Given its height, many miles above sea level, the stratosphere is typically a barren place. Suspended above the weather, this dry atmospheric layer rarely houses anything more tangible than gases. Occasionally a vast volcanic eruption—like Mount Pinatubo, in 1991—might inject a load of sun-reflecting particles into the layer, cooling the planet for a couple of years. But those rare eruptions were the only ticket for those particles, called aerosols, to reach the otherwise pristine stratosphere.
Or so climate scientists once thought.
Since 2000, however, researchers have discovered a steady uptick in the stratosphere’s aerosol layer, its optical depth—the satellite-measured proxy scientists use to track such particles—up 4 percent to 10 percent a year. Yet there have been no planet-heaving, Pinatubo-style eruptions. It’s been a puzzle, and researchers have flagged those aerosols as one possible reason the planet, while still warming, has not heated up as quickly as some scientists once expected, a phenomenon I detailed in 2011.
There are many reasons for short-term temperature fluctuations, with the climate’s natural variability first among equals. But one line of inquiry has focused on stratospheric aerosols. What are they doing, and where did they come from?
Two ideas have competed: First, aerosols from smaller volcanic events, like the Soufrière Hills eruption, in Montserrat, buoyed by tropical air currents or monsoons, must be capable of sneaking up into the stratosphere and loitering for several years. Second, and more controversially, the increase could stem from the vast uptick in coal burning, and attendant particle pollution, seen in China and elsewhere in Asia over the past decade.
To this day, scientists still haven’t nailed down the origins of those mystery aerosols. But much circumstantial evidence now points toward smaller volcanic eruptions as the primary source, evidence that was bolstered this week by research accepted for publication in Geophysical Research Letters, a premier geoscience journal.
Using an updated atmospheric model primed separately with man-made and volcanic sulphur emissions from 2000 to 2010, the researchers, led by Ryan R. Neely III, a Ph.D. student at the University of Colorado at Boulder, found that the volcanic simulation closely matched reality, while the Asian coal emissions barely increased the particles found in the stratosphere. It wasn’t even close.
The “simulations unambiguously show that moderate volcanic eruptions are the main drivers” of stratospheric aerosol variability from 2000 to 2010, they concluded. While there may be a slight human-caused signal in the lower part of the stratosphere, they said, it is dwarfed by those moderate volcanic emissions.
While far from the final word on the subject—much uncertainty remains about how aerosols travel in the atmosphere—the team lays out what is very likely an important distinction: Those increased aerosols, and the possible cooling they caused, should not be seen as a man-made trend. Instead, they are part of the planet’s normal fluctuations, like the Pacific’s flopping between El Niño and La Niña weather patterns.
In other words, climate variability is even more important in explaining the noisy warming record of the last decade. Try as we might, humanity just can’t loft much of its aerosol pollution into the stratosphere. Unless, that is, we intentionally pump it up there.
And then we’d call it geoengineering.
[Image by Barry Voight, National Science Foundation, Wikimedia Commons]