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Impact of geoengineered aerosols on the troposphere and stratosphere

Author(s): Tilmes S, Garcia RR, Kinnison DE, Gettelman A, Rasch PJ

Published: June, 2009

Publisher: Journal of Geophysical Research

DOI: 10.1029/2008JD011420

Tags: Stratospheric Aerosols, Environmental Side-Effects, Climate Modelling

URL: http://www.agu.org/journals/ABS/2009/2008JD011420.shtml

Abstract: A coupled chemistry climate model, the Whole Atmosphere Community Climate Model was used to perform a transient climate simulation to quantify the impact of geoengineered aerosols on atmospheric processes. In contrast to previous model studies, the impact on stratospheric chemistry, including heterogeneous chemistry in the polar regions, is considered in this simulation. In the geoengineering simulation, a constant stratospheric distribution of volcanic-sized, liquid sulfate aerosols is imposed in the period 2020–2050, corresponding to an injection of 2 Tg S/a. The aerosol cools the troposphere compared to a baseline simulation. Assuming an Intergovernmental Panel on Climate Change A1B emission scenario, global warming is delayed by about 40 years in the troposphere with respect to the baseline scenario. Large local changes of precipitation and temperatures may occur as a result of geoengineering. Comparison with simulations carried out with the Community Atmosphere Model indicates the importance of stratospheric processes for estimating the impact of stratospheric aerosols on the Earth's climate. Changes in stratospheric dynamics and chemistry, especially faster heterogeneous reactions, reduce the recovery of the ozone layer in middle and high latitudes for the Southern Hemisphere. In the geoengineering case, the recovery of the Antarctic ozone hole is delayed by about 30 years on the basis of this model simulation. For the Northern Hemisphere, a onefold to twofold increase of the chemical ozone depletion occurs owing to a simulated stronger polar vortex and colder temperatures compared to the baseline simulation, in agreement with observational estimates.


An overview of geoengineering of climate using stratospheric sulphate aerosols

Author(s): Rasch PJ, Tilmes S, Turco RP, Robock A, Oman L, Chen CC, Stenchikov GL, Garcia RR

Published: November, 2008

Publisher: Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences

DOI: 10.1098/rsta.2008.0131

Tags: Stratospheric Aerosols

URL: http://www.ncbi.nlm.nih.gov/pubmed/18757276

Abstract: We provide an overview of geoengineering by stratospheric sulphate aerosols. The state of understanding about this topic as of early 2008 is reviewed, summarizing the past 30 years of work in the area, highlighting some very recent studies using climate models, and discussing methods used to deliver sulphur species to the stratosphere. The studies reviewed here suggest that sulphate aerosols can counteract the globally averaged temperature increase associated with increasing greenhouse gases, and reduce changes to some other components of the Earth system. There are likely to be remaining regional climate changes after geoengineering, with some regions experiencing significant changes in temperature or precipitation. The aerosols also serve as surfaces for heterogeneous chemistry resulting in increased ozone depletion. The delivery of sulphur species to the stratosphere in a way that will produce particles of the right size is shown to be a complex and potentially very difficult task. Two simple delivery scenarios are explored, but similar exercises will be needed for other suggested delivery mechanisms. While the introduction of the geoengineering source of sulphate aerosol will perturb the sulphur cycle of the stratosphere signicantly, it is a small perturbation to the total (stratosphere and troposphere) sulphur cycle. The geoengineering source would thus be a small contributor to the total global source of 'acid rain' that could be compensated for through improved pollution control of anthropogenic tropospheric sources. Some areas of research remain unexplored. Although ozone may be depleted, with a consequent increase to solar ultraviolet-B (UVB) energy reaching the surface and a potential impact on health and biological populations, the aerosols will also scatter and attenuate this part of the energy spectrum, and this may compensate the UVB enhancement associated with ozone depletion. The aerosol will also change the ratio of diffuse to direct energy reaching the surface, and this may influence ecosystems. The impact of geoengineering on these components of the Earth system has not yet been studied. Representations for the formation, evolution and removal of aerosol and distribution of particle size are still very crude, and more work will be needed to gain confidence in our understanding of the deliberate production of this class of aerosols and their role in the climate system.


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