So with recent anthropogenic warming of the globe, has a
rise in hurricane activity been witnessed, or is it expected to increase?
This question is difficult to answer reliably, as there is
not an accurate record of hurricane activity in the Atlantic before 1944, with
continual satellite coverage only accessible from the year 1966 (Goldenberg etal. 2001). Therefore, although past records do exist, periods which appear
quieter in earlier years could be attributed to limited data coverage.
Easterling et al. (2000) suggest that the biggest setback in analysing whether
or not the frequency of extreme weather events has indeed changed is due to the
limited amount of climatic data available.
It is thought, however, that hurricane activity over the
Atlantic Ocean is increasing, with double the hurricane activity observed from
1995-2000 compared to that of the period from 1971-1994 (Goldenberg et al.
2001). In 1995 alone, there were 11 hurricanes, and 19 tropical storms, which
according to Saunders et al. (1997) is double the 50 year average.
The image taken from the NOAA shows a slight increase in hurricanes over the years, although there are apparent oscillations in the trend. In the above graph, named storms are shown in yellow, hurricanes are depicted in green and hurricanes category 3 or above are in red. Looking at the graph, it appears that stronger hurricanes have been witnessed more recently in the last 2 decades, though this could be due to better monitoring techniques.
It seems that hurricane formation over the Atlantic is
mainly influenced by sea surface temperatures (SST), sea level pressure and
vertical wind shear (Saunders et al. 1997). With anthropogenic warming
(noticeably starting from the Industrial Revolution in the late 1700s (Crutzen2002), there has been an observed increase in the SST which positively
correlates with the number of hurricanes and tropical storms (Wang et al. 2007).
According to Saunders et al (1997) the SST directly influenced at least 42% of
hurricane numbers in the MDR from 1979-1995. The Atlantic Ocean, however,
boasts a large body of warm water at ~28.5°C
in the summer and autumn months, known as the Atlantic Warm Pool (AWP);
therefore the potential energy for hurricane formation exists there with or
without human interaction with the climate (Wang et al. 2007). It was the large
extent of the AWP in 2005 that was associated with the 28 named storms on
record (Wang et al. 2007).
Seeing as the number of hurricanes varies over both an
annual and multidecadal timescale, however, it is hard to justify that the observed
increase in storm activity is a feedback to recent climate change, or just part
of a longer scale of variability (Goldenberg et al. 2001).
IS THERE EVIDENCE TO SUGGEST THAT HURRICANE ACTIVITY IS
INCREASING?
Although the evidence of Goldenberg et al (2001) seems to
suggest that increasing SSTs has a direct implication on the number of hurricanes,
they reason that this observation could be due to longer scale variability, or
even due to the fact that coverage of hurricane activity has significantly
improved over recent years. Therefore, it is reasonable that increased
hurricane activity can be due (although not solely) to improved monitoring
techniques. It has also been suggested by Easterling et al. (1999) that increased
media coverage could be attributed to the public perception that hurricane
activity was on the increase, although this was found to be true only for the
USA.
After reading these articles, I believe that it is
inconclusive whether or not recent Atlantic storm activity is a product of
anthropogenic global warming or just enhanced monitoring. I do, however,
believe that there is sufficient evidence that SSTs are rising (Maslin 2009),
which increases the potential for further hurricane formation (as it is likely
that the 26.5°C threshold is going to be exceeded more frequently) and with the
Earth's system of positive and negative feedbacks to compensate for atmospheric
changes, hurricanes could become more common. Hurricanes remove heat from the
ocean surface through mixing of waters and upwelling during and after
formation, and as the SST increases, the atmosphere’s latent heat content also
increases exponentially; which could mean that future hurricanes won’t just be
able to form more frequently, but will also release more energy when they do
(Saunders et al. 1997). This ocean-atmosphere energy exchange would act as a
potential cooling system, helping to restore atmospheric and oceanic
temperature differences towards equilibrium.
FURTHER REFERENCE
Maslin, M. A.
(2009) ‘Global Warming: A Very Short Introduction’, Oxford University Press:
Oxford.
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