Expansion of Earth's Tropical Regions Bringing Cyclones Closer To Home

By Matthew Mientka on May 14, 2014 12:45 PM EDT

A new study in the journal Nature suggests human activities may be causing not only global climate change but a possibly related expansion of tropical weather patterns toward northern and southern poles. Photo courtesy of NOAA.
A new study in the journal Nature suggests human activities may be causing not only global climate change but a possibly related expansion of tropical weather patterns toward northern and southern poles. Photo courtesy of NOAA.

As some accept the inevitability of global climate change, scientists say a new and possibly related environmental threat is emerging as the tropical regions expand toward the poles and bring powerful cyclones closer to home for many along the coasts.

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Though not yet conclusive, researchers say the 35 mile-per-decade pace of the tropical expansion observed for the past 30 years may be related to increases in greenhouse gases and atmospheric pollution, as well as the continuing depletion of the stratospheric ozone layer. In a paper published Tuesday in Nature, Jim Kossin of the National Climatic Data Center says the expansion is bringing tropical cyclones away from equatorial populations and toward the northern cities producing much of the pollution some say is driving the change.

"The rate at which tropical cyclones are moving toward the poles is consistent with the observed rates of tropical expansion," Kossin said in a press statement. "The expansion of the tropics appears to be influencing the environmental factors that control tropical cyclone formation and intensification, which is apparently driving their migration toward the poles."

The movement is bringing devastating winds and flooding to northern populations, worsening weather phenomena scientists already attribute to global climate change. But the change also brings problems for tropical countries dependent on such heavy rainfalls to replenish freshwater sources drained continuously by the hot sun. Interestingly, the tropical expansion lacks the symmetry one might expect while looking at a globular depiction of the planet. The expansion is occurring at a much quicker pace in the northern and southern Pacific and South Indian Oceans, with no evidence yet of heightened cyclone activity in the Atlantic.

Still, Kossin says he's confident the data supports conclusions about the tropical expansion after focussing on areas where cyclones typically reach their maximum strength.

"Historical intensity estimates can be very inconsistent over time, but the location where a tropical cyclone reaches its maximum intensity is a more reliable value and less likely to be influenced by data discrepancies or uncertainties," Kossin says.

Although the tropical expansion and migration of cyclones have been observed as independent phenomena, scientists believe the two are related with similar variability and trends. Yet, they cannot yet say whether the poleward migration of tropical cyclones is caused by human-related activity, says Gabriel Vecchi, a researcher at the Geophysical Fluid Dynamics Laboratory who helped with the study.

"Now that we see this clear trend, it is crucial that we understand what has caused it -- so we can understand what is likely to occur in the years and decades to come," she said.

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