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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 balance 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 chlorofluorocarbons (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 possibility 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
Page 664
FIGURE A.1 Earth's radiation balance. The solar
radiation is set at 100 percent; all other values are in relation
to it. About 25 percent of incident solar radiation is reflected
back into space by the atmosphere, about 25 percent is absorbed by
gases in the atmosphere, and about 5 percent is reflected into
space from the earth's surface, leaving 45 percent to be absorbed
by the oceans, land, and biotic material (white arrows).
Evaporation and mechanical heat transfer inject
energy into the atmosphere equal to about 29 percent of incident
radiation (grey arrow). Radiative energy emissions from the earth's
surface and from the atmosphere (straight black arrows) are
determined by the temperatures of the earth's surface and the
atmosphere, respectively. Upward energy radiation from the earth's
surface is about 104 percent of incident solar radiation.
Atmospheric gases absorb part (25 percent) of the solar radiation
penetrating the top of the atmosphere and all of the mechanical
heat transferred from the earth's surface and the outbound
radiation from the earth's surface. The downward radiation from the
atmosphere is about 88 percent and outgoing radiation about 70
percent of incident solar radiation.
Note that the amounts of outgoing and incoming
radiation balance at the top of the atmosphere, at 100 percent of
incoming solar radiation (which is balanced by 5 percent reflected
from the surface, 25 percent reflected from the top of the
atmosphere, and 70 percent outgoing radiation), and at the earth's
surface, at 133 percent (45 percent absorbed solar radiation plus
88 percent downward radiation from the atmosphere balanced by 29
percent evaporation and mechanical heat transfer and 104 percent
upward radiation). Energy transfers into and away from the
atmosphere also balance, at the atmosphere line, at 208 percent of
incident solar radiation (75 percent transmitted solar radiation
plus 29 percent mechanical transfer from the 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 absorbed 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 unchanging); 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.
Page 665
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 production began a few decades ago.
Continued increases in atmospheric concentrations 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 atmospheric concentrations of
greenhouse gases or to other natural or human causes. The
temperature record much before 1900 is not reliable for estimates
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
Page 666
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 current 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. However,
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 contrast,
decomposes in the atmosphere in about 10 years. The greenhouse gas
with the longest atmospheric lifetime (except for CO2), CFC-115, has an average atmospheric
lifetime of about 400 years. The overall contribution of greenhouse
gases to global warming depends on their atmospheric lifetime as
well as their ability to trap radiation. Table A.1 shows the
relevant characteristics 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
Page 667
TABLE A.1 Key Greenhouse Gases Influenced by Human
Activity
CO2
CH4
CPC-11
CFC-12
N2O
Preindustrial atmospheric concentration
280 ppmv
0.8 ppmv
0
0
288 ppbv
Current atmospheric concentration (1990)a
353 ppmv
1.72 ppmv
280 pptv
484pptv
310 ppbv
Current rate of annual atmospheric
accumulationb
1.8 ppmv (0.5%)
0.015 ppmv (0.9%)
9.5 pptv (4%)
17 pptv (4%)
0.8 ppbv (0.25%)
Atmospheric lifetime (years)c
(50–200)
10
65
130
150
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.
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 characterizes
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.
SOURCE: Intergovernment Panel on Climate Change.
1990. Climate Change: The IPCC Scientific Assessment, J. T.
Houghton, G. J. Jenkins, and J. J. Ephraums, eds. New York:
Cambridge University Press. 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 effects 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
Page 668
FIGURE A.2 Additional radiative forcing of
principal greenhouse gases from 1990 to 2030 for different emission
rates. The horizontal axis shows changes in greenhouse 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 estimates 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 thecurvelabeled "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 0.2W/m2.
SOURCE: Courtesy
of Michael C. MacCracken.
Page 669
gas is plotted for different emissions of each gas compared to
1990 emission levels. The figure shows the impact of different
percentage changes in emissions (compared to 1990 emission rates)
on the radiative forcing. Figure 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
initial 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 surface. Increasing the geographical coverage
of low-level clouds would reduce greenhouse warming, whereas
increasing the amount of high, convective 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.
Satellite 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
greenhouse 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 greenhouse 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 equivalent to a doubling of the
preindustrial level of atmospheric CO2. The midpoint of this range corresponds
to an average global climate warmer than
Page 670
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 0 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.
SOURCE: Courtesy
of Michael C. MacCracken.
Page 671
any in the last 1 million years. The consequences of this amount
of warming 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 redistribute 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 concentrations is called the
"equilibrium temperature.'' Since atmospheric concentrations of
greenhouse gases are constantly changing, the temperature measured
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 greenhouse warming is
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 numerical models will help
characterize the climatic system, including the atmosphere, oceans,
and land-based elements like forests and ice fields. However, only
careful interpretation of actual measurements can reveal what has
occurred and when.
15. How can credible estimates of future 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
improvements 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
Page 672
• Atmospheric lifetimes of greenhouse gases until they
decompose or transfer into sinks
• Effectiveness of positive or negative feedback mechanisms
that enhance or reduce warming
• Human actions, which effect 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 CO2 and other greenhouse gases, especially
CH4
• Interactions between temperature change and cloud
formation and the resulting feedbacks
• Effects of global warming on biological sources of
greenhouse gases
• Interactions between changing climate and ice cover and
the resulting feedbacks
• 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 differences in regional
precipitation and reduction of snow and ice fields at high
latitudes.
19. Are these 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, projections 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
Page 673
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?
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 pressure, 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 temperatures 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.
21. 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 understood, 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 greenhouse gases) or
potentially very risk (if we use climate engineering). However, a
comprehensive action program could slow or reduce the onset of
greenhouse warming.
Page 681
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 natural 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 offsetting emissions:
• ''Direct" reduction or offsetting of emissions through
altering equipment, 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 incentives 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 implementation 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,
atmospheric lifetime of the gas, and other relevant factors
affecting the total contribution of that gas to greenhouse warming.
Second, the amount of
Page 682
TABLE A.4 Estimated 1985 Global Greenhouse Gas Emissions
from Human Activities
Greenhouse Gas Emissions (Mt/yr)
CO2-equivalent
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
0.5
150
(>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)
NOTE: Mt/yr = million (106) metric tons (t) per year. All entries
are rounded because the exact values are controversial.
aCO2-equivalent emissions are calculated from
the Greenhouse Gas Emissions column by using the following
multipliers:
CO2
1
CH4
21
CFC-11 and -12
5,400
N2
290
Numbers in parentheses are percentages of
total.
bTotal
does not sum due to rounding errors.
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.
Page 683
CO2 that would produce the same
amount of forcing at the surface is calculated. 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 lifetimes
of these gases in the atmosphere.
35. What mitigation options are most cost-effective?
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
removed 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 potential environmental
impacts.
37. How much would it cost to significantly reduce current
U.S. greenhouse 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 energy: efficiency improvements in lighting and
appliances, white roofs and paving to enhance reflectivity, and
improvement in building and construction 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 addition, there is a danger of some "double
counting." For example, in the area of energy supply both nuclear
and natural gas energy options assume replacement
Page 684
TABLE A.5 Comparison of Selected Mitigation Options in
the United States
Mitigation Option
Net Implementation Costa
Potential Emission Reductionb (t CO2 equivalent per year)
Building energy efficiency
Net benefit
900 millionc
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 costd
200 million
Electricity supply
Low to moderate costd
1000 millione
NOTE: Here and throughout this report, tons are
metric.
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.
dSome
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.
of the same coal-fired power plants. Table A.5, however,
presents only options that avoid double counting. Finally, although
there is evidence that efficiency programs can pay, there is no
field evidence showing 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.
Page 685
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.
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 information 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 infrastructure,
and major structures and facilities) or requiring high volumes of
water may require special attention. Coastal urban settlements
would be
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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 generally 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 economic value. Loss of species, in their view, is
undesirable whether or not those species have any commercial value.
They generally hold that preservation 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
cultural 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 sectors 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. Finally, some possible climatic
changes like shifts in ocean currents have consequences that could
be extremely severe, and thus the costs of adaptation 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
anticipated climatic changes on a global basis.
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42. How much should be spent in response to greenhouse
warming?
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 options?
A wide array of policy instruments of two different types are
available: regulation and incentives. Regulatory instruments
mandate action, and include controls on consumption (bans, quotas,
required product attributes), production (quotas on products or
substances), factors in design or production (efficiency,
durability, processes), and provision of services (mass transit,
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 transferable 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 consumption, 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 incentives. 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.
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45. Is international action necessary?
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 climate 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 adverse 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 regions
better. Countries that currently face difficulty coping with
extreme 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 specific locales. In addition, this analysis
applies to the next 30 to 50 years. The situation may be different
beyond that time horizon.
Page 689
Natural communities of plants and animals, however, face much
greater difficulties. Greenhouse warming would likely stress such
ecosystems sufficiently 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 "realized" 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 investments 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.
Page 690
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 anticipated.
50. What should be done now?
The panel developed a set of recommended options in five areas:
reducing or offsetting emissions, enhancing adaptation to
greenhouse warming, improving 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 utilizing forest offsets.
Eliminating CFC emissions has the biggest single contribution.
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 conservation 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 maintaining agricultural basic,
applied, and experimental 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.
Page 691
53. What can be done to improve knowledge for future
decisions?
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 expanded. 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
quantification 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 potential to mitigate greenhouse warming and are relatively
cost-effective in comparison to other mitigation options. However,
their feasibility and especially the side-effects associated with
them need to be carefully examined. 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
atmosphere, 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, including representation by
officials at an appropriate level.
