Scientists have long thought that the aerosols derived from sulfur dioxide helped land vegetation in a big way to absorb the greenhouse gas carbon dioxide (CO2) during photosynthesis. But a new study examining how vegetation across the eastern United States responded to decreasing levels of sulfur dioxide (SO2) from 1995 to 2013 suggests that the link between SO2 and photosynthesis—and thus CO2 levels—may not be so strong after all.
“Over 19 years, sulfur dioxide emissions fell by about 70% … while photosynthesis only decreased by 1%,” said Gretchen Keppel-Aleks, an atmospheric scientist at the University of Michigan and lead author of a recent study in Geophysical Research Letters, a publication of the American Geophysical Union. “This is good news, since it means that we can go ahead and improve air quality by regulating sulfur dioxide, all while having a minimal impact on the climate,” she added.
The new finding, although revealing more about how SO2 behaves, deepens a climate mystery, Keppel-Aleks explained: Each year, land ecosystems absorb about 25% of the CO2 humans emit, even though our CO2 output increases more and more each year.
Why the land carbon “sink” can keep pace with skyrocketing CO2 levels eludes explanation.
Now, with the role of SO2 in doubt, researchers will need to explore new possibilities in the “hunt for where anthropogenic carbon is going in the terrestrial biosphere,” Keppel-Aleks said. They also might need to carry out new observations because the field is “observationally poor compared to what we would like in order to actually unpack what’s driving the global carbon sink,” she added.
Sulfur Dioxide’s Scattering Effects
In the air, SO2 reacts with oxygen to form sulfate ions, or SO42-. In many humid, eastern U.S. states, intense hazes form when sulfate particles take up water, making them swell and boosting their efficiency at scattering sunlight, said Jenny Hand, an atmospheric scientist at Colorado State University in Fort Collins, who was not involved in the new work.
Apart from scattering light and creating haze, SO2, emitted from coal-fired power plants as well as during other industrial processes, can cause acid rain and respiratory health problems. These are a couple of reasons the United States’ 1990 Clean Air Act amendments mandated SO2 emission curbs, which brought on the 70% drop in emissions from 1995 to 2013, Keppel-Aleks explained.
This decline provided “a great test case,” Keppel-Aleks said, which gave her and her coauthor, Rebecca Washenfelder, an atmospheric scientist at the University of Colorado Boulder, the chance to run a climate model using real-world data. Previous studies, she explained, mostly just used modeling, but “if you’re going to run a model, you should probably try to evaluate it against something you can see in the Earth.” So the team incorporated the eastern U.S. SO2 data into the Community Earth System Model (CESM), which can simulate the climatic effects of the pollutant’s decline.
Less sulfate means less scattered—or diffuse—light. This scattering was thought to boost photosynthesis. “If you’ve got a nice parallel beam of light coming toward trees, then the leaves at the top of the canopy will be the only ones absorbing photons, while leaves deeper in the canopy are just sitting there doing nothing,” Keppel-Aleks said. But when light scatters, “you’ve got photons that are coming at the canopy from multiple different angles,” and the leaves below the canopy surface can now also photosynthesize, she added, which was believed to help the land carbon sink keep pace with the rise of anthropogenic CO2 emissions.
To quantify sulfate’s effect on incoming light, the team measured the aerosol optical depth (AOD), or the extent to which aerosols like sulfate absorb or scatter light. They found that AOD “decreased by almost 50%” from 1995 to 2013, said Keppel-Aleks, whereas photosynthesis decreased very little. These numbers show that in the kinds of temperate ecosystems she and Washenfelder examined, at least, “there’s a very weak link between diffuse light and carbon uptake,” Keppel-Aleks explained.
A More Realistic Picture
Each year between 2000 and 2009, vegetation removed about 27% of the roughly 7.8 petagrams—or 7,800,000,000 metric tons—of carbon then emitted annually, on average, by humans. Before the new paper, SO2 was thought to help remove as much as 5 petagrams of carbon every year by helping stimulate photosynthesis.
“That’s an insane number, and a lot of people were questioning it,” said Natalie Mahowald, an atmospheric scientist at Cornell University in Ithaca, N.Y., who was not involved in the new work. CESM, though, “very carefully constrains the relationship between incoming solar radiation” and how much photosynthesis occurs, so “the new number they devised in this paper makes a lot more sense,” she said. Now, according to the new study, it seems photosynthesis stimulated through SO2 removed just 0.5 petagram of carbon in total between 1995 and 2013.
Rooted in Field Data
In a 2015 field study, other scientists recorded how photosynthesis varied under both diffuse and direct light conditions. Strengthened by that prior research, the new findings are “realistic and definitively show that the link between scattered light and photosynthesis is not important in temperate ecosystems,” Keppel-Aleks said.
But the field data came from only four sites in the eastern United States, so the team used data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Aqua and Terra satellites to extrapolate the observed relationships between diffuse light and photosynthesis to the whole eastern United States. The researchers then compared those results to their CESM simulation. “The results agreed with the climate model results to within 20%, which is quite good,” Keppel-Aleks said.
The new findings, published on 28 September, also mean that cleaning SO2 out of the skies will not cause CO2 levels in the atmosphere to then spike, as SO2 does little to help draw CO2 out of the air, Mahowald said. But sulfate, when it scatters solar radiation, can cause some light to escape back out into space, which helps cool the planet. “There’s this double whammy that’s going on: As we clean up the air quality, we may well see slightly warmer temperatures.”
—Lucas Joel, Freelance Writer; email: firstname.lastname@example.org
Correction, 28 November 2016: An earlier version of this article misidentified sulfate ions as sulfate and incorrectly reported amounts of carbon emitted to the atmosphere and of carbon absorbed by vegetation as amounts of carbon dioxide gas. This article has been updated to correct those inaccuracies.