One of the pre-conditions for the formation of a tropical cyclone is that the ocean should be warm enough (27 deg C or higher). Tropical storms therefore tend to form only over certain ocean basins and in certain preferred seasons where and when this condition could get satisfied. An increase in sea surface temperature brought about by global warming should, logically speaking, lead to enhanced cyclonic activity. However, it should be remembered here that a warm ocean is only one of the pre-conditions for the formation of a tropical cyclone, and not the only one. Tropical storms do not exist by themselves but are embedded in the general atmospheric flow which does influence them a great deal. And it is not only the number of tropical storms that is important, but also the peak intensity that they reach, and the length and orientation of the tracks that they follow. Thus statistical correlations between global warming and the frequency of occurrence of tropical storms cannot be viewed in isolation and without due regard to these other aspects.

Nowadays, whenever there is a major tropical cyclone or hurricane, heightened scientific and media attention gets focused upon global warming as a cause behind the extreme event. Thus Atlantic hurricanes like Katrina, Rita, or the recent Dean, or major typhoons elsewhere, at once result in a “we-told-you-so” stand being taken by climate change enthusiasts. In this context, it is worthwhile to take a look at what tropical cyclone experts from around the world have to say about it. An International Workshop on Tropical Cyclones (IWTC-VI) was organized by the World Meteorological Organization at San Jose, Costa Rica, in November 2006. At the end of the Workshop, the participants issued a statement on the linkage between anthropogenic (human-induced) climate change and tropical cyclones. Since there were 125 delegates from 34 different countries and regions, and since the process was overseen by a committee of the WMO Tropical Meteorology Research Programme TMRP Committee TC2, the statement can be regarded as an authoritative and consensus view of the global community of tropical cyclone researchers and forecasters. The statement is remarkably balanced in its approach and findings and it is very categorical in what it says.

First of all, the consensus statement makes it very clear that no individual tropical cyclone can be directly attributed to climate change. The increasing socio-economic impact that tropical cyclones have been making in recent years is largely because of rising concentrations of population and infrastructure in coastal regions.

Another important point that the statement makes is that as of now, no firm conclusions can be drawn about the influence of global warming on tropical cyclones as there is equal evidence both for and against it.

The statement draws attention to the various difficulties in determining accurate long-term trends in the characteristics of tropical cyclones. The observed multi-decadal variability of tropical cyclones in some regions could be natural or anthropogenic or both. Methods of estimating wind speeds associated with tropical cyclone have undergone changes in recent years and different practices are followed in different regions. In most regions there are no observations from instrumented aircraft flying into tropical cyclones.

The statement accepts that if the climate continues to warm, some increase in tropical cyclone peak wind speed and rainfall is likely to occur. There is, however, an inconsistency between models which project small changes in wind speed and some observational studies which suggest large changes. Also, how tropical cyclone tracks or areas of impact may change in the future cannot be foreseen now.

The statement also refers to the increased vulnerability of coastal areas due to cyclone-related storm surge, if the sea level were also to rise because of global warming.

The text of the Summary Statement on Tropical Cyclones and Climate Change can be read on the web site of the World Meteorological Organization at http://www.wmo.ch/pages/prog/arep/tmrp/documents/iwtc_summary.pdf and the text of the complete statement is available at http://www.wmo.ch/pages/prog/arep/tmrp/documents/iwtc_statement.pdf.

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