The annual number of tropical cyclones forming globally has decreased by approximately 13% during the 20th century, and scientists say the main cause is a rise in global warming, according to a new study in Nature Climate Change by a group of international scientists including NOAA scientists. The annual number of tropical cyclones forming globally has decreased by approximately 13% during the 20th century, and scientists say the main cause is a rise in global warming, according to a new study in Nature Climate Change by a group of international scientists including NOAA scientists. Scientists used reconstructed observation data, including the Twentieth Century Reanalysis dataset developed by NOAA’s Physical Sciences Laboratory and partners, as well as high-resolution climate model experiments to reveal the declining trend in the annual number of tropical cyclones since 1850 at both global and regional scales.
The global annual number decreased by 13% in the 20th century when compared to the period between 1850 and 1900. For most ocean basins, the decline accelerated since the 1950s, when climate warming has been unprecedented. “Human emissions have warmed tropical oceans above pre-industrial levels, with most warming occurring since the mid-twentieth century,” said lead author Savin Chand, Ph.D., of the Federation University of Australia. “While such changes in sea surface temperature are expected to intensify storms, some associated changes in atmospheric circulations in the tropics are thought to prevent storm formation.” Despite this global longterm declining trend, there was one exception in the North Atlantic basin, where the number of tropical cyclones or hurricanes have increased in recent decades. “We attribute this increase primarily to decreasing human-caused aerosols,” said Hiroyuki Murakami, Ph.D., a NOAA scientist and a co-author of the new paper. Murakami recently published research showing that the significant decline in aerosols or air pollution in Europe and North America from 1980 to 2020 contributed to increasing tropical cyclones in the North Atlantic region.
Aerosol pollution had served to reflect the sun’s heat back into space, allowing a cooler North Atlantic ocean to fuel fewer tropical cyclones. But when the pollution was reduced through strong environmental regulations in Europe and North America, the number of annual storms increased. Despite this recent increase in tropical cyclones, the number of storms in the late 20th century were still lower than estimates for pre-industrial times, according to the new study. “It may be good news that fewer cyclones are forming because of anthropogenic global warming,” said Chand. “It should be noted, however, that frequency is only one aspect controlling the risks with tropical cyclones.” He added that geographical distributions of tropical cyclones are shifting, tropical cyclones are getting more intense, rain rates are increasing and some storms are slowing down as they travel over land, all this potentially amplifying the damages to humans and coastal communities. Murakami said the new study was important because it is the first time that scientists have been able to estimate the numbers of annual tropical cyclones for an entire century for the global ocean. This has been difficult because reliable observational tropical cyclone data from satellites has only been available since 1980. Prior to that the observational record was incomplete and based largely on observations from ships and aircraft.
NOAA’s development of the 20th Century Reanalysis project led by scientists at NOAA’s Physical Sciences Laboratory was essential to this research. “The reanalysis using longterm sea surface temperature, sea level pressure and sea ice distribution created a three dimensional atmosphere, allowing the reconstruction of proxy observations,” explained Murakami. “Using the reanalysis and climate model experiments, we were able to see the larger longterm trend and better determine the climate forces that have contributed to this trend.”
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