previous fifty years
does have “a strong statistical connection between tropical Atlantic sea surface
temperatures and Atlantic hurricane activity”
(http://www.usgcrp.gov/usgcrp/links/hurricanes.htm). While we have better equipment
than fifty years ago, some scientists say that the trends in increased temperatures and
their affect on hurricane intensity and severity are still undetectable. Along with that, it
still may be difficult to tell, even with our modern equipment, whether or not the increase
in tropical storms is due to human activity or natural causes
(http://www.gfdl.noaa.gov/global-warming-and-hurricanes).
Turn on any television station during hurricane/tropical storm season and there will be
discussion about global warming and increases in hurricanes/tropical storms. At first
glance, it may appear that global warming must be the cause of the increased numbers of
hurricanes and tropical storms, but is there, in actuality, a scientifically sounded
connection between the rise in hurricane/tropical storm and global warming?
It may appear that if a cause and effect relationship is apparent between global warming
trends and hurricanes/tropical storms, than it would be an easy task to produce data and
analysis that supports this relationship. As stated by Arpe and Leroy (2008), “A
controversy has arisen because hurricane decadal variability is very large and therefore
trends in changes cannot be extracted from short observational data sets with
confidence.” In other words, there has been such variability in hurricane trends from
decade to decade that attempting to associate trends is difficult. Another point presented
by Arpe and Leroy is that although computer modeling is effective in many regards, it is
still a computer model. When viewing the time-series of hurricane frequencies and
hurricane energy since 1850, as described by Arpe and Leroy (2008), “It is clearly
dangerous to draw any conclusion concerning trends when looking only at the last 20–30
years.” From this figure it is indeed hard to suggest that there has been an increase of
hurricane activity in the recent decades, especially if one has to assume that in the early
years the amounts shown here may be underestimated.
Fig. 1. Time-series of hurricane frequency and energy during the last 150 years. The plot is based on data
from http://www.aoml.noaa.gov/hrd/tcfaq/E11.html (prepared by C.W. Landsea). ener: hurricane energy, MH: major
hurricane frequencies, HU: hurricane frequencies, NS: named tropical storm frequencies.
L. Bengtson et al., in his publication, How may tropical cyclones change in a warmer
climate?, used computer modeling to run three ensemble runs for the last 30 years of the
19th, 20th, and 21st century (2007). These Max Plank Institute computer model runs
examined SST (Sea Surface Temperature) and atmospheric conditions. These simulations
have the ability of introducing specific atmospheric changes, such as increased green
house gasses and global warming effects. With data collected from the simulated runs,
Bengston reported that there was no significant difference in the 19th and 20th century
storms, although there was a considerable reduction by 20% from tropical cyclones in the
21st century, but there was an increase in the intensity of the storms. The following tables
present findings (19C = 19th Century, 20C = 20th Century, etc.)
Kerry Emanuel found data supporting that net hurricane power dissipation was highly
correlated with SST, reflecting well-documented climate signals in the North Atlantic and
North Pacific, and global warming (2005, Emanuel). Results from Emanuel’s research
(2005) suggests that “future warming may lead to an upward trend in tropical cyclone
destructive potential and a substantial increase in hurricane-related losses in the twentyfirst century.” Using the equation,
,
Emanuel defined an index, Potential Destructiveness Index (PDI), of the potential
destructiveness of hurricanes based on the total dissipation of power, integrated over the
lifetime of the cyclone (2005). The following graph presents Emanuel’s findings.
The PDI has been multiplied by 2.1 10-12 and the SST, obtained from the Hadley Centre Sea Ice and SST
data set (HadISST)22, is averaged over a box bounded in latitude by 6° N and 18° N, and in longitude by
20° W and 60° W. Both quantities have been smoothed twice using equation (3), and a constant offset has
been added to the temperature data for ease of comparison. Note that total Atlantic hurricane power
dissipation has more than doubled in the past 30 yr.
While viewing the immense amount of data regarding hurricanes, cyclones, and tropical
storms, it was apparent that there still exists a great deal of discrepancy over trends
associating such storms and global warming. Those individuals that use computer
modeling “see” and associate a trend, but others may or may not. In conclusion, scientific
data and residual data are present for many events that occur on Earth. Weather happens
and then it is completed. Unless humans are there to observe and record, the physical
event in mainly lost after the event. Therefore attempting to collect data from the past is
difficult if not impossible. Therefore, we look for computer models to aid us in this area
of research, which is only as reliable and affective as the scientists programming the
programmed simulations.
Is the fact that more affluent property is located in the regions where these storms
are hitting the US?
According to NOAA, as of 1998 coastal lands which comprise less than 20 percent of the
land of the United States is home to over 53% of the population. At the time of the
report, population growth in these areas was increasing at a rate of approximately 3,600
people a day. The projected growth was an additional 27 million people by 2015. As the
population has grown, so has the density of the population in the coastal areas. In 1960,
the population density was 187 people per square mile which had increased to 273 people
per square mile by 1994. Since 1970, over 40% of the growth of Florida and California
construction, the fastest growing places in the United States, within the coastal region.
Florida alone could be its own topic for research. In 1940, the population of Florida was
1.8 million people. By 1996, the population of Florida had increased 700% to a total of
14.3 million residents. The construction of housing in those areas also dramatically
changed the landscape and natural watershed impacting the ways water is handled during
precipitation events. When we examine the top hurricanes to strike the United States, we
see that half impacted Florida, and almost off of those occurred within the past 20 years.
This would tend to support the idea that it is the growth of building in hurricane prone
areas that is causing the perceived intensity of the storms to increase.
To further examine this question, we need to look at the frequency of hurricanes,
particularly in places such as Florida. When we look at the number of hurricanes to
strike the United States by decade, we do not see any noticeable increase in the past 20
years. Although 2005 was a particularly intense year for hurricane activity, it appears we
need to investigate much further into the future to determine if there is a trend, because
past data would suggest there is not an overwhelming trend.
Figure 3: Hurricanes by Decade, through 2004
Saffir-Simpson Category1
1
2
3
4
5
All
1,2,3,4,5
1851-1860
8
5
5
1
0
19
6
1861-1870
8
6
1
0
0
15
1
1871-1880
7
6
7
0
0
20
7
1881-1890
8
9
4
1
0
22
5
1891-1900
8
5
5
3
0
21
8
1901-1910
10
4
4
0
0
18
4
1911-1920
10
4
4
3
0
21
7
1921-1930
5
3
3
2
0
13
5
1931-1940
4
7
6
1
1
19
8
1941-1950
8
6
9
1
0
24
10
1951-1960
8
1
5
3
0
17
8
1961-1970
3
5
4
1
1
14
6
1971-1980
6
2
4
0
0
12
4
1981-1990
9
1
4
1
0
15
5
1991-2000
3
6
4
0
1
14
5
2001-2004
4
2
2
1
0
9
3
1851-2004
109
72
71
18
3
273
92
Average Per Decade
7.1
4.7
4.6
1.2
0.2
17.7
6.0
Decade
Major
3,4,5
Figure 4: Hurricanes by decade 1911-2005, graphical
The National Climate Center has further supports the concept that the location of
construction has a bigger influence over the measured impact of hurricanes than number
also. They state it is “changes in society…particularly the explosion of high-density
development along our alluring but vulnerable beachfronts.” They note that coastal
development over the past 50 years has not only accelerated, but in the last 20 years its
character shifted from the occasional inexpensive home or vacation property to luxury
hotels, high-rise condominiums and multi-million dollar homes that comprise an entire
community.
Development not only places these properties in areas where the perils are more likely to
arrive. Construction eliminates the wetlands that could absorb some waters and protect
properties from the devastating storm surge. Storm water management practices also
differ from the natural handling methods of precipitation, leading to flooding that might
have otherwise been handled naturally.
There is evidence to suggest that it is human activity may be causing the storms to
become more significant based on the damage that they cause, but it is because of
constructing in hurricane prone areas that is of concern, not climate change.
Bibliography
Nature 463, 1066-1070 (25 February 2010), Tropical cyclones and permanent El Niño in
the early Pliocene epoch; Alexey V. Fedorov, Christopher M. Brierley & Kerry Emanuel
Sriver, R. L., Goes, M., Mann, M. E., and Keller, K., Climate response to realistic
tropical cyclone-induced ocean mixing in an Earth system model of intermediate
complexity, Journal of Geophysical Research-Oceans(in review).
Sciencenow, http://news.sciencemag.org/sciencenow/2010/02/tropical-cyclones-turn-upthe-he.htmlTropical Cyclones Turn Up the Heat by Phil Berardelli on February 24, 2010
Arpe, K. and Leroy, S. A. G., Atlantic hurricanes—Testing impacts of local SSTs, ENSO,
stratospheric QBO—Implications for global warming, Quaternary International, 2008, V.
195, Issues 1-2, p. 4-14
L. Bengston et al., How may tropical cyclones change in a warmer climate?, Tellus A, V.
59, Issue 59 A, p. 539-561
Emanuel, K., Increasing destructiveness of cyclones over the past 30 years, Nature, V.
436, p. 686-688
http://www.nydailynews.com/news/national/2009/08/13/2009-08
13_rock_you_like_a_hurricane_research_parallels_surge_in_atlantic_storm_activity_to.h
tml#ixzz0jWa5ByZo
Has Global Warming Affected Atlantic Hurricane Activity? Thomas R. Knutson,
Geophysical Fluid Dynamics Laboratory/NOAA Sept. 3, 2008; Last Revised Mar. 26,
2010
http://www.climatecentral.org/library/faqs/is_global_warming_making_hurricanes_worse
^ a b c d e Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research
Division.
"Frequently Asked Questions: When is hurricane season?". National Oceanic and
Atmospheric Administration. http://www.aoml.noaa.gov/hrd/tcfaq/G1.html.
http://www.nhc.noaa.gov/HAW2/english/history.shtml
http://www.usgcrp.gov/usgcrp/links/hurricanes.htm.
http://www.pewclimate.org/hurricanes.cfm#freq.
http://myweb.fsu.edu/jelsner/index.html.
http://www.gfdl.noaa.gov/global-warming-and-hurricanes.
http://www.sciencemag.org/cgi/content/full/309/5742/1844.
http://oceanservice.noaa.gov/websites/retiredsites/sotc_pdf/POP.PDF
http://www.nhc.noaa.gov/pastdec.shtml
http://www.ncpa.org/pub/ba530