For the past 40 years, as far back as the satellite record extends, the number of tropical cyclones that form around the world has held relatively steady, at about 90 per year. As climate change feeds heat energy into the atmosphere and oceans, these storms have become fiercer, but not more frequent.
That could change in coming decades, according to a new model developed by a Massachusetts Institute of Technology meteorologist.
Professor Kerry Emanuel said he was surprised by his latest findings. "In our earlier studies, our results had been largely in line with what other groups were saying: that cyclones would get stronger, but you wouldn't necessarily see more of them," he said. "But when we ran it with new data, we saw the global frequency go up.
"It's a bit of a mystery, honestly."
While the models themselves can't identify the cause of this increase, Emanuel said he and other climate scientists were exploring the possibility that a wide-scale reduction in aerosols, rather than an increase in global greenhouse gases, might be the culprit.
Aerosols are solid particles suspended in a gas, and they can take many forms, from clouds to smog to aerosol sprays. Industrial pollution is a significant contributor to global aerosol concentrations, dispersing nitrates, sulfates and other particulates into the atmosphere.
While some of these aerosols act as short-term climate forcers, others -- particularly sulfates -- mitigate global warming by reflecting solar radiation back out of the atmosphere. Efforts by East Asian nations to reduce their aerosol output would mean less of a reflective shield in the future, Emanuel said.
"Once China and India start to clean up their acts, I think that's going to have a big impact on the oceans," he said. "We know, for example, that cyclones are affected much more by solar radiation" -- which sulfates reflect -- "than by infrared," which they allow to pass through.
There is significant evidence that the United States' own efforts to reduce industrial pollution in the 1970s and 1980s led to an increase in cyclone activity in the Pacific, he said.
A storm's origins, in high definition
Cyclones come into being where heat and moisture from the surface of the ocean interact with storm systems, forming a funnel around a warm, low-pressure core. Heat energy from the ocean's surface acts as the cyclones' fuel, part of the reason these systems tend to originate in warmer equatorial regions.
Cyclones typically start off as small-scale disturbance events, making them extremely difficult to map in conventional climate models such as those used by the Intergovernmental Panel on Climate Change (IPCC), Emanuel said. These models simulate climate systems in a three-dimensional lattice, with each point in the lattice representing a point in space. Because the systems being mapped are themselves so enormous, each data point is set 100 miles apart -- too widely spaced to catch the early formation of most cyclones.
To track cyclone formation on a finer scale, Emanuel created his own program, one that could be embedded into the models used in the IPCC. The program watches for indications of cyclone formation, then "zooms in" to a progressively finer scale as the cyclone intensifies.
For the raw data to plug into his model, Emanuel turned to the data sets compiled by the Coupled Model Intercomparison Project (CMIP), a synthesis of many climate modeling projects that backs the IPCC's various assessment reports.
Emanuel had used his "downscaling" technique to crunch CMIP data for years without diverging from the general consensus on future cyclone frequency. However, scientific understanding of the role of aerosols has evolved significantly over the past several years, possibly explaining the discrepancy between earlier findings and the MIT study's newest conclusions.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500