Kicking up a storm

Increased air pollution in South Asia including India is pushing up cyclone intensity in the Arabian Sea putting West Asia at the receiving end of catastrophic natural calamities.

Exploiting observational, reanalysis and model data, a team of US and Korean scientists have now blamed high level of air pollution in South Asia for a sharp rise in the intensity of tropical cyclones over the Arabian Sea during and before the monsoon season.

All powerful cyclones in the Arabian Sea that occurred in the past 30 years made landfalls in India, Pakistan, Oman or Iran causing considerable loss of life and substantial damage.

Increase in cyclone intensity is the most notable in the last decade. In 1998, a major cyclone resulted in more than 1,100 deaths in western India and Cyclone Gonu in 2007 caused more than $ four billion in collective damage to Oman, United Arab Emirates and Iran.

“We suggest that the increasing intensity of land-falling tropical cyclones is a consequence of regional emissions of pollution aerosols,” the scientists reported in the November 3 issue of Nature. They show an increase in the intensity of pre-monsoon Arabian Sea tropical cyclones during the 1979-2010 period, which appears to be a direct consequence of a simultaneous upward trend in an anthropogenic black carbon and sulphate emission.

The pollution creates a conducive environment for cyclone-intensification and since most Arabian Sea cyclones make landfall, the results suggest an additional impact on human health and economic damage. Aerosols are air pollutants originating from natural sources like volcanic ash, dust and sea salt as well as man-made sources like burning of woods and fossil fuels, emission of thermal power plants and vehicular pollution.

Biomass burning

Over much of South Asia, anthropogenic air pollution has led to the formation of thick layers of haze known as atmospheric brown clouds. The main source is vehicle and industrial pollution as well as biomass burning in rural areas, which deposits black carbon in the atmosphere with serious negative consequences for human health.

South Asia includes India, Pakistan, Bangladesh, Nepal and Thailand. The pollution produces brownish clouds of aerosol particles that can be several kilometres thick. The hazy conditions spread out over the Arabian Sea block some of the sun’s energy and prevent it from reaching the sea surface. While the three-km thick pollution haze hangs over India and adjoining areas for months in the winter, they have a relative short-life of a couple of weeks in the summers.

By inhibiting the amount of incoming solar energy at the sea surface, brown clouds effectively reduce the warmest Arabian Sea temperatures as against the equatorial Indian Ocean.

The smaller temperature difference between the north and southern part of the Indian Ocean leads to a key change in wind speed pattern and intensifying tropical cyclones. This would not have happened in an environment without pollution. “Our research shows air pollution needs to be curbed for the benefit of health, agriculture and water security of the region. This paper adds another new dimension to the hazards of air pollution,” V Ramanathan, one of the authors of the study at the University of California told Deccan Herald.

Is there a time lag?

Reduction of emissions could have a nearly immediate effect on the tendency of pre-monsoon tropical cyclones to reach their maximum potential intensity, they suggest.

But is there a time lag between pollution accumulation and cyclone or does it have to be simultaneous? For instance, will extra pollution in December-March lead to intensification of cyclones in May-June? Or does the pollution have to occur around the same time?

Explains team member Amato Evan from the University of Virginia, “There is a lag due to the thermal inertia of the ocean. If you draw a bath with very cold water at first, and then switch to very warm water half way through, the bath will be lukewarm because the water still has a memory of the cold water you started out with.

“For the Arabian Sea, ocean temperatures in April and May still have a memory of the temperature from the proceeding months. I don’t know exactly how many, but based on other ocean basins, May-June temperatures being related to December-March temperatures would be a very reasonable assumption.”

More studies needed?

A senior Indian climate scientist, however, said many more research studies and climate modeling were needed before the air pollution could conclusively be blamed for more tropical cyclones in the Arabian Sea as the relationship between wind speed pattern and tropical cyclone is poorly understood. Asked about the Indian objection, one of the team members James Kossin from US National Ocean and Atmospheric Administration said, “Regarding the relationship between cyclones and wind shear, however, this is probably one of the most well-studied and well-corroborated aspects of the paper’s main hypothesis, and has been recognised for many decades.

The US-Korean team admitted that conclusions were based on a limited number of intense tropical cyclones that occurred over the study period and further work would be required to confirm the proposed relationship between aerosols and cyclone intensity.

“The author’s results shed much-needed light on the relatively unexplored topic of Arabian Sea cyclones and climate. They imply that fascinating connections exist between tropical cyclone activity, air pollution in south Asia and global warming that have strong social and economic implications,” said Ryan Sriver at the Pennsylvania State University who is not connected to the study.

But interpretation of the results, he warned, must be treated with caution. Particularly so because of the timing as the paper comes out weeks before Durban Climate Change summit.

For instance, the authors’ analysis focuses on about 20 cyclones during the past 30 years corresponding to events occurring before the annual onset of monsoon. Out of these 20 events, the five most powerful storms occurred between 1998 and 2010 and it is this small fraction that determines the positive shift in cyclone intensity.

“This is a very small number of events to use as a basis for estimating climate trends. Given the small sample size and handful of anomalous intense storms, it is difficult to disentangle meaningful climate signals from random variability of the sample,” Sriver cautioned.