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pdf file: Dr. Teller's Paper download

Dr, Edward Teller


Global Warming and Ice Ages:

I.  Prospects for Physics-Based Modulation of Global Change


This paper was prepared for submittal to the

22nd International Seminar on Planetary Emergencies


Erice (Sicily), Italy

August 20-23, 1997


August 15, 1997


This is a preprint of a paper intended for publication in a journal or proceedings. Since

changes may be made before publication, this preprint is made available with the

understanding that it will not be cited or reproduced without the permission of the






This document was prepared as an account of work sponsored by an agency of  the United States Government. Neither the United States Government nor the  University of California nor any of their employees, makes any warranty, express  or implied, or assumes any legal liability or responsibility for the accuracy,  completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade  name,  trademark, manufacturer, or otherwise, does not necessarily constitute or  imply its endorsement, recommendation, or favoring by the United States  Government or the University of California. The views and opinions of authors  expressed herein do not necessarily state or reflect those of the United States  Government or the University of California, and shall not be used for advertising or product endorsement purposes.



I. Prospects For Physics-Based Modulation Of Global Change*


Edward Teller& and Lowell Wood#

Hoover Institution, Stanford University, Stanford, CA 94305-6010




Roderick Hyde

University of California Lawrence Livermore National Laboratory

Livermore CA 94551-0808



It has been suggested that large-scale climate changes, mostly due to atmospheric injection of  "greenhouse gases" connected with fossil-fired energy production, should be forestalled by  internationally-agreed reductions in, e.g., electricity generation. The potential economic impacts of  such limitations are obviously large: $1011/year. We propose that for far smaller — <1% —  costs, the mean thermal effects of "greenhouse gases" may be obviated in any of several distinct  ways, some of them novel. These suggestions are all based on scatterers that prevent a small  fraction of solar radiation from reaching all or part of the Earth. We propose research directed to  quite near-term realization of one or more of these inexpensive approaches to cancel the effects of  the "greenhouse gas" injection.


While the magnitude of the climatic impact of "greenhouse gases" is currently uncertain, the prospect of severe failure of the climate, for instance at the onset of the next Ice Age, is undeniable. The proposals in this paper may lead to quite practical methods to reduce or eliminate all climate failures.


Global Warming and Ice Ages:

I. Prospects for Physics-Based Modulation of Global Change


August 15, 1997

Lawrence     Livermore National   Laboratory


Edward Teller


Abstract 2:


1 million tons of scattering materials ~ 1%

refresh about 20% per year


discusses lifting masses into air or space


Indeed, some types of   stratospheric deployment of oxide particulates – e.g., SO 2 or Al 2 O 3 – might be accomplished simply by   operating one or more well-engineered combustors – e.g., of elemental S or Al – at high-altitude, near-equatorial  ground-sites, from which  stratospheric injection of warm gas is intrinsically advantaged.   (Combustor engineering would focus on mass-efficient, optimal-sized scatterer particle generation in the  vertically-directed exhaust, which likely would have a rocket nozzle character in order to facilitate  swift manipulation of the temperature and density of combustion products  across usefully large ranges.)






In recent years, consideration of the possible warming of the climate due to the injection into the atmosphere of ''greenhouse gases,'' particularly carbon dioxide, CO2,1 has motivated proposals to impose international limitations of the burning of fossil fuels, particularly ones yielding less heating-value per gram of CO2 released, such as coal. The starting point of the present paper is the widely-appreciated fact2 that increases in average world-wide temperature of the magnitude currently predicted can be canceled3 by preventing about 1% of incoming solar radiation – insolation – from reaching the Earth.4,5 This could be done by scattering into space from the vicinity of the Earth an appropriately small fraction of total insolation. If performed nearoptimally, 6 we believe that the total cost of such an enhanced scattering operation would probably be at most $1 billion per year, an expenditure that is two orders of magnitude smaller in economic  terms than those underlying currently proposed limitations on fossil-fired energy production.7,8,9 Some of these insolation-modulating scattering systems may be re-configured to effectively increase insolation by an amount – perhaps 3% – sufficient to prevent another Ice Age.10


We first survey various physical processes to accomplish this scattering. We then compare the various ways in which these scatterers may be deployed. Next, we propose that particular attention be given to three possible realizations of this technology-based program. We note geographical variability aspects of insolation modulation. We conclude by suggesting that the problem of possible changes in climate may be better solved by cooperative application of modern technologies rather than by international measures focused on prohibitions. Scattering Fundamentals. In general, three basic types of scatterers exist, for scattering any type of electromagnetic radiation, including sunlight. The simplest type is based on any material in which the electric fields of light cause a displacement of electric charges; thus, any material at all can be used. The magnitude of the displacement of charges by an electric field of unit strength is measured by the dielectric constant e, where e=1 means there is no displacement. The scattering is proportional to (e-1)2, that is, highly polarizable materials generally will be more useful. This class of scatterer requires the near-optimal deployment11 of an estimated several million tons of  scattering material in order to prevent an estimated (global- and time-)average temperature increase of 31.5C associated with a doubling of atmospheric CO2 during the coming century;12 the corresponding cost is ~$0.5 billion/year.


More effective scatterers can be realized by employing that subset of materials which exhibit high electrical conductivity. In this special case, electrons may be separated from their original locations by any distance, and it is the magnitude of the optical-frequency current carried by these electrons that characterize the effectiveness of such scattering materials – which are generally metals.


Employed near-optimally, tens of thousands of tons of high-conductivity metal – roughly 1% of the required mass of dielectric materials – are required to scatter 1% of the Earth's total insolation; the corresponding costs are $0.07-0.14 billion/year. In principle, the most effective of all possible scatterers are atoms or molecules that scatter light in resonance.


Such extremely strong scattering can be obtained for light of a frequency adapted to a

specific atom or molecule. The simplest example would be scattering of a narrow band of red light by lithium atoms or of yellow light by sodium atoms. Unfortunately, such exceptionally strong scatterings occur only in the immediate neighborhood of an atomic transition-frequency, and the atom will selectively interact with light of frequencies which deviates from the resonant one by about one part in ten million (for visible light). This difficulty can be overcome by broadening the resonance (accompanied by a proportionate weakening of the scattering-strength) or by using scatterers that have many separate resonances – or, most effectively, by a combination of these two approaches. Of the order of 1 million tons of such resonant-type scattering material are estimated  to suffice to remove 1% of the total insolation of the Earth; the corresponding cost may be $0.3- 0.75 billion/year.


The intrinsic scattering strengths of dielectrics, electrical conductors, and resonant scatterers are in the approximate ratio of 1 to 104 to 106, respectively,13 for visible light; in practical implementations14 useful for insolation modulation, however, these ratios may be much different.



How Should Scatterers Be Deployed?


There are three obvious choices for deployment-sites for scatterers on scales of interest for insolation modulation.15 One is the terrestrial stratosphere, the second is in a low-Earth orbit (i.e., an orbit whose radius may be as much as twice the radius of the Earth), and the third is a position along the line between the centers of the Earth and the Sun (approximately one hundred times the Earth's radius distant from the Earth).


Of the three deployments, the stratospheric location is by far the least expensive on a pound-for pound basis; positioning mass in the stratosphere currently is at least 104 times less costly than putting it into low Earth orbit.16 Moreover, the mid-stratospheric residence time of sub microscopic scattering particles of anthropogenic17 and natural18 origins is comparable to the half decade residence time of its molecular components,19 so that appropriately fine-scale particulate loadings of the middle stratosphere will persist for five-year intervals. However, the stratosphere is a chemically uncongenial location due to the high flux of ultraviolet radiation from the Sun and the presence of oxygen, particularly in the more reactive form of ozone.


Ideally, we would prefer to deploy scattering systems – or their principal components – that would remain in place and retain their performance-pertinent properties for a century, which is of the  order of the interval20 required for a CO2 emission pulse to be effectively sunk into the deep ocean.


However, we consider the half-decade mid-stratosphere residence time to be sufficiently long for practical deployments. We may re-constitute the deployment of a scattering system twice per decade (or 20% per year), and we even consider such a short duration to constitute a relatively rapid, naturally-operating means of disposing of possible unwanted side-effects of insolation modulation. Chemical stability in the stratosphere, even for material with readily available electrons on its surface, is a tractable issue for present purposes.


Deployment in low-Earth orbit is an obvious alternative,21 one which offers potentially very longterm  positional stability combined with excellent durability of many materials. Technologies that  could greatly decrease the cost of space-launch could make a telling difference in the practicality of  all types of space-deployed scattering systems of scales appropriate to insolation modulation.22


Light pressure arising from the momentum imparted to the scatterer by sunlight may significantly  perturb the orbital elements of the scatterer, and managing this momentum poses an additional  technical challenge to LEO-deployed scatterers.  An interesting though not necessarily a practical case comprises the third alternative.23 Terrestrial  insolation has an angular definition of one part in 120. Thus, if the scattering system is deployed  ~102 times the Earth's diameter distant from the Earth, small-angle (1o) scattering will suffice for  an appropriate deflection of the Earth-directed sunlight (either toward the Earth, if warming is  desired, or away from it, if cooling is sought). This small angle permits the use of relatively very  modest quantities of either conductors or dielectrics to comprise the scattering system –  approximately 102-fold smaller than those needed to effect insolation modulation of the same  magnitude when deployed near the Earth.24 The management of the radial and angular momenta of  the sunlight scattered poses basic, albeit quite tractable, issues with respect to position  maintenance.25 


Some Specific Proposals. There are obviously numerous ways in which the potentialities and  difficulties mentioned above can give rise to a workable scattering system, one of a scale adequate  to modulate the total insolation of the Earth by 1%. In the following, we shall provide some  details regarding specific possibilities, ones selected to illustrate basic features of each of the major  classes of scattering systems.


Sub-Microscopic Oxide Particulates.


During the present decade, the eruption of Mt.  Pinatubo in the Philippines induced a transient drop in the global mean  temperature of ~0.5 K,   apparently due to insolation modulation by volcanic particulates.26 It is believed that this cooling  was induced predominantly by scattering of sunlight by SO2-based particulates of sub-micron  scale, ones which may have grown into more effective scatterers by scavenging residual  stratospheric water and cations, resulting in myriad still-sub-micron droplets of high-concentration  sulfur acids and salts. Indeed, it has been suggested that the advent of marked greenhouse effects  due to CO2 emissions has been delayed through the present time by the simultaneous emission of   large quantities of sulfate particulates (primarily arising from the ~21% sulfur content by weight  of typical fuel-grade coal), resulting in significant tropospheric scattering of sunlight.27 To these extents, the case of dielectric scattering-based insolation modulation already has some  empirical  basis.


It may well be feasible to transport and disperse enough SO2 (or SO3 or H2SO4) into the stratosphere to produce the desired insolation modulation effect28,29 – and even to do so partly on the  basis of existing experience, as well as much prior analysis. It has also been suggested that alumina  injected into the stratosphere by the exhaust of solid-rocket motors might scatter non-negligible  amounts of sunlight.30 We expect that introduction of scattering-optimized alumina particles into the  stratosphere may well be overall competitive with use of sulfur oxides;31 alumina particles offer a  distinctly different environmental impact profile.


Conducting Sheets.


The reference 1% reduction in insolation might be obtained by deploying electrically-conducting sheeting, either in the stratosphere or in low Earth orbit. Three  quite different physical mechanisms comprise the foundations of the three distinct approaches  which we consider. In the first, mixtures of suitable metals are deposited in ultra-thin layers and  convenient area, and are then protectively coated.32 Platelets of such material are then deployed in  the stratosphere – or perhaps in low Earth orbit – and act to absorb sunlight by the photoelectric  effect; the absorbed energy is then thermally re-radiated, with ~half escaping into space. In the  second, metallic ''nets'' of ultra-fine mesh-spacing are employed to reflect solar photons of optical  wavelengths into space.33 In the third, optimized metallic-walled balloons – similar in concept to ones with which children play –  are self-deployed into the stratosphere from ground level, where  they serve to scatter insolation.34  Each of these approaches involves total masses of the order of 105 tons, although the detailed mass budgets are quite different. The total cost-to-own any one of these three metallic systems at full-scale appears to lie in the range of $0.07-0.14 billion/year.

Quasi-Resonant Scatterers.


A third approach involves the use of resonant – actually  quasi -resonant – scatterers, also deployed in the stratosphere.35 While the potential mass efficiency  of this class of scatterers is singularly high, practical considerations centered on photochemical  durability in the stratosphere indicate that total masses of 106 tons of material may have to be  deployed36 – still modest-scale systems37 which moreover may have to be replaced only twice per decade. For near-term, relatively low-risk insolation modulation studies, we propose the use of  sub-microscopic particulates composed of frozen perfluorohydrocarbon ''nano-droplets'' loaded  with embedded molecules of selected organic dyes.38 The total cost-to-own such a full-scale  insolation modulation system may be quite competitive – of the order of $0.3-0.75 billion/year.


Fine-Grained Insolation Modulation.


We note technical possibilities of modulating insolation  in a latitude-dependent manner. Consistent with the slow latitudinal mixing-time of the  stratosphere well above the tropopause, different amounts of scattering material might be deployed  (e.g., at middle stratospheric altitudes, ~25 km) at different latitudes, so as to vary the magnitude  of insolation modulation for relatively narrow latitudinal bands around the Earth, e.g., to reduce  heating of the tropics by preferential loading of the mid-stratospheric tropical reservoir with  insolation scatterer.


Indeed, scatterers of sunlight could be deployed at some latitudes to decrement insolation, while  scatterers of Earth-emitted long-wavelength infrared radiation (which effectively increment  insolation) could be deployed at other latitudes.39 Differential cooling and heating, respectively, of  underlying land-and-ocean latitudinal bands could thereby be accomplished. Furthermore, use of  scatterers of varying stratospheric residence times to simultaneously modulate insolation and  LWIR radiative losses in a specified latitude band might be employed to fine-tune, e.g., diurnal or  seasonal temperature variability.40




We have reviewed all the approaches known to us which appear to be of  practical  significance with respect to addressing the large-scale thermal effects of climate failure – both  global warming and Ice Ages – from the perspective of insolation modulation. In the course of this review, we have applied fundamental physical design principles to mass-optimize several previous proposals in order to enhance their practicality, and we have been able to remove more than an order-of-magnitude of superfluous mass from some earlier conceptual designs. Two insolation modulation systems which we have considered – quasi-resonant scatterers for intraatmospheric applications and the small-angle-scattering system for deep space use – are apparently novel. These involve total system masses of the order of 103-106 tons – which is 2-5 orders of magnitude less mass than that of the most interesting previous proposals. We conclude that the insolation modulation approach to prevention of climate failure is certainly technically feasible-inprinciple, and that the total costs-to-own its best examples may be de minimis.


We believe that research along several lines to study the deployment and operation in sub-scale – perhaps 10-3 of a full-scale, 1% insolation-equivalent system – of appropriate scatterers of sunlight is justified immediately by considerations of basic technical feasibility and possible cost-to-benefit.


Summary discussions such as those sketched here can only outline the directions to consider.  However, even very preliminary estimates of performance and practicality suffice to make us  optimistic about ultimate workability and utility.


Success can be expected to be more significant than merely counteracting the global climate  modifications arising from large-scale injection of greenhouse gases.


Straightforward  modifications of what we have discussed including the scattering not of incoming sunlight but of  the long-wavelength infrared radiation emitted by the Earth could be effective in preventing onset  of both ''little'' and full-sized Ice Ages.41


These may occur with little warning, seemingly at any  time, and could severely impact human affairs on notably brief time-scales.42


Indeed, the Earth's climate system may manifest finite-amplitude instability along several axes, with small  perturbations occasionally resulting in large shifts, some swiftly executed.43


Greenhouse warming of the Earth due to human activities is a possibility,44 moreover one for  which mitigative/remedial actions of the types proposed here can be at once deliberate and  effective.45 In contrast, Ice Age-severity cooling, another in the series of events that have occurred  quasi-periodically many times during the last 1.2 million years,46 is a practical certainty.


Moreover, a several-decade duration ''cold snap'' of Ice Age Maximum temperature-drop is known to have occurred in the Northern Hemisphere with essentially no warning during the last interglacial period, under precursor climatic conditions only slightly warmer than the present-day



Today, our scientific knowledge and our technological capability already are likely sufficient to provide solutions to these problems; both knowledge and capability in time-to-come will certainly  be greater. Whether exercising of present capability can be done in an internationally acceptable  way is an undeniably difficult issue, but one seemingly far simpler than securing international  consensus on near-term, large-scale reductions in fossil fuel-based energy production48 –  especially in a world exhibiting very large geographical and cultural differences in per capita  energy use, past, present and future.


We believe that, prior to any actual deployment of any scattering system aimed at full-scale 1%  insolation modulation, completely transparent and fully international research in sub-scale  could result in public opinion conducive to a reasonable technology-based approach

to prevention of large-scale climatic failures of all types. International cooperation  in the research phase, based on complete openness, is necessary and may be sufficient to secure the understanding and support without which any of these  approaches will fail.



The purpose of this paper is not to advocate definite solutions. It is only to augment  the scientific effort to find solutions of general acceptability and benefit.


The blame for ''bad weather'' may be too heavy for any human to bear. But, we  hope, thinking before acting might be acceptable. 48


While we claim no particular expertise in policy matters, we note expert opinion to the effect that  ''. . .stabilization [of atmospheric CO2 levels] requires an eventual and sustained reduction of emissions to  substantially below current levels'' [Wigley TML, Richels R and Edmonds J, Economic and environmental  choices in the stabilization of atmospheric CO2 concentrations, Nature 379, 240-3 (1996)] and we estimate  that ''substantial'' worldwide reduction in fossil fuel usage over the next several decades will not occur  without substantial likelihood of inducing major conflict.








1  Intergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change ,  JT Houghton, et al, eds. (Cambridge Univ. Press, Cambridge, 1996).


2    E.g., Hansen JE and Lacis AA, Sun and dust versus greenhouse gases: An assessment of their relative  roles in global climate change, Nature 346, 713-9 (1990).


3  See, e.g., MacCracken M, Geoengineering the Climate, Proceedings of the Workshop on the  Engineering Response to Global Climate Change , Palm Coast FL, June 1-6, 1991, and UCRL-JC-  108014 (1991), Lawrence Livermore National Laboratory, Livermore CA, 1991.


4  Ref. 1, ibid. Also, Wigley, TML, ''The Contribution From Emissions of Different Gases to the Enhanced  Greenhouse Effect'' in Climate Change and the Agenda for Research , T. Hanisch, ed. (Westview,  Boulder, CO, 1994) estimates the present-day excess positive radiative forcing to be ~2 W/m2, roughly  three-fourths of which is due to CO2 and one-quarter to CH4.


5  While it may be argued that uncertainties in the accuracy and fidelity of modeling tools are sufficiently great that they should not be relied upon to forecast reliably the effects of enhanced  sunlight scattering on the climate [or as does, e.g., Lindzen RS, Ann. Rev. Fluid Mech. 26, 353 (1994),  that they are fundamentally unverified predictive tools], it is these same modeling tools – not the  presently quite ambiguous observations of the actual climate – that are considered sufficiently robust to  motivate the present level of concern about man-made climate change.


6   One basic point-of-departure of the present paper from the large body of excellent previous work in  similar directions [see, e.g., work surveyed in Panel on Policy Implications of Greenhouse Warming,  Policy Implications of Greenhouse Warming: Mitigation, Adaptation and the Science Base , U.S.  National Academy of Sciences (National Academy Press, Washington, DC, 1992) and in Working Group  II, Climate Change 1995 Impacts, Adaptations and Mitigation of Climate Change: Scientific-  Technical Analysis , Second Assessment Report of the Intergovernmental Panel on Climate Change, RT  Watson, et al, eds. (Cambridge University Press, 1995)] is the emphasis which we have placed on  reducing deployed scattering system masses down to rather fundamental physical limits, consistent with contemporary engineering realities, in order to realize conceptual designs which may be the most practical to study in sub-scale and then to deploy in full-scale. We expect that all cost-effective mitigation efforts may require such optimization-focused design work. With respect to the fundamental utility of such efforts, we note that the section of the cited NAS report addressing ''mitigation'' options of the general type which we now consider concluded with the statement (on p. 460) ''Perhaps one of the surprises of this analysis is the relatively low costs at which some of the geoengineering options [aimed at offsetting global warming] might be implemented.''


7  See, e.g., testimony by Janet Yellen (Chair, Council of Economic Advisers to the President of the U.S.) before the House Commerce Subcommittee on Energy and Power, July 15, 1997, and press reports thereof (e.g., Fialka JJ, ''Effort to Curb Global Warming Is Tied To Higher Energy Prices in Two Studies,'' W a l l Street Journal , 16 July 1997, p. A2), in which estimates of ~40% increases in bulk fossil-energy prices in  order to attain price-rationing of fossil fuel-derived energy sufficient to suppress greenhouse emissions below ''dangerous levels'' were characterized as ''mid-level ones.'' These fractional price increases  translate to ~$4x1011/year world-wide, or ~$1011/year in the U.S.


8   All cost estimates in this paper should be regarded as scoping in character.


9   We note that efforts directed to cost minimization of mitigation technologies is specifically supportive of the UN Framework Convention on Climate Change, whose Article 3 states that  ''policies and measures to deal with climate change should be cost-effective so as to ensure  global benefits at the lowest possible cost.''


10  'A doubling of atmospheric CO2 during the coming century is IGCC-estimated to result in a 2.5 1.5o C change in mean temperature, while the mean temperature decrease from the present level which prevailed at the middle of the last Ice Age is estimated to be ~10o C.


11  The large variance in previous estimates of the amount of dielectric scatterer required for full-scale  insolation modulation appears to arise primarily from references to dimensionally non-optimized  scattering materials, which can be quite mass-wasteful. (The optimal choice is to match the size of the  scatterer to the reduced wavelength of the peak of the solar spectral radiance, and to maximize the  squared difference from unity of the scatterer's dielectric constant divided by its specific gravity.)


Also, it is necessary to select scattering materials which scatter only with quite small losses, in order to  minimize possibly undesirable heating of the portion of the atmosphere in which the scatterer is deployed.


12  Intergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change , JT Houghton, et al, eds. (Cambridge Univ. Press, Cambridge, 1996).


13   In evaluating the mass efficiency of any scattering unit or a scattering system, consideration of the spaceand  time-averaged optical-frequency current density in the matter comprising the system gives a generalpurpose  figure-of-merit for the mass budget of the system. When multiplied by the ‘optical leverage’ figure-ofmerit,  it generates an index of effectiveness of mass utilization in creating a sunlight-deflecting system. Note  that  this average (optical-frequency) current density-per-gram metric permits the comparison of metallic, dielectric and atomic-resonant scattering materials, moreover in a manner independent of particular scattering system geometry. It merely looks at the quantity which scatters the sunlight’s optical field: the space- and time-averaged current density driven up by the electric fields of the solar photons in the matter constituting the scattering unit, and the outgoing-wave optical-frequency fields which this density generates  Most dielectric materials of interest have optical dielectric coefficients e 2, and we generally must  consider (e-1)9x10-13 farads/cm. Available metals (e.g., Al) have electrical resistivities 3x10-6 ohm-c (taking some degradation from best bulk properties for very thin layers), i.e., conductivities s3x105 mho/cm. Optical frequencies of resonant transitions are ~1015 Hz, and these transitions correspond t electron travel over orbital distances ~3 , i.e., across atoms of ~3 diameter. Take a reference midvisible optical angular frequency of ~4x1015 sec-1. Then the optical current density I in a unit volume (whose greatest dimension is assumed to be l/2p) of such a dielectric at unit electric field strength is I~E/Z~wC, or 4x1015 x 9x10-13, or 3.6x103 amp/cm2. The current density in the same circumstances in   good metal is just I~sE~s, or 3x105 amps/cm2. Unit optical electric field strength of 1 V/cm corresponds to an optical flux of 10-4 W/cm2, so that sunlight’s intensity at 1 AU of 0.1 W/cm2 implies a mean frequency averaged optical electric field of ~30 V/cm. This optical field strength will drive ~30 transitions/sec in a typical full-strength-dipole atomic oscillator, as noted above; unit optical electric field strength thus will drive ~3x10-2 transitions/second in such an atom. Each atom is assumed to occupy a volume of ~3x10-23 cm3, so that ~1021 transitions/sec are driven by a unit-strength solar-spectral optical electric field in a cm3 volume of such material. Each such transition corresponds to a optical-frequency current of a single electron’s charge – 1.6x10-19 coulombs – moving through a distance ~3x10-8 cm every ~10-15 sec. This corresponds to a optical frequency current density of ~5x109 amp/cm2 (!).  For constituting scattering units, then, the relative figures-of-merit of best-available dielectrics, metals and resonant-scatterers are roughly 1:102:106.


14  The ''packaging mass overhead'' for quasi-resonant scatterers typically is much larger than that for metallic and dielectric scatterers, so that much of the former's huge intrinsic mass advantage is lost in scattering systems prepared for multi-year durability-in-service.


15  The Earth's surface is not considered for reasons of land-use and local microclimate impacts, while  the ocean surface poses stability/durability/navigation compatibility concerns, and tropospheric  residence times are not usefully long for the types of scattering systems which we consider.


16  We estimate a total cost of lifting mass into the stratosphere on wide-body commercial aircraft to be  ~$0.3/pound, whereas the current cost of putting a pound-mass of payload into low Earth orbit by  contract with commercial space-launch services is ~$5,000, for 5-15 ton payloads. Indeed, some types of  stratospheric deployment of oxide particulates – e.g., SO2 or Al2O3 – might be accomplished simply by  operating one or more well-engineered combustors – e.g., of elemental S or Al – at high-altitude, nearequatorial  ground-sites, from which stratospheric injection of warm gas is intrinsically advantaged.  (Combustor engineering would focus on mass-efficient, optimal-sized scatterer particle generation in the  vertically-directed exhaust, which likely would have a rocket nozzle character in order to facilitate  swift manipulation of the temperature and density of combustion products across usefully large ranges.)


17  Feely HW and Spar J, Tungsten-185 From Nuclear Bomb Tests As a Tracer For Stratospheric  Meteorology, Nature 188,1062-4 (1960); Telegadas K and List RJ, Atmospheric Radioactivity  along the HASL Ground-Level Sampling Network, 1968 to mid-70, as an Indicator of Tropospheric and Stratospheric Sources, J. Geophys. Res. 74, 1339(1969); Telegadas K, Report  243 (U.S. Atomic Energy Commission, Washington, DC, 1971);


18  Trepte CR and Hitchman MH, Tropical stratospheric circulation deduced from satellite aerosol data, Nature 355, 626-8 (1992); Trepte CR, Veiga RE, and McCormick MP, The Poleward  Dispersal of Mount Pinatubo Volcanic Aerosol, J. Geophys. Res. 98, 18563-73 (1993); Grant WB  et al, Use of volcanic aerosols to study the tropical stratospheric reservoir, J. Geophys. Res. 101,  3973-88 (1996).


19  Boering KA, Wofsy SC, Daube BC et al, Stratospheric Mean Ages and Transport Rates from Observations of Carbon Dioxide and Nitrous Oxide, Science 274, 1340-3 (1996).


20 See, e.g., Manabe S and Stouffer RJ, Century-scale effects of increased atmospheric CO2 on the oceanatmosphere  system, Nature 364, 215-7 (1993).


21  National Academy of Sciences, National Academy of Engineering and Institute of Medicine, Policy  Implication of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (National  Academy Press, Washington, D.C., 1992).


22 Since space-deployed scatterers could in principle last indefinitely, they have a several dozen-fold  advantage in time-integrated mass budget relative to stratospherically-deployed ones, which must be  re-constituted twice per decade. This durability-in-service saving trades off interestingly, albeit not  compellingly, against the present ~104-fold disadvantage in cost of transportation to deployment-site.


Space-launch service costs will have to decrease to a few dozen dollars per pound in order to become  competitive for present purposes. High-acceleration payload launchers are potentially of interest, as  scatterer payloads are likely to be very acceleration-tolerant – and perhaps can be segmented into quite  modest sizes and masses, as well.


23  Positioning a sunlight-shade or Snell's Law-refractor (i.e., a 1-D Fresnel phase plate) composed of  1014 gms of lunar glass near the Earth-Sun interior Lagrange point (L1) has been suggested in Early JT,   Space-Based Solar Shield To Offset Greenhouse Effect, J. Brit. Interplanet. Soc., 42, 567-9 (1989). The  present proposal positions a metallic small-angle-scatterer of sunlight of comparable area but ~105-fold  smaller mass Sunward of L1. A system of this type likely would be assembled quite close to the Earth,  e.g., in LEO, and then rapidly ''flown'' into its deployment location as a solar sail, exploiting its very  small mass-to-optical cross-section and its active radiation momentum management capabilities.


24  The total areal size of such a scattering system must remain the same as scattering systems deployed in close  proximity to the Earth, but its total mass potentially may be much more modest, as it must scatter sunlight only  through ~102-fold smaller angles. In particular, the conducting elements of the scattering units need carry only  ~102-fold smaller currents, so that they can be of 102-fold smaller cross-sectional area (for a given optical  electric field strength, which is effectively constant for all scattering system deployments in ~1 AU sunlight),  and thus can have 102-fold smaller mass. The mass of such a scattering system can be estimated by noting that  the total area must be ~1% of the Earth’s disc, while it must have good metallic conductors of ~3 x10-11 cm2   cross-sectional area (~600 thick by ~1000 in transverse width) spaced every 100l/2~3 x 10-3 cm in both  transverse dimensions. If the density of the conductor is taken to be 3 gm/cm3 (e.g., Al), then the mass density  is ~ 1.2x10-7 gm/cm2, and the total mass is just this density times the 1% of the Earth’s disc area of ~1.2x1018  cm2 which is to be sun-shaded, or ~1.2x1016 cm2: ~1.4x109 gms. The total mass of the ideal scattering system  thus is ~1.5x109 gm or 1,500 ton – plus whatever overhead mass is required to deploy and operate these  (literally) diaphanous scattering screens; the actual scattering system has about twice this mass – ~3400 tons –  for reasons which are discussed below.  The possibilities of incrementing or decrementing terrestrial insolation in manners which are both geographically  and spectrally selective by use of such a distant scattering system seem obvious.


25  The momentum carried by sunlight, while very small, is assuredly not negligible: at 1 AU, it amounts to ~4x10-5 dynes/cm2. Over a day, this accumulates to ~3 dyne-sec/cm2 and, over a year, to ~103 dynesec/ cm2 of impulse fluence. If exactly backscattered (toward the sun), only radial momentum is gained by the scattering unit; however, if deflected at any angle <p, it will also impart angular momentum to the scatterer: the Poynting-Robertson effect. Since scattering units typically have an areal mass <<10-3 gm/cm2,  they can undergo many gee-sec of acceleration even during a day, and thousands of gee-second annually (i.e.,  can see Dv of >>10 km/sec). Such velocity changes in essentially all orbits of interest are almost invariably  intolerable, and either deployment or operational means must be devised to avoid them.


Unwanted angular momentum may be readily jettisoned by time-varying the orientation of a relatively very  small reflector capable of scattering sunlight through a high mean angle. However, the radial momentum

imparted by sunlight must be sustained for intervals as long as a half-year, until the Earth’s orbital motion

about the sun will dissipate it; in most cases-of-interest, this is a very long time-interval over which to tolerate  radial momentum absorption. However, if the scattering unit is deployed sunward of the Earth’s orbit but  quasi-orbits the sun at the same angular rate as does the Earth (so as to continually shadow-shield the  designated fraction of the Earth), then it may in principle sink as large fraction as may be desired of the

incident sunlight’s radial momentum into the solar gravitational field, i.e., it may position itself closer to the  Sun than the L1 point so as to sink the scattered radiation's radial momentum into the solar gravitational field;  the smaller its areal mass density, the more Sunward of L1 it must position itself. Now the radial radiation  momentum flux is ~4x10-5 dynes/cm2, only 5x10-5 of which is deposited on the scatterer as it deflects the  incident sunlight by 10-2 rad, so that the mass density corresponding to deposited radiation radial momentum balancing the gravitational ‘deficiency’ is ~1.5x10-7 gm/cm2. This is only a few times greater than the areal  mass density of the ideal scattering system positioned at L1. Thus, by moving ~1% inward from L1 – and  increasing the total area of the scattering system by ~2-fold, in order to continue to shadow the Earth while  moving further Sunward from it, we can simultaneously sink the residual radial momentum of the scattered  sunlight into the solar gravitational field and maintain the desired shading of the Earth.  Residuals in both radial and angular momentum flux from solar gravitation and from the Poynting-Robertson  effect, respectively, are probably best sunk by sunlight retroreflection action performed by a relatively small area (i.e., 0.3%) of high-angle reflector. However, this necessarily will have a ~50X greater areal mass-density  than does the small-angle-scattering screens of greatest interest, so that a ~15% overhead cost is thereby  incurred by the scattering system.



26  Trepte CR and Hitchman MH, Tropical stratospheric circulation deduced from satellite aerosol data, Nature 355, 626-8 (1992); Trepte CR, Veiga RE, and McCormick MP, The Poleward Dispersal of Mount  Pinatubo Volcanic Aerosol, J. Geophys. Res. 98, 18563-73 (1993); Grant WB et al, Use of volcanic  aerosols to study the tropical stratospheric reservoir, J. Geophys. Res. 101, 3973-88 (1996).


27  See, e.g., Taylor KE and Penner JE, Response of the climate system to atmospheric aerosols and greenhouse gases, Nature 369, 734-7 (1994); Chuang CC, et al, An Assessment of the Radiative Effects of  Anthropogenic Sulfate, J. Geophys. Res. 102, 3761-78 (1997); and Intergovernmental Panel on Climate  Change, Climate Change 1995: The Science of Climate Change , JT Houghton, et al, eds. (Cambridge  Univ. Press, Cambridge, 1996).


28  See, e.g., Dyson FJ and Marland G, Technical fixes for the climatic effects of CO2. Workshop on the  Global Effects of Carbon Dioxide from Fossil Fuels. USDoE Report CONF-770385 (USDoE, Washington  DC, 1979); Budyko MI, The Earth's Climate: Past and Future. (Academic Press, New York, 1982);  Broecker WS, How to Build a Habitable Planet (Eldigio Press, Palisades, NY, 1985).


29 In this approach, advantage is taken of the stronger Rayleigh scattering of the shorter wavelengths of optical  sunlight as compared to those of terrestrial thermal emissions. A performance-optimized scattering system of  present interest consists of a world-wide, ultra-thin cloud of dielectric particles, each of 0.05-0.1 mm diameter, deployed in the Earth’s stratosphere. The Rayleigh scattering cross-section S of a particle of volume V whose  polarizability a=3(e-1)/4p(e+2) for photons of angular frequency w is S = (8p/3)(w/c)4a2V2. [See, e.g.,  Landau LD and Lifshitz EM, Electrodynamics of Continuous Media (Pergamon, Oxford, 1984).] For such a spherical particle whose diameter is equal to a quarter-wavelength of 0.5 mm radiation which has a dielectric  coefficient e~1.7 (e.g., a water-rich microdrop of refractive index ~1.3), S may be estimated to be ~5x10-12  cm2. Thus, ~1028 of these particles, uniformly distributed in the stratosphere, would be required to scattershield  the 5x1018 cm2 of terrestrial surface with optical density 10-2 at the sub-solar point. (While only the  bluer portion of the visible spectrum, l0.5 mm, would be scattered with this cross-section, the effective scattering in the earlier half of local morning and the latter half of local afternoon in the tropics and all day at  higher latitudes is considerably greater than this value, due to the greater atmospheric path traversed by  incident sunlight, so that the net required energetic effect of panchromatic scattering is attained, in first  approximation). Each of these scattering particles may be estimated to have an average mass of ~10-15 gm  (i.e., be ~0.12 mm in diameter and of unit density), so that the required quantity of them would imply a total  mass of the scattering system of ~1013 gms, or 10,000,000 tons. A mean stratospheric lifetime of each scattering particle of 5 years would imply a required injection rate of 2x106 tons annually, or a time-averaged  injection rate of 60 kg/second, which is feasible to maintain e.g., with highly parallel exercising of existing fine-aerosol-dispersion technology.


30 E.g., Brady BB, Fournier EW, Martin LR and Cohen RB, Stratospheric Ozone Reactive Chemicals Generated by Space Launches Worldwide. Aerospace Report No. TS-94(4231)-6. (The Aerospace  Corporation, El Segundo, CA, 1994).


31  Because of the strong dependence of the Rayleigh scattering cross-section on e, [(e-1)/(e+2)]2, when e is  not much greater than unity, it would be more somewhat mass-efficient to deploy alumina microspheres,  instead of SO2/SO3/H2SO4 ones: the significantly greater density of alumina (~3.5 gm/cm3) is more than  compensated by its greater dielectric coefficient, 3.10. (Alumina, like sulfate, is ubiquitous in the terrestrial  biosphere, and its stratospheric injection seemingly poses no significant environment issues. However, its  refractory character makes it more challenging to deploy, at least in some modes. A notable exception might  be a high-average-power combustor of aluminum powder deployed at a high-altitude, near-equatorial groundsite,  one whose carefully-engineered exhaust-stream could hydrodynamically penetrate the overlying tropopause and inject 0.1 mm-diameter alumina particles into the mid-stratosphere with high mass-efficiency. ) Worth noting in passing is the fact that the annual tonnages of either sulfur or aluminum oxides presently  proposed for stratospheric deployment are tiny compared to the quantities of these materials which are either  naturally lofted into the atmosphere (e.g., by dust storms) or are already injected by human activities (e.g., burning of fossil fuels of all types).

32Absorption of solar photons with thermal (quasi-isotropic) re-radiation of the absorbed energy may be reasonably mass-efficient in some circumstances, e.g., when using photoelectric-effect (bound-free) absorbers.


For instance, the photoelectric cross-section of good metals just above the work-function-edge for near-optical  radiation is ~3x10-17 cm2, and is ~3x10-18 cm2 at three times this photon energy (i.e., a 107- and 108-fold reduction below the line-center bound-bound resonant cross-section, corresponding to the relative final-state  phase-space volumes of the two types of transitions). For scattering unit design purposes, an effective  photoelectric cross-section of 10-17 cm2 is a reasonable estimate (taking into account finite solar spectral  width requirements and considering that the solar spectral intensity falls rapidly above even the lowest-energy  single-element photo-edge, that of Cs metal at a photon wavelength ~0.37 mm), and assuming that all photons  with energies above the photo-edge interact with this cross-section. Likewise, note that a multiple alkalimetal  alloy system can lower this photoedge to an effective 2 eV threshold (e.g., as the tri- and quad-alkalimetal- alloy photocathodes of commercial photodetection devices routinely do). A sheet of this material of a  few dozen atoms thicknesses – ~100 – would then represent an extinction length for blue-green photons, and  would have a mass-density of 3x10-6 gm/cm2. Then, assuming that scattering units comprised of such 10-6  cm-thick sheeting would absorb half the incident solar spectrum and that the corresponding scattering system would sun-shield 0.01 of the Earth’s disc continuously, the minimum mass-budget of an efficiently implemented scattering system comprised of such ultra-thin sheets of photoelectric absorber/thermal re-radiator would be (2)(10-2)(1.2x1018 cm2)(3x10-6 gm/cm2)~7x1010 gms ~ 70,000 tons. If deployed either in the  atmosphere or in LEO, individual scattering units could be effective only 50% of the time, so that 140,000  tons would be required to fully implement such a scattering system.


The constituent materials of every efficient photoelectric absorber for solar-spectrum radiation inherently are  readily oxidizable, particularly in the highly (photo)reactive upper atmosphere, so that only LEO deployment  of such systems appears feasible – unless two-fold mass penalties are paid for protective jacketing, e.g., SiO. In addition to having a relatively large mass budget for a space-deployed system, such an absorption/reradiation module in space would have the active momentum-management requirements characteristic of any space-based scattering system; this additional complexity would trade off against the multi-decade lifetimes  (and thus far lower annualized costs) implicit in LEO deployment.


33  The scattering system of this approach is comprised of a set of single-layer screens which scatter incident  sunlight through ~p/2 rad, and whose conducting elements must be ~1 skin-depth in longitudinal extent, or  200 , by ~300 in transverse width, spaced at 3x10-5 cm distances in both dimensions; packaging mass for protection against stratospheric oxidation would be comparatively small, if metal having a thin protective  oxide, such as Al, is used. Such a high-angle scattering system would have electrically conducting ‘wire’ elements massing a total of ~15,000 tons; averaging random orientation and diurnal availabilities drives this up by ~2p, to ~90,000 tons. (Overhead mass for deployment and operation would, of course, be additional,  although, in fractional terms, it would be smaller, as the scattering screens would be much stronger, due to  their thicker conducting members.)


34  Thin-walled helium-filled balloons are routinely employed to lift multi-ton payloads to 30-50 km altitudes,  and maintain them there for multi-day intervals (limited mostly by the durability-against-photooxidation of the balloon's plastic wall). It thus seems entirely reasonable to consider ultra-thin metallic-walled balloons, partially inflated with a suitable lifting-gas, e.g., H2, as sunlight-backscattering ‘stratostats’, for they can be expected to have very long stratosphere residence times (probably limited by micrometeoritic puncture-rates).  A wall thickness of ~0.02 mm of e.g., Al, is ~1 skin-depth at the 8x1014 Hz frequencies characteristic of midvisible solar radiation and thus suffices to (predominantly) backscatter the blue end of the incident solar spectrum; however, it is 0.2 skin-depth at 2-3x1013 Hz and thus is nearly transparent to Earth thermal emissions in the 8-14 mm wavelength atmospheric pass-band. Each of these balloons thus constitutes a ‘micro anti-greenhouse’: it passes space-directed low-frequency photons emitted by the Earth but reflectively backscatters-into-space the high-frequency ones incoming from the Sun.


Since the thickness of the balloon's metallic wall is determined by electrodynamic skin-depth considerations,  the balloon diameter is selected so as to position it at the desired stratospheric altitude when it is fully  expanded, subject to the constraint that it must be buoyant at the launch-site altitude. An Al wall thickness  of 0.02 mm, a balloon radius of 4 mm, a fully-inflated volume of 0.25 cm3, a mass of 10-5 grams and an optical scattering cross-section (between ~0.25 and ~1 mm wavelength) of 0.5 cm2 correspond to a balloon  deployment altitude of 25 km. (Likely different choices of deployment altitude involve 2-fold adjustment of the balloon diameter while keeping its wall thickness invariant.) Since it is required to scatter-shield  ~5x1016 cm2 (recalling that 1% of the Earth’s total area is to be so sun-shielded), ~1017 of these ‘‘selflofting  blue-UV chaff’’ objects will be required, with an associated mass of ~1012 grams, or 1,000,000 tons.


If they are deployed uniformly over a 50 year interval (i.e., quickly compared to a doubling-time of  atmospheric CO2), then ~2x1010 grams/year – ~6x107 balloons/sec, or 108 cm2/sec, or 1 hectare/sec of  these scatterers – must be created and deployed. (Newspaper printing-press technology seems a particularly likely basis for ‘printing’ these balloons on a H2-loaded Al bilayer-sandwich substrate. High-circulation major newspapers print ~106 copies of a ~10-2 hectare-area newspaper daily, for a round-the-clock mean rate of  ~0.1 hectare printed per second; the peak printing rate is ~0.3 hectare/sec. Thus, the time-averaged balloon  creation-and-deployment rate is comparable to the peak output rate of a large newspaper printing-facility.)


This deployment rate will require 4.5x107 lbs/year of Al-as-foil (at a current annual cost of $40 M for ingot, likely a few times this for mass-produced ultra-thin foil), and a ~15-fold lower mass of several-fold cheaper H2 gas: ~1,500 tons. These ‘‘self-lofting blue-UV chaff’’ balloons constitute one of the lowest-technology approaches, in that, once created and released at ground-level, they self-deploy and automatically-operate. The feedstream requirement is only ~1% of the current total annual production of aluminum metal. Since 6% (by mass) of the Earth’s crust is Al, the quantity of ultra-thin Al foil drifting down from the skies during the mid-21st century wouldn’t discernibly impact the environment, the more so as such very thin foils would photooxidize to the hydrated oxide quite rapidly in the wet troposphere: at ~290oK, Al exposed to dry oxygen forms a stable oxide  layer of 10-30 thickness, but wet oxidizing atmospheres can form >1 mm hydrated oxide layers on bulk Al in a matter of hours – and hydrated AlOx films crumble to powder very rapidly. (It therefore should be feasible to deploy such ‘‘self-lofting blue-UV chaff’’ from desert sited-factories – or from high-latitude ground-sites – and have them quickly rise into the stratosphere intact, while they would invariably be wet-oxidized into invisiblyfine hydrated alumina dust during their slow, end-of-life descent through the troposphere, obviating ''littering'' concerns raised in connection with return-to-Earth of far larger, non-optimized balloons). It's obviously feasible to employ a metallic overcoating of the required thickness applied to an appropriate thickness of a stronger dielectric, e.g., a modern plastic film such as polyaramid or polybenzoxazole, in order to gain durability. (The metallic overcoating would then serve its sunlight-scattering function and would also protect the super-strength plastic film from stratospheric photochemical damage. )


This particular insolation-scattering solution has a much less pronounced dispersion than does the Rayleigh  scattering one: it only scatters more strongly with the half-power of the photon frequency (rather than the

fourth). Its two distinctive features are that it's an approach which manifestly can be implemented without  any human machinery leaving the surface of the Earth and it's a scattering system which quickly assumes and thereafter maintains a designed-in altitude in the stratosphere, e.g., so that its meridional transport features will be known a priori. Moreover, its effectiveness could be nearly doubled by applying a somewhat heavier  coating of LWIR-reflecting material (e.g., a small band-gap semiconductor) to one of the two sheets of film  used in balloon manufacture, so that the upper metal-coated hemisphere of each balloon would scatter sunlight back into space, while its lower semiconductor-coated hemisphere would scatter Earth-emitted LWIR radiation back Earth-ward.


35 Resonant absorption and (quasi-isotropic) fluorescent re-radiation of solar (near-)optical photons is an incompletely-compelling candidate mechanism for scattering units primarily because even atoms with fulldipole-  transition oscillator strengths in the (near-)visible spectrum absorb with maximum radiative strength  only over relatively very small wavelength intervals (Dw/w ~ 10-7) and secondarily because such strong  absorption is typically seen only in metallic gases (and similarly low-density, effectively-collisionless  circumstances, in which the natural width of the transition is a regrettably large fraction of its full width).   However, the intrinsic mass efficiency of scattering units comprised of a set of resonant absorbers could be  expected to be extremely high, and it likely is worth considerable applied photophysics experimental effort to  spectrally broaden such resonant transitions to cover ~2% of the solar spectrum. Normal matter never works  harder in sustainable electromagnetic terms than when it’s scattering radiation on a full electric-dipole-strength  transition at the maximum rate given by that transition’s Einstein A coefficient (~108 sec-1, for the full-dipolestrength  optical transitions of present interest): i.e., an alkali-metal atom (e.g., Li) will process ~3x10-11 W of  resonant radiation (108 photons/sec, each of 1.8 eV or 3x10-12 ergs energy) for a mass-cost of 10-23 grams  (e.g., an Li6 atom, scattering on its 6708 resonant transition) – which corresponds to a specific scattering  power of 3x1012 W/gm(!). If it were feasible to exercise matter this vigorously in scattering units, then the  working-mass of an entire scattering system would be only ~1 kg. However, the 1018 photons/cm2-sec of  (near-)optical solar flux at 1 AU only present ~1011 photons/cm2-sec within the ~30 MHz absorption-line of a  typical full electric-dipole-strength optical resonant transition, which has a characteristic peak absorption  cross-section s ~ 3x10-10 cm2 (i.e., s ~ l2/4p, with l ~ 6x10-5 cm). Thus, such an atom only processes ~30  photons/sec when hung-in-space in 1 AU sunlight, i.e., it works at only 3x10-7 of its maximum feasible  scattering-rate. Instead of 1 kg of scattering unit-mass, 3x109 grams, or 3000 tons, would have to be employed  – moreover, as a scattering-disc of free atoms positioned on the Sun-Earth line. Nonetheless, this is a small  mass for the ''active'' component of a full-scale insolation modulation system; it motivates serious  consideration of options based on resonant scattering physics.


36 Intercalation-loading any of the new nanometer-scale poly-carbon structures with fully-enclosed interiors (e.g.,  C60 buckyballs, graphitic nanotubes, etc.) with alkali metal atoms (e.g., as has already been done, using K and  Rb, in order to generate moderately high-temperature superconductors) might constitute a useful  [packaging+spectral broadening] means which would simultaneously protect the contained species from  oxidation and would appropriately broaden its resonant transition. At least some of these structures, e.g.,  nanotubes, can be sized to ''semi-snugly fit'' the atom being so caged, so that its outer-electronic-shell-based  resonant transition could be broadened to the required extent by either a RMS environmental variation from one  caged atom to another of ~4x10-3 Ry or an equivalent variability-in-time of the caged atom’s environment.  If the scattering system so constituted were to be made of ~unity optical thickness on these broadened resonant  transitions, then this unit-optical-depth system would effectively reflect ~50% of the incident in-band solar  radiation back into space; thus, there is a requirement for an effective width of 2% of the solar spectrum near  the Planck peak for a full-scale insolation modulation system. These scattering units might be positioned in  the stratosphere; however, atmospheric positioning imposes a diurnal efficiency penalty, relative to positioning  in a scattering-disc on the Earth-Sun line. Also, since a cage for a Li6 atom may have a mass ~120 times that  of the caged atom itself, the 3000 tons of full electric dipole strength optical scatterer-in-a-disc intrinsically  required to scatter 2% of the solar spectral radiance at 1 AU might be increased to ~750,000 tons with the  concatenated diurnal efficiency penalty and the [packaging+spectral broadening] ''mass overhead'' of a C60  buckyball.



A distinctly different alternative route to realizing such high-strength, broadband quasi-resonant scattering of  (near-)optical radiation is based upon organic dye molecules which, in solution, exhibit rather ideally the high   oscillator strengths and broadband absorption desired in the present application. While isolated dye molecules  exhibit fine structure-rich absorption/fluorescence on their electronic transitions in the (near-)visible spectrum  due to concatenated effects of myriad rotational and vibrational splittings, solvent-broadening effects smear  the spectrum of dye molecules-in-solution into a featureless continuum – without significantly diminishing the  frequency-integrated radiative strengths of the principal interband electronic transitions. Highly concentrated  gels of such dyes – e.g., ones derived from low vapor pressure solvents which are ''glasses'' at stratospheric  temperatures, such as the higher molecular weight perfluoroalkanes, may be expected to serve aptly as  scattering units of still quite high mass-efficiency, ones for which the corresponding scattering system mass  may be not much greater than that whose scattering units are caged alkali/alkaline-earth metal atoms: ~106  tons. Materials such as Al or Si, which auto-coat with durable, oxygen-impervious, high-integrity oxide-skins  of only a few monolayers thickness, might be aptly employed in lieu of graphitic nanotubes for transparently  jacketing such dye-loaded-glasses against stratospheric conditions.


Alternatively, use of  perfluorohydrocarbons as the dye-bearing material may obviate the need for any protective jacketing, as well  as simplify the mass-production of such scattering units. The corresponding scattering systems may be the  ones of choice within this preferred class of quasi-resonant scattering units, simply because the dye-bearing  fluid could be stratospherically dispersed from an airplane tank as a suitably fine aerosol, the individual nanoparticles  of which would quick-freeze at stratospheric temperatures and thereupon become photochemically  inert over multi-year time-scales.


While significantly greater total mass might have to be deployed to  constitute such a scattering system, the simplicity and relatively low technical risk with which the system  could be created and deployed might be an overriding consideration.  How much total mass would be required for each of these alternatives can be reliably estimated from  spectroscopic measurements of such metal atom- or dye molecule-loaded nano-containers, i.e., what the  absorption spectra and oscillator strengths of the as-packaged materials actually are.


After such scoping-level  measurements, it will likely be productive to prepare ton-quantities of such quasi-resonant scattering material,  disperse it into the stratosphere and then measure the residence time and global distribution of the scattering  units, e.g., with ground-based range-gated, spectrally-resolved lidar systems. Such modest quantities should be  very readily prepared and dispersed, and can be argued from first principles to not have any discernible global  effects.


Such measurements, in turn, should provide a reliable basis for predicting the effects of 103-104  greater quantities of such scattering units, i.e., their utility as a full-scale insolation modulation system. It’s  also worth noting-in-passing that the resonant transitions chosen to be intercalation-broadened – or those  glassed-in dyes chosen to absorb-&-fluoresce – likely could be selected to lie exclusively in the near-UV or -  IR portions of the solar spectral radiance on the Earth’s atmosphere, so that the resulting ‘spectral notching’ of  sunlight as seen at or near the Earth’s surface would be invisible to people, just as the near-IR solar spectral  notchings due to absorption by atmospheric H2O already are. The as-perceived ‘‘environmental impact’’ of  such spectrally-notched insolation subtraction would thereby be essentially zero.


37 We estimate that an as-deployed caged metal atom-based full-scale scattering system may have a total  mass of 7.5x105 tons. Due both to higher molecular weight and lower unit radiative strength of dye  molecules relative to Li atoms – offset somewhat by proportionately lower ''mass overhead'' – an organic  dye-based system may have a total system mass of ~106 tons. (In spite of its greater mass, a dye-based  system may be preferred because its spectrally broadened features are readily attained and maintained. )  The $500 million deployment cost of such a dye-based system would likely be considerably less than its  materials cost, which we estimate as ~$1 billion.


38 We term this approach ''quasi-resonant'' because it involves spreading out the exceedingly intense,  pure hue arising from resonant absorption and fluorescent emission of light by selected organic  molecules into broad bands of still quite intense absorption-fluorescence by these molecules when  intimately mixed with other materials. We provisionally choose the perfluorohydrocarbons as the ''other materials,'' because their durability in the even more demanding solar UV/EUV and atomic  oxygen environments of low Earth orbit has been demonstrated on the Long-Duration Exposure Facility.


We propose to load one or more of these dyes, selected for high radiative strength in either the near-  UV or near-IR spectral bands, in a high-boiling fluorocarbon liquid and then to disperse ultra-small  droplets of this liquid in the stratosphere, where they will freeze and thereafter provide effective  protection against oxidation of the dye molecules which they carry – in addition to not perturbing  stratospheric photochemistry.


39 Such a system might have to be comprised of scatterers designed to have stratospheric residence  times significantly less than five years, so that they would e.g., vertically, exit the stratosphere before they had migrated in latitude to unacceptable extents. It appears feasible-in-principle to  exploit the stiffly altitude-dependent meridional transport in the lower-to-middle stratosphere to  move scattering material poleward from equatorial deployments at most any desired mean rate  between 10 yr-1 and 0.2 yr-1 .


40 The significance of design optimization to minimize deployed system masses becomes more apparent  when considering deployment options featuring post-deployment operational intervals much less than  the greatest attainable (~5 years). Deployment mass-rates and costs may   become quite burdensome for,  e.g., single-month residence times just above the tropopause, unless near-optimally designed scatterers  are employed.





41  Both types of space-based scattering systems – high-angle-scattering ones in LEO and small-anglescattering  ones on the Earth-Sun line – may be used to scatter sunlight onto the Earth that otherwise  would have passed nearby it. ''Self-lofting blue-UV chaff'' could be transformed into ''self-lofting  LWIR chaff'' by replacing its metal shell with a semiconductor one chosen to have a direct (for reasons  of mass efficiency) band-gap of a few tenths of an eV – energetic enough to reflect LWIR radiation  coming up from the Earth's surface and lower atmosphere but of sufficiently low energy to pass  virtually all incoming solar photons without significant attenuation (e.g., InSb). The very low mass  small-angle-scattering system in deep space can be readily converted to direct additional sunlight onto  the Earth from a position slightly offset from the Earth-Sun axis, rather than scatter it away from an   on-axis location.


42 See, e.g., Greenland Ice Core Project (GRIP) Members, Climate instability during the last interglacial  period recorded in the GRIP ice core, Nature 364, 203-7 (1993) for a discussion of observed ''few decade to  centuries'' large-amplitude temperature variability of the Northern Hemisphere during the   interglacial period immediately preceding the present one. A repetition of the 7-decade ''cold snap''  of the ~14o C peak magnitude inferred from examination of the Greenland cores might reduce arable   land world-wide by – at the very least – 1% per year following its abrupt onset, a rate which would  result in large-scale famine on single-decade time-frames, as planetary food reserves become  exhausted. (The inferred time-scale of the associated shifts in atmospheric circulation occurring  during these ''fast'' events is 1-3 decades, which is comparable with that inferred to have occurred  during termination of the most recent Ice Age. )


43 E.g., Broecker WS, Cooling the tropics, Nature 376, 213-4 (1995) concludes ''The palaeoclimate record  shouts out to us that, far from being self-stabilizing, the Earth's climate system is an ornery beast  which overreacts even to small nudges.''


44 Hasselmann K, Are We Seeing Greenhouse Warming? Science 276, 914 (1997).


45  See, e.g., Wigley TML, Richels R and Edmonds J, Economic and environmental choices in the  stabilization of atmospheric CO2 concentrations, Nature 379, 240-3 (1996), where it is demonstrated  that ''business as usual'' in fossil fuel-based energy production for the next three decades does not doom  the Earth to global warming in excess of 2oC, if reasonable measures are taken thereafter. We note that  insolation modulation systems can be studied-in-subscale and then deployed on single-decade timescales,  short compared to the onset of any global warming (or cooling) currently considered.


46 Imbrie J et al., On the Structure and Origin of Major Glaciation Cycles 2. The 100,000-Year Cycle,  Paleoceanography 8, 699-735 (1993).


47 Greenland Ice Core Project (GRIP) Members, Climate instability during the last interglacial  period recorded in the GRIP ice core, Nature 364, 203-7 (1993)


48  While we claim no particular expertise in policy matters, we note expert opinion to the effect that   ''. . .stabilization [of atmospheric CO2 levels] requires an eventual and sustained reduction of emissions to   substantially below current levels'' [Wigley TML, Richels R and Edmonds J, Economic and environmental  choices in the stabilization of atmospheric CO2 concentrations, Nature 379, 240-3 (1996)] and we estimate  that ''substantial'' worldwide reduction in fossil fuel usage over the next several decades will not occur  without substantial likelihood of inducing major conflict.


Advanced technology paths to global climate stability- Energy for a greenhouse planet
MI Hoffert, K Caldeira, G Benford, DR Criswell, C … - Science, 2002 - nrel.gov
... in the latest “Sum- mary for Policymakers” by the “Mitigation” Working Group ... Thus,
the decarbonization of fuels alone will not mitigate global warming. ...
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Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet
I Efficiency - Science, 2002 - sciencemag.org
... No discussion of global warming mitigation is complete without mentioning
"geoengineering" (78, 79), also called climate engineering or planetary engineering ...
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The cumulative effects of climate warming and other human stresses on Canadian freshwaters in the …
DW Schindler - Can. J. Fish. Aquat. Sci, 2001 - article.pubs.nrc-cnrc.gc.ca
... grasses embedded in lake sediments (Teller and Last ... are generically referred to as
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Earth systems
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DW KEITH - ucalgary.ca
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... 1989; Seifritz, 1989; Flannery et al., 1997; Teller et al ... pronounced in high latitudes
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IAF-02-U. 1.01 Earth Rings for Planetary Environment Control
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Annual Review of Energy and the Environment
DW Keith - Annual Review of Energy and the Environment, 2000 - energy.annualreviews.org
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Systems-Level Considerations for Mitigating the NEO Impact Hazard
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Risk Management of Planetary Defense
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‘Update on the progress of the Brazilian wood BIG-GT demonstration project
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II II GG GG CC CC PP PP II II tt tt aa aa ll ll ii ii aa aa nn nn RR RR ee ee pp pp oo oo rr rr tt …
S Board - iugs.cnr.it
... help respective governments, earthquake engineering and disaster mitigation planning
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Interim Report IR-04-055
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surface warming (which influences ... 2 (Teller et al., 2002) in the topographic ...
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Running Head: What Would Have Happened?
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Intergovernmental Panel on Climate Change
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The role of the oceans in climate
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mitigation of climate ... level rise than it will on reducing climate warming. ...
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CL Authors, L Authors, C Authors - amap.grida.no
... Social acceptance at the local, national, and global scale may also influence
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Earth systems engineering and management
B Allenby - IEEE Technology and Society Magazine, 2001 - ieeexplore.ieee.org
... is limited to the individual engineer and a particular artifact (eg, an accurate
timepiece), it does not make sense to say that the global restructuring of ...
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S Houser, V Teller, M MacCracken, R Gough, P … - Native peoples - sedac.ciesin.org
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JJ MAGNUSON, KE Webster, RA Assel, CJ Bowser, PJ … - Hydrological Processes, 1997 - doi.wiley.com
... the potential changes in aquatic ecosystems related to climatic warming (JB Smith ...
BP, when the Laurentide Ice Sheet migrated northwards (Teller and Thorliefson ...
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Post-Hypsithermal plant disjunctions in western Alberta, Canada
WL Strong, LV Hills - Journal of Biogeography, 2003 - blackwell-synergy.com
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Climate stabilisation and “dangerous” climate change: A review of the relevant issues
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Hydrology and Water Resources
L Authors, C Author - duet.cfr.washington.edu
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it has been widely recognized that changes in the cycling of water between ...
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Inevitable Surprises
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Disasters as Extreme Events and the Importance of Networks for Disaster Response Management
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Department of Civil Engineering
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... p. 13 Mitigation techniques? ... With respect to the much-debated issue of global warming,
it is key to understand how ice sheets and climate interact (Oppenheimer ...
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Assessment of Glacial Hazards based on Remote Sensing and GIS Modeling
SP Geographie - dissertationen.unizh.ch
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CR Chapman - Case Study for, 1999 - boulder.swri.edu
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what is, obviously, a global issue ... 1994, Morrison & Teller 1994); and self ...
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IGCP Italian Report
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Forecasts and outcomes
A Skaburskis, MB Teitz - Planning Theory and Practice, 2003 - taylorandfrancis.metapress.com
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A Thesis, L Liu, S Regina - env.uregina.ca
... 1992 & 1994). These methods have been extensively applied for the planning of
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