New Approach to Geoengineering

Using CESM, scientists have demonstrated for the first time that a new approach to geoengineering could potentially be used to limit Earth’s warming to a specific target while reducing some of the risks and concerns identified in past studies, including uneven cooling of the globe.

The findings from the new research, led by scientists from the National Center for Atmospheric Research (NCAR), Pacific Northwest National Laboratory (PNNL), and Cornell University, represent a significant step forward in the field of geoengineering. Whereas past modeling studies have typically sought to answer the question "What happens if we do geoengineering?" this new approach turned the question around, to ask, "How might geoengineering be used to meet specific climate objectives?” The findings are detailed in a series of papers published in a special issue of the Journal of Geophysical Research – Atmospheres.

In short, the team studied injecting sulfur dioxide into the upper atmosphere, above the cloud layer. The idea of combating global warming with these injections is inspired by history's most massive volcanic eruptions. When volcanoes erupt, they loft sulfur dioxide high into the atmosphere, where it's chemically converted into light-scattering sulfate particles called aerosols. These sulfates, which can linger in the atmosphere for a few years, are spread around the Earth by stratospheric winds, forming a reflective layer that cools the planet. To mimic these effects, sulfur dioxide could be injected directly into the stratosphere, perhaps with the help of high-flying aircraft. But while the injections would counter global warming, they would not address all the problems associated with climate change, and they would likely have their own negative side effects.

The scientists used the NCAR-based Community Earth System Model with its extended atmospheric component, the Whole Atmosphere Community Climate Model. WACCM includes detailed chemistry and physics of the upper atmosphere and was recently updated to simulate stratospheric aerosol evolution from source gases, including geoengineering. "It was critical for this study that our model be able to accurately capture the chemistry in the atmosphere so we could understand how quickly sulfur dioxide would be converted into aerosols and how long those aerosols would stick around" said NCAR scientist Michael Mills, also a lead author. "Most global climate models do not include this interactive atmospheric chemistry.

The scientists also significantly improved how the model simulates tropical stratospheric winds, which change direction every few years. Accurately representing these winds is critical to understanding how aerosols are blown around the planet. They tested their model by seeing how well it could simulate the 1991 eruption of Mount Pinatubo, including the amount and rate of aerosol formation, how those aerosols were transported around the globe and how long they stayed in the atmosphere. Fourteen possible injection sites at seven different latitudes and two different altitudes were examined— something never before tried in geoengineering research. The researchers found that they could spread the cooling more evenly across the globe by choosing injection sites on either side of the equator.

Global mean temperature deviation

The simulations on the left represent how global temperatures are expected to change if greenhouse gas emissions continue on a "business as usual" trajectory. The simulations on the right show how temperature could be stabilized in a model by injecting sulfur dioxide high into the atmosphere at four separate locations. Because greenhouse gases are being emitted at the same rate in the simulations on the left and the right, stopping geoengineering would result in a drastic spike in global temperatures.

The researchers then pieced together all their work into a single model simulation with specific objectives: to limit average global warming to 2020 levels through the end of the century and to minimize the difference in cooling between the equator and the poles as well as between the northern and southern hemispheres. They gave the model four choices of injection sites — at 15 degrees and 30 degrees North and South in latitude — and then implemented an algorithm that determines, for each year, the best injection sites and the quantity of sulfur dioxide needed at those sites. The model's ability to reformulate the amount of geoengineering needed each year, based on that year's conditions, also allowed the simulation to respond to natural fluctuations in the climate.

The model successfully kept the surface temperatures near 2020 levels against a background of increasing greenhouse gas emissions that would be consistent with a business-as-usual scenario. The algorithm’s ability to choose injection sites cooled the Earth more evenly than in previous studies, because it could inject more sulfur dioxide in regions that were warming too quickly and less in areas that had over-cooled. However, by the end of the century, the amount of sulfur dioxide that would need to be injected each year to offset human-caused global warming would be enormous: almost five times the amount spewed into the air by Mount Pinatubo on June 15, 1991.

We are still a long way from understanding all the interactions in the climate system that could be triggered by geoengineering, which means we don’t yet understand the full range of possible side effects" said NCAR scientist Simone Tilmes, a lead author. "But climate change also poses risks. Continuing research into geoengineering is critical to assess benefits and side effects and to inform decision makers and society."

The research was funded by the Defense Advanced Research Projects Agency and the National Science Foundation.