PNNL

Abstract

Climate model studies of the consequences of solar geoengineering are central to evaluating whether such approaches may help to reduce the harmful impacts of global warming. In this study we compare the sunshade solar geoengineering response a perturbed parameter ensemble (PPE) of the HadCM3 model and a multi-model ensemble (MME) by analyzing results for experiment G1 from the Geoengineering Model Intercomparison Project (GeoMIP). The PPE only perturbed a small number of parameters and shares a common structure with the unperturbed HadCM3 model, so model independence is limited, as is the additional weight the PPE adds to the robustness of the common climate response features in the MME. However, the PPE indicates some of the factors that drive the spread within the MME. We isolate the role of global mean temperature biases for both ensembles and find that these biases have little effect on the ensemble spread in the hydrological response but do reduce the spread in surface air temperature response, particularly at high latitudes. We identify three key uncertainties in the response of surface variables to solar geoengineering: the circulation consequences of spatial imbalances in solar and CO2 forcing, the dynamic response to the vertical balance of solar and CO2 forcing, and the CO2 physiological response of plants. Of these three, the CO2 physiological response is identified as the most significant due to its connection to the terrestrial vegetation response and to the surface hydrological response, both of which have been highlighted as being key concerns for solar geoengineering.