Hurricanes Part III: Frequency and Global Warming
Over the last several decades the media have consistently reported that global warming would result in more frequent and more powerful hurricanes. The graph below shows the data for total frequency of hurricanes in the Atlantic since 1944, as well as the number of strong hurricanes, defined as category three and above. While I have seen analyses of these data which have attempted to tease out an increasing trend, there has not been a noteworthy change in either the frequency or the strength of hurricanes during this period, despite rising air and water temperatures.
The lack of change in hurricane frequency and intensity is consistently referenced by climate change deniers as evidence that scientists have been wrong, biased, or participating in some massive conspiracy. The problem with both the news stories and the accusations of malfeasance is that there has never been a scientific consensus that global warming would result in either more frequent or stronger storms.
The inaccurate news coverage of the potential effects of global warming on hurricanes is another example of poor science reporting in the mainstream media. I believe that this failure stems from an unwillingness to spend the time and to commit sufficient news bandwidth to grapple with complex scientific issues. Fortunately here at Chapelboro.com, we can delve into these matters at a level of detail sufficient to understand what is really happening.
Given that one of the prerequisites for a hurricane is a water temperature of at least 79.7 °F, it is tempting to assume that warmer oceans would result in more and perhaps stronger storms. If water temperature were the only parameter which affected hurricanes, then this assumption might turn out to be correct. However, as is often the case with phenomena in nature, the situation is a bit more nuanced. Let’s consider the parameters other than water temperature which effect hurricanes and how these may be impacted by global warming.
In order for a tropical cyclone – the precursor to a hurricane – to get started, a mass of cold air must be positioned above an area of warm water. Furthermore, since the strength of the storm is dependent on the rate of condensation of water droplets in the upper atmosphere (which happens faster as the air gets colder) the initial strength of the storm is dependent on air temperature. Therefore, to the extent that global warming is increasing the temperature of the air above the ocean, both the probability of hurricanes forming and the expected storm strengths are directionally reduced.
Another key parameter in the formation of a tropical cyclone is the need for low wind shear, which sounds counter intuitive at first. In order for the circulating air flows of a cyclone to form and stabilize, the other air movement in the area must be limited; otherwise the cyclone never becomes “organized”. The research I have read on the potential impact of global warming on winds in the Atlantic is inconclusive. However, to the extent that warmer temperatures may result in stronger winds above the oceans (a reasonably likely scenario), the probability of hurricanes being formed would be reduced.
Despite not having a noticeable effect on frequency or strength, global warming is having two definite effects on hurricanes. The warming of the oceans does result in more days per year when the surface water temperature is at or above 79.7 °F, which is lengthening hurricane season. According to the Pew Center on Global Climate Change, the duration of hurricane season has grown by five days per decade since 1915. Therefore, in coming decades we should anticipate hurricanes starting in May and continuing through December. Global warming is also causing sea level rise from the thermal expansion of ocean water as well as the melting of the polar ice caps. Increases in the baseline level of the ocean reduce the amount of storm surge necessary to cause damage along the coast line, as evidenced recently in Manhattan by Superstorm Sandy.
I believe that part of the problem regarding news reports on the impact of global warming on hurricanes is that headlines about an impending series of superstorms create more of a splash than a thoughtful review of the slow steady lengthening of hurricane season and the number of inches that sea level is rising each decade. Perhaps a hurricane on Christmas will increase the buzz-worthiness of the actual science.
There is still no sign of Barry out over the Atlantic, so he must still be working away on Vilcom Circle. Have a comment or question? Use the interface below or send me an email to firstname.lastname@example.org.