Here's a little article...

It appears that I'm not the only one thinking of the possible impacts that our actions will have on future climate. The BBC reported on the effects of climate change already being evident in Europe, ahead of the UN Climate Convention in Qatar starting on Monday.

In agreement that the frequency and intensity of extreme weather events are going to increase (with some of the areas to be hit hardest also those less than able to cope) the report acknowledges that the cost of damages caused by extreme events is already increasing, in part due to larger populations and more infrastructure in vulnerable areas. But it begs the question, how are we going to cope in our future world? The UN convention hopes to address the current and future state of our climate as well as assessing possible adaptation and mitigation techniques. Have we passed the point where we can curb our carbon emissions, meaning that mitigation and adaptation are our sole defences? 

Have a think, read the article, and I'll follow this with another post shortly.

http://www.bbc.co.uk/news/science-environment-20408350

Will hurricane frequency decrease in the future?

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.

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.

http://www.sciencemag.org/content/327/5964/454.full.pdf

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.

http://www.nature.com/ngeo/journal/v1/n6/full/ngeo202.html

Hurricane trends and climate change: a brief overview

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.

The wind's picking up a bit...

Here's a short introductory video to hurricane formation:

Hurricanes, otherwise known as cyclones in the Northern Indian Ocean, or typhoons off the eastern coast of Asia, mainly form in the months of August, September and October. In the Atlantic, most of the major hurricanes are formed in what is termed the MDR (Main Development Region) which is situated between 10° and 20°N, so any changes in the temperature or atmospheric conditions in the Tropics can greatly influence rates of hurricane formation (Goldenberg et al. 2001).

Hurricanes are fuelled by heat, so in order to form, there needs to be a large expanse of ocean where the sea surface temperature (SST) is 26.5°C or above (National Geographic 2012). Hurricanes begin as tropical storm disturbances (Wu et al. 2006), and often remain at this status, but every so often these low pressure storm cells develop into the hurricane systems which can bring devastation. These formations occur when the ocean releases its heat into the atmosphere through latent heat transportation, which forms large cumulonimbus towers. When this tower expands into the troposphere, it is influenced by the converging anticyclonic winds which exist there, driving the storm system and transferring the heat release into kinetic energy. The larger the temperature differences between the ocean and the atmosphere, the larger the driving force for the hurricane.

One of the instantly recognisable features of the hurricane is the eye which resides in the centre of the storm. This warm core intensifies the upper anticyclone in the troposphere, in turn driving greater heat and moisture uptake from the ocean which feeds the storm further. Air is sucked into the low pressure system, rises close to the eye wall and is heated adiabatically whilst being uptaken to the troposphere; when it cools it then descends outside of the storm formation. This is known as a Carnot energy cycle and is known for its efficiency. The dry air descending from the troposphere (blown in from across the Sahara desert (Wu et al. 2006)) is drawn into the eye and heated, again adiabatically, to intensify the heat supply to the hurricane (National Geographic 2012).

According to Chorley et al (2010), the main driving forces for hurricane formation are:

• A large supply of heat and moisture from the ocean surface, as well as having a low frictional drag on the surface waters
•  The release of latent heat through condensation, which aids in forming the towering cumulonimbus clouds and also provides potential energy for kinetic movement.  
•  Divergent winds in the upper troposphere

Inhibiting factors include:

•        Decrease in SST or when the hurricane system makes landfall, as there is no further supply of heat energy which can drive the storm system.
•        Cold air can also be drawn into the vortex which again decreases the energy input into the storm system.
•        A strong vertical wind shear (such as an area beneath a Jet stream), as it is difficult for the vortex to form.

FURTHER REFERENCE:

Chorley, R.J. and R. G. Barry (2010) ‘Atmosphere, Weather and Climate (Ninth Edition)’, Routledge: New York.