Geo Engineering Climate Change with Sulfate Aerosols
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Raupach et al, 2007, Global and regional drivers of accelerating CO2 emissions, PNAS Preliminaries • Geoengineering:The intentional large-scale manipulation of the global environment. The term is usually been applied to proposals to manipulate the climate with the primary intention of reducing undesired climatic change caused by human influences. • Why do this study? – A worry that we are transforming our energy system far too slowly to avoid the risk of catastrophic climate change. – What might be deployable in a planetary emergency to mitigate some of the effects of greenhouse gas warming? – We are not proposing that geo-engineering be done! We are proposing that the implications be explored (see Crutzen study and Cicerone commentary, Climatic Change, 2006) Exploring Geo-Engineering strategies to Counter Climate Change Phil Rasch • Tidbits from 3 studies (with extras) – Rasch, Crutzen, Coleman, Exploring the geoengineering of climate using stratospheric sulfate aerosols, 2008, Geo. Res. Letters – Rasch, et al, An overview of geoengineering climate using stratospheric sulfate aerosols, 2008, special issue on geoengineering, Phil. Trans. A., submitted – Latham et al, Global Temperature Stabilization via Controlled Albedo Enhancement of Low-Level Maritime Clouds, 2008, special issue on geoengineering, Phil. Trans. A., submitted with thanks to D. B. Coleman, P. Crutzen, J. Latham, A. Robock, S. Salter, G. Stenchikov, S. Tilmes, R. Turco, C. Amman, K. Caldeira, J. Kazil, D. Keith, M. Mills, O.B. Toon, T.M. Wigley Quotes from George Bernard Shaw (with tongue firmly in cheek) • "A reasonable man adapts himself to suit his environment. An unreasonable man persists in attempting to adapt his environment to suit himself. Therefore, all progress depends on the unreasonable man “ • "People are always blaming their circumstances for what they are. I don't believe in circumstances. The people who get on in this world are the people who get up and look for the circumstances they want, and if they can't find them, make them." • "A life spent making mistakes is not only more honorable but more useful than a life spent doing nothing." • “It is dangerous to be sincere unless you are also stupid.” • Was GBS a “geoengineer before his time”? An incomplete history of Geo-engineering • Apologies for leaving out many contributors! Good reviews in – Keith (2000, Ann. Rev. Energy Environ.) – American Institute of Physics Website by Spencer Weart, at http://www.aip.org/history/climate/RainMake.htm • In 1905 Arrhenius discussed a “virtuous circle” in which CO2 emissions would warm the climate, changing the northern limits of agriculture and enhancing productivity. • Cloud seeding efforts started in 30s and 40s • Modern concept goes back at least to 1945 in a meeting organized by John Von Neumann at Princeton on “ deliberate modification of weather”. – 1953 Presidents Advisory Committee on weather control with focus on “rainmaking” – 1955 in interview in Fortune magazine JVN speculated that “Microscopic layers of colored matter spread on an icy surface, or in the atmosphere above one, could inhibit the reflection-radiation process, melt the ice, and change the local climate Geoengineering history II • By 1970s US gov spending $20M/yr on weather modification research. Substantial amounts also spent in USSR on this. • Circa 1974, ... Budyko calculated that if global warming ever became a serious threat, we could counter it with just a few airplane flights a day in the stratosphere, burning sulfur to make aerosols that would reflect sunlight away. • 1977, National Academy Report on Geo-engineering, ... • Lamb, Hubert H. (1971). "Climate-Engineering Schemes to Meet a Climatic Emergency." Earth-Science Reviews said – "an essential precaution is to wait until a scientific system for forecasting the behavior of the natural climate... has been devised and operated successfully for, perhaps, a hundred years." Geo-engineering history III • More recent studies – NAS (1992) – Teller (1997) – Govindaswami and Caldeira (2000, 2002, 2003) reduced solar flux by 1.8% (4.2W/m2) – Crutzen (2006, Climatic Change) Theoretical, Back of the envelope – Wigley (2006, Science) Upwelling diffusion Energy Balance Model • mitigation in combination with sulfate aerosols equivalent to about 1 Pinatubo every 2 years would go a long ways towards ameliorating global warming Alternate Strategies (not discussed today) • Space mirrors, (L. Wood, R. Angel) • Sequestration of CO2 • Iron Fertilization, ... • Painting rooftops white, ... Cautions • There are obviously very important Moral, Ethical, Legal issues to be considered here • Arguments have been made that we shouldn’t even be considering these types of “solutions”. – Geoengineering will undercut society’s resolve to deal with emissions – Action would change climate in different ways for different nations. People in threatened regions most likely to use intervention • Worst case outcomes – unanticipated impacts (e.g. CFCs and ozone hole) – forestall global warming only to find we had triggered a new ice age Geoengineering by Sulfate Aerosols • What would be the impact of injecting precursors of sulfate aerosols into the middle atmosphere, where they would act to increase the planetary albedo, and thus counter some of the effects of greenhouse gas forcing? • This geo-engineering approach may be a natural (but imperfect) analogue to a volcanic eruption • Follow up to a study by Crutzen (Climatic Change, 2006) – Back of the envelope calculation – A more detailed and comprehensive exploration of Budyko’s idea • This study uses a relatively sophisticated General Circulation Model for a somewhat more quantitative, and comprehensive look at the problem, but it is still far too simple to be a believable characterization Commentaries on Crutzen's paper • Five comments appeared on the paper – Cicerone, Kiehl, Bengtsson, MacCracken, Lawrence • Comments emphasized issues of science, caution • All were constructive, Cicerone’s struck a chord in me: – Supported Crutzen’s call for research – Proposed a framework for progress • Research considered separately from implementation • Refereed papers are to be encouraged: they permit poor or dangerous ideas to be seen as such and meritorious ones to develop further • Develop mathematical models based on scientific principles • State assumptions and equations • Produce quantitative computations, evaluation of system sensitivity • Explore side effects, look for irreversible features. • Moratorium should be considered on experimentation Fundamental idea • Injection of SO2 at 25km in tropics will form sulfate aerosol. This will act to cool the planet • Estimates based on Crutzen (2006) suggest 1-2Tg S/year (as sulfate) would suffice – 2-4% of current anthropogenic surface emissions – Cost ~$25Billion/yr ($25/capita/yr in the affluent world) – 1-3 W/m2 reduction in incoming solar radiation Important processes for stratospheric aerosols (from SPARC Assessment of Stratospheric Aerosols, 2006 Global Sulfur Budget (rough) What size are the aerosols? • May look like tropospheric or background stratospheric sulfate – reff < 0.15 um (e.g. Bauman etal 2003) – primarily scattering in Solar part of spectrum • May look like Volcanic aerosol – reff ~ 0.45 um – absorption in • Near IR of Solar energy spectrum • Terrestrial longwave spectrum CCSM Model configuration Used for the Rasch etal 2008a • Model configuration optimized for studying physical feedbacks in a middle atmosphere configuration – Model top at about 80km, 52 layers – 2x2.5 Degree Horizontal resolution – Finite Volume solution for dynamics with desirable properties for transport – Sulfur Photochemistry includes only that relevant to the oxidation of DMS and SO2 –> SO4 (Oxidants are prescribed) • This version includes a relatively comprehensive “physical” characterization of the atmosphere and land, but: – No Biogeochemistry (particularly as it contributes to Carbon and Nitrogen Cycles, Ocean Ecosystems) – Prescribed Ocean and Sea Ice Dynamics (but does include thermodynamics) --- So called “slab ocean model” + “thermodynamic sea ice model” – No Aerosol/Cloud Microphysical formulations relevant to the “indirect aerosol forcing effect” Experimental setup • Simulations performed – Fixed aerosol and greenhouse forcing at present day values (Control) – Doubled CO2 (2XCO2) – Injection of SO2 at 25km, 10N - 10S • Pinatubo thought to inject 10-30 Tg S (over a few days) • 1 Tg S/yr assuming a small (or background) aerosol size distribution forms • 2 Tg S/yr small particles • 2 Tg S/yr as large (or volcanic) aerosol forms – Doubled CO2 + the above permutations of emission amount and aerosol size Global Averaged Annual Averaged Surface Temperature change (flawed representation for evap and sedimentation) Time series of Annually Avg’d Surface Temperature Global Precipitation Sulfate during June, July, August Impact on Sfc Temperature (JJA) NCAR (Slab Ocean) Rutgers/GISS (fully coupled) Ozone Changes • Ozone losses following volcanic eruptions are well documented – Empirical relationships (e.g. Tilmes 2007) can estimate ozone loss accurately as a function of vortex temperature, aerosol sfc area (SAD), halogen loading – Chemistry also reasonably well understood. Chemistry-climate models should be able to predict ozone depletion • Tilmes et al (2008) used the aerosol distributions of Rasch et al (2008) to estimate ozone depletion from geo-engineered sulfate aerosol – WACCM (Whole Atmosphere Community Climate Model) • Slab Ocean Model • Sfc-150km • Relatively comprehensive interactive reactive photochemistry for the middle atmosphere • Simulations from 2010-2050 were performed with changing CO2 and CFCs, with and without geoengineering Springtime Ozone loss from geoengineering (2 Tg S/yr, volcanic sized) Summary, Discussion, Conclusions (for geoengineering by sulfate aerosols) • The model suggests the earth cools, with many components return to a state more like the unperturbed earth • Delivery of aerosol (or precursor) a formidable task (1M flights/year with a fighter jet or Concorde payload) • Unlikely that all aspects of climate will match present day (e.g. precip) • Increase in aerosols will deplete more ozone (until chlorine is gone) • Increase in UV from ozone depletion balanced to some degree by aerosol attenuation & extinction • Change in direct/diffuse radiation with impact on photosynthesis (-> ecosystem studies, and solar energy production) • These studies have explored the tip of the iceberg. Much more work is needed before identifying this strategy as potentially viable. • Some consequences (e.g. Ocean Acidification) are not dealt with at all by this strategy. Geoengineering by boundary layer cloud seeding • Deliberate introduction of CCN to enhance the Droplet Concentration in Low-level clouds increasing Cloud Albedo & Longevity - i.e. a cooling effect. • This geo-engineering approach may be considered an analogue to Natural droplet creation at ocean surface – Bubble-bursting, white-capping – NaCl droplets effective CCN • Ship tracks, fires, industrial pollution sources • (Idea based on: -Twomey (1977), Charlson et al. (1987), Wigley (1989), Slingo (1990)) • CO2 doubling compensated by (Slingo, 1990): – 120% increase in droplet concentrations – 40% decrease in effective radius – 12% increase in oceanic cloud cover • Follow up to studies of Latham, 1990, 2000, Bower et al., 2006 • This study uses a relatively sophisticated General Circulation Model for a somewhat more quantitative, and comprehensive look at the problem, but it is still far too simple to be a believable characterization The cloud seeding paradigm • Natural sea salt is estimated to be ~10% of CCN over the ocean. If one increased sea salt mass but maintained the same naturally broad size distribution, then a 10x increase in seasalt particles might double CCN, but might also produce more giant nuclei, increasing precipitation, and decreasing cloud. • Instead, assume is that the particle size distribution of a geoengineered aerosol could be optimized (e.g. monodisperse, initial droplet size of 0.8 micron) • These CCN activate preferentially over naturally occurring aerosol. Doubling of droplet number N could be ensured by adding 2N seawater CCN. Model Setup • Community Atmosphere Model version 3.5+ – Revised convection (Neale et al 2007, Richter & Rasch 2007) – Cloud microphysics tweaks, • Drop Activation tweaked between sfc and 850mb • Rasch & Kristjansson (2001, bulk single moment scheme predicting mass, drop radius and number are prescribed) OR • Morrison, Gettelman, Ghan (2007a, b; internally consistent 2 moment scheme predicting mass and number) • 2x2.5o, 26 layer, 5 year runs with fixed SST • Changes near surface reach 5km • Changes to Liquid Water Path • Annual Averages of Control SWCF Short Wave Cloud Forcing SWCF from increment ->375/cm3 SWCF from increment ->1000/cm3 Cloud forcing as a function of areal extent (most susceptible clouds seeded first) Optimizing cloud seeding (Sampling over a 5 year simulation) DJF JJA Cloud Seeding feedbacks Fixed SST Interactive SST Summary • Emission reductions are still the desired choice to ameliorating climate change • Aerosol cloud interactions are extremely complex (e.g. IPCC AR4) • Nevertheless: The short lifetime of aerosols, short response time of clouds to aerosol perturbations, the perturbation is to existing components of the natural system, the strong influence on climate, cost --> make cloud seeding interesting to consider as a means to geo- engineering • Assuming that this model provides insight into forcing of the climate system: – Extra-tropical clouds might play a role in the geo-engineering – Pristine clouds more susceptible --> Southern Hemisphere easier to brighten – Changing CDNC at higher altitudes could also play a role in forcing – Study suggests that seeding perhaps 25% of the globe would counteract much of the forcing associated with a doubling of CO2 --> quite strong local radiative forcing – Response of coupled system important to explore (feedbacks, ocean circulations, ocean ecosystems, land/ocean precipitation redistribution, ?) – If interesting, then more work required to consider technology development, strategies for assessment, ramifications Extra Slides Latitude Height Solar Heating rates (K/day) Heating rates for geosulfate (2 Tg S/yr) Total Heating rates Volcanic-like aerosol Background aerosol Latitude Height Longwave Heating rates (K/day) Total Heating rate Volcanic-like Size distribution (2 Tg S) Change in Solar Radiative Fluxes by geoengineering 2 Tg/yr emissions as background-like aerosol (forcing) Surface Top of Atmosphere Change in Solar Radiative Fluxes by geoengineering 2 Tg S/yr as Volcanic-like aerosol (forcing) Surface Top of Atmosphere Change in Longwave Radiative Fluxes by geoengineering Volcanic-like aerosol (forcing) Surface Top of Atmosphere Effect of GeoSulfate on zonal avg Temperature (1 Tg S/Bkg) 2xCO2+GeoSul - Control 2xCO2+GeoSul - 2xCO2 Temperature Changes at tropical tropopause Angell (1993) Model (2 Tg/yr Volc) Flux changes from geo-engineering TOA Surface Atmosphere Longwave Background ~0 ~0 ~0 Longwave Volcanic -0.9 -0.1 0.8 (more energy into atm) Shortwave Background -5.1 -4.1 -1.0 (less energy into atm) Shortwave Volcanic -3.2 -2.6 -0.6 (less energy into atm) • For a given emission rate background small particle aerosol – Scatters more solar back to space – Absorbs less longwave from surface • Small particles are more effective for geo- engineering Summary • Geo-engineering by Sulfate in a model world will act to cool planet, return some features to “present day”, e.g. Surface temperature over much of the globe – 1.5 Tg S works if particles are small, more like 3Tg S if particles are large • A number of features are “different” from either present day, or 2xCO2 world – Precipitation – Polar winter surface temperatures • Demonstrable interactions exist between the greenhouse forcing and geo-engineering forcing, e.g., burden of geosulfate in presence of CO2 forcing. Feebacks are important. • Numerous obvious remaining topics for exploration in this simple framework – Subtleties of hydrologic cycle (e.g. Trenberth Study) – Seasonal and higher frequency transient aspect of simulation – Sea Ice • Numerous obvious issues to explore with model augmentation – Aerosol formulation improvements • Microphysics, size and number resolution • Independence of geo-sulfate from other aerosol components • Interaction with clouds (cirrus?) – Chemistry (particularly ozone depletion) – Dynamical Ocean and Sea-ice Models – Biogeochemistry (Land Ecosystems, and Ocean pH!) Closing thoughts • We have a long way to go – How would injection really manifest itself in aerosol evolution (would it look like volcanic size distribution or background)? – What happens to sulfate entering upper troposphere (effect on cirrus?)? – Chemistry (effects both stratosphere and troposphere)? – Many, many unanswered questions regarding feasibility (cost, delivery, etc) • Some manifestations of CO2 emissions cannot be treated by this type of geoengineering, e.g. changes to ocean pH. • Still need to reduce CO2 emissions!