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Appendix A
Questions and Answers About
Greenhouse Warming
THE GREENHOUSE EFFECT: WHAT IS KNOWN,
WHAT CAN BE PREDICTED
1. What is the "greenhouse effect?"
In simplest terms, "greenhouse gases" let sunlight through to the earth's
surface while trapping "outbound" radiation. This alters the radiative bal-
ance of the earth (see Figure A.1) and results in a warming of the earth's
surface. The major greenhouse gases are water vapor, carbon dioxide (CO2),
methane (CH4), chlorofluorocarbons (CFCs) and hydrogenated chlorofluo-
rocarbons (HCFCs), tropospheric ozone (O3), and nitrous oxide (N2O).
Without the naturally occurring greenhouse gases (principally water vapor
and CO2), the earth's average temperature would be nearly 35°C (63 °F)
colder, and the planet would be much less suitable for human life.
2. Why is it called the "greenhouse" effect?
The greenhouse gases in the atmosphere act in much the same way as the
glass panels of a greenhouse, which allow sunlight through and trap heat
. .
Inside.
3. Why have experts become worried about the greenhouse effect now?
Rising atmospheric concentrations of CO2, CH4, and CFCs suggest the pos-
sibility of additional warming of the global climate. The panel refers to
warming due to increased atmospheric concentrations of greenhouse gases
as "greenhouse warming." Measurements of atmospheric CO2 show that the
1990 concentration of 353 parts per million by volume (ppmv) is about one-
quarter larger than the concentration before the Industrial Revolution (prior
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Representative terms from entire chapter:
climatic changes
86
Reflected \ \
(5%) \ \
Atmosphere\ \
Earth's Surface \~l
Absorbed (45%)
APPENDIX A
Reflected
(25%) Incident Solar Radiation (100%)
Outgoing Radiation (70%)
Top of Atmosphere
~ vvvvv i.1~.
I_ Transmitted (75%)
(25%)
~ 7^ ~ ~ ~ ~ '_ ~
______
Transmitted (50%)
Evaporation
and Mechanical
Heat Transfer
(29%)
(80%)
Upward
· Radiation
I (104%)
Downward
Radiation
(88%)
~_ _
(45%)
APPENDIX A
87
to 1750~. Atmospheric CO2 is increasing at about 0.5 percent per year. The
concentration of CH4 is about 1.72 ppmv, or slightly more than twice that
before 1750. It is rising at a rate of 0.9 percent per year. CFCs do not
occur naturally, and so they were not found in the atmosphere until produc-
tion began a few decades ago. Continued increases in atmospheric concen-
trations of greenhouse gases would affect the earth's radiative balance and
could cause a large amount of additional greenhouse warming. Increasing
the capture of energy in this fashion is also called "radiative forcing." Other
factors, such as variation in incoming solar radiation, could be involved.
4. Has there been greenhouse warming in the recent past?
Best estimates are that the average global temperature rose between 0.3°
and 0.6°C over about the last 100 years. However, it is not possible to say
with a high degree of confidence whether this is due to increased atmo-
spheric concentrations of greenhouse gases or to other natural or human
causes. The temperature record much before 1900 is not reliable for esti-
mates of changes smaller than 1°C (1.8°F).
5. What about CO2 and temperature in the prehistoric past?
According to best estimates based on analysis of air bubbles trapped in ice
sheets, ocean and lake sediments, and other records from the geologic past,
there have been three especially "warm" periods in the last 4 million years.
The Holocene optimum occurred from 6,000 to 5,000 years ago. During
that period, atmospheric concentrations of CO2 were about 270 to 280 ppmv,
and average air temperatures about 1°C (1.8°F) warmer than modern times.
The Eemian interglacial period happened with its midpoint about 125,000
years ago. Atmospheric concentrations of CO2 were 280 to 300 ppmv, and
temperatures up to 2°C (3.6°F) warmer than now. The Pliocene climate
optimum occurred between 4.3 and 3.3 million years ago. Atmospheric
concentrations of CO2 have been estimated for that period to be about 450
ppmv, with temperatures 3° to 4°C (5.4° to 7.2°F) warmer than modern
times. The prehistoric temperature estimates are from evidence dependent
surface plus 104 percent upward radiation balanced by 50 percent of incoming solar
continuing to the earth's surface, 70 percent outgoing radiation, and 88 percent
downward radiation). These different energy transfers are due to the heat-trapping
effects of the greenhouse gases in the atmosphere, the reemission of energy ab-
sorbed by these gases, and the cycling of energy through the various components in
the diagram. The accuracy of the numbers in the diagram is typically +5.
This diagram pertains to a period during which the climate is steady (or unchang-
ing); that is, there is no net change in heat transfers into earth's surface, no net
change in heat transfers into the atmosphere, and no net radiation change into the
atmosphere-earth system from beyond the atmosphere.
88
APPENDIX A
on conditions during growing seasons and probably are better proxies for
summer than winter temperatures. The estimate for the Pliocene period is
especially controversial.
6. What natural things affect climate in the long run?
On the geologic time scale, many things affect climate:
· Changes in solar output
· Changes in the earth's orbital path
· Changes in land and ocean distribution (tectonic plate movements and
the associated changes in mountain geography, ocean circulation, and sea
level)
· Changes in the reflectivity of the earth's surface
· Changes in atmospheric concentrations of trace gases (especially CO2
and CH4)
· Changes of a catastrophic nature (such as meteor impacts or extended
volcanic eruptions)
7. What is meant by "atmospheric lifetime" and "sinks?"
These concepts can be illustrated by referring to what is called the "carbon
cycle." When CO2 is emitted into the atmosphere, it moves among four
main sinks, or pools, of stored carbon: the atmosphere, the oceans, the soil,
and the earth's biomass (plants and animals). The movement of CO2 among
these sinks is not well understood. About 45 percent of the total emissions
of CO2 from human activity since preindustrial times is missing in the cur-
rent accounting of CO2 in the atmosphere, oceans, soil, and biomass. Three
possible sinks for this missing CO2 have been suggested. First, more CO2
may have been absorbed into the oceans than was thought. Second, the
storage of CO2 in terrestrial plant life may be greater than estimated. Third,
more CO2 may have been absorbed directly into soil than is thought. How-
ever, there is no direct evidence for any of these explanations accounting
for all the missing CO2. CO2 in the atmosphere is relatively "long-lived" in
that it does not easily break down into its constituent parts. CH4, by con-
trast, decomposes in the atmosphere in about 10 years. The greenhouse gas
with the longest atmospheric lifetime (except for CO2), CFC-1 15, has an average
atmospheric lifetime of about 400 years. The overall contribution of green-
house gases to global warming depends on their atmospheric lifetime as
well as their ability to trap radiation. Table A.1 shows the relevant charac-
teristics of the principal greenhouse gases.
8. Do all greenhouse gases have the same effect?
Each gas has different radiative properties, atmospheric chemistry, typical
atmospheric lifetime, and atmospheric concentration. For example, CFC-12 is
roughly 15,800 times more efficient molecule for molecule at trapping heat
than CO2. Because CFC-12 is a large, heavy molecule with many atoms and a
APPENDIX A
TABLE A.1 Key Greenhouse Gases Influenced by Human Activity
89
co2 CH4 CFC- 1 1 CFC- 12 N2O
Preindustrial 280 ppmv 0.8 ppmv 0 0 288 ppbv
atmospheric
concentration
Current atmospheric 353 ppmv 1.72 ppmv 280 pptv
concentration ( 1 990)a
Current rate of annual
484 pptv 310 ppbv
1.8 ppmv 0.015 ppmv 9.5 pptv
atmospheric (0.5%) (0.9%) (4%)
accumulations
17 pptv 0.8 ppbv
(4%) (0.25%)
Atmospheric lifetime (50-200) 10 65 130 150
(years)C
aThe 1990 concentrations have been estimated on the basis of an extrapolation of
measurements reported for earlier years, assuming that the recent trends remained
approximately constant.
bNet annual emissions of CO2 from the biosphere not affected by human activity,
such as volcanic emissions, are assumed to be small. Estimates of human-induced
emissions from the biosphere are controversial.
CFor each gas in the table, except CO2, the "lifetime" is defined as the ratio of the
atmospheric concentration to the total rate of removal. This time scale also charac-
terizes the rate of adjustment of the atmospheric concentrations if the emission rates
are changed abruptly. CO2 is a special case because it is merely circulated among
various reservoirs (atmosphere, ocean, biota). The "lifetime" of CO2 given in the table
is a rough indication of the time it would take for the CO2 concentration to adjust to
changes in the emissions.
NOTES: Ozone has not been included in the table because of lack of precise data.
Here ppmv = parts per million by volume, ppbv = parts per billion by volume, and
pptv = parts per trillion by volume.
SOURCE: World Meteorological Organization. 1990. Climate Change, the IPCC
Scientific Assessment. Cambridge, United Kingdom: Cambridge University Press.
Table 1.1. Reprinted by permission of Cambridge University Press.
CO2 molecule is small and light in comparison, there are fewer molecules of
CFC-12 in each ton of CFC-12 emissions than CO2 molecules in each ton of
CO2 emissions. Each ton of CFC-12 emissions is about 5,750 times more
efficient at trapping heat than each ton of CO2. The comparatively greater amount
of CO2 in the atmosphere, however, means that it accounts for roughly half of
the radiative forcing associated with the greenhouse effect.
9. Do greenhouse gases have different e~ects over time?
Yes. Figure A.2 shows projected changes in radiative forcing for different
greenhouse gases between now and 2030. The potential increase for each
gas is plotted for different emissions of each gas compared to 1990 emis
9o
APPENDIX A
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CHANGE IN ANTHROPOGENIC EMISSIONS
FROM 1 990 TO 2030 (percent)
FIGURE A.2 Additional radiative forcing of principal greenhouse gases from 1990
to 2030 for different emission rates. The horizontal axis shows changes in green-
house gas emissions ranging from completely eliminating emissions (-100 percent)
to doubling current emissions (+100 percent). Emission changes are assumed to be
linear from 1990 levels to the 2030 level selected. The vertical axis shows the
change in radiative forcing in watts per square meter at the earth's surface in 2030.
Each asterisk indicates the projected emissions of that gas assuming no additional
regulatory policies, based on the Intergovernmental Panel on Climate Change esti-
mates and the original restrictions agreed to under the Montreal Protocol, which
limits emissions of CFCs. Chemical interactions among greenhouse gas species are
not included.
For CO2 emissions remaining at 1990 levels through 2030, the resulting change
in radiative forcing can be determined in two steps: (1) Find the point on the curve
labeled "CO2" that is vertically above 0 percent change on the bottom scale. (2) The
radiative forcing on the surface-troposphere system can be read in watts per square
meter by moving horizontally to the left-hand scale, or about 1 W/m2. These steps
must be repeated for each gas. For example, the radiative forcing for continued
1990-level emissions of CH4 through 2030 would be about 0.2 W/m2.
SOURCE: Chapter 3 of the report of the Effects Panel.
APPENDIX A
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Climate Sensitivity to
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CHANGE IN ANTHROPOGENIC EMISSIONS
FROM 1990 TO 2030 (percent)
FIGURE A.3 Commitment to future warming. An incremental change in radiative
forcing between 1990 and 2030 due to emissions of greenhouse gases implies a
change in global average equilibrium temperature (see text). The scales on the
right-hand side show two ranges of global average temperature responses. The first
corresponds to a climate whose temperature response to an equivalent of doubling of
the preindustrial level of CO2 is 1°C; the second corresponds to a rise of 5°C for an
equivalent doubling of CO2. These scales indicate the equilibrium commitment to
future warming caused by emissions from 1990 through 2030. Assumptions are as
in Figure A.2.
To determine equilibrium warming in 2030 due to continued emissions of CO2 at
the 1990 level, find the point on the curve labeled "CO2" that is vertically above
O percent change on the bottom scale. The equilibrium warming on the right-hand
scales is about 0.23°C (0.4°F) for a climate system with 1° sensitivity and about
1.2°C (2.2°F) for a system with 5° sensitivity. For CH4 emissions continuing at 1990
levels through 2030, the equilibrium warming would be about 0.04°C (0.07°F) at 1°
sensitivity and about 0.25°C (0.5°F) at 5° sensitivity. These steps must be repeated
for each gas. Total warming associated with 1990-level emissions of the gases
shown until 2030 would be about 0.41 °C (0.7°F) at 1° sensitivity and about 2.2°C
(4°F) at 5° sensitivity.
Scenarios of changes in committed future warming accompanying different greenhouse
gas emission rates can be constructed by repeating this process for given emission
rates and adding up the results.
92
APPENDIX A
sion levels. The figure shows the impact of different percentage changes in
emissions (compared to 1990 emission rates) on the radiative forcing. Fig-
ure A.3 extends this to show the impact on equilibrium temperature for
different sensitivities of the climatic system (in degrees Celsius).
10. What is meant by a "feedback" mechanism?
One example of a greenhouse warming feedback mechanism involves water
vapor. As air warms, each cubic meter of air can hold more water vapor.
Since water vapor is a greenhouse gas, this increased concentration of water
vapor further enhances greenhouse warming. In turn, the warmer air can
hold more water, and so on. This is an example of a positive feedback,
providing a physical mechanism for "multiplying" the original impetus for
change beyond its initial force.
Some mechanisms provide a negative feedback, which decreases the ini-
tial impetus. For example, increasing the amount of water vapor in the air
may lead to forming more clouds. Low-level, white clouds reflect sunlight,
thereby preventing sunlight from reaching the earth and warming the sur-
face. Increasing the geographical coverage of low-level clouds would re-
duce greenhouse warming, whereas increasing the amount of high, convec-
tive clouds could enhance greenhouse warming. This is because high, convective
clouds absorb energy from below at higher temperatures than they radiate
energy into space from their tops, thereby effectively trapping energy. Sat-
ellite measurements indicate that clouds currently have a slightly negative
effect on current planetary temperature. It is not known whether increased
temperatures would lead to more low-level clouds or more high, convective
clouds.
11. Can the temperature record be used to show whether or not green-
house warming is occurring?
The estimated warming of between 0.3° and 0.6°C (0.5° and 1.1°F) over the
last 100 years is roughly consistent with increased concentrations of green-
house gases, but it is also within the bounds of "natural" variability for
weather and climate. It cannot be proven to a high degree of confidence
that this warming is the result of the increased atmospheric concentrations
of greenhouse gases. There may be an underlying increase or decrease in
average temperature from other, as yet undetected, causes.
12. What is the basis for predictions of global warming?
General circulation models (GCMs) are the principal tools for projecting
climatic changes. GCMs project equilibrium temperature increases between
1.9° and 5.2°C (3.4° and 9.4°F) for greenhouse gas concentrations equiva-
lent to a doubling of the preindustrial level of atmospheric CO2. The mid-
point of this range corresponds to an average global climate warmer than
APPENDIX A
93
any in the last 1 million years. The consequences of this amount of warm-
ing are unknown and may include extremely unpleasant surprises.
13. What is "equilibrium temperature''?
The oceans, covering roughly 70 percent of the earth's surface, absorb heat
from the sun and redistribute it to the deep oceans slowly. It will be
decades, perhaps centuries, before the oceans and the atmosphere fully re-
distribute the absorbed energy and the currently "committed" temperature
rise is actually "realized." The temperature at which the system would
ultimately come to rest given a particular level of greenhouse gas concen-
trations is called the "equilibrium temperature." Since atmospheric concen-
trations of greenhouse gases are constantly changing, the temperature mea-
sured at any time is the "transient" temperature, which lags behind the
committed equilibrium warming. The lag depends in part on the sensitivity
of the climate system and is believed to be between 10 and 100 years. This
phenomenon makes it difficult to use temperature alone to "prove" that
. . .
green nouse warming 1S occurring.
14. How can we know when greenhouse warming is occurring?
The only tools we have for trying to produce credible scientific results are
observations combined with theoretical calculation. Detecting additional
greenhouse warming will require careful monitoring of temperature and
other variables over years or even decades. Further development of numeri-
cal models will help characterize the climatic system, including the atmo-
sphere, oceans, and land-based elements like forests and ice fields. How-
ever, only careful interpretation of actual measurements can reveal what has
occurred and when.
15. flow can credible estimates offuture global warming be made?
Several approaches can be used. Scientific "first principles" can be used to
estimate physical bounds on future trends. GCMs can be used to conduct
"what if" experiments under differing conditions. Comparisons can be made
with paleoclimatic data of previous interglacial periods. None of these
methods is absolutely conclusive, but it is generally agreed that GCMs are
the best available tools for predicting climatic changes. Substantial im-
provements in GCM capabilities are needed, however, for GCM forecasts to
increase their credibility.
16. What influences future warming ?
The amount of climatic warming depends on several things:
· The amount of sunlight reaching the earth
· Emission rates of greenhouse gases
· Chemical interactions of greenhouse gases in the atmosphere
94
APPENDIX A
.
Atmospheric lifetimes of greenhouse gases until they decompose or
transfer into sinks
· Effectiveness of positive or negative feedback mechanisms that en-
hance or reduce warming
· Human actions, which affect radiative forcing in both positive and
. .
negative directions
17. What are the major "unknowns" in predictions?
Major uncertainties include:
· Future emissions of greenhouse gases
· Role of the oceans and biosphere in uptake of heat and CO2
Amount of CO2 and carbon in the atmosphere, oceans, biota, and soils
Effectiveness of sinks for CO and other greenhouse gases, especially
2
CH
· Interactions between temperature change and cloud formation and the
resulting feedbacks
feedbacks
Effects of global warming on biological sources of greenhouse gases
Interactions between changing climate and ice cover and the resulting
Amount and regional distribution of precipitation
Other factors, like variation in solar radiation
18. How can the uncertainties best be handled?
Data can be arrayed to validate components of the models. Increasing the
number of data sets can also help. In addition, the variation in GCM results
can be compared to provide a sense of their "robustness." A major
"intercomparison" of GCMs is being conducted, and has shown large differ-
ences in regional precipitation and reduction of snow and ice fields at high
latitudes.
_ 1 _ _ ~ 1 , , 1 ~
19. Are there changes associated with an equivalent doubling of the
preindustrial level of atmospheric CO2 that can be stated with confidence?
Because of the uncertainty in our understanding of various factors, projec-
tions reflect different levels of confidence.
Highly plausible: Global average surface warming
Global average precipitation increase
Reduction in sea ice
High-latitude surface winter warming
Plausible: Global sea level rise
Intensification of summer mid-latitude, mid-continental
drying
High-latitude precipitation increase
APPENDIX A
Highly uncertain: Local details of climate change
Regional distribution of precipitation
Regional vegetation changes
Increase in tropical storm intensity or frequency
20. What about storms and other extreme weather events?
95
The factors governing tropical storms are different from those governing
mid-latitude storms and need to be considered separately.
One of the conditions for formation of typhoons or hurricanes today is a
sea surface temperature of 26°C (79°F) or greater. With higher global
average surface temperature, the area of sea with this temperature should be
larger. Thus the number of hurricanes could increase. However, air pres-
sure, humidity, and a number of other conditions also govern the creation
and propagation of tropical cyclones. The critical temperature for their
creation may increase as climate changes these other factors. There is no
consistent indication whether tropical storms will increase in number or
intensity as climate changes. Nor is there any evidence of change over the
past several decades.
Mid-latitude storms are driven by equator-to-pole temperature contrast.
In a warmer world, this contrast will probably weaken since surface tem-
peratures in high latitudes are projected to increase more than at the equator
(at least in the northern hemisphere). Higher in the atmosphere, however,
the temperature contrast strengthens. Increased atmospheric water vapor
could also supply extra energy to storm development. We do not currently
know which of these factors would be more important and how mid-latitude
storms would change in frequency, intensity, or location.
Can projections be improved?
Better computers alone will not solve the problems associated with positive
and negative feedbacks. - Better understanding of atmospheric physics and
chemistry and better mathematical descriptions of relevant mechanisms in
the models are also needed, as are data to validate models and their
subcomponents. Significant improvements may require decades.
22. Is it possible to avoid the projected warming?
It is possible only at great expense or by incurring risks not now under-
stood, unless the earth is itself self-correcting. Continued increases in
atmospheric concentrations of greenhouse gases would probably result in
additional global warming. Avoiding all future warming either would be
very costly (if we significantly reduce atmospheric concentrations of green-
house gases) or potentially very risky (if we use climate engineering). However,
a comprehensive action program could slow or reduce the onset of green-
house warming.
APPENDIX A
103
to assess how much to spend on emission reductions or offsets. However,
all estimates are approximations with very little precision. The amount to
allocate to prevent additional greenhouse warming depends significantly on
the preferred degree of risk aversion.
PREVENTING OR REDUCING ADDITIONAL
GREENHOUSE WARMING
32. What are the sources of greenhouse gas emissions?
All of the major greenhouse gases except CFCs are produced by both natu-
ral processes and human activity. Table A.4 summarizes the principal sources
of greenhouse gases associated with human activity.
33. What interventions could reduce greenhouse warming?
It is useful to examine two different aspects of reducing emissions or offset
. . .
tong emissions:
.
.
"Direct" reduction or offsetting of emissions through altering equip-
ment, products, physical processes, or behaviors
· "Indirect" reduction or offsetting of emissions through altering the
behavior of people in their economic or private lives and thus affecting the
overall level of activity leading to emissions
It is much easier to estimate potential effectiveness and costs of direct
reductions than of indirect incentives on human behavior. This is mostly
because of the many factors that affect behavior in addition to the incen
lives in any particular program.
34. How can specific mitigation options be compared?
Mitigation options can be compared quantitatively in terms of their cost-
effectiveness and qualitatively in terms of the obstacles to their implemen-
tation and in terms of other benefits and costs.
The standard quantitative unit used to compare mitigation options is the
cost per metric ton of carbon emissions reduced or per metric ton of carbon
removed from the atmosphere. The amount of carbon can be converted to
the amount of CO2 in the atmosphere by multiplying by 3.67, which is the
ratio of the molecular weights of carbon and CO2. Other greenhouse gases
can be "translated" to CO2 equivalency by using two calculations. First, the
amount of radiative forcing caused by a specific concentration of the gas is
estimated in terms of the change in energy reaching the surface (in watts
per square meter). This estimate accounts for atmospheric chemistry, atmo-
spheric lifetime of the gas, and other relevant factors affecting the total
contribution of that gas to greenhouse warming. Second, the amount of
104
APPENDIX A
TABLE A.4 Estimated 1985 Global Greenhouse Gas Emissions from
Human Activities
Greenhouse Gas CO2-equivalent
Emissions (Mt/yr) Emissionsa (Mt/yr)
CO2 Emissions
Commercial energy 18,800 18,800(57)
Tropical deforestation 2,600 2,600(8)
Other 400 400(1 ~
TOTAL 21,800 21,800(66)
CH4 Emissions
Fuel production 60 1,300(4)
Enteric fermentation 70 1,500(5)
Rice cultivation 110 2,300(7)
Landfills 30 600(2)
Tropical deforestation 20 400(1)
Other 30 600(2)
TOTAL 320 6,700(20)b
CFC- 11 and CFC- 12 Emissions
TOTAL 0.6 3,200(10)
N2O Emissions
Coal combustion 1 290(>1)
Fertilizer use 1.5 440(1)
Gain of cultivated land 0.4 120(>1)
Tropical deforestation 0.5 150(> 1)
Fuel wood and industrial biomass 0.2 60(>1)
Agricultural wastes 0.4 120(>1)
TOTAL 4 1,180(4)
TOTAL 32,880(100)
aCO2-equivalent emissions are calculated from the Greenhouse Gas Emissions
column by using the following multipliers:
co2
CH4
CFC-ll and-12
N2O
Numbers in parentheses are percentages of total.
bTotal does not sum due to rounding errors.
21
5,400
290
NOTE: Mt/yr = million (106) metric tons (t) per year. All entries are rounded be-
cause the exact values are controversial.
SOURCE: Adapted from U.S. Department of Energy. 1990. The Economics of Long-
Term Global Climate Change: A Preliminary Assessment- Report of an Interagency
Task Force. Springfield, Va.: National Technical Information Service.
APPENDIX A
105
CO2 that would produce the same amount of forcing at the surface is calcu-
lated. This is the CO2 equivalent for that specific concentration of the other
greenhouse gas. The respective costs per ton for different options can then
be compared directly. It is important to recognize, however, that these
calculations allow comparison only of initial contributions. They do not
account for changes in energy-trapping effectiveness over the various life-
times of these gases in the atmosphere.
35. What mitigation options are most cost-e~ective ?
The panel ranks options for reducing greenhouse gas emissions or removing
greenhouse gases from the atmosphere according to their cost-effectiveness.
Some of these options have net savings or very low net implementation
costs compared to other investments. The options range from net savings to
more than $100 per metric ton of CO2-equivalent emissions avoided or re-
moved from the atmosphere. The most cost-effective mitigation options are
presented in Table A.5.
36. What are examples of options with large potential to reduce or offset
emissions ?
The so-called geoengineering options have the potential of substantially
affecting atmospheric concentrations of greenhouse gases. They have the
ability to screen incoming sunlight, stimulate uptake of CO2 by plants and
animals in the oceans, or remove CO2 from the atmosphere. Although they
appear feasible, they require additional investigation because of their poten-
tial environmental impacts.
37. How much would it cost to significantly reduce current U.S. green-
house gas emissions?
It depends on the level of emission reduction desired and how it is done.
The most cost-effective options are those that enhance efficient use of en-
ergy: efficiency improvements in lighting and appliances, white roofs and
paving to enhance reflectivity, and improvement in building and construc-
tion practices.
Figure A.4 compares mitigation options, and Table A.5 gives the panel's
estimates of net cost and emission reductions for several options. It must
be emphasized that the table presents the panel's estimates of the maximum
technical potential for each option. The calculation of cost-effectiveness of
lighting efficiency, for example, does not consider whether the supply of
light bulbs could meet the demand with current production capacities. Nor
does it consider the trade-off between expenditures on light bulbs and on
health care, education, or basic shelter for low-income families. In addi-
tion, there is a danger of some "double counting." For example, in the area
of energy supply both nuclear and natural gas energy options assume re
106
APPENDIX A
TABLE A.5 Comparison of Selected Mitigation Options in the United States
Mitigation Option
Net Implementation
Costa
Potential Emissionb
Reduction
(t CO2 equivalent
per year)
Building energy efficiency Net benefit 900 millions
Vehicle efficiency (no fleet change) Net benefit 300 million
Industrial energy management Net benefit to low cost 500 million
Transportation system management Net benefit to low cost 50 million
Power plant heat rate improvements Net benefit to low cost 50 million
Landfill gas collection Low cost 200 million
Halocarbon-CFC usage reduction Low cost 1400 million
Agriculture Low cost 200 million
Reforestation Low to moderate costs 200 million
Electricity supply Low to moderate costs 1000 millions
.,
aNet benefit = cost less than or equal to zero
Low cost = cost between $1 and $9 per ton of CO2 equivalent
Moderate cost = cost between $10 and $99 per ton of CO2 equivalent
High cost = cost of $100 or more per ton of CO2 equivalent
bThis "maximum feasible" potential emission reduction assumes 100 percent
implementation of each option in reasonable applications and is an optimistic "upper
bound" on emission reductions.
CThis depends on the actual implementation level and is controversial. This
represents a middle value of possible rates.
Some portions do fall in low cost, but it is not possible to determine the amount
of reductions obtainable at that cost.
eThe potential emission reduction for electricity supply options is actually 1700
Mt CO2 equivalent per year, but 1000 Mt is shown here to remove the double-
counting effect.
NOTE: Here and throughout this report, tons are metric.
SOURCE: Chapter 11 of the Mitigation Panel report.
placement of the same coal-fired power plants. Table A.5, however, pre-
sents only options that avoid double counting. Finally, although there is
evidence that efficiency programs can pay, there is no field evidence show-
ing success with programs on the massive scale suggested here. Thus there
may be very good reasons why "negative cost options" on the figure are not
implemented today.
The United States could reduce its greenhouse gas emissions by between
10 and 40 percent of the 1990 levels at low cost, or perhaps some net
savings, if proper policies are implemented.
APPENDIX A
00
80
60
-
a)
.>
a,
o
-
cn
8 -20
40
20
a
-40
-60
-80
-1 00
107
25% Implementation/High Cost
100% Implementation/Low Cost
100% Annual U.S. CO2
equivalent emissions
0 2 4 6 8
EMISSION REDUCTION (billion tons CO2 equivalent per year)
FIGURE A.4 Comparison of mitigation options. Total potential reduction of CO2-
equivalent emissions is compared to the cost in dollars per ton of CO2 reduction. Options
are ranked from left to right in CO2 emissions according to cost. Some options show
the possibility of reductions of CO2 emissions at a net savings.
SOURCE: Chapter 11 of the report of the Mitigation Panel.
ADAPTING TO ADDITIONAL GREENHOUSE WARMING
38. Will human and natural systems adapt without assistance?
Farmers adjust their crops and cultivation practices in response to weather
patterns over time. Natural ecosystems also adapt to changing conditions.
The real issue is the rate at which human and natural systems will be able to
adjust.
39. At what rates can human and natural systems adapt?
Many human systems have decision and investment cycles that are shorter
than the time in which impacts of climate change would become manifest.
These systems in the United States should be able to adjust to climate
change without governmental intervention, as long as it is gradual and in-
formation about the rates of change is widely available. This applies to
agriculture, commercial forestry, and most of industry. Industrial sectors
with extremely long investment cycles (e.g., transport systems, urban infra-
structure, and major structures and facilities) or requiring high volumes of
water may require special attention. Coastal urban settlements would be
108
APPENDIX A
able to react quickly (within 3 to 5 years) if sea level rises. Response
would be much more difficult, however, where financial and other resources
are limited, such as in many developing countries.
Some natural systems adjust at rates an order of magnitude or more
slower than those anticipated for global-scale temperature changes. For
example, the observed and theoretical migration of large trees with heavy
seeds is an order of magnitude slower than the anticipated change in climate
zones. Furthermore, natural ecosystems cannot anticipate climate change
but must wait until after conditions have changed to respond.
40. What is the value of the vulnerable natural ecosystems?
Natural ecosystems contribute commercial products, but their value is gen-
erally considered to exceed this contribution to the economy. For example,
genetic resources are generally undervalued because people cannot capture
the benefits of investments they might make in preserving biodiversity.
Many species are unlikely to ever have commercial value, and it is virtually
impossible to predict which ones will become marketable.
In addition, some people value natural systems regardless of their eco-
nomic value. Loss of species, in their view, is undesirable whether or not
those species have any commercial value. They generally hold that preser-
vation of the potential for evolutionary change is a desirable goal in and of
itself. Humanity, they claim, should not do things that alter the course of
natural evolution. This view is sometimes also applied to humanity's cul-
tural heritage to buildings, music, art, and other cultural artifacts.
41. How much would it cost to adapt to the anticipated climatic changes?
The panel's analysis suggests that some human and natural systems are not
very sensitive to the anticipated climatic changes." These include most sec-
tors of industry. Other systems are sensitive to climatic changes but can be
adapted at a cost whose present value is small in comparison to the overall
level of economic activity. These include agriculture, commercial forestry,
urban coastal infrastructure, and tourism. Some systems are sensitive, and
their adaptation is questionable. The unmanaged systems of plants and
animals that occupy much of our lands and oceans adapt at a pace slower
than the anticipated rate of climatic change. Their"future under climate
change would be problematic. Poor nations may also adapt painfully. Fi-
nally, some possible climatic changes like shifts in ocean currents have
consequences that could be extremely severe, and thus the costs of adapta-
tion might be very large. However, it is not currently possible to assess the
likelihood of such cataclysmic changes.
No attempt has been made to comprehensively assess the costs of antici-
pated climatic changes on a global basis.
APPENDIX A
42. How much should be spent in response to greenhouse warming?
109
The answer depends on the estimated costs of prevention and the estimated
damages from greenhouse warming. In addition, the likelihood and severity
of extreme events, the discount rate, and the degree of risk aversion will
modify this first-order approximation.
The appropriate level of expenditure depends on the value attached to the
adverse outcomes compared to other allocations of available funds, human
resources, and so on. In essence, the answer depends on the degree of risk
aversion attached to adverse outcomes of climate change. The fact that less
is known about the more adverse outcomes makes this a classic example of
dealing with high-consequence, low-probability events. Programs that truly
increase our knowledge and monitor relevant changes are especially needed.
IMPLEMENTING RESPONSE PROGRAMS
43. What policy instruments could be used to implement response op-
tions ?
A wide array of policy instruments of two different types are available:
regulation and incentives. Regulatory instruments mandate action, and in-
clude controls on consumption (bans, quotas, required product attributes),
production (quotas on products or substances), factors in design or produc-
tion (efficiency, durability, processes), and provision of services (mass tran-
sit, land use). Incentive instruments are designed to influence decisions by
individuals and organizations and include taxes and subsidies on production
factors (carbon tax, fuel tax), on products and other outputs (emission taxes,
product taxes), financial inducements (tax credits, subsidies), and transfer-
able emission rights (tradable emission reductions, tradable credits). The
choice of policy instrument depends on the objective to be served.
44. At what level of society should actions be taken?
Interventions at all levels of human aggregation could effectively reduce
greenhouse warming. For example, individuals could reduce energy con-
sumption, recycle goods, and reduce consumption of deleterious materials.
Local governments could control emissions from buildings, transport fleets,
waste processing plants, and landfill dumps. State governments could restructure
electric utility pricing structures and stimulate a variety of efficiency incen-
tives. National governments could pursue action in most of the policy areas
of relevance. International organizations could coordinate programs in various
parts of the world, manage transfers of resources and technologies, and
facilitate exchange of monitoring and other relevant data.
. . ~. . . . .
110
45. Is international action necessary?
APPENDIX A
The greenhouse phenomenon is global. Unilateral actions can contribute
significantly, but national efforts alone would not be sufficient to eliminate
the problem. The United States is the largest contributor of CO2 emissions
(with estimates ranging from 17 to 21 percent of the global total). But even
if this country were to totally eliminate or offset its emissions, the effect on
overall greenhouse warming might be lost if no other countries acted in
concert with that aim.
46. What about differences between rich and poor countries?
Poor and developing countries are likely to be the most vulnerable to cli-
mate change. In addition, many developing countries today are sorely pressed
in a variety of other ways. They may conclude that other issues have more
immediate consequences for their citizens. Incentives in all parts of the
world for intervention in the area of greenhouse warming may thus draw
heavily on the industrialized nations. They may be called upon to help poor
countries stimulate economic development and thus become better able to
cope with climate change. They may also be asked to provide expertise and
technologies to help poor countries adapt to the conditions they face.
ACTIONS TO BE TAKEN
47. Do scientific assessments of greenhouse warming tell us what to do?
Current scientific understanding of greenhouse warming is both incomplete
and uncertain. Response depends in part on the degree of risk aversion
attached to poorly understood, low-probability events with extremely ad-
verse outcomes. Lack of scientific understanding should not be used as a
justification for avoiding reasoned decisions about responses to possible
additional greenhouse warming.
48. Is it better to prevent greenhouse warming now or wait and adapt to
the consequences?
This complicated question has several parts.
· First, will it be possible to live with the consequences if nothing is
done now? The panel's analysis suggests that advanced, industrialized countries
will be able to adapt to most of the anticipated consequences of additional
greenhouse warming without great economic hardship. In some regions,
climate and related conditions may be noticeably worse, but in other re-
gions better. Countries that currently face difficulty coping with extreme
APPENDIX A
111
climatic events, or whose traditional coping mechanisms are breaking down,
may be sorely pressed by the climatic changes accompanying an equivalent
doubling of atmospheric CO2 concentrations. It is important to recognize
that there may be dramatic improvement or disastrous deterioration in spe-
cific locales. In addition, this analysis applies to the next 30 to 50 years.
The situation may be different beyond that time horizon.
Natural communities of plants and animals, however, face much greater
difficulties. Greenhouse warming would likely stress such ecosystems suf-
ficiently to break them apart, resulting in a restructuring of the community
in any given locale. New species would be likely to gain dominance, with a
different overall mix of species. Some individual species would migrate to
new, more livable locations. Greenhouse warming would most likely change
the face of the natural landscape. Similar changes would occur in lakes and
oceans.
In addition, there are possible extremely adverse consequences, such as
changing ocean currents, that are poorly understood today. The response to
such possibilities depends on the degree of risk aversion concerning those
outcomes. The greater the degree of risk aversion, the greater the impetus
~ . .
for preventive action.
· Second, does it matter when interventions are made? Yes, for three
different kinds of reasons. Because greenhouse gases have relatively long
lifetimes in the atmosphere, and because of lags in the response of the
system, their effect builds up over time. These time-dependent phenomena
lead to the long-term "equilibrium" warming being greater than the "real-
ized" warming at any given point in time. These dynamic aspects of the
climate system show the importance of acting now to change traditional
patterns of behavior that we have recently recognized to be detrimental,
such as heavy reliance on fossil fuels. In addition, the implications of
intervention programs for the overall economy vary with time. Gradual
imposition of restraints is much less disruptive to the overall economy than
their sudden application. Finally, the length of investment cycles can be
crucial in determining the costs of intervention. In addition, some invest-
ments can be thought of as insurance, or payments now to avoid undesirable
outcomes in the future. The choice is made more complicated by the fact
that the outcomes are highly uncertain.
· Third, what discount rate should be used? The selection of a discount
rate is very controversial. Macroeconomic calculations for the United States
show a return on capital investment of 12 percent. The choice of discount
rate reflects time preference. The panel has used discount rates of 3, 6, and
10 percent in its analysis. Finally, consumers often behave as if they have
used a discount rate closer to 30 percent. The panel has also included this
rate for comparison when options involve individual action.
2
APPENDIX A
49. Are there special attributes of programs appropriate for response to
greenhouse warming?
Yes. The uncertainties present in all aspects of climate change and our
understanding of response to potential greenhouse warming place a high
premium on information. Small-scale interventions that are both reversible
and yield information about key aspects of the relevant phenomena are
especially attractive for both mitigation and adaptation options. Monitoring
of emission rates, climatic changes, and human and ecologic responses should
yield considerable payoffs.
Perhaps the most important attribute of preferred policies is that they be
able to accommodate surprises. They should be constructed so that they are
flexible and can change if the nature or speed of stress is different than
. .
antlclpated.
50. What should be done now ?
The panel developed a set of recommended options
or . . ·
~. . ~.
in five areas: reducing
or offsetting emissions, enhancing adaptation to greenhouse warming, im-
proving knowledge for future decisions, evaluating geoengineering options,
and exercising international leadership. The panel recommends moving
decisively to undertake all of the actions described under questions 51 through
55 below.
51. What can be done to reduce or offset emissions of greenhouse gases?
Three areas dominate the panel's analysis of reducing or offsetting current
emissions: eliminating CFC emissions and developing substitutes that minimize
or eliminate greenhouse gas emissions, changing energy policy, and utiliz-
ing forest offsets. Eliminating CFC emissions has the biggest single contri-
bution. Recommendations concerning energy policy are to examine how to
make the price of energy reflect all health, environmental, and other social
costs with a goal of gradual introduction of such a system; to make conser-
vation and efficiency the chief element in energy policy; and to consider the
full range of supply, conversion, end use, and external effects in planning
future energy supply. Global deforestation should be reduced, and a moderate
domestic reforestation program should be explored.
52. What can be done now to help people and natural systems of plants
and animals adapt to future greenhouse warming?
Most of the actions that can be taken today improve the capability of the
affected systems to deal with current climatic variability. Options include
. . . . . . .. . . . .
1 research; making
water supplies more robust by coping with present variability; taking into
consideration possible climate change in the margins of safety for long-
lived structures; and reducing present rates of loss in biodiversity.
maintaining agricultural basic, applied, and experimenta
APPENDIX a
53. What can be done to improve knowledge for future decisions?
113
Action is needed in several areas. Collection and dissemination of data that
provide an uninterrupted record of the evolving climate and of data that are
needed for the improvement and testing of climate models should be ex-
panded. Weather forecasts should be improved, especially of extremes, for
weeks and seasons to ease adaptation to climate change. The mechanisms
that play a significant role in the responses of the climate to changing
concentrations of greenhouse gases need further identification, and quantifi-
cation at scales appropriate for climate models. Field research should be
conducted on entire systems of species over many years to learn how CO2
enrichment and other facets of greenhouse warming alter the mix of species
and changes in total production or quality of biomass. Research on social
and economic aspects of global change and greenhouse warming should be
strengthened.
54. Do geoengineering options really have potential?
Preliminary assessments of these options suggest that they have large po-
tential to mitigate greenhouse warming and are relatively cost-effective in
comparison to other mitigation options. However, their feasibility and es-
pecially the side-effects associated with them need to be carefully exam-
ined. Because the geoengineering options have the potential to affect greenhouse
warming on a substantial scale, because there is convincing evidence that
some of these cause or alter a variety of chemical reactions in the atmo-
sphere, and because the climate system is poorly understood, such options
must be considered extremely carefully. If greenhouse warming occurs,
and the climate system turns out to be highly sensitive to radiative forcing,
they may be needed.
55. What should the United States do at the international level?
The United States should resume full participation in international programs
to slow population growth and contribute its share to their financial and
other support. In addition, the United States should participate fully in
international agreements and programs to address greenhouse warming, in-
cluding representation by officials at an appropriate level.
