The definition of a greenhouse gas: gas that transmits solar (ie, short wavelength) radiation, but absorbs infrared (ie, thermal, long wavelength) radiation.

The greenhouse gases are:

• Carbon dioxide, CO₂
• Methane, CH₄
• Nitrous oxide, N₂O
• Chlorofluorocarbons, CFCs – the new refrigerants, with hydrogen added to the molecular structure to promote destruction in the troposphere (so they don’t reach the stratosphere and destroy the ozone), are also greenhouse gases
• Water vapor, H₂O
• Ozone, O₃, in the troposphere
• Aerosol particles of organic carbon (commonly called soot) – although not a gas, these particles exhibit greenhouse gas behavior.

The mass of air in the earth’s atmosphere is closely approximated from the simple equation:

where M = mass of air = 5.14 x 10¹⁸ kg

g = acceleration of gravity

P = sea level air pressure = 101.325 kN/m²

A = surface area of earth

The concentration by volume (or mole) of a gas (such as CO₂) in the atmosphere is expressed as:

where C_I = concentration of the gas

M_I = mass of the gas in atmosphere (kg)

MW_I = molecular weight of the gas (kg/kmol)

M = mass of the atmosphere = 5.14 x 10¹⁸ kg

MW = molecular weight of air @ 29 kg/kmol

By this equation, 7.83 Gte of CO₂ corresponds to C_CO2 = 1 x 10^-6 (ie, 1 part per million by volume, ppmv). Gte = giga tonne = 10⁹ tonne = 10¹² kg.

Since the mass fraction of C in CO₂ is 12/44:

Table 13.2 in Bodansky shows greenhouse gas concentrations in the atmosphere:

CO₂ 280 ppmv 353 ppmv 1.8 ppmv 0.5%

CH₄ 0.8 1.7 0.015 0.9

N₂O 0.29 0.31 0.0008 0.25

CFCs 0 0.001 4.0

Figure 14.1, p 14:3

Note the increase in CO2 concentration in the atmosphere.

What causes the annual fluctuation of about 5 ppmv?

Article 14.7, p 14:15

Annual emission of CO₂ from fossil fuel combustion 5.4 ± 0.5 GteC

Annual emission of CO₂ and other C from wildland burning 1.6 ± 1.0 GteC

Annual gross emission 7.0 ± 1.5 GteC

Annual ocean uptake 2.0 ± 0.8 GteC

Annual net emission 5.0 ± 2.3 GteC

Greenhouse warming potential (GWP) is defined on p 15:1.

GWP is calculated by considering the absorption strength of the molecule and its lifetime in the atmosphere. CO₂ serves as the reference point and has a GWP = 1.

GWP values are given in Table 15.1, p 15:3 for a 100 yr time period:

CO₂ 1 ~100 yr 1

CH₄ 58 10.5 yr 11

N₂O 206 132 yr 270

CFC-11 3970 55 yr 4100

CFC-12 5750 116 yr 7400

GWP expresses the potency of the different molecules. Since there is a much greater amount of CO₂ in the atmosphere than the other gases, CO₂ has the greatest overall effect. However, on the basis of the same amount of gas, the rank-order is CFCs > N₂O > CH₄ > CO₂.

Radiative forcing is defined in article 12.3, p. 12:9, of Bodansky, and further discussed in Chapter 15.

Radiative forcing (w/m²) = increased net downward flux of infrared radiation, taken at the top of the troposphere. The radiative forcing is based on the GWP of the gas and its concentration in the atmosphere.

Values for radiative forcing are given in Table 15.2, p. 15:5.

1765-1990 1.5 0.56 0.10 0.29 2.45

projection for:

1990-2100 5.34 0.91 0.37 0.83 7.45

What is the impact of the 2.45 w/m² of radiative forcing on the surface temperature of the earth? A simple model for this is given in article, pp 15:6-8 of the text. This model is based on radiative feedback. That is, as more IR energy strikes the earth’s surface, in order to maintain equilibrium, the earth’s surface responds by becoming warmer, so that more IR energy can be emitted from the earth’s surface. The result of the modeling (shown at the end of this section) is:

where DQ = radiative forcing (2.45 w/m²)

l_o = feedback parameter (3.3 w/m²-K)

DT_s = increase in earth’s surface T (0.8 deg C)

The DT_s of 0.8 deg C estimated for the temperature increase of the earth over the industrial age is close to value inferred from surface temperature measurements and estimates (see article 15.6 of text). However, several feedback mechanisms are neglected in the simple radiative feedback model. Other feedback mechanisms are discussed in article 15.3 of the text. They are suggested by Figure 12.1, p 12:7.

Other feedback mechanisms are:

• Increased water vapor as a result of increased evaporation by warming: positive feedback since water vapor is a greenhouse gas.
• Changes in cloud cover as a result of warming. This is very complex. It is uncertain whether this feedback is positive or negative. A negative scenario would involve an increase in the albedo of the earth due to greater reflectivity of solar radiation by clouds.
• Decrease in snow and ice by warming. This is a positive feedback, since the albedo of the earth would decrease.
• Methane release as a result of warming. One scenario is a release from methane hydrates. This is a positive feedback.

Another effect is decreased sulfate particulate in the atmosphere from improved control of sulfur emissions from power-plants and other combustion sources. This will promote warming, since less reflectivity of solar radiation will occur. See section 13.3.6, p 13:15.

The text gives and discusses four approaches to addressing the greenhouse problem:

• Further Study
• Adaptation
• Compensation
• Removal
• Reduction

Determining the impact of increasing greenhouse gas emissions is a very formidable task, which should be pursued. For example, at present one cannot place confidence in the prediction of the local details of climate change.

Although there appears to be agreement that more study and R&D should be undertaken, there are major differences regarding action on the greenhouse effect. Positions at the opposite end of the "spectrum" are as follows:

• No action until further study justifies the action.
• Action before the studies are completed - precautionary principle.

The intermediate position is as follows:

Actions which benefit the greenhouse problem, yet have other important benefits, such as energy conservation and energy efficiency - no regrets policy.

This can be both anticipatory (e.g., move to higher ground before the sea rises) and responsive (e.g., move to higher ground after the sea rises).

It appears that adaptation will involve both winners and losers. Some societies will have more resources and flexibility to adapt than others. Some societies will gain by adapting, and some will not.

Regarding sea level: A sea level rise of 30 to 110 cm is predicted over the next century. A 100 cm rise will affect 5 million square kilometers of land and 1 billion people. Thus, a very great amount of adaptation will be required if the prediction of sea level rise is valid. Sea level rise is a major concern of some island nations and nations with highly populated major river deltas. The Netherlands already has a sea wall 16 meters above the mean sea level, and can probably afford to increase its height. A poorer nation might not be able to do this.

Regarding agriculture and forests: If change is rapid, managed forests can probably adapt better than natural ecosystems. Forest management could become a key issue regarding global climate change - this is important for the Pacific Northwest. Farmers may need to adapt to different growing seasons, different moisture patterns, and a different climate. On average, a 7 to 15% increase in precipitation is expected over the next century, though the variation from region to region is expected to be high.

Regarding the response of humans to temperature rise: There will likely be more use of air conditioning. Regarding energy for environmental control of the built environment, will there be a net gain or loss in per capita energy use? That is, will a decrease in space heating occur and cancel the increase in energy use for air conditioning?

It has been proposed to add particles to the atmosphere to reflect the incident solar radiation, i.e., to increase the earth's albedo.

Uncontrolled compensation occurs when ash and sulfate particles enter the atmosphere from volcanic eruptions.

Incidental compensation occurs when sulfate particles from coal (and oil) burning enter the atmosphere. This is thought to be causing cooling of industrialized parts of the world.

Regarding removal of CO₂ from emission sources: Removal of CO₂ from the stacks of coal-fired electrical generating stations has been studied preliminarily. Serious study will probably take place, since the US Dept of Energy has issued requests-for-proposals in this area. The task is huge: an average coal burning power-plant emits about 100 times as much CO₂ as SO₂ by mass. Sulfur dioxide is used as a benchmark because it is currently removed from stack gases. The cost of SO₂ removal is significant - e.g., about 30% of the installed capital cost of a power-plant.

Techniques for CO₂ removal from gas streams are known and are in everyday use (but not for stack gas treatment). An important application is CO₂ removal from well-head natural gas. This is practiced using chemical absorption by amine solvents (see the chemical equation on page 6 of Chapter 16) and by membrane separation.

The major classifications of commercial gas separation are:

• Cryogenic separation
• Membrane separation
• Adsorption
• Absorption

Of these, adsorption and absorption may be best suited to removal of CO₂ from stack gases. Cryogenic separation would require reheating of the gas for plume buoyancy. Membrane separation would benefit from the development of membranes able to withstand stack gas temperature levels.

In the case discussed in Bodansky, CO₂ is absorbed from the stack gas by an amine compound. This occurs at 100 degrees F (thus, stack gas cooling is required). The amine solvent is regenerated by heating to 300 degrees F, leaving the CO₂ to be disposed. Disposal in underground caverns or in the ocean has been considered.

Estimates of the energy required to remove the CO₂ have been made. In order to operate the absorption system at the power-plant, the overall power-plant efficiency will drop about 3% absolute (e.g., from 38 to 35%). In another study, an overall energy loss of 17% (relative) was estimated for the CO₂ removal and storage system.

The cost will probably be high. In one study, a 75% increase in the cost of the electricity is estimated. Another study estimates the increase in installed capital cost as a factor three, and the increase in operations and maintenance (O&M cost) as a factor two.

It is likely that CO₂ removal from power-plant exhaust stacks will be demonstrated in the US and overseas.

Removal of CO₂ by the biosphere: This is an interesting area. It is not certain if the terrestrial biosphere is a net source or sink of CO₂ - probably it is a net source at present because of deforestation in the southern hemisphere. However, with significant reforestation, the terrestrial biosphere could become a net sink of CO₂.

About 50 Gte C per year is removed from the atmosphere by photosynthesis. This is balanced approximately by decay of organic matter. A 10% increase in global photosynthesis without a corresponding increase in decay could balance the 5 to 6 GteC per year emitted into the atmosphere from fossil fuel burning. How much forest would be needed? A 1990 IPCC study indicates that a high growth forest of size 1000 km by 1000 km could absorb 1 Gte C per year from the atmosphere. Thus, 5 to 6 million square kilometers of high growth forest would be required to balance the fossil fuel emission. There are some problems with this approach:

• 5 million square kilometers is a huge area.
• Good quality soil and climate for this amount of high growth forest may not be available.
• The land available for reforestation will probably grow trees at a significantly lower rate than accomplished by a high growth forest.
• As the forest matures, equilibrium may be re-established between photosynthesis and decay, leading to little net removal of CO₂.

Thus, managed forestry may be required, with removal and sequestering of the wood. Or the wood could be burned, replacing the use of fossil fuels - thus, the CO₂ would simply be recycled between growth and burning.

Will deforestation out-run reforestation? This might occur given that about 2 million people are seeking new arable land and firewood.

The cost of reforestation will be significant. A cost of $200 to $400 billion has been estimated for reforestation of 5 million square kilometers.

What about small-scale efforts? What if the US planted 100 million trees in urban areas as has been proposed? About 100 pounds C would be taken up by each growing tree per year. The 100 million trees would remove about 5.5 Mte C per year. In addition, it is estimated that shade provided by the trees would reduce the primary energy used for air conditioning by 0.5 Quads per year. A "back-of-the-envelope" calculation shows that this would reduce C emissions by 10 Mte per year. Thus, the total removal/reduction of CO₂ would be about 15 to 16 Mte C - which is about 0.3% of the annual global CO₂ emission into the atmosphere. This approach, while not having a big effect, has several side benefits, and thus, fits under the intermediate (no regrets) approach.

Enhanced ocean uptake of CO₂ also has been studied. See the text for a case.

This involves reduction in greenhouse gas emissions, for example, through reductions in the burning of fossil fuels, through switching from coal to natural gas, and through reductions in deforestation.

In addition to CO₂ reduction, opportunities exist for reductions in methane emissions. The actions are changes in rice cultivation, changes in livestock feed and handling, and burning of land-fill gas. Note, burning of methane eliminates a high-GWP gas and creates a low-GWP gas. And if the methane is burned in an engine running an electrical generator, energy is produced, off-setting fossil fuel use.

Regarding CFCs, will the new, low-ozone-depleting refrigerants be better or worse than CFCs in their impact on global warming? Some of the IPCC data indicates strong IR absorption by the new refrigerants, though lifetimes in the atmosphere are much less than the current CFCs.