Cyclones and climate change
The severity of cyclones in Bay of Bengal, like Phailin, has triggered speculations about whether it is linked to climate change. But the impact of climate change on intensity and frequency of cyclones is not understood well and so far not been proved conclusively.
It has been observed that there is an increase in numbers and proportion of hurricanes of the category 4 and 5 globally since 1970 with a simultaneous decrease in the total number of cyclones and cyclone days. But there are very few historical records of tropical storms and this makes it difficult to understand whether the changes seen now exceed the variability expected from natural causes. Changes in observational capabilities also make it difficult to compare the cyclones then and now.
Modelling studies suggest that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11 per cent along with 34 per cent decrease in the average frequency of tropical cyclones by 2100, say researchers in a study published in Nature Geoscience in 2010.
A study may prove crucial in understanding cyclone Phailin; it focuses on environmental conditions leading to cyclones in the Bay of Bengal during 13 cyclones. The researchers from National Institute of Oceanography in Visakhapatnam found that upper ocean heat content plays an important role in intensification of cyclones. These researchers had tracked 13 cyclones, which included Nargis and Sidr, using data from National Oceanic and Atmospheric Administration (NOAA) in US. The study was published in Current Science in November 2010 but the researchers point out that the sample size they took was small and further studies need to be carried out.
A modelling study published a few months ago provides a better understanding, but of cyclones in the Atlantic Ocean. The storm surges generated during the cyclones are the most harmful aspect of tropical cyclones. The researchers developed a storm surge index from six long high-frequency tide-gauge records from south-eastern United States, which provide information about Atlantic cyclone activity from 1923.
Linking this data to changes in global temperature patterns suggest that the extreme events are especially sensitive to temperature changes. Researchers from China, Denmark, Finland, Sweden, United Kingdom show that there would be a doubling of Katrina magnitude events due to increasing temperature over the 20th century. This study was published in April in journal PNAS. The researchers used warming patterns predicted by six climate models and found that in the 21st century there could be a twofold to sevenfold increase in the frequency of Katrina magnitude events for a 1°C rise in global temperature.