The Effect of Global Warming on Hurricanes

What is Global Warming?
Global Warming is a term denoting a gradual warming of the earth's average temperature. The term has become synonymous to the earth's warming exclusively due to man-made effects (anthropogenic) especially from carbon dioxide. The term "Global Warming" was changed several years to "Climate Change". IMO this was done as it became apparent there was a pause in the warming.

Carbon dioxide concentrations have increased from 300 parts per million from 1900, to 419 parts per million by 2021. The most accelerated increase has been during the past 40 years. Some scientists have drawn the conclusion that the increase in the earth's average temperature (about 1°F since 1975) is essentially solely based on man-made activities such as the burning of fossil fuels, biomass burning, and deforestation. It is interesting to note that during the autumn of 2013, the United Nations confirmed that the earth has nearly stopped warming since 1998. The cause for the pause continues to be investigated.

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Causes of Global Warming
The earths atmosphere is made up of different gases. Nitrogen makes up 78% of the total composition, oxygen 21%, argon 1% and traces gases .1%. Carbon dioxide makes up most of the volume of the rare gases. Water vapor varies from almost 0% over the deserts to 4% over the oceans. Water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons are the gases of interest since these are greenhouse gases. Greenhouse gases can reradiate heat or trap heat absorbed by the earth from the sun. Water vapor is by far the largest mechanism by percentage for heat retention out of the greenhouse gases, but changes little over time in atmospheric concentrations. Carbon dioxide and other greenhouse gases have been increasing. The largest contributor is China which produces nearly 1/4 of the world's carbon dioxide emissions.

Greenhouse gases

In the past, the earths orbital eccentricity, variance of the tilt and wobble have led to periods of warmer weather. Even though the solar energy from the sun is considered a constant, a slight change could cause temperature fluctuations. Recently, Mars has also shown a decrease in ice coverage without greenhouse gas changes (Fenton, 2007). The cause for the reduction in aerial coverage and possibly warming may be from dust (Kahn, 1992).

In summary, there are four main mechanisms which would cause global warming, or in other words allow for a warming (or even cooling) of the earth's average surface temperature. These are: (1) solar energy variance, (2) gradual changes in the earth's orbit, tilt, and wobble, (3) change in the earth's albedo, and (4) greenhouse gas changes. It is difficult to calculate what percentage of the recent warming could be attributed to human activities. We can eliminate changes in the earths orbit, tilt, and wobble causing a temperature increase since these are long term effects.

What are the Effects of Global Warming on Hurricanes?
There is a common misconception that because the global temperature has increased, hurricanes also must increase in number and intensity. The primary factor in the ability of a hurricane to strengthen or weaken after formation is the wind shear profile of the atmosphere - not ocean water temperature. Another major factor is the amount of moisture in the lower and middle atmosphere. It has been warm enough in the tropics to produce hurricanes for a very long time. The temperature for tropical cyclone formation equates to a sea surface temperature of about 80°F (26.5°C). If the behavior of hurricanes are studied, cases can be found where hurricanes have strengthened over cooler water and have weakened over warmer water. There maybe a small "boost" to a hurricane's strength as it moves over the Gulf Stream if upper air wind profiles do not change much. Some of the computer generated hurricane models have this bias built in and will rapidly strengthen a tropical cyclone if predicted to go over a warm pool of water.

The recent upturn in tropical cyclone activity was predicted long before Global Warming became a household name. Tropical cyclone activity in the tropical north Atlantic has been known to vary in cycles and an increase was anticipated. For the 20th century, there were nine tropical storms of which five became hurricanes on average. Since 1995, there has been a marked increase to fifteen tropical storms of which eight became hurricanes on average. The 2013 season showed a slow down in numbers of tropical cyclones. In fact, only two hurricanes formed. This is the lowest number since 1982. Looking at the past century, there were othere active periods such as the early to mid 1930's. In general the 1930's through the 1960's were an above average period.

Other considerations should take into account medium to long-term atmospheric - oceananic processes. ENSO is a well know process that has distinct effects on north Atlantic tropical cyclone activity. Kelvin waves and the Madden-Julian Oscillation are also currently being studied on their affects of tropical cyclone activity. The PDO or Pacific Decadal Oscillation also may be connected. There are many different types of waves or oscillations in the atmosphere and ocean. There is much research to be done on how these processes contribute to tropical cyclone formation and intensification.

The Effects of Global Warming on Hurricanes: Summary
There has been a noted increase in the number of hurricanes in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico since the mid-1990's. The long term average is that nine tropical storms will form per year, of which about five will become hurricanes. Since the mid-1990's the average is fifteen tropical storms, of which eight will become hurricanes. There also has been an upward trend in the concentration of the greenhouse gas carbon dioxide in the atmosphere and oceans due to man's activities over the past 100 years. Some scientists have drawn a one to one correlation between the earth's recent temperature increase and the number of hurricanes and their intensity. If the bigger picture is observed - world-wide tropical cyclone activity - no trend is seen.

As it was noted, the number one factor in tropical cyclone intensity is related to the atmospheric wind shear profile. Research is not conclusive that a small increase in the earth's temperature would correlate with increased intensity and number of tropical cyclones. If the current trend of above average tropical cyclone activity were to continue for a considerably longer period, only then could the conclusion be drawn that warmer temperatures have played some part to cause an increase in tropical cyclone formation.

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