Global warming continues to heat up the earth, but volcanoes are keeping us just a little cooler.
A new paper published in Geophysical Research Letters shows thatvolcanic eruptions may be part of the reason why the earth isn’t heating up quite as fast as climate models predict. Sneaky sulfur dioxide emissions coming from smaller volcanoes that weren’t previously factored into climate models are temporarily cooling surface temperatures, said research from MIT atmospheric scientist David Ridley.
“If an eruption is powerful enough, the sulfur dioxide can reach the upper atmosphere, the stratosphere, where it forms literally liquid sulfuric acid droplets,” said Benjamin Santer, a research scientist at Lawrence Livermore National Laboratory and co-author on the study. “Those droplets reflect some fraction of incoming sunlight back to space, preventing that sunlight from penetrating deeper into the atmosphere. That’s the primary cooling mechanism.”
According to Santer and Ridley’s research, that light-refracting cooling effect is strong enough to bring global temperatures down anywhere from 0.09˚ to 0.22˚ F since 2000. Unfortunately the cooling won’t do much to counteract global warming in the long term—Ridley said that the amount of sulfur dioxide released in a small eruption generally dissipates after about one year. But these emissions may be part of the reason why over the last ten to 15 years, average global temperatures haven’t increased as rapidly as they have in decades past. The Intergovernmental Panel on Climate Change estimates that average worldwide temperatures are currently increasing at about one-third the rate that they were between 1951 and 2012.
“I think there’s quite a good case now that volcanoes are at least able to explain about a third of that,” Ridley said.
On top of providing volcano emissions data, Ridley’s study also offers scientists a new way to explore the lower stratosphere. Both current research and climate models rely on data derived from satellite observations to measure what’s happening in the stratosphere. That works well until around nine to ten miles above the earth’s surface, where clouds contaminate the data and make it difficult to discern exactly what’s happening. The problem becomes even more complex around the poles where the stratosphere dips lower than it does in the tropics and creates “this kind of wedge of stratosphere that we’re missing when just using the satellites” Ridley said.
Instead of making estimates based on satellite observations alone, Ridley’s team also used data from a balloon-borne particle counter and from measurement devices on the ground. These included four lidar systems, which measure atmospheric particles using laser light pulses, and data from a series of robotic solar photometers called AERONET that use sunlight to measure how effective aerosol particles are at blocking light. The ground and air-based measurements gave researchers a clearer picture of the chemical makeup of the lower stratosphere.
“Even though it’s a small part of the atmosphere that we were able to include that hadn’t been included before, it probably has a majority of the aerosols that are important” in the short term, said Ryan R. Neely III, a co-author on the study and lecturer of observational atmospheric science at the University of Leeds.
Alan Robock, a climate scientist who was not involved in the study but was quoted in the journal’s press release, commended Ridley’s team for using ground and air-based instruments to examine the lower stratosphere in a way that satellite data simply can’t. He said that the new observational methods can potentially help scientists make better climate predictions and create more accurate models in the future.
Creating accurate climate models hasn’t been easy in the middle of a so-called global warming pause or “hiatus,” especially one that’s controversial among scientists. While some attribute the slowdown to the ocean storing heat, others chalk it up to solar cycles or temperature fluctuations from El Niño and La Niña weather patterns.
“The hiatus, the pause, it’s a little misleading,” said Todd Sanford, a climate scientist with Climate Central. “We’re still setting global [temperature] records. Really what this is talking about is how quickly temperatures are increasing, not that they have stopped increasing.”
Even with the pause, global warming is still a major environmental problem, one so large that some researchers are investigating whether a strategy like spraying sulfur dioxide into the stratosphere to mimic the cooling effects from volcanoes is a viable temporary solution.
“We know that if this were to be done, we could get fairly rapid reductions in temperatures but there are issues with it,” Sanford said. “You’re masking the effect of CO2 in some ways. That’s good as long as you’re doing it, but if for any reason you stopped injecting these particles up into the atmosphere, you’re now very quickly unmasking all of that CO2 warming. You’d get all of that warming back. It’s one of these things where if you start it and you’re not doing anything else on CO2, you’ve got to keep it going.”
Besides, Sanford added, simply cooling the atmosphere without reducing CO2 won’t address other problems caused by carbon, like increasing ocean acidification.
Sulfur dioxide injections could also deteriorate the ozone, produce uneven temperature and precipitation patterns, completely obscure our view of the sky, and create global political issues as the world decides “what temperature to set the thermostat” Robock said. He added that the technology to execute this type of geoengineering doesn’t yet exist, though others like Harvard climate scientist David Keith argue that it does. Even if we were able to get a critical mass of sulfuric acid into the stratosphere, there’s no way to control it once its there.
“If you have an existing cloud up there and you start spraying more sulfur, theory tells us the particles will grow and you’ll get larger particles rather than more particles and they’ll be much less effective at scattering sunlight,” Robock said. “They’ll also fall out faster so you have to put a lot more up there.”
Kicking our carbon habit is the real solution to global warming, Robock added, but until that happens, creating more accurate climate models can help us better understand how the atmosphere is changing.
Ridley warns against the dangers of placing too much emphasis on volcanic cooling. While volcanoes are playing a small but significant role in keeping rising temperatures a little in check, sulfur dioxide cooling isn’t a safeguard against the effects of global warming. “This is really just a bit of an offset on the warming rather than a change in the expected trend on warming,” he said. Besides, he added, no good global hiatus lasts forever. “We’ve got no reason to believe that that will continue.”