It seems counterintuitive, but all of those hot sulfur particles heaved high into the Earth’s atmosphere by volcanic eruptions in the past few years may actually have reduced the amount of global warming expected during the 2000s, according to the findings of a new study.
The research, conducted by the National Oceanic and Atmospheric Administration in Boulder, Colorado, determined that a recent increase in the abundance of particles high in the sky has offset about one-third of the current-climate warming impact of carbon dioxide (CO2) during the past decade.
In the stratosphere, miles above Earth’s surface, small, airborne particles reflect sunlight back into space, which leads to a cooling effect at the ground level. These particles are also called “aerosols.” Sulfur dioxide (SO2), carbonyl sulfide (OCS), and dimethyl sulfide (DMS) are the dominant surface emissions that contribute to aerosol formation. These aerosols can be produced not only by smaller volcanic eruptions, but by such “demonized” industries as coal-burning power plants.
“Since the year 2000, stratospheric aerosols have caused a slower rate of climate warming than we would have seen without them,” said John Daniel, a physicist at the NOAA Earth System Research Laboratory (ESRL) and an author of the new study.
Scientists had known that aerosols cool the planet, and that big eruptions spew lots of aerosols into the upper atmosphere. But nobody had calculated how smaller recent eruptions might affect climate. For example, many had assumed that stratospheric aerosols had dropped essentially to zero after particles from the 1991 eruption of the Philippines’ Mt. Pinatubo fell out of the atmosphere.
Daniel and his colleagues, led by recently retired NOAA atmospheric scientist Susan Solomon, looked at aerosol measurements taken from satellites and from the Mauna Loa observing station in Hawaii. The researchers traced aerosols to several small eruptions, including those of Soufrière Hills in Montserrat and Tavurvur in Papua New Guinea, both in 2006.
The reasons for the 10-year increase in stratospheric aerosols are not fully understood and are the subject of ongoing research, said coauthor Ryan Neely, with the University of Colorado and the Cooperative Institute for Research in Environmental Sciences (CIRES). Likely suspects are natural sources – smaller volcanic eruptions – and/or human activities, which could have emitted the sulfur-containing gases, such as sulfur dioxide, that react in the atmosphere to form reflective aerosol particles.
Moreover, future global temperatures will depend on stratospheric aerosol. The warming from greenhouse gases and aerosols calculated for the coming decade can vary by almost a factor of two — depending on whether aerosols continue to increase at the same rate as over the past decade, or if instead they decrease to very low levels, such as those experienced in 1960.
If stratospheric aerosol levels continue to increase, temperatures will not rise as quickly as they would otherwise, said Ellsworth Dutton, also with NOAA ESRL and a co-author on the paper. Conversely, if stratospheric aerosol levels decrease, temperatures would increase faster. Dutton and his colleagues use the term persistently variable to describe how the background levels of aerosol in Earth’s stratosphere can change from one decade to the next, even in the absence of major volcanic activity.
“The ‘background’ stratospheric aerosols are more of a player than we thought,” said Daniel. “The last decade has shown us that it doesn’t take an extremely large volcanic eruption for these aerosols to be important to climate.”
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Cheryl Kaften is an accomplished communicator who has written for consumer and corporate audiences. She has worked extensively for MasterCard (News - Alert) Worldwide, Philip Morris USA (Altria), and KPMG, and has consulted for Estee Lauder and the Philadelphia Inquirer Newspapers. To read more of her articles, please visit her columnist page.Edited by
Jennifer Russell
