## The Planetary Energy Balance Levers Available for Climate Manipulation

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The earth's climate system is a dynamic and very complex system, but it obeys the basic principles of physical and chemical thermodynamics and kinetics. The climate system at equilibrium is a balance between the incoming solar radiative energy and outgoing long-wave terrestrial radiative energy. The "levers" that have the capacity to substantially shift the earth's energy balance are: (1) altering the flux (quantity) of solar radiation entering the earth's atmosphere; (2) altering the fraction of solar radiation that is reflected, unchanged, back into space, and; (3) altering the radiative emissivity of the earth's atmosphere, e.g., its capacity for absorbing infrared radiation. Geoengineering proposes to apply these large levers to intentionally shift (manage) the earth's energy balance.

The Stefan-Boltzmann Law succinctly describes the relationship among these levers,

Here, S0 is the average solar irradiance outside Earth's atmosphere; a is Earth's average albedo, e is the emissivity of Earth's atmosphere; s is the Stefan-Boltzmann constant (equal to 5.67 x 10-8 W m-2 K-4), and T is Earth's "black body" temperature. The left side of Eq. 9.1 provides a simple estimate of the solar energy that is absorbed, while the right side of the equation is an estimate of the quantity of energy that radiates back to space by the earth system. When the solar energy absorbed is equal to the energy re-emitted by the earth system, Earth's climate system is at thermodynamic equilibrium. The scientific understanding concerning Earth's energy budget on decadal to century timescales is incomplete. However, over the long term, this fundamental thermodynamic balance will hold.

Geoengineering, as a strategy for mitigating climate warming, involves purposefully altering some of the values in this energy balance equation - modifying the solar irradiance (S0), Earth's average albedo (a), and the emissivity of Earth's atmosphere (e).

Solar irradiance, S0 is the average quantity of solar energy that falls upon one square meter of Earth's atmosphere per second. If the earth were flat, S0 would average 1,366 W/m2 [12]. However, the earth's curvature combined with the fact that only half of the planet is illuminated by the sun at any time substantially reduces the actual energy absorbed - down to a mean of 343 W/m2. A number of astrophysi-cal processes, from the size and ellipticity of the earth's orbit around the sun, to cyclic changes in the sun's magnetic fields, lead to variability in S0 [13]. Over the past 50 years, the 11-year solar sunspot cycle has been the chief cause of this variability. Analyses of the available solar luminosity data over this period show a variance in the range of 2 W/m2 or less than 0.1% [14, 15]. While these cycles in luminosity can be seen in the temperature record, there has been no upward trend that might explain the observed climate warming (IPCC 2007).

Earth's albedo (a) is a measure of the reflectivity of its surface and atmosphere. Multiplying the solar irradiance (S0) by (1-a) gives the fraction of solar energy that is absorbed into the system. Albedo is a function of the fraction of white or light-colored surfaces, such as sea ice, glaciers, or deserts, or in the atmosphere, clouds and scattering (non-absorbing) aerosol. For simple calculations like Eq. 9.1, the earth's albedo is usually estimated to be around 0.3, indicating that approximately a third of the sun's radiation is reflected into space by these light scattering surfaces. However, in practice, these reflective features over or at the earth's surface vary over time scales ranging from seconds to millennia. Shifting continents, and ice ages have altered global albedo on geological time scales. Over centuries, and more recently, on decadal and annual time scales, human agriculture and development has altered the albedo of the earth's land surfaces. Recent rapid deforestation has dramatically altered surface albedo (as well has the biosphere's capacity for carbon sequestration). The cloud and aerosol composition of the atmosphere changes on scales from years down to seconds. Thus, in practice, the earth's albedo varies widely and changes constantly.

If Earth were a true "black body," the energy it re-emits to space would simply be a function of its absolute temperature. The ideal version of the Stefan-Boltzmann equation, j=ctT4, where j is the energy flux per surface area, s is the Stefan-

Boltzmann constant, and T is Earth's black body temperature in Kelvin units, quantifies this energy flux. However, Earth's atmosphere interferes to a degree with the emission of terrestrial (IR) radiation. The emissivity (e), in this context, is a measure of our atmosphere's opacity to infrared radiation. If the atmosphere were perfectly transparent to IR radiation, e would equal unity (1). Values of e that are less than unity indicate that the atmosphere absorbs IR radiation to some degree.

Earth's black body temperature, as measured above the atmosphere by satellite, is -19°C - well below the freezing point of water - while the average surface temperature is 15°C. The strong thermal gradient that exists between the surface and the top of the earth's troposphere is a consequence of the non-ideal emissivity of the atmosphere. We know this as the familiar "Greenhouse Effect." The greater the atmospheric concentration of greenhouse gases, the lower the emissivity of the atmosphere. The lower the emissivity of the atmosphere, the higher the earth's surface temperature (T, in Eq. 9.1) must rise to achieve thermal equilibrium.

A number of factors complicate the absorption and dispersion of energy at the earth's surface. These factors include the planet's near-spherical shape; the heterogeneous mix of surface types with a wide range of albedo values; the earth's photo-chemically active atmosphere; the vast world ocean, and; a biosphere that absorbs solar radiation and CO2, and subsequently re-emits CO2 and other GHGs (CH4, methane; and N2O, nitrous oxide). The ocean not only absorbs solar radiation, but also contains 50 times the concentration of CO2 as the atmosphere. On geologic time scales, changes in any of these climate system components have induced ice ages and warm interglacial periods, along with climate variability at shorter time scales.

The perturbation represented by rapidly increasing atmospheric GHG concentrations due to human activities has disrupted the earth's energy balance. While the global atmospheric burden of GHGs continues to grow, the equilibrium state towards which the earth system is moving will include higher average surface temperatures. The climate science community has projected increasingly dramatic changes in meteorological patterns for the future, as the additional heat energy trapped by the enhanced Greenhouse Effect disperses within the Earth system [1] .

The geoengineering strategy is to apply the climate system levers implied by the variables in Eq. 9.1: reducing solar irradiance; increasing the earth's albedo; increasing the emissivity of the atmosphere. In the following sections, we describe several examples of how geoengineering proponents have proposed to apply these levers. While the list of proposals is not exhaustive, from a physical standpoint, all geoengineering proposals seek to employ one of the three available levers. Any difference amongst them is in the details. A summary of the options discussed here is provided in Table 9.1, which qualitatively compares the current level of understanding on the potential environmental outcomes (side effects) associated with successful implementation of each of the suggested geoengineering strategies. Other metrics included for comparison include implementation cost and the expected timeframe when action would be required to maintain climate forcing. Each proposal's timeframe-to-maintain can also be interpreted as a reversibility-timeframe, assuming the geoengineering action would only be halted by natural mechanisms.

Table 9.1 Proposed methods to geoengineer climate

Proposal

Forcing mechanism

Cost to implement3

Timescale to maintain

Stratospheric aerosol injection

Years

Tropospheric marine cloud-seeding

Weeks

Space-based reflectors

Surface albedo Reflecting shortwave \$\$-\$\$\$ Months modification radiation

Potential co-benefits

Potential undesirable consequences

Level of understanding

Brilliant sunrises and sunsets

None

None

Reduced urban heat island effects

Loss of stratospheric ozone

Whitening of the sky Unknown effects of manufactured novel aerosols

Altered hydrological cycles

Increased aerosol pollution of ocean and ocean-bordering regions

Altered hydrological cycles

Altered primary productivity due to altered PAR flux Altered hydrological cycles

Significant interference with ocean ecosystem Ocean surface modification likely to have poor aesthetics and affect shipping industries

Feasibility: High Co-benefits: High Consequences: Low

Feasibility: Low Co-benefits: High Consequences: moderate

Feasibility: Low Co-benefits: Low Consequences: Low

Feasibility: Low Co-benefits: Low Consequences: Low

Ocean pH

modification

Phytoplankton fertilization

Reforestation and afforestation

Chemical weathering

Greenhouse gas drawdown

Greenhouse gas drawdown

Greenhouse gas drawdown

Greenhouse gas drawdown

Unknown

Unknown

Years

Months

Unknown

Years

Unknown

Years

Mitigation of CO,-driven ocean acidification

Restoration of depleted fish populations

• Replenishment of depleted forests

• Urban heat island mitigation for urban forestry

Unknown

Large-scale manipulation of ocean surface chemistry likely to impact ocean ecosystems

Algae overgrowth and creation of dead zones in ocean

CH4 and N,0 production below fertilized euphotic zones

Impacts of increased water demand, soil run-off, and fertilizer. Changes in biodiversity with wide-scale forest monoculture Environmental impacts of large-scale reagent and waste production.

Feasibility: Low Co-benefits: Low Consequences: Low

Feasibility: Low Co-benefits: Low Consequences: Low

Feasibility: Moderate Co-benefits: High Consequences: Low

Feasibility: Low Co-benefits: Low Consequences: Low

o aCost approximated using existing published estimates and assuming scale of implementation would be enough to compensate for forcing equivalent of doubling CO, (from preindustrial leaves) and held for 50 years. \$ = 10-100 billion USD, \$\$ = 100 billion - 1 trillion USD, \$\$\$ = 1-10+ trillion USD bSulfate aerosols have projected costs at 2-7 trillion USD [16], while novel manufactured aerosols optimized to reflect solar radiation were projected to be considerably less costly at 15-40 billion USD [17]