Abstract
Abstract. A new set of stratospheric aerosol geoengineering (SAG) model experiments has been performed with Community Earth System Model version 2 (CESM2) with the Whole Atmosphere Community Climate Model (WACCM6) that are based on the Coupled Model Intercomparison Project phase 6 (CMIP6) overshoot scenario (SSP5-34-OS) as a baseline scenario to limit global warming to 1.5 or 2.0 ∘C above 1850–1900 conditions. The overshoot scenario allows us to applying a peak-shaving scenario that reduces the needed duration and amount of SAG application compared to a high forcing scenario. In addition, a feedback algorithm identifies the needed amount of sulfur dioxide injections in the stratosphere at four pre-defined latitudes, 30∘ N, 15∘ N, 15∘ S, and 30∘ S, to reach three surface temperature targets: global mean temperature, and interhemispheric and pole-to-Equator temperature gradients. These targets further help to reduce side effects, including overcooling in the tropics, warming of high latitudes, and large shifts in precipitation patterns. These experiments are therefore relevant for investigating the impacts on society and ecosystems. Comparisons to SAG simulations based on a high emission pathway baseline scenario (SSP5-85) are also performed to investigate the dependency of impacts using different injection amounts to offset surface warming by SAG. We find that changes from present-day conditions around 2020 in some variables depend strongly on the defined temperature target (1.5 ∘C vs. 2.0 ∘C). These include surface air temperature and related impacts, the Atlantic Meridional Overturning Circulation, which impacts ocean net primary productivity, and changes in ice sheet surface mass balance, which impacts sea level rise. Others, including global precipitation changes and the recovery of the Antarctic ozone hole, depend strongly on the amount of SAG application. Furthermore, land net primary productivity as well as ocean acidification depend mostly on the global atmospheric CO2 concentration and therefore the baseline scenario. Multi-model comparisons of experiments that include strong mitigation and carbon dioxide removal with some SAG application are proposed to assess the robustness of impacts on societies and ecosystems.
Highlights
Large-scale mitigation efforts to phase out anthropogenic emissions are likely no longer sufficient to keep global mean surface temperature from rising less than 2 ◦C above preindustrial levels, which is required to avoid significant impacts on societies and ecosystems (IPCC, 2018)
Various uniformly defined stratospheric aerosol geoengineering (SAG) modeling experiments of different complexity have been designed within the Geoengineering Model Intercomparison Project (GeoMIP) to be performed by different modeling groups within phase 5 of the Coupled Model Intercomparison Project (CMIP5) (Kravitz et al, 2011) and Coupled Model Intercomparison Project phase 6 (CMIP6) (Kravitz et al, 2015)
This paper describes a set of new SAG simulations using WACCM6, which aim to keep global warming to less than 1.5 or 2.0 ◦C above pre-industrial
Summary
Large-scale mitigation efforts to phase out anthropogenic emissions are likely no longer sufficient to keep global mean surface temperature from rising less than 2 ◦C above preindustrial levels, which is required to avoid significant impacts on societies and ecosystems (IPCC, 2018). Various uniformly defined SAG modeling experiments of different complexity have been designed within the Geoengineering Model Intercomparison Project (GeoMIP) to be performed by different modeling groups within phase 5 of the Coupled Model Intercomparison Project (CMIP5) (Kravitz et al, 2011) and CMIP6 (Kravitz et al, 2015) These simulations involve either injecting sulfur dioxide at the Equator or using earlier derived prescribed aerosol distributions to reach the described goals (e.g., Pitari et al, 2014). To facilitate baseline scenarios that allow similar peak-shaving geoengineering experiments, as described by Tilmes et al (2016), CMIP6 designed the overshoot scenario (OS) SSP5-34-OS (O’Neill et al, 2016) This scenario follows the high forcing scenario SSP5-85 until 2040 and applies drastic decarbonization efforts, including mitigation and active carbon dioxide removal to produce net-negative emissions after 2070.
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