Study: Reducing Man-Caused Air Pollution In North America And Europe Brings Surprising Finding: More Hurricanes
Research has also found that increased pollution in Asia can reduce tropical cyclones
A new NOAA study published in the journal today scientific advances Over four decades of tropical cyclones shows the surprising finding that reducing particulate air pollution in Europe and North America has contributed to an increase in the number of tropical cyclones in the North Atlantic Basin and a decrease in the number of such storms in the Southern Hemisphere. The study also found that increases in particulate matter pollution in Asia contributed to fewer tropical cyclones in the western North Pacific basin.
“Air pollution is a major environmental risk to human health, and we have made great strides in reducing health risks by reducing particulate air pollution,” said Hiroyuki Murakami, a physicist at NOAA’s Geophysical Fluid Dynamics Laboratory and author of the study. “But reducing air pollution does not always reduce the risk of tropical cyclone hazards.”
While a number of recent studies have examined how increasing greenhouse gas emissions affect global tropical cyclone activity, Murakami addresses the less studied and highly complex area of how particulate matter pollution combined with climate change affects tropical cyclones in different areas of the planet. Murakami arrives at these conclusions using the state-of-the-art climate model developed at NOAA GFDL.
How does less pollution in the northern hemisphere increase tropical storms in the Atlantic?
Animation: Increase in tropical cyclones in the North Atlantic
Click on this image to play an animation showing the results of new research by Hiroyuki Murakami, which concluded that 40 years of reductions in man-made pollution from factories, transportation and other sources in North America and Europe are increasing have contributed to an increase in tropical cyclones in the North Atlantic. In contrast, increased air pollution from China and India contributed to a decline in tropical cyclones in the western North Pacific over the same 40-year period from 1980 to 2020. Credit: NOAA
Over the past 40 years, Europe and North America have led the way in reducing air pollution from particulate matter from industry, automobiles, energy and other sources. The increasing absence of man-made air pollution in the northern hemisphere, which is estimated to drop by 50 percent from 1980 to 2020, has led to surface warming over the tropical Atlantic contributing to more frequent tropical cyclones. Without significant amounts of particulate pollution to reflect sunlight, the ocean absorbs more heat and warms faster. A warming Atlantic Ocean was a key factor in a 33 percent increase in tropical cyclone numbers over that 40-year period, Murakami said.
The reduction in pollution has also resulted in warming in the mid- and high-latitude regions of the northern hemisphere. This warming of land and ocean causes the polar stars to move steadily toward the pole jet stream from the tropics to the arctic. The jet stream shift resulted in weakening westerly winds in the upper troposphere in the tropical Atlantic basin, a region of the atmosphere about 10 to 12 miles from Earth’s surface. In turn, weaker winds mean there is less difference between wind speeds in the lower and upper troposphere, or less wind shear. When wind shear is low, tropical cyclones can develop and increase in strength over the Atlantic.
How does more pollution in Asia reduce tropical storms in the western North Pacific?
The Earth system processes at work in the western North Pacific—an area where powerful tropical cyclones are called typhoons—are the flip side of what’s happening in the Atlantic basin. Air pollution is also a key factor behind the decline in tropical cyclones in the western North Pacific, according to the new study. In this case, a 40 percent increase in particulate air pollution concentration was one of several factors that contributed to a 14 percent decline in tropical cyclones, Murakami said. Other factors include natural fluctuations and increased greenhouse gases.
Over the western North Pacific, increasing air pollution from the rapidly developing economies of China and India has reduced the strength of the Indian summer monsoon winds. The increasing pollution is cooling the land in East Asia and serving to reduce the difference between the temperature of the land and the ocean. Without this temperature contrast, monsoon winds weaken. In general, summer tropical cyclones in Asia form in what is known as the monsoon trough, where the west Indian monsoon winds meet the western Pacific trade winds. With weaker monsoon winds there are fewer tropical cyclones.
How does less pollution in the northern hemisphere lead to fewer tropical cyclones in the southern hemisphere?
The warming trend in the northern hemisphere’s mid- and high-latitudes has altered large-scale global circulation patterns, Murakami said. These changes have resulted in increased upward airflow in the northern hemisphere. This causes downward airflow in the southern hemisphere. This downward flow of air is at high pressure, which inhibits tropical cyclone formation.
What is the implication of the new research?
“This study shows that decreasing air pollution leads to an increased risk of tropical cyclones, which occur in the North Atlantic and could also occur in Asia if air pollution is reduced rapidly,” Murakami said. “The ironic result points to the need for careful policy-making in the future that takes into account the pros and cons of multiple impacts.”
Murakami added that the projection over the next few decades is that human-caused particulate air pollution in the North Atlantic will remain stable and that increases in greenhouse gases will have a more significant impact on tropical cyclones. The forecast is for fewer tropical cyclone numbers, but those that do occur are likely to be more intense.
For more information, see NOAA Geophysical Fluid Dynamics Laboratory Research Highlight
Go online to read the research in Science Advances, “Significant global impact of anthropogenic aerosols on tropical cyclones over the past 40 years.”
For more information, please contact Monica Allen, NOAA Communications, at firstname.lastname@example.org or by phone at 202-379-6693