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1994 > Chapter 2

Alternatives to Traditional
Transportation Fuels 1994

Volume 2

Greenhouse Gas Emissions

2. The
Greenhouse Effect and Greenhouse Gases: An Overview

The term “greenhouse
effect” was coined by the 19th century scientist Jean Fourier. The
greenhouse effect occurs when GHGs allow sunlight to reach the Earth's
surface but absorb or scatter most of the heat emanating from the Earth's
surface, thus retaining heat in the atmosphere. Without the natural greenhouse
effect, the average temperature would be about -18oC, or -0.4oF
[23]
[24].
Although the terms “greenhouse effect” and “global
warming” are used due to their familiarity, the concept at issue here
is actually “infrared absorptivity” of greenhouse gases [25].

Scientists have been concerned with the greenhouse effect regarding not
only the quality of life on Earth, but also its effect on other planets.
Twenty-six million miles from Earth, in an orbit much closer to the Sun,
Venus spins through space with a furnace-like surface temperature of more
than 800o Fahrenheit (F) (426.5o Celsius [C]), which is
much hotter than its proximity to the Sun would explain. Scientists used to
believe that Venus fell victim to the greenhouse effect because 96 percent of
its atmosphere is carbon dioxide, with nitrogen accounting for almost all the
remainder [26].
It is now generally agreed within the planetary atmospheres community that
carbon dioxide alone would lead to an average temperature of less than 25oC.
The primary reason that Venus is warmer than this is the presence of sulfuric
acid cloud cover over the entire planet, extending from about 50 kilometers
to 70 kilometers from the surface.

Unlike Venus, Earth is blessed with an atmosphere rich in nitrogen and
oxygen. Dry air near Earth's surface currently contains 78.08 percent
nitrogen, 20.95 percent oxygen, 0.93 percent argon, 0.0353 percent carbon
dioxide, and trace amounts of methane, nitrous oxide, carbon monoxide, ozone,
hydrogen, helium, neon, ammonia, hydrogen sulfide, and sulfur dioxide (Table
2) [27]
[28].
Water can vary roughly from 0 percent to 4 percent of the atmosphere (by
volume) depending on location, altitude, and atmospheric conditions, with the
global average being around 3 percent.

Table 2. Composition of the Atmosphere

Constituent

Fraction by Volume in Dry Air

Total Amount

moles

grams

Nitrogen

78.08%

1.38 x 1020

3.87 x 1021

Oxygen

20.95%

3.7 x 1019

1.19 x 1021

Argon

0.93%

1.65 x 1019

6.59 x 1019

Water
Vapor

variable

9.4 x 1017

1.7 x 1018

Carbon
Dioxide

353 ppmv

6.27 x 1016

2.76 x 1018

Neon

a18.18 ppmv

3.2 x 1015

6.48 x 1016

Helium

a5.24 ppmv

9.28 x 1014

3.71 x 1015

Methane

a1.72 ppbv

3.1 x 1014

4.9 x 1015

Krypton

a1.14 ppmv

2.0 x 1014

1.69 x 1016

Hydrogen

a500 ppbv

9.0 x 1013

1.8 x 1014

Ozone

avariable

6.88 x 1013

3.3 x 1015

Nitrous
Oxide

a310 ppbv

5.2 x 1013

2.3 x 1015

Carbon
Monoxide

a120 ppbv

2.1 x 1013

5.9 x 1014

Xenon

a87 ppbv

1.54 x 1013

2.02 x 1015

Ammonia

a100 ppbv

1.76 x 1012

3.0 x 1013

Nitrogen
Dioxide

a1 pptv

1.76 x 1011

8.1 x 1012

CFC-12

a480 pptv

8.3 x 1010

1.0 x 1013

CFC-11

a280 pptv

4.95 x 1010

6.8 x 1012

Sulfur
Dioxide

a200 pptv

3.59 x 1010

2.3 x 1012

Hydrogen
Sulfide

a200 pptv

3.5 x 1010

1.2 x 1012

aValues of trace constituents
valid in 1990.

CFC-12 = CCl2F2. CFC-11 = CCl3F.

ppmv = Parts per million by volume. ppbv = Parts per billion by volume.
pptv = Parts per trillion by volume.

Note: One mole of a gas is equal to the amount of a substance that contains
as many elementary units (6.023 x 1023 molecules or atoms) as
there are atoms in 12 grams of carbon-12.

Source: D.C. Hartmann, Global Physical Climatology (San Diego:
Academic Press, 1994), p. 8.

The Earth's atmosphere has been transformed slowly, as human activity has
pumped into it billions of tons of greenhouse gases (GHGs) such as carbon
dioxide, water vapor, and large amounts of other gases that absorb the heat
energy emitted from Earth's surface, not to mention the addition of
anthropogenic heat (i.e., direct heat generated by human activities) from
burning of fossil fuels, including transportation fuels, and operation of
almost all equipment.

The principal force driving Earth's weather and climate change comes from
the Sun. Although the Earth receives only about one two-billionth of the
energy emitted by the Sun, it is this energy that heats the Earth, drives
ocean currents, and creates weather patterns. The heat output of the Sun has
varied by about one-third since life on Earth began, and continues to vary during
the solar cycles. Despite this change, global temperatures have remained in a
narrow range suitable for life. It appears that some form of climate
regulation is in operation.

During the past decade or so, people have become concerned with how human
activity may be affecting the world's climate. This concern has focused
largely on anthropogenic GHGs--that is, GHGs generated by human activity such
as the combustion of fuel for transportation. Anthropogenic [29]
GHGs intensify the natural greenhouse effect because they absorb infrared
radiation emitted from the Earth's surface, increasing the heat trapped
inside the atmosphere (Figure 2). GHGs occur naturally in the atmosphere, and
they are essential to life on Earth in its present form. The concern is that
human activity may be increasing the concentration of atmospheric GHGs enough
to alter the climate worldwide.

D

Most of the radiation reaching the Earth's surface and atmosphere is
visible and infrared light. About 70 percent of the radiation reaching the Earth's
atmosphere and surface is absorbed. The Earth's atmosphere and surface
reflect the remainder. Molecules always emit lower energy than they absorb.
Therefore, when the visible light of solar radiation is absorbed, it is
emitted as long wavelength infrared heat waves. Part of the total heat
emitted as a result of visible and infrared light absorption is absorbed by
GHG molecules and clouds and re-emitted in all directions. Some of the
re-emitted radiation is absorbed by the surface. The remainder of the total
heat escapes through the atmosphere and into space (Figure 2).

The major GHGs are water vapor (H2O), carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), fluorocarbons [30],
and ozone (O3). Of these, carbon dioxide is the most commonly
discussed. However, water vapor is the most important GHG due to its
abundance (it represents about 3 percent of the gases in the Earth's
atmosphere). Carbon dioxide and water vapor are the two major products of all
hydrocarbon fuel combustion.

Water vapor is the predominant absorber of incoming solar radiation and a
major contributor to the natural greenhouse effect. Scientists at the
National Oceanic and Atmospheric Administration have reported that the
atmospheric water vapor content in the stratosphere at mid-latitudes in the
northern hemisphere has been increasing over the last 14 years [31].
Water vapor in the stratosphere can harm the ozone layer by stimulating the
formation of polar clouds, which help pollutants such as oxides of nitrogen
and halocarbons destroy ozone. At the tropopause, a rather distinct boundary
between the troposphere and the stratosphere located at an altitude
fluctuating around 15 kilometers, there is a sharp change in the
concentration of water vapor (that is, the variation in concentration within
the stratosphere is minimal) (Table 3). Lindzen and others [32]
argue that water vapor between 2 kilometers (800 millibars) and 16 kilometers
from Earth's surface (the tropopause) is the primary determinant of the
greenhouse effect. However, Shine and others [33]
argue that water vapor concentration in the lower troposphere is an equally
important contributor to the greenhouse effect. Currently, it is believed
that the impact of anthropogenic water vapor from the surface sources such as
fuel combustion is minimal on the atmospheric water vapor concentrations.

Table
3. Atmospheric Water Vapor Concentrations, 1981-1994

Altitude

(kilometers)

Concentration

(parts per million)

Average Annual Increase

(percent)

10-12

59.2

1.03

12-14

11.88

0.49

14-16

4.66

0.54

16-18

3.87

0.73

18-20

3.85

0.84

20-22

4.07

0.54

22-24

4.21

0.38

24-26

4.29

0.34

Source:
S.J. Oltmans and D.J. Hoffman, “Increase in Lower-Stratospheric Water
Vapor at Mid-Latitude Northern Hemisphere Site from 1981-1994,” Nature,
374 (1995): 146-149.

Methane (CH4) is a product of organic decay. The largest
natural source of methane is the world's wetlands, although it is also the
major constituent of natural gas and a potent GHG. Although methane occurs in
the atmosphere in one two-hundredth of the quantity of carbon dioxide, it has
5-10 times the heat-trapping potential per molecule [34]
[35].
Methane is increasing in the atmosphere at an annual rate of 1 percent,
double the rate of increase for carbon dioxide. Activities that release
methane are rice-paddy agriculture, waste treatment, biomass burning,
livestock production, and venting during natural gas and coal exploration and
production activities. Methane is also released during the transport of
natural gas.

Approximately 90 percent of atmospheric methane is chemically destroyed in
the troposphere. Chemical destruction of methane through oxidation occurs by
hydroxyl radicals. Although the concentration of hydroxyl radicals is very
small (0.04 parts per trillion by volume) [36],
they are the main oxidants of atmospheric methane, carbon monoxide [37],
oxides of nitrogen, and non-methane hydrocarbons. Atmospheric hydroxyl
radicals are produced by dissociation of water vapor and reaction between
water vapor and other trace gases in the atmosphere. These reactions are
controlled by pressure, temperature, atmospheric pH, altitude, and reactants
concentration.

Nitrous oxide (N2O) is also a powerful GHG. It stays in the
atmosphere for 150-180 years, eventually floating up into the stratosphere
where it helps destroy the ozone layer. Its concentration is increasing by
0.2 percent to 0.3 percent per year. Its main source is the tropics, but
roughly 20 percent of nitrous oxide emissions result from manufacturing and
using chemical fertilizers and from burning fossil fuels. The increased use
of emission control devices like catalytic converters in internal combustion
engines contributes further to these emissions. The use of fertilizers in
growing corn for ethanol is the major component of the ethanol fuel cycle's
high nitrous oxide emission.

Although chlorofluorocarbons (CFCs) are not products of engine combustion,
these chemicals are associated with vehicle activity because they result from
the production and use of air cooling devices. Automotive CFCs are being
phased out of new models. Concentrations of CFCs are rising approximately 0.5
percent per year, and these chemicals typically persist in the atmosphere
from 75 to 180 years. CFCs are 20,000 times more potent than carbon dioxide
in trapping Earth's thermal radiation. However, some scientists believe that
CFCs might have countervailing effects that cause cooling, so on balance, the
effect of CFCs is not clear.

Ozone is not produced directly from the combustion of transportation
fuels; however, combustion products like NOx, hydrocarbons, and
water vapor play a major role in its formation. Ozone is a beneficial GHG in
the stratosphere and a harmful pollutant in the troposphere. Ozone survives
anywhere from a few hours to a few days in the upper troposphere and for only
an hour in the stratosphere [38]
(see Appendix B for a discussion of ozone's stability). Thinning the
stratospheric ozone layer increases the amount of harmful UV-C radiation
reaching the Earth's surface. This will not only increase UV-induced
diseases, but also aid the production of ozone in the troposphere. It is
beneficial when ozone stays in the stratosphere because ozone shields the
Earth's surface from harmful ultraviolet rays of the Sun. Because of its
oxidizing power, ozone is hazardous to health. Therefore, ozone is classified
as a criteria pollutant in the troposphere. Throughout the atmosphere,
however, ozone acts as a greenhouse gas.

Greenhouse gases have the ability to absorb infrared radiation (radiation
with a wavelength of 1 micrometer or more) and also have, in general, longer
residence times in the atmosphere than criteria pollutants. Although some
compounds (e.g., carbon monoxide, oxides of nitrogen [except nitrous oxide],
and nonmethane volatile organic compounds) have properties of greenhouse
gases in terms of absorption of infrared radiation, they are also called
criteria pollutants because they are hazardous to health.

Proceed to Chapter
3