Shortwave radiative forcing, rapid adjustment, and feedback to the surface by sulfate geoengineering: analysis of the Geoengineering Model Intercomparison Project G4 scenario

This study evaluates the forcing, rapid adjustment, and feedback of net shortwave radiation at the surface in the G4 experiment of the Geoengineering Model Intercomparison Project by analysing outputs from six participating models. G4 involves injection of 5 Tg yr?1 of SO2, a sulfate aerosol precursor, into the lower stratosphere from year 2020 to 2069 against a background scenario of RCP4.5. A single-layer atmospheric model for shortwave radiative transfer is used to estimate the direct forcing of solar radiation management (SRM), and rapid adjustment and feedbacks from changes in the water vapour amount, cloud amount, and surface albedo (compared with RCP4.5). The analysis shows that the globally and temporally averaged SRM forcing ranges from ?3.6 to ?1.6 W m?2, depending on the model. The sum of the rapid adjustments and feedback effects due to changes in the water vapour and cloud amounts increase the downwelling shortwave radiation at the surface by approximately 0.4 to 1.5 W m?2 and hence weaken the effect of SRM by around 50 %. The surface albedo changes decrease the net shortwave radiation at the surface; it is locally strong (~ ?4 W m?2) in snow and sea ice melting regions, but minor for the global average. The analyses show that the results of the G4 experiment, which simulates sulfate geoengineering, include large inter-model variability both in the direct SRM forcing and the shortwave rapid adjustment from change in the cloud amount, and imply a high uncertainty in modelled processes of sulfate aerosols and clouds

Impact of the GeoMIP G1 sunshade geoengineering experiment on the Atlantic meridional overturning circulation

We analyze the multi-earth system model responses of ocean temperatures and the Atlantic Meridional Overturning Circulation (AMOC) under an idealized solar radiation management scenario (G1) from the Geoengineering Model Intercomparison Project. All models simulate warming of the northern North Atlantic relative to no geoengineering, despite geoengineering substantially offsetting the increases in mean global ocean temperatures. Increases in the temperature of the North Atlantic Ocean at the surface (~0.25 K) and at a depth of 500 m (~0.10 K) are mainly due to a 10 Wm−2 reduction of total heat flux from ocean to atmosphere. Although the AMOC is slightly reduced under the solar dimming scenario, G1, relative to piControl, it is about 37% stronger than under abrupt4 × CO2 . The reduction of the AMOC under G1 is mainly a response to the heat flux change at the northern North Atlantic rather than to changes in the water flux and the wind stress. The AMOC transfers heat from tropics to high latitudes, helping to warm the high latitudes, and its strength is maintained under solar dimming rather than weakened by greenhouse gas forcing acting alone. Hence the relative reduction in high latitude ocean temperatures provided by solar radiation geoengineering, would tend to be counteracted by the correspondingly active AMOC circulation which furthermore transports warm surface waters towards the Greenland ice sheet, warming Arctic sea ice and permafrost

Shortwave radiative forcing, rapid adjustment, and feedback to the surface by sulfate geoengineering: analysis of the Geoengineering Model Intercomparison Project G4 scenario

This study evaluates the forcing, rapid adjustment, and feedback of net shortwave radiation at the surface in the G4 experiment of the Geoengineering Model Intercomparison Project by analysing outputs from six participating models. G4 involves injection of 5 Tg yr(-1) of SO2, a sulfate aerosol precursor, into the lower stratosphere from year 2020 to 2069 against a background scenario of RCP4.5. A single-layer atmospheric model for shortwave radiative transfer is used to estimate the direct forcing of solar radiation management (SRM), and rapid adjustment and feedbacks from changes in the water vapour amount, cloud amount, and surface albedo (compared with RCP4.5). The analysis shows that the globally and temporally averaged SRM forcing ranges from -3.6 to -1.6 W m(-2), depending on the model. The sum of the rapid adjustments and feedback effects due to changes in the water vapour and cloud amounts increase the downwelling shortwave radiation at the surface by approximately 0.4 to 1.5 W m(-2) and hence weaken the effect of SRM by around 50 %. The surface albedo changes decrease the net shortwave radiation at the surface; it is locally strong (∼ −4 W m(-2)) in snow and sea ice melting regions, but minor for the global average. The analyses show that the results of the G4 experiment, which simulates sulfate geoengineering, include large inter-model variability both in the direct SRM forcing and the shortwave rapid adjustment from change in the cloud amount, and imply a high uncertainty in modelled processes of sulfate aerosols and clouds

Atlantic hurricane surge response to geoengineering

Devastating floods due to Atlantic hurricanes are relatively rare events. However, the frequency of the most intense storms is likely to increase with rises in sea surface temperatures. Geoengineering by stratospheric sulfate aerosol injection cools the tropics relative to the polar regions, including the hurricane Main Development Region in the Atlantic, suggesting that geoengineering may mitigate hurricanes. We examine this hypothesis using eight earth system model simulations of climate under the Geoengineering Model Intercomparison Project (GeoMIP) G3 and G4 schemes that use stratospheric aerosols to reduce the radiative forcing under the Representative Concentration Pathway (RCP) 4.5 scenario. Global mean temperature increases are greatly ameliorated by geoengineering, and tropical temperature increases are at most half of those temperature increases in the RCP4.5. However, sulfate injection would have to double (to nearly 10 teragrams of SO2 per year) between 2020 and 2070 to balance the RCP4.5, approximately the equivalent of a 1991 Pinatubo eruption every 2 y, with consequent implications for stratospheric ozone. We project changes in storm frequencies using a temperature-dependent generalized extreme value statistical model calibrated by historical storm surges and observed temperatures since 1923. The number of storm surge events as big as the one caused by the 2005 Katrina hurricane are reduced by about 50% compared with no geoengineering, but this reduction is only marginally statistically significant. Nevertheless, when sea level rise differences in 2070 between the RCP4.5 and geoengineering are factored into coastal flood risk, we find that expected flood levels are reduced by about 40 cm for 5-y events and about halved for 50-y surges

Arctic sea ice and atmospheric circulation under the abrupt4xCO2 scenario

We analyze sea ice changes from eight different earth system models that have conducted experiment abrupt4xCO2 of the Coupled Model Intercomparison Project Phase 5 (CMIP5). In response to abrupt quadrupling of CO2 from preindustrial levels, Arctic temperatures dramatically rise by about 10°C—16°C in winter and the seasonal sea ice cycle and sea ice concentration are significantly changed compared with the pre-industrial control simulations (piControl). Changes of Arctic sea ice concentration are spatially correlated with temperature patterns in all seasons and highest in autumn. Changes in sea ice are associated with changes in atmospheric circulation patterns at heights up to the jet stream. While the pattern of sea level pressure changes is generally similar to the surface air temperature change pattern, the wintertime 500 hPa circulation displays a positive Pacific North America (PNA) anomaly under abrupt4xCO2-piControl. This large scale teleconnection may contribute to, or feedback on, the simulated sea ice cover change and is associated with an intensification of the jet stream over East Asia and the north Pacific in winter

Simulated responses and feedbacks of permafrost carbon under future emissions pathways and idealized solar geoengineering scenarios

The carbon-rich northern high-latitude permafrost is a potential climate tipping point. Once triggered, its thawing and release of carbon dioxide and methane might unleash irreversible changes in the Earth’s climate system. We investigate the response of permafrost under three Shared Socioeconomic Pathways (SSPs) with no mitigation (SSP5-8.5), moderate mitigation (SSP2-4.5) and delayed mitigation (SSP5-3.4-OS), and three solar geoengineering scenarios applied to each experiment to prevent global warming from exceeding 2 °C above pre-industrial. The long-term negative emissions in SSP5-3.4-OS preserves much more frozen soil than SSP5-8.5, but shows nearly as much permafrost carbon loss this century as SSP2-4.5 due to its mid-century temperature overshoot. Solar geoengineering to meet the 2 °C target above pre-industrial effectively suppresses permafrost thawing and reduces subsequent carbon release from the soil. However, the carbon emission from permafrost still continues after the temperature is stabilized, due to the decomposition of thawed permafrost carbon. More solar insolation reduction is required to compensate the positive permafrost carbon feedback, which exerts greater impacts on the efficiency of solar geoengineering under a scenario with strong climate policy and lower carbon emissions