U41E-01 INVITED
Terraforming the Planets and Climate Change Mitigation on Earth
Hopefully, purposeful geo-engineering of the Earth will remain a theoretical concept. Of course, we have already inadvertently changed the Earth, and over geologic history life has left an indelible imprint on our planet. We can learn about geo-engineering schemes by reference to Earth history, for example climate changes after volcanic eruptions provide important clues to using sulfates to modify the climate. The terrestrial planets and Titan offer additional insights. For instance, Mars and Venus both have carbon dioxide dominated greenhouses. Both have more than 10 times as much carbon dioxide in their atmospheres as Earth, and both absorb less sunlight than Earth, yet one is much colder than Earth and one is much hotter. These facts provide important insights into carbon dioxide greenhouses that I will review. Mars cools dramatically following planet wide dust storms, and Titan has what is referred to as an anti- greenhouse climate driven by aerosols. These data can be used to reassure us that we can model aerosol caused changes to the climate of a planet, and also provide examples of aerosols offsetting a gas-driven greenhouse effect. People have long considered whether we might make the other planets habitable. While most of the schemes considered belong in the realm of science fiction, it is possible that some schemes might be practical. Terraforming brings to mind a number of issues that are thought provoking, but not so politically charged as geo-engineering. For example: What criteria define habitability, is it enough for people to live in isolated glass enclosures, or do we need to walk freely on the planet? Different creatures have different needs. Is a planet habitable if plants can thrive in the open, or do animals also need to be free? Are the raw materials present on any planet to make it habitable? If not, can we make the materials, or do we have to import them? Is it ethical to change a planetary climate? What if there are already primitive creatures hidden somewhere, does that change the ethics? Or perhaps, if left alone, the planet would later evolve to have life, is it ethical to alter that path? Many of these questions have parallels to things we need to consider when we talk about geo-engineering Earth. I will touch upon some of these subjects in this talk.
U41E-02
Preparing Climate Engineering Responses to Climate Emergencies I: Aerosol Particle Design and Stratospheric Delivery Options
Although international efforts to stabilize CO2 concentrations may well prove sufficient to prevent or delay severe climate impacts, there is already a non-negligible possibility that the climate will respond rapidly and non-linearly to present concentrations. If climate sensitivity high, it may be too late to avert dramatic consequences for human societies or natural ecosystems even with immediate and aggressive mitigation efforts. Climate engineering that induced rapid changes in the climate system might limit the risks posed by such "climate emergencies," although uncertainty in the climatic response to such interventions makes it difficult to estimate the risk-effectiveness of such engineering. The authors of this abstract thus gathered for a one-week intensive study to explore the question: What program of comprehensive technical research over the next decade would maximally reduce the uncertainties associated with climate engineering responses to climate emergencies? We focused on the possible injection of aerosols into the stratosphere. This presentation (see also Blackstock et al.) focuses on technical evaluation of aerosol particle and stratospheric deployment alternatives. First, the five core (multidimensional) "control variables" for stratospheric aerosol interventions are identified that could be engineered to tailor its climatic impacts: 1) aerosol material composition; 2) aerosol particle size (and shape); 3) amount of aerosol dispersed; 4) geographic and vertical dispersion location; and 5) temporal sequencing of aerosol dispersion. Correlations between other relevant intervention characteristics (e.g. aerosol spectral scattering properties and lifetimes) and these control variables are identified (along with sources of uncertainty therein). We examine fundamental and practical constraints on the range and precision with which these variables can be controlled. Limits imposed by both natural physical and chemical processes (e.g. chemical reactivity and particle agglomeration) and feasible deployment technologies (e.g. high-altitude lofting capabilities and aerosol dispersion mechanisms) are evaluated. This analysis yields insights into new avenues of research in particular, viable aerosol material possibilities and stratospheric dispersion methods (e.g. methods for dispersing mass-efficient volatile hydride compounds which oxidize and coagulate to form aerosols)—along with a better appreciation of the limits and uncertainty governing all such options.
U41E-03 INVITED
Sulphate Geoengineering in the UT/LS: Some Relevant Processes
We consider the potential effects of meteorological dynamics, the physics and chemistry of aerosols and the photodissociation of sulphuric acid upon the posited maintenance of a 'parasol' of geoengineered sulphate aerosol in the lower stratosphere. Specific observational and experimental results include the spread of tungsten-185 from the Hardtack series of nuclear weapon tests in 1958, satellite observations of the spread of volcanic eruptions, tracer and water profiles in the tropical UT/LS, the organic coating of surfactants on aerosols, the observed distributions of aerosols and the overtone driven photodissociation of sulphuric acid in the stratosphere. A few implications for the logistics of any possible future geoengineering injection are considered briefly. The uncertainties arising from the analysis subtract significantly from the predictability of any supposed amelioration of the effects of global warming from continued increases in carbon dioxide from fossil fuel combustion.
U41E-04 INVITED
Estimating efficiency of the controlled sulphur emissions in the stratosphere to mitigate global warming
An attempt is made to estimate an efficiency of sulphur loading in the stratosphere to mitigate global warming employing a large ensemble of numerical experiments with the climate model of intermediate complexity developed at the A.M.Obukhov Institute of Atmospheric Physics RAS (IAP RAS CM). In this ensemble, the model is forced by the historical+SRES A1B anthropogenical greenhouse gases+tropospheric sulphates scenario for 1860--2100 with an additional sulphur emissions in the stratosphere started in 2012. Different ensemble members were constructed by varying emission intensity, residence time and optical properites of stratospheric sulphur. Given global loading of the sulphates in the stratosphere, at the global basis the most efficient latitudinal distribution of geoengineering aerosols is that peaked between 50° N and 70° N. At regional scale other latitudinal distributions may be superior. In particular, the distributions peaked in the tropics are the most efficient to reduce warming in the subtropics and the distrbutions peaked at 50° N is the superior to mitigate annual warming in Siberia. However, an approach of geoengineering has inherent flaws. First, it results in a widespread dryness. The second, and perhaps more dangerous, issue is due to the fast removal of geoengineering climatic effect if the corresponding emissions are stopped. After this stop, climate trajectory returns to the non--mitigated one within few decades. This results in a necessity to continue a geoengineering mitigation very long in future. Third, estimated sulphur emissions amount 5-10 TgS/yr in 2050 and 10-14 TgS/yr in 2100 which is not a small part of the current emissions of tropospheric sulphates. The latter may lead to marked enhancement of the tropospheric sulphates pollution. The results obtained with the IAP RAS CM are further interpreted by making use of an energy--balance climate model. As a whole, the results obtained with this simpler model support conclusions made on the basis of the IAP RAS CM simulations.
U41E-05
Stratospheric Aerosol Injection for Geoengineering Purposes
A number of studies have focused on the large-scale aspects of massive stratospheric aerosol injections for the purpose of modifying global climate to counterbalance current and future greenhouse warming effects. However, no descriptions of actual injection schemes have been presented at any level of detail; it is generally assumed that the procedure would be straightforward. Approaches mentioned include direct injection of dispersed microparticles of sulfates or other mineral particles, or the emission of precursor vapors, such as sulfur dioxide or hydrogen sulfide, that lead to particle formation. Using earlier aircraft plume research as a guide, we investigate the fate of injected aerosols/precursors from a stratospheric platform in terms of the chemical and microphysical evolution occurring in a mixing plume. We utilize an advanced microphysics model that treats nucleation, coagulation, condensation and other processes relevant to the injection of particulates at high altitudes, as well as the influence of plume dilution. The requirements of particle size and concentration for producing the desired engineered radiative forcing place significant constraints on the injection system. Here, we focus on the effects of early microphysical processing on the formation of a suitable aerosol layer, and consider strategies to overcome potential hurdles. Among the problems explicitly addressed are: the propensity for emitted particles to coagulate to sizes that are optically inefficient at solar wavelengths, accelerated scavenging by an enhanced background aerosol layer, the evolution of size dispersion leading to significant infrared effects, and total mass injection rates implied by stratospheric residence times. We also investigate variability in aerosol properties owing to uncertain nucleation rates in evolving plumes. In the context of the microphysical simulations, we discuss infrastructure requirements in terms of the scale of the intervention and, hence, the overall feasibility of such an approach to climate change mitigation.
U41E-06
Use of Volcanic Eruptions as a Natural Analog for Evaluating Effects of Stratospheric Geoengineering on the Hydrological Cycle, Ocean Heat Content, and Sea Level
Large-scale human intervention into natural systems, geoengineering, is considered as a means to counterforce global warming. Among the discussed geoengineering schemes one of the most feasible (because of its relatively low cost and existing natural analog) is based on injection of sulfur aerosols or their precursors into the stratosphere (therefore here we call it "stratospheric geoengineering") to increase the Earth's planetary albedo and cool the Earth. Recent model studies, however, indicated reduction of precipitation as a side effect of injection of scattering aerosols in the lower stratosphere, and did not assess the forced long-term effect on ocean circulation and thermal structure. In this study we take advantage of the analogy between stratospheric geoengineering and volcanic impacts to better quantify the effects of geoengineering on hydrological cycle and the ocean that are crucial for assessing biospheric and economic consequences of geoengineering. We employ the coupled climate model CM2.1, developed at NOAA's Geophysical Fluid Dynamics Laboratory, and simulate responses to quasi-permanent geoengineering forcing, as well as transient impacts of the 1991 Pinatubo and 1815 Tambora eruptions. Testing volcanic model simulations against observations allows us to more reliably estimate the range of climate system responses to stratospheric aerosols, their dependence on the magnitude of forcing, and associated characteristic times. We found that stratospheric aerosol cooling intensifies ocean vertical mixing and tends to strengthen the meridional overturning circulation. Sea ice appears to be sensitive to volcanic forcing, especially during the warm season. Volcanic ocean temperature signals scale roughly linearly with respect to radiative forcing, but ocean overturning circulation response is less than linear. In two-three years after injection of aerosols, while ocean temperatures decrease and the global hydrological cycle remains suppressed, precipitation over land tends to recover. The quasi-permanent cooling from geoengineering aerosols penetrate into the deep ocean more slowly than from sporadic volcanic cooling, which more vigorously intensifies ocean vertical mixing. Ocean subsurface temperature, sea level, and overturning circulation have an extremely long relaxation time of about a century. Therefore geoengineering consequences in the ocean, despite a quicker atmospheric temperature recovery, will be felt for at least a century after geoengineering forcing is turned off.
U41E-07
The Practicality of Geoengineering
Injecting sulfate aerosol precursors into the stratosphere to produce an artificial sulfate aerosol cloud has been suggested as a technique to geoengineer the climate to reduce global warming. Advocates have suggested that this would be easy and inexpensive, but to date there has been no detailed estimate of the actual costs or practicality. Here we evaluate several possible means of injecting the equivalent of 3-5 Tg SO2 per year into the lower Arctic or tropical stratosphere. We assess airplanes, balloons, artillery shells, and space elevators from the viewpoint of cost and possible speed of application. No such systems currently exist, and it would take a major manufacturing effort and deployment experiments to develop any of the proposed mechanisms. We estimate the costs of building the systems and of annual operation and maintenance, and evaluate the environmental impacts at the location of deployment and globally.
U41E-08 INVITED
Geoengineering by seeding boundary layer clouds using two climate modeling paradigms
We explore the Earth system climate response to geoengineering by seeding maritime boundary layer clouds. We contrast the response of the system using an atmospheric GCM coupled to two different formulations for sea ice and ocean dynamics: 1) a full ocean and dynamic sea ice model; 2) a slab ocean model with a thermodynamic sea ice model. We show that the climate response is quite different in the two formulations and explore the reason for the difference.
U41E-09
Geoengineering of stratocumulus decks to counterbalance global warming: Pros, Cons and Side-Effects
Anthropogenic emissions of carbon dioxide from fossil-fuel burning are the primary cause of global warming and the projected rate of temperature change is very likely to increase in the future. Many geoengineering solutions have recently been suggested to reduce global warming. Here we use a state-of-the-science atmospheric general circulation model coupled to a mixed-layer ocean model to investigate the climatic impact of geoengineering via modifying stratocumulus decks. The model's climate is more than twice as sensitive to modification of the South Pacific stratocumulus area compared with the North Pacific or South Atlantic stratocumulus area. This strong sensitivity is associated with the cooling of sea-surface temperatures in the southern Pacific inducing patterns of temperature and precipitation response resembling La Niña conditions. If the southern Atlantic stratocumulus sheet is geoengineered, the model suggests an El Niño-like response with precipitation reduced by 15percent over South America as a whole, but a 30percent reduction over Amazonia. If all stratocumulus decks were geoengineered, the model suggests precipitation reductions of 15percent over South America, 9percent over North America, and 5percent over Europe and Asia. Thus the use of geographically inhomogenous radiative forcing mechanisms to counterbalance global warming induces distinct geographic responses in temperature and precipitation that may be very detrimental to some areas of the Earth.
U41E-10
Geoengineering and the Problem of Harm
Suppose geoengineering "works" and that there are no competing antagonistic interventions. Even on this
rosy scenario, it is extremely unlikely that it will "work" for everyone since (as Schneider pointed out many
years ago) there is no reason to think the effects of geoengineering will offset the effects of global warming
locally. But as Robock et al have argued (Robock, Alan, Luke Oman, and Georgiy Stenchikov, 2008:
Regional climate responses to geoengineering with tropical and arctic SO2 injections. J. Geophys. Res., in
press), things may even worse than that. Suppose in some places (e.g. India), climate change +
geoengineering leaves you worse off (hotter and drier) than climate change alone. Might it nonetheless be
fair to proceed on the basis of the numbers – that is if many more would benefit than those who would not? I
consider under what circumstances we may "let the numbers count", and where it is wrong to do so,
irrespective of the numbers. I argue that the case of geoengineering is one in which we may let the numbers
count, but only under conditions that are extremely difficult to satisfy.
http://www.csp.rutgers.edu
U41E-11
International Collective Governance and the Need to Reduce Scientific Uncertainty about Geoengineering
Geoengineering has been discussed for decades around the edges of the climate science community. However, today limited progress in abating emissions of GHGs makes the subject ever more salient. There is a growing need both for the foreign policy community to begin to consider how best to develop a framework for collective international governance of this issue, and to undertake research that will reduce uncertainty about the likely effectiveness and consequences of specific geoengineering technologies. In this paper we briefly outline insights on global governance gained from a workshop we to start a conversation about international governance that we convened at the Council on Foreign Relations in May 2008. This meeting's deliberations concluded that it is time to move studies of geoengineering and its impacts into mainstream research agendas. We outline some of the elements that such research should cover and then report on a study we are conducting based on simulations run using the climateprediction.net version of the coupled atmosphere-ocean general circulation model (AOGCM), Hadley Centre Coupled Model, version 3 (HadCM3) developed by the UK National Centre for Atmospheric Science. Past geoengineering modeling studies using AOGCMs have generally examined the effects of applying a constant geoengineering forcing. In this study we apply a range of 135 transient forcing scenarios designed to span the range of plausible uncertainties associated with countering anthropogenic GHG and sulfur aerosol forcings. Twin ensembles are being investigated for responses to IPCC A1B emissions scenario between 2000 and 2080, both with and without geoengineering activities starting in 2005. In our initial runs all models use identical parameter inputs, with the exception of an initial condition parameter. Geoengineering activities were mimicked in the models by modifying the volcanic aerosol radiative inputs, applied as variations in stratospheric optical depth over four zonal bands bounded by the equator, 30°N and 30°S. This work was supported by NSF cooperative agreement SES-034578.