There are two sides to every story, and with conditions in
the future looking uncertain there will always be disagreements within the scientific
community as to what will happen to hurricane frequency trends.
Knutson et al (2008), like many others, noticed the correlation
between the increase in Atlantic SST and hurricane event frequency since the
1950s. With current predictions of GHGs, they agreed that modelling hurricane
frequencies with future conditions should generate more hurricanes with
increased PDIs (measure of the potential destructive power) from present day
figures.
Based on this logic, they conducted a study using a model
simulation which had mirrored present trends when run with past data; although the
intensity of the simulated hurricanes was less than had been observed. To
simulate the future hurricane scenario, CMIP3 models were used, with future
climate data based on the IPCCs A1B scenario. This data was run with the
hurricane frequency data from 1980-2006. The outcome of the study was that a
decrease in hurricanes (-18%) and tropical storms (-27%) was observed, although
precipitation rates in near proximity of hurricanes were higher than predicted
in other studies.
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| Present day and predicted storm tracks as predicted by Benson et al (2010) : Note the increase of the red showing the increase of Category 4 and 5 storms |
These results led Knutson et al. (2008) to believe that rising
SST is not the only factor to influence the frequency of hurricanes. They acknowledge
that some environmental factors are not present within the IPCC scenarios, such
as the presence of aerosols; but came to the conclusion that it has been the
warming of Atlantic waters in comparison to other basins that has influenced
the rise in hurricane formation, NOT just the increase in temperature.
They reason that this is because:
When the North Atlantic warms more rapidly than other ocean basins, as it has since 1980, the changing SST gradients favour northward displacement of the Atlantic Intertropical Convergence Zone, and – in the main development region for Atlantic storms- reduced vertical shear, reduced stability of the tropospheric thermodynamic profile and increased potential intensity.…all these favour tropical storm genesis and probably produce the positive correlation between Atlantic SST and hurricane frequency in both SST and our model.
A study carried out by Bender et al. (2010) re-ran this data
using different models in order to test the previous study findings. Once again,
they came to the conclusion that the number of hurricane days are set to
decrease, but the intensity of the storms are set to increase.
Links to both the articles are posted below, and I suggest you
take a read of them before deciding on the significance of the studies’
conclusions. After I read through the articles, however, I felt that the
conclusions are justified, although both experiments are limited by the
capacity of the model. There are many environmental parameters that can’t be
modelled in the future scenarios, and as we all know, the environment is a
network of complicated teleconnections: leaving out one parameter could alter
the output trend significantly. It also struck me that Bender et al (2010) don’t
seem too confident in their results and urge others to reassess their work
using better GCMs and hurricane simulation models. Until modelling reaches a
higher level of sophistication, however, I don’t think it’s going to be possible
to accurately predict future trends.
But please, leave me your comments and let me know your take
on this!
REFERENCES:
Bender, M.A., T. R. Knutson, R. E.
Tuleya, J.J. Sirutis, G. A. Vecchi, S.T. Garner, I. M. Held (2010), 'Modelled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes', SCIENCE 327, 454.
Knutson, T. R., J.S. Sirutis, S.T.
Garner, G.A. Vecchi and I.M. Held (2008) 'Simulated Reduction in Atlantic Hurricane Frequency under Twenty-First-Century Warming Conditions', NATURE GEOSCIENCE, Vol 1.
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