Date Due: 03/28/2010 Problem Statement

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-by-year 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)

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 19^th, 20^th, and 21^st 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 19^th and 20^th century storms, although there was a considerable reduction by 20% from tropical cyclones in the 21^st century, but there was an increase in the intensity of the storms. The following tables present findings (19C = 19^th Century, 20C = 20^th 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 twenty-first 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.

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.

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

Sciencenow, http://news.sciencemag.org/sciencenow/2010/02/tropical-cyclones-turn-up-the-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

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^ ^{a b c d e} Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division.

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.