Blame it on Asia.
Researchers from the United States have said that Asian countries' man-made pollution could be one cause of changing weather patterns around the world.
The team took a United Nations climate change panel's pollution estimates for 1850 and 2000, and fed them into a climate simulation computer model.
It found that the pollutants caused stronger storms over the Pacific Ocean.
The scientists' paper was published in the latest issue of the US journal Proceedings of the National Academy of Sciences.
"The effects are quite dramatic," said lead author Wang Yuan, from the California Institute of Technology. "The pollution results in thicker and taller clouds and heavier precipitation."
Fellow researcher Zhang Renyi from the Texas A&M University added: "There appears to be little doubt that these particles from Asia affect storms sweeping across the Pacific, and subsequently the weather patterns in North America and the rest of the world."
But local researchers pointed to several limitations in the study, despite agreeing that the findings are plausible.
First, the findings seen in the computer model were not compared to relevant real-life observations, partly due to a lack of measurements.
Second, the scientists chose to hold other variables constant, such as sea surface temperatures, when these have actually changed.
Even if storms over the Pacific Ocean were becoming stronger, this could have also been caused by other factors like higher sea surface temperatures or an increase in greenhouse gases such as carbon dioxide.
"The problem of attribution is a very tricky one," said National University of Singapore (NUS) Assistant Professor Jason Cohen, who is studying the interaction between climate systems and aerosol pollutants similar to those used in the study.
However, he noted that the study could also be underestimating the impact of Asia's pollution. This is because the estimates used are in fact "considerably lower" than the actual pollution.
Dr Cohen found that one type of aerosol pollution called black carbon was globally underestimated by 200 to 300 per cent, and East, South and South-east Asia's emissions in particular were severely underestimated.
NUS senior lecturer Adrian Lee also said the study's findings should not be linked to the extreme weather events in recent years, such as the heatwaves in Australia and the cold snap in the US. "The study is looking at the difference between 1850 and 2000," said the atmospheric science researcher. "It is not looking at what has happened in the last 14 years."
The Singapore researchers said one benefit of the study could be to spur a greater movement to rein in man-made aerosol pollution. Dr Lee said: "This is something that we can control and have a real impact on."
This article was published on April 16 in The Straits Times.
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