Modelling radiative and climate effects of aerosols: from Anthropogenic emissions to geoengineering

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Title: Modelling radiative and climate effects of aerosols: from Anthropogenic emissions to geoengineering
Author: Laakso, Anton
Belongs to series: Finnish Meteorological Institute Contributions 119
ISSN: 0782-6117
ISBN: 978-951-697-3
Abstract: Atmospheric aerosols have been shown to exert a cooling effect on climate by scattering incoming solar radiation back to space or increasing cloud albedo by increasing the cloud droplet number concentration in the clouds. If greenhouse gas emissions cannot be reduced to slow down climate warming, it has been postulated that climate could be artificially cooled by increasing atmospheric aerosol concentrations. These methods are called solar radiation management or geoengineering. This work evaluated two potential solar radiation management techniques; 1) where marine aerosol concentrations would be elevated to enhance marine cloud albedo and 2) a technique where stratospheric sulphur concentration would be increased. The key objectives of this thesis were to: 1) Investigate the potential of aerosols to cool the climate at the global scale, 2) Evaluate the role of the simulation of the aerosol microphysics in the global climate models and 3) Identify the possible limits in the effectiveness of the Solar Radiation Management techniques as well as the risks related to these techniques. Climate is already being affected by our current aerosols emissions. This work also examined how the geographical change in aerosol emissions has affected the climate and evaluated how the climate would change due to aerosols if all electrical energy were to be generated by nuclear power is instead of by coal combustion. The global aerosol-climate model ECHAM-HAMMOZ was used to study radiation and the climatic effects of aerosols. The model simulates the formation, growth, transportation and deposition of aerosols and their interactions with clouds. In addition, the effects on the climate, assessed as temperature changes, were studied using a mixed layer ocean model coupled to the atmospheric model ECHAM and Max Planck Institute's Earth System Model (MPI-ESM). The results revealed that the geoengineering techniques which were studied in this work, have the potential to significantly cool climate and thus slow down global warming. However, the cooling effect has limitations. Stratospheric sulphur injections would lead to relatively less cooling, should the amount of injected sulphur need to be increased. Thus for example, a large volcanic eruption would also lead to a clearly smaller and shorter period cooling if the volcano were to erupt during the stratospheric sulphur injections than if the eruption took place in an unperturbed atmosphere. This work also studied if ship or air traffic were to be used for geoengineering by increasing sulphur concentration in fuel. This would lead to a significant cooling effect but would require changes in current legislation. Even if this were to occur, the cooling effect would be concentrated around the vicinity of routes of the traffic and would thus lead to a regionally uneven cooling effect. Furthermore, the cooling effect would be clearly smaller compared to injection strategies which were intended to maximize the cooling effect of aerosols. The global mean cooling effect attributable to aerosols would be rather small due to the geographical change in tropospheric aerosol emissions or change in energy production studied here when compared to the warming due to the increased greenhouse gas emissions. This work shows the necessity of including aerosol microphysics into climate modelling since most of the conclusions in this work could not be obtained without aerosol microphysics or taking into account the interactions between clouds and aerosols. Thus, micrometer scale physical phenomena would influence the climatic impact on a global scale. Simulating aerosol microphysics led also to many unpredictable results.
Date: 16-04
Subject: geoengineering
aerosol radiative effects
global climate modelling

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