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The myriad proposals to geoengineer the climate may usefully be classified by their mode of action (Figure 10.3). The root division is between alteration of

Figure 10.3 Taxonomy of climate modification. The taxonomy organizes the modes of climate modification - equivalently, possibilities for anthropogenic forcing of climate - both deliberate and inadvertent. The modes of climate modification listed as geoengineering have been proposed with the primary aim of climate modification. Note that some modes appear both as geoengineering and as inadvertent climate modification.

Figure 10.3 Taxonomy of climate modification. The taxonomy organizes the modes of climate modification - equivalently, possibilities for anthropogenic forcing of climate - both deliberate and inadvertent. The modes of climate modification listed as geoengineering have been proposed with the primary aim of climate modification. Note that some modes appear both as geoengineering and as inadvertent climate modification.

radiative fluxes to regulate the net thermal energy input and alteration of the internal dynamics that redistribute energy in the climate system.8 The overwhelming majority of geoengineering proposals aim to alter radiative energy fluxes, either by increasing the amount of outgoing infrared radiation through reduction of atmospheric CO2, or by decreasing the amount of absorbed solar radiation through an increase in albedo. With more generality we subdivide alteration of radiative energy fluxes into alteration of thermal (long-wave) radiation or solar (short-wave) radiation. Proposals to alter internal dynamics have focused on the oceans or on surface/atmosphere interaction, and are subdivided accordingly in Figure 10.3. Here we focus on the means of climate modification in general - either deliberate or inadvertent - whereas the categor

8 Some treatments use a forcing/feedback division in place of the energy-inputs/internal-dynamics division used here (Watson, Zinyowera et al., 1996), however this is not as precise since internal feedbacks (e.g., ice-albedo feedback) modify the energy input.

ization illustrated in Figure 10.2 describes responses to anthropogenic climate change. Figure 10.3 emphasizes this point by including a classification of human impacts on climate to stress the strong relationship between impacts and geoengineering.

With respect to geoengineering aimed at countering CO2-induced global change, there is a fundamental difference between controlling CO2 and controlling its effects. Albedo modification schemes aim to offset the effect of increasing CO2 on the global radiative balance, and thus on average surface temperatures; climatic change may, however, still occur owing to changed vertical and latitudinal distributions of atmospheric heating (Section 10.4.2.1). Moreover, increased CO2 has substantial effects on plant growth independent of its effect on climate - an effect that cannot be offset by an increase in albedo. In addition, modification of albedo using shields in space or in the stratosphere would reduce the sunlight incident on the surface. The possible effects of this reduction on ecosystem productivity have not been examined.

10.4.2 Energy balance: albedo

10.4.2.1 Aim and effect of albedo modification

It has long been suggested that albedo geoengineering aimed at countering the climatic effects of increased CO2 would produce significant alterations in climate even if perfect control of mean surface temperature were achieved (Keith and Dowlatabadi, 1992; Dickinson, 1996; Schneider, 1996). A recent numerical experiment, however, has demonstrated that modification of albedo can compensate for increased CO2 with remarkable fidelity.

Govindasamy and Caldeira (2000) tested the effects of albedo geoengineering using a high quality model known to do a good job of simulating the global radiative balance.9 They compared a control case with two tests cases, one with 2 X CO2 and the other a "geoengineering" case with 2 X CO2 and a reduction of solar constant10 by 1.8%. By design, the geoengineering case had (almost) the same mean surface temperature as the control. Surprisingly, the spatial pattern of surface temperature showed little change despite the changed vertical and latitudinal distributions of atmospheric heating. Compared to the

9 The model was version three of the Community Climate Model at a horizontal resolution of T42 with 18 vertical layers, run with interactive sea ice coupled to a slab ocean. The 2 X CO2 climate sensitivity was 1.75°C in this configuration. The statistics presented below are derived from the last 15 years of a 40 year model run.

10 Uniform modification of planetary albedo accomplished using scattering systems in space or in the stratosphere would produce a climatic effect equivalent to an alteration of the solar constant (the solar flux at the top of the atmosphere).

control, the geoengineering case produced statistically significant temperature changes over only 15% of the globe as compared to 97% for the 2 X CO2 case. Contrary to expectations, there was very little change in the amplitude of the diurnal and seasonal cycles in the geoengineering case.

Although a single numerical experiment does not prove the case, it nevertheless suggests that the climate is less sensitive to changes in the meridional distribution of heating than is often assumed, and therefore the assumption that albedo geoengineering could not do an effective job of countering CO2-induced climate change must be reexamined.

10.4.2.2 Atmospheric aerosols Aerosols can increase albedo either directly by optical scattering or indirectly by acting as cloud condensation nuclei that increase the albedo and lifetime of clouds by decreasing the mean droplet size. The modification of climate via alteration of cloud and aerosol properties was first proposed in the 1950s (Section 10.3). The most famous early proposal was by Budyko who suggested increasing the albedo to counter CO2-induced climate change by injecting SO2 into the stratosphere where it would mimic the action of large volcanoes on the climate (Budyko, 1982). He calculated that injection of about 1071 per annum into the stratosphere would roughly counter the effect of doubled CO2 on the global radiative balance. The NAS92 study showed that several technologically straightforward alternatives exist for injecting the required mass into the stratosphere at low cost.

As with other geoengineering proposals, deliberate and inadvertent climate modification are closely linked: anthropogenic sulfate aerosols in the troposphere currently influence the global radiation budget by ~ 1 Wm~2 - enough to counter much of the effect of current anthropogenic CO2.

Addition of aerosol to the stratosphere could have serious impacts, most notably, depletion of stratospheric ozone. Recent polar ozone depletions have clearly demonstrated the complexity of chemical dynamics in the stratosphere and the resulting susceptibility of ozone concentrations to aerosols. Although the possibility of this side effect has long been noted (Budyko, 1982; MacCracken, 1991; Keith and Dowlatabadi, 1992), no serious analysis has been published. In addition, depending on the size of particles used, the aerosol layer might cause significant whitening of the daytime sky. This side effect raises one of the many interesting valuation problems posed by geoengi-neering: How much is a blue sky worth?

Recent work by Teller et al. (Teller, Wood et al., 1997; Teller, Caldeira et al., 1999) has reexamined albedo geoengineering. In agreement with NAS92, Teller et al. found that 1071 of dielectric aerosols of ~ 100 nm diameter are sufficient to increase the albedo by —1%, and suggested that use of alumina particles could minimize the impact on ozone chemistry. In addition, Teller et al. demonstrated that use of metallic or optically resonant scatterers could greatly reduce the total mass of scatterers required. Two configurations of metallic scatterers were analyzed: mesh microstructures and micro-balloons. Conductive metal mesh is the most mass efficient configuration.11 In principle, only —1051 of such mesh structures is required to achieve the benchmark 1% albedo increase. The proposed metal balloons have diameters of —4 mm, are hydrogen filled, and are designed to float at altitudes of —25 km. The required mass is —1061. Because of the much longer stratospheric residence time of the balloon system, the required mass flux (t/yr) to sustain the two systems is comparable. Finally, Teller et al. show that either system, if fabricated with aluminum, can be designed to have long stratospheric lifetimes yet oxidizes rapidly in the troposphere, ensuring that few particles are deposited on the surface.

It is unclear whether the cost of the novel scattering systems will be less than that of the older proposals, as is claimed by Teller et al., because although the system mass is less, the scatterers will be more costly to fabricate. However, it is unlikely that cost would play any significant role in a decision to deploy stratospheric scatterers, because the cost of any such system is trivial compared to the cost of other mitigation options. The importance of the novel scattering systems is not in minimizing cost, but in their potential to minimize risk. Two of the key problems with earlier proposals were the potential impact on atmospheric chemistry, and the change in the ratio of direct to diffuse solar radiation with the associated visual whitening of the sky. The new proposals suggest that the location, scattering properties, and chemical reactivity of the scatterers could, in principle, be tuned to minimize both of these impacts.

10.4.2.3 Planetary engineering from space Proposals to modify the climate using space-based technology reflect an extreme of confidence in human technological prowess. Fittingly, some of the grandest and earliest such proposals arose in the USSR immediately following the launch of Sputnik (Section 10.3.2). During the 1970s, proposals to generate solar power in space and beam it to terrestrial receivers generated substantial interest at NASA and among space technology advocates. Interest in the technology waned under the light of realistic cost estimates, such as the 1981 NAS analysis (Committee on Satellite Power Systems, 1981).

In principle, the use of space-based solar shields has significant advantages

11 The thickness of the mesh wires is determined by the skin-depth of optical radiation in the metal (about

20 nm). The spacing of wires (—300 nm) must be —1/2 the wavelength of scattered light.

over other geoengineering options. Because solar shields effect a "clean" alteration of the solar constant, their side effects would be both less significant and more predictable than for other albedo modification schemes. For most plausible shield geometries, the effect could be eliminated at will. Additionally, steerable shields might be used to direct radiation at specific areas, offering the possibility of weather control.

The obvious geometry is a fleet of shields in low-earth orbit (NAS92). However, solar shields act as solar sails and would be pushed out of orbit by the sunlight they were designed to block. The problem gets worse as the mass density is decreased in order to reduce launch costs. A series of studies published in 1989-92 proposed locating the shield(s) just sunward of the L1 Lagrange point between the Earth and the Sun where they would be stable with weak active control (Early, 1989; Seifritz, 1989).

Teller et al. (Teller, Wood et al., 1997) note that a scattering system at the L1 point need only deflect light through the small angle required for it to miss the earth, about 0.01 rad as compared to —1 rad for scatterers in near earth orbit or in the stratosphere. For appropriately designed scattering systems, such as the metal mesh described above, the reduced angular deflection requirement allows the mass of the system to decrease by the same ratio. Thus, while a shield at the L1 point requires roughly the same area as a near-earth shield, its mass can be —102 times smaller. Teller et al. estimate the required mass at —3 X 103 t. The quantitative decrease in mass requirement suggested by this proposal is sufficient to warrant a qualitative change in assessments of the economic feasibility of space-based albedo modification.

The cost of this proposal has not been seriously analyzed. An optimistic order-of-magnitude estimate is 50-500 billion dollars.12 Arguably, the assumptions about space technology that underlie this estimate could also make space solar power competitive.

10.4.2.4 Surface albedo The most persistent modern proposals for large-scale engineering of surface albedo were the proposals to create an ice free Arctic Ocean to the supposed benefit of northern nations (Section 10.3.2).

Modification of surface albedo was among the first geoengineering meas

12 Cost assessment is heavily dependent on expectations about the future launch costs. Current costs for large payloads to low earth orbit (LEO) are just under 10 k$/kg. Saturn V launches (the largest launcher ever used successfully) cost 6 k$/kg. The stated goal of NASA's current access to space efforts is to lower costs to 2 k$/kg by 2010. This proposal requires —30 launches of a Saturn V - approximately equal to the cumulative total of payload lifted to LEO since Sputnik. We assume (i) that the transit to L1 can be accomplished without large mass penalty (perhaps by solar sailing), and (ii) that average cost of hardware is less than 10k$/kg.

ures proposed to counter CO2-induced warming. For example, the possibility for increasing the oceanic albedo was considered in a series of US assessments (PSAC65, NAS77, and NAS92). Proposals typically involved floating reflective objects, however, "the disadvantages of such a scheme are obvious" (NAS77). They include contamination of shorelines, damage to fisheries, and serious aesthetic impacts.

Local modification of surface albedo accomplished by whitening of urban areas, can however, play an important role in reducing energy used by air conditioning and in adapting to warmer conditions.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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