Did the Clean Air Act unleash Atlantic hurricanes?


Hurricane Andrew, 1992. (NASA)

Emerging research theorizes the decline of polluting sulfur-based particles in the atmosphere, which block sunlight and cool the Earth beneath, have allowed Atlantic hurricane activity to  increase in recent decades.  Could it be our efforts to clean the air have, paradoxically, made it more stormy?

One of the great success stories of U.S. environmental policy is the implementation of the Clean Air Act, and the subsequent dramatic decreases in atmospheric pollutants.

Concentrations of sulfur dioxide (SO2), a largely power plant-based pollutant that has adverse respiratory effects and contributes to acid rain, plummeted 83 percent between 1980-2010  according to the U.S. EPA.

Sulfur dioxide concentrations from 121 U.S. monitoring stations 1980-2010. (US EPA)
Sulfur dioxide concentrations from 121 U.S. monitoring stations 1980-2010. (U.S. EPA)

But a consequence of the SO2 reductions is the simultaneous drop in the concentrations of sulfate particles or aerosols, which form in the atmosphere through chemical reactions after SO2 is emitted.

Sulfate aerosols, while pollutants themselves and unhealthy to breathe, have an atmospheric cooling effect, as the particles are white and highly reflective.  The sun’s energy bounces off of them, rather than heating the atmosphere and the Earth’s surface.

For hurricanes, heat is fuel.  And if you rob them of this fuel, all other things being equal, they do not grow as intense. Hurricane activity was depressed from the 1960s to 1980s, when both the concentrations of these sulfate aerosols in the U.S. were highest and sea surface temperatures were relatively low.

But from the late 1980s onwards, when clean air legislation began to cleanse the atmosphere of these sun-blocking particles, temperatures and hurricane activity shot up in the tropical Atlantic.

Via U.S. EPA: "This figure presents annual values of the Power Dissipation Index (PDI). Tropical North Atlantic sea surface temperature trends are provided for reference. Note that sea surface temperature is measured in different units, but the values have been plotted alongside the PDI to show how they compare. The lines have been smoothed using a five-year weighted average, plotted at the middle year. The most recent average (2007–2011) is plotted at 2009." More info.
Via U.S. EPA and Kerry Emanuel: “This figure presents annual values of the Power Dissipation Index (PDI). Tropical North Atlantic sea surface temperature trends are provided for reference. Note that sea surface temperature is measured in different units, but the values have been plotted alongside the PDI to show how they compare. The lines have been smoothed using a five-year weighted average, plotted at the middle year. The most recent average (2007–2011) is plotted at 2009.” More info.

In effect, by ridding the atmosphere of sulfate aerosols,  the Clean Air Act may have primed the atmosphere and ocean in the tropical Atlantic for more/stronger storms, especially as greenhouse gas concentrations were rising which added even more heat to the system.

A much-publicized study from the journal Nature Geoscience last month, which used sophisticated computer models to explore the relationship between aerosols and historic hurricane activity in the Atlantic, concluded aerosols are likely a driving force.

“Our results raise the possibility that external factors, particularly anthropogenic [manmade] aerosols, could be the dominant cause of historical tropical storm variability, and highlight the potential importance of future changes in aerosol emissions,” the authors Dunstone et al. wrote.

This study reached a similar conclusion to a 2006 study published by climate scientist Michael Mann and renowned MIT hurricane expert Kerry Emanuel.

“This study provides confirmatory evidence, using model simulations, of what Kerry Emanuel and I demonstrated in our 2006 Eos article based purely on statistical arguments,” Mann said in statement posted on his Facebook page.

Other scientists agree that there is a relationship between aerosols and Atlantic hurricane activity, but are not convinced aerosols are the key driver.

Gabriel Vecchi, a climate scientist at NOAA, says hurricanes are not as sensitive to changes in aerosols as the June Nature Geoscience study claims.

“The climate model experiments they analyze lead me to question their claim that aerosols are “the dominant” cause of historical tropical storm variability: their model has been shown to have an excessive response to changes in aerosols,” Vecchi said in an email.

Vecchi and a colleague published a study in May estimating aerosols can explain about half the variability in hurricanes.

Climate scientist Kevin Trenberth suspects variations in ocean currents and circulation, which he “doubts” models simulate well, play a major role in hurricane trends.

“My take on this is that [aerosols] may be part of the explanation but not all of it because of the role of the oceans,” Trenberth said in an email.

Tom Knutson, another NOAA climate scientist, also expressed reservations about a dominating aerosol role.

“The question is, how much of the hurricane variation is due to this process [aerosols], versus other factors, such as internal climate variability, increasing greenhouse gases, etc.,” Knutson said in an email. “I don’t think we can presently conclude very much with confidence.”

One thing most active climate and hurricane researchers agree on is that as aerosols continue to decline and greenhouse gas concentrations grow, the atmosphere and oceans will continue warming and more powerful storms are likely in the tropical Atlantic.

“Going forward [decreases in] aerosols and [increases in] greenhouse gases are both projected to lead to an intensification,” NOAA’s Vecchi said.

If the Clean Air Act helped to bring about the surge in Atlantic hurricane activity by removing the cooling aerosols, the looming question is: can new measures under this act to reduce heat-infusing greenhouse gases, in time, reverse what it got started?

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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