Global modeling of impact relevant stratospheric aerosol climate intervention scenarios

Simone Tilmes, Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, Douglas E. MacMartin, Mechanical and Aerospace Engineering, Cornell University, Jan T. M. Lenaerts, University of Colorado Boulder, Leo van Kampenhout, Utrecht University, Laura Muntjewerf, Delft University of Technology, Lili Xia and Alan Robock, Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, Cheryl Harrison, Port Isabel Lab, University of Texas Rio Grand Valley, TX, Kristen M. Krumhardt and Michael J. Mills, National Center for Atmospheric Research, Ben Kravitz, Indiana University

Background/Question/Methods
Different climate intervention strategies (also called geoengineering) have been proposed as a mean to potentially mitigate some or the worst effects of climate change. One of these methods, stratospheric aerosol interventions (SAI), would require increasing the stratospheric aerosol burden in order to reflect some part of the sunlight and counter future warming. Large volcanic eruptions that have injected sulfur into the stratosphere are analogues of these approaches in nature. They reduce surface temperatures for some years but can also result in droughts and other impacts to climate and ecosystems. Climate intervention approaches would require annual injections of sulfur or aerosols to sustain cooling. The effects of these interventions have been studied with global Earth System Models. However, those often do not aim towards reaching surface temperature targets well below 2.0oC, a requirement for avoiding large impacts and risks.
Results/Conclusions
The Geoengineering Large Ensemble (GLENS) project performed model simulations designed to reduce earlier identified side effects of SAI, including overcooling in the tropics, failure to sufficiently cool high latitudes (to prevent reductions of summer sea-ice), and large shifts in precipitation patterns. However, the GLENS simulations are based on a high emission future pathway and therefore require an increasing amount of annual sulfur injection with time. This would impose a large risk of a termination of the application with potential detrimental effects for ecosystems. A more reasonable SAI model experiment, the “peak-shaving” experiment, is designed to overcome many of these problems when aligned with serious emission reductions that alone are not sufficient to stave off severe climate effects. Small or moderate applications of SAI, slowly phased in and out, could be used to limit global warming to 1.5oC or 2.0oC compared to 1850-1900 conditions. Depending on the setup of the experiment, including temperature targets and emissions reductions, benefits and impacts relevant to society and ecosystems can differ substantially. Some measures, like heat waves, Arctic sea-ice, land-ice, and ocean net primary productivity (NPP), more strongly depend on the defined temperature target, while others, including land NPP, and ocean acidification, depend more strongly on the amount of carbon in the atmosphere. Other measures, including regional precipitation changes and reductions of the Antarctic ozone hole further depend on the injection amount. Based on these findings, small or moderate applications of SAI in addition to large-scale mitigation efforts show the lowest climate impacts and may limit the risks of reaching climate and ecological tipping points.