Fuelled by air pollution, storms in South-east Asia are becoming stronger: Study

SINGAPORE - Air pollution from forest fires in South-east Asia may be one reason why

storms in the region are becoming stronger

, new research has found.

The study, led by Professor Steve Yim, who heads the NTU Centre for Climate Change and Environmental Health, analysed 10 years of satellite and weather data from across South-east Asia.

The data shows that powerful storms are becoming more frequent and intense, especially over maritime South-east Asia. This region includes countries such as Singapore, Malaysia, Indonesia and the Philippines.

The researchers also quantified storm intensity by the size and concentration of raindrops. They found that raindrops produced in polluted areas – regions where pollutant particles blocked sunshine from reaching the ground – were up to 1.8 times larger than those in places with clean air.

Aside from biomass burning, other forms of human-caused emissions in South-east Asia come from urban and industrial sources, such as vehicular emissions and household fuel combustion. The burning of fossil fuels for energy generation is another significant source of pollution in the region.

Polluted storms increase rainfall by up to 50 per cent, found the study, which was published on Oct 3 in the Geophysical Research Letters scientific journal.

The researchers determined this by studying the rain rate – total rainfall per unit time – in environments with different levels of air pollution, eventually observing that polluted environments yielded more intense rainfall.

These findings show that South-east Asian countries could be at risk of more intense and frequent storms, due to the double whammy of climate change and air pollution, said Prof Yim.

Climate change intensifies storms, as warming oceans increase heat and moisture in the atmosphere, creating conditions ripe for the formation of storm clouds.

But air pollution supercharges storms further by increasing the number of particles on which water vapour can condense, Prof Yim explained. This finding is especially salient as South-east Asia is one of the top biomass burning regions in the world, he added.

Prof Yim said: “In Indonesia, for example, the farmers burn the land for agricultural purposes, and the smoke may be blown to neighbouring countries like Singapore, resulting in air pollution.”

Biomass burning in South-east Asia is also often done over an open fire, which means the gas produced is not filtered, and results in a lot of smoke being released into the natural environment, he explained.

Tiny particles, also known as aerosols, from dust, smoke or fog serve as seeds for the formation of clouds, when water vapour in the atmosphere condenses around them.

More pollutants in the air would thus yield a larger number of clouds, each of which is smaller in size – since the same amount of water vapour will condense around many more “seeds”.

These small clouds are not substantial enough to trigger rainfall, hence prolonging their lifespan. And the longer clouds stay in the atmosphere, the larger they grow, increasing the amount of rain that eventually falls.

Air pollution also makes tropical storms more dangerous by increasing the number of lightning strikes, the study found.

Storms in polluted conditions trigger up to 27 per cent more lightning. This is because smaller rain clouds generated in pollution-heavy environments will rise to higher altitudes, where the temperature is very low.

This causes the water vapour within the clouds to freeze, forming ice crystals.

The friction and collisions between these crystals are essential to lightning formation, Prof Yim explained.

According to the Meteorological Service Singapore, the Republic has one of the highest rates of lightning activity globally, with an average of 167 thunderstorm days each year.

“Lightning activity is very high in Singapore, and may pose a risk to pedestrians and the aviation industry,” said Prof Yim.

Finally, researchers also found that polluted storms have a wider geographical range, with the reach of storms almost doubling in the presence of aerosols.

The researchers have two hypotheses why this is so.

With more aerosols in the air, more small clouds will form. The total area of coverage by these small clouds tends to be higher than that of a single large cloud, Prof Yim explained.

Also, since the lifespan of small clouds is longer, they can spread out and move to different locations, widening the reach of the storm far beyond its centre.

This heightens health risks associated with rainfall and flooding, he said.

“When the spatial coverage of rainfall is large, there are more chances to accumulate run-off at the ground level, which will create good environments for mosquitoes to survive.”

This could potentially fuel the

spread of vector-borne diseases like dengue

, Prof Yim added.

The findings from this study may be used to strengthen forecasting capabilities, which currently depend on climate models that are limited in their ability to predict rainfall.

There are two mechanisms for rainfall in Singapore, Prof Yim explained, and though current systems are effective for forecasting rain originating from Sumatra – since rain clouds can be spotted moving towards the Republic from afar – they are less attuned to ones caused by local circulation patterns.

“Forecast systems are very critical because if we are able to predict heavy rain, it will help the public to better protect their property,” said Prof Yim.

He said his team is now developing artificial intelligence models to better forecast rain. Aerosols could also be factored into Singapore’s weather models, which presently do not take them into account, he added.

Asked about the significance of this research, Professor Matthias Roth from NUS’ Department of Geography, who was not involved in the study, said it could help to inform severe weather forecasting, climate model accuracy and air quality policy.