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Page 12
3
The Greenhouse Gases and their Effects
Atmospheric concentrations of greenhouse gases are increasingly
well known. Current concentrations, emission accumulation rates,
and atmospheric lifetimes of key gases are summarized in Table 3.1.
Past releases of these gases are less well documented. As shown in
Figure 3.1, atmospheric CO2 began
increasing in the eighteenth century. Regular monitoring, begun in
1958, shows an accelerated increase in atmospheric CO2. About a decade of data also documents
rapidly increasing atmospheric concentrations of CH4. Indirect evidence from tree rings, air
bubbles trapped in glacial ice as it formed, and other sources has
been used to reconstruct past concentrations of these gases.
The dispersion and transformation of greenhouse gases in the
atmosphere are also fairly well understood. There is, however, one
important exception: CO2. Recent
measurements indicate that about 40 percent of the CO2 released into the atmosphere stays there
for decades at least, and about 15 percent seems to be incorporated
into the upper layers of the oceans. The location of the remaining
45 percent of the CO2 from human
activity is not known. Until the redistribution of newly emitted
CO2 is more thoroughly understood,
reliable projections of the rate of increase of atmospheric CO2 will lack credibility even for precisely
estimated emission rates. Even so, it is probably sensible during
the next decade or two to use 40 percent of CO2 emissions as an estimate of the
atmospheric accumulation rate. On a longer time scale, there is, as
of now, no estimation procedure that merits confidence.
Each greenhouse gas is subject to different chemical reactions
in the atmosphere and to different mechanisms of alteration or
removal. Thus projections of future concentrations must account not
only for emissions but also for transformations in the atmosphere.
In addition, the various greenhouse gases have different
energy-absorbing properties. For example, each molecule
Page 13
TABLE 3.1 Key Greenhouse Gases Influenced by Human
Activity
CO2
CH4
CFC-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
484 pptv
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
NOTE: 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: Intergovernmental 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.
of CH4 absorbs radiative energy
25 times more effectively than each molecule of CO2, and CFC-12 is 15,800 times more
effective than CO2 on a per molecule
basis and, since molecules of the two gases have different mass,
5,750 times more effective on a per mass basis. Figure 3.2
incorporates a simple extrapolation of current atmospheric
transformation rates. It displays the incremental energy absorption
rates that would accompany various emission scenarios. The energy
absorption is given in watts per square meter (W/m2) and, in accord with the vocabulary of
this subject, changes in the absorption
Page 14
FIGURE 3.1 Atmospheric concentrations of CO2. Atmospheric CO2 began increasing in the eighteenth
century.
Direct measurements made at the Mauna Loa Observatory in Hawaii
since 1958 indicate that the increase has accelerated.
SOURCE: Adapted
from W. M. Post, T.-H. Peng, W. R. Emanuel, A. W.King, V. H.Dale,
and D. DeAngelis.
1990. The global carbon cycle. American Scientist
78(4):310–326, Figure 3b.
are called "radiative forcing." The curves show the aggregate
contribution of each gas for the period from 1990 to 2030.
Earth's Radiation Balance
The climatic system of the earth is driven by radiant energy
from the sun. Incoming solar radiation at the top of the earth's
atmosphere has an average intensity, over the year and over the
globe, of 340 W/m2. Over the long
time
Page 15
FIGURE 3.2 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 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 0.2W/m2.
SOURCE: Courtesy
of Michael C. MacCracken.
Page 16
FIGURE 3.3 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 17
periods during which the climate is steady, the radiation from
the top of the atmosphere to space has, again on average, the same
intensity. As can be seen in Figure 3.3, the incoming arrows,
representing the incoming intensity or energy flux, balance the
outgoing arrows at the top of the atmosphere. The figure shows a
similar balance at the earth's surface. The downward flow of energy
at the earth's surface is 133 percent of the incident solar
radiation (the 45 percent of the incident solar radiation absorbed
from the incoming energy flow plus the 88 percent downward infrared
radiation). The combined downward transfer of energy at the earth's
surface is greater than that arriving at the top of the atmosphere
because the atmosphere, since it has a temperature greater than
absolute zero, also emits energy. The energy emitted by the
atmosphere adds to that arriving at the surface. The energy
arriving at the earth's surface is balanced by that leaving the
surface (the 29 percent evaporation and mechanical heat transfer
and the 104 percent upward infrared radiation). Similarly, the flow
of energy into the atmosphere (incoming solar radiation not
reflected from the top of the atmosphere, outbound evaporation and
mechanical heat transfer, and upward infrared radiation from the
earth's surface) balances the flow of energy away from the
atmosphere (incoming solar radiation transmitted to the earth's
surface, outgoing infrared radiation, and downward infrared
radiation).
Some of the numbers shown in Figure 3.3 depend on the state of
the atmosphere, for example, its temperature, greenhouse gas
content, cloud distribution, and wind distribution. Others depend
on the temperature of the land and ocean surfaces and/or on the ice
cover. Changes in any and all of these characterizing features can
produce changes in the individual heat fluxes and, in particular,
changes in atmospheric and/or oceanic temperature. These can lead
to changes in cloud cover and humidity that, in turn, induce
further changes in the state of the atmosphere. In addition, both
the interdependencies of the individual heat transfer contributions
illustrated in Figure 3.2 and the (partial) list of possible
changes in characterizing features just mentioned imply that
increases in greenhouse gas concentrations will lead to
modifications of the climate.
Page 18
FIGURE 3.4 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 3.2
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 19
It is important to recognize that these climate modifications
are not instantaneous responses to the gas concentration changes
that produce them. There is always a transient period, or "lag,"
before the equilibrium temperature is reached. In an equilibrium
condition, all of the incoming energy is radiated back to space.
During the transient period, however, some of that incoming heat is
still being used to heat up the deep oceans, which warm more slowly
than the atmosphere. So the surface temperature of the planet is
not yet at the temperature required to balance all of the incoming
energy. Accordingly, the full commitment to temperature rise
corresponding to the greenhouse gas accumulations at a given time
may not become fully apparent for several decades (or more). The
ultimate increase in global average temperature corresponding to a
given increase in greenhouse gas concentration is called the
equilibrium global average temperature.
Figure 3.4 shows possible impacts on the global equilibrium
temperature of changes in atmospheric concentrations of greenhouse
gases. Two scales have been added to the right-hand side of the
figure describing the radiative forcing properties of greenhouse
gases (Figure 3.2). The scale labeled 5°C is associated with
the hypothesis that the equivalent of doubling CO2 would produce a 5° increase in the
equilibrium global average temperature, and the 1°C scale
accompanies the hypothesis that such a doubling would imply a
1° increase.
Figure 3.4 can be used to construct scenarios of changes in
committed future warming resulting from policies that lead to
different greenhouse gas emission rates. In particular, it can be
used to produce a first approximation of the implications for
greenhouse warming of policies resulting in specified emission
rates. This could be very helpful in establishing priorities for
action. For example, the effect of reducing N2O emissions by 10 percent is much smaller
than that of reducing CH4 by 10
percent.
Because it is so difficult to determine the extent of global
warming from temperature measurements alone, it would be very
helpful to monitor the radiation balance of the earth. There are
currently, however, no functioning satellites capable of directly
measuring outbound infrared radiation.
What we can Learn from Climate
Models
The climate extremely variable. Temperature, humidity,
precipitation, and wind vary markedly from week to week and season
to season. These natural variations are commonly much larger than
the changes associated with greenhouse warming. There are also
patterns to these natural variations, and it is these patterns that
we think of as "climate."
The importance of greenhouse warming will be determined by the
magnitude and abruptness of the associated climatic changes. Useful
prediction requires credible quantitative estimates of those
changes. Numerical computer simulations using general circulation
models (GCMs) are generally considered
Page 20
the best available tools for anticipating climatic changes. Data
from previous interglacial periods can be compared to results from
computer models. Past conditions, however, are inexact metaphors
for current increases in atmospheric concentrations of trace
gases.
In order to simulate the intricate climatic system, GCMs
themselves are complicated. They are complex computational schemes
incorporating well-established scientific laws, empirical
knowledge, and implicit representations. Mechanisms occurring on
scales smaller than the smallest elements of the atmosphere, land,
or oceans resolved in the GCM (i.e., "subgrid" scales) are
represented by mathematical characterizations called
"parameterizations." A typical GCM involves hundreds of thousands
of equations and dozens of variables. About half a dozen different
model types exist, and others are being developed.
One major drawback common to all current GCMs is that they lack
adequately validated representations of important factors like
cloud cover feedback, ocean circulation, and hydrologic
interactions. Therefore it is unreasonable to expect the models to
provide precise predictions, decades into the future, of global
average temperature. This is especially so given that the expected
global temperature rise is smaller than current naturally occurring
regional temperature fluctuations on all time scales, daily,
seasonal, and decadal.
General circulation models most commonly simulate the
equilibrium climatic conditions associated with doubling
atmospheric concentrations of CO2
compared to preindustrial levels. Current GCM simulations based on
these assumptions show a range of global average equilibrium
temperature increases of 1.9° to 5.2°C (3.4° to
9.4°F). Many other calculations and simulations have been
conducted; some with no cloud interactions, some with only a simple
heat sink in place of oceans, some with no distinction between day
and night. For the most part, these calculations also provide
predictions within or close to this range.
The GCM results have been interpreted in slightly different ways
by groups with differing perspectives. The Intergovernmental Panel
on Climate Change (IPCC) used a range of 2° to 4°C
(3.6° to 7.2°F) accompanying an equivalent doubling of
preindustrial CO2. The National
Research Council's Board on Atmospheric Sciences and Climate used a
range of 1.5° to 4.5°C (2.7° to 8.1°F), numbers
receiving slightly greater usage among atmospheric scientists.
For the purposes of informed policy choice, it is crucial to
acknowledge the limited capability of the GCMs. This is especially
true because there is no clear connection between temperature
records of the last century and the atmospheric accumulation of
greenhouse gases. The temperature record for the northern
hemisphere, for example, shows some rise until about 1940, a slight
decrease from 1940 until the mid-1970s, followed by another
rise.
Page 21
There currently is no persuasive evidence that these variations
were driven by growing atmospheric concentrations of greenhouse
gases. The 100-year temperature record is not inconsistent with the
range of climate sensitivity predicted by the GCMs, but neither is
it inconsistent with the natural variability of the earth's
climate.
There is another key limitation on the knowledge acquired from
GCMs. In essence there are fewer than two dozen GCM simulation runs
with five independent models on which to base conclusions. Every
one incorporates untested and unvalidated hypotheses. They may be
sensitive to changes in ways that current calculations have not yet
revealed. For example, a recent examination of available computer
runs shows considerable difference in the treatment of clouds.
Although all runs yield similar results for a "clear sky" without
clouds, their results vary substantially when clouds are included.
The limited number of GCM simulations has two important
consequences. First, there are too few runs to scientifically
determine "most likely" values within the range. Second, it is not
strictly possible to eliminate temperature changes of less than
1°C (1.8°F) or greater than 5°C (9°F).
Although GCMs cannot produce scientific "proof" in their
predictions, they do map seasonal cycles of surface temperature
quite well. GCMs also reasonably simulate daily and annual
variability in air pressure patterns over large areas. In addition,
most models represent the broad features of wind patterns, and the
most recent models provide realistic simulations of winter and
summer jet streams in the lower stratosphere. GCM simulations of
other climate variables, such as precipitation, soil moisture, and
north-south energy transport, are much less satisfactory. They do
not provide credible quantitative estimates of the longer-term
changes in global climate that might be driven by greenhouse gas
accumulations.
The panel believes that prudent policy choices should be based
on conservative assumptions in the face of large uncertainty. The
panel uses a range of 1° to 5°C (1.8° to 9°F) and
notes that it is broader than ranges adopted by other groups. In
the panel's view, this range expresses much less unwarranted faith
in the numbers produced by GCMs than does a narrower range.
Simply looking at the global average temperature associated with
an equivalent doubling of preindustrial levels of CO2 does not convey some important aspects
of climate change. For example, there is no particular significance
to exactly that level of greenhouse gas concentrations. In fact,
unless serious efforts to limit releases of greenhouse gases are
undertaken, atmospheric concentrations will exceed this level
during the next century. In addition, GCMs may not produce reliable
information about regional or local aspects of climate change that
are of greatest interest to decision makers. These include amounts
and timing of precipitation, frequency and timing of floods and
temperature extremes, and wind extremes. Soil moisture content,
dates
Page 22
of first and last frost, and timing of exceptionally hot days
are all more important for plant life than is average
temperature.
What we can Learn from the Temperature
Record
Global temperature data are available for the period 1890 to
1990, but those from the earlier half of the century are difficult
to interpret with confidence. The most comprehensive assessment of
the record of surface temperature, depicted in Figure 3.5, reveals
a warming since the late nineteenth century of between 0.3° and
0.6°C (0.5° and 1.1°F). This warming is supported by
several different kinds of information. Adjustments have been
attempted to negate known complicating factors such as the biases
introduced by the location of long-term measurement stations near
urban areas with their attendant local warming.
To some extent the natural temperature variation in the climatic
system makes it difficult to interpret the observational record. In
particular, it is not possible to determine how much, if any, of
the average global temperature rise over the last century might be
attributed to greenhouse warming.
Increasing atmospheric concentrations of greenhouse gases may
produce changes in both the magnitude and the rate of change of
global average temperature that have few or no precedents in the
earth's recent history. Figure 3.6 depicts estimates of the ranges
of temperature in various periods of the past. A range of less than
1°C (1.8°F) was experienced in the last century, less than
2°C (3.6°F) in the last 10,000 years, and perhaps 7°C
(13°F) in the last million years. Figure 3.6 shows these
temperatures compared to a line representing an average global
temperature of about 15°C (59°F), which is the global
average temperature for the period 1951 to 1980. During this period
the largest number of temperature recording stations were
operating, and the averages for this period are commonly used as a
base against which to assess global temperatures. Despite the
modest decline in the average temperature in the northern
hemisphere between about 1940 and 1975, we are still in an
unusually warm period of earth's history. Thus the temperature
increases of a few degrees projected for the next century are not
only large in recent historical terms, but could also carry the
planet into largely unknown territory.
Recent analyses, however, raise the possibility that some
greenhouse warming could be offset by the cooling effect of sulfate
aerosol emissions. Such emissions may have contributed to regional
temperature variations and to differences in the temperature
records of the northern and southern hemispheres.
On the geologic time scale, many things affect climate in
addition to trace gases in the atmosphere: changes in solar output,
changes in the
Page 23
FIGURE 3.5 Combined land air and sea surface
temperature relative to 1951–1980 average temperatures. Land
air temperatures are typically measured 1 to 2 m above ground
level. Sea surface temperatures are typically measured in the layer
from the ocean's surface to several meters below.
SOURCES: Land air
temperatures are updated from P. D. Jones, S. C.B. Raper, R. S.
Bradley, H. F. Diaz, P. M. Kelly, and T. M. L. Wigley. 1986.
Southern hemisphere surface air temperature variations,
1851–1984. Journal of Climate and Applied Meteorology
25:1213–1230. P. D. Jones, S. C. B. Raper, R. S. Bradley, H.
F. Diaz, P. M. Kelly, and T. M. L. Wigley. 1986. Northern
hemisphere surface air temperature variations, 1851–1984.
Journal of Climate and Applied Meteorology 25:161–179.
Sea surface temperatures are from the U.K. Meteorological Office
and the COADS as adjusted by G. Farmer, T. M. L. Wigley, P. D.
Jones, and M. Salmon. 1989. Documenting and explaining recent
global-mean temperature changes. Final Report to NERC,
Contract GR/3/6565. Norwich, United Kingdom: Climate Research
Unit.
Page 24
FIGURE 3.6 An approximate temperature history of
the northern hemisphere for the last 850,000 years. The panels are
at the same vertical scale. The top panel shows the last million
years, the second panel amplifies the last 100,000 years, the third
panel the last 10,000 years, and the bottom panel the last 1,000
years. The horizontal line at 15°C is included for reference
and is the approximate average global temperature for the period
1951 to 1980. Considerable uncertainty attaches to the record in
each panel, and the temperature records are derived from a variety
of sources, for example, ice volume, as well as more direct data.
Spatial and temporal (e.g., seasonal) variation of data sources is
also considerable.
SOURCE: National
Research Council. 1983. Changing Climate:Report of the Carbon
Dioxide Assessment Committee. Washington, D.C.: National
Academy Press .Figure 1.14.
Page 25
earth's orbital path, changes in land and ocean distribution,
changes in the reflectivity of the earth, and cataclysmic events
like meteor impacts or extended volcanic eruptions.
These and other contributors to the earth's climate make it
difficult to interpret the temperature history. Just as it is
impossible to rule out natural variability, it is also impossible
to rule out an underlying trend, so that the observed rise of
0.3° to 0.6°C (0.5° to 1.1°F) may be superimposed
on a long-term (but nonuniform) rise or fall in global
temperature.
Sea Level
Average sea level of the oceans has varied throughout earth's
history, and it is changing slightly today. Global sea level was
about 100 m (328 feet) lower than current levels at the coldest
point of the last ice age about 18,000 years ago. During the
geologic past, there have been repeated variations from present sea
level of more than this amount, both during times of intense
glaciation and during periods in which the earth was free of ice.
All of human civilization, however, has lived in a period when the
average sea level was roughly as it is today.
Tide gauges measure sea level variations in relation to a fixed
point on land and thus record ''relative sea level" (RSL). RSL at
any particular place varies over time and space. The direct causes
of these variations include vertical motions of land to which the
tide gauge or other measuring device is attached and changes in the
volume of sea water in which the gauge is immersed. Differences in
atmospheric pressure, water runoff from land, winds, ocean
currents, and the density of sea water all cause variations in sea
level in comparison to the global average sea surface.
Climate-related contributions to sea level change are of two
kinds: variations in the actual amount or mass of water in the
ocean basins (due mostly to changes in precipitation and runoff)
and thermal expansion or contraction (changes in the density of
water, due to variations of temperature and salinity).
The melting of the northern continental ice sheets between
15,000 and 7,000 years before the present probably accounts for
most of the rise of the sea to present levels. Some have suggested
that global warming due to increased atmospheric concentrations of
greenhouse gases could lead to disintegration of the West Antarctic
Ice Sheet, most of which is grounded below sea level. If climate
warms and warmer ocean water intrudes under the ice sheet, the
release of ice from the sheet would accelerate. The melting of the
West Antarctic Ice Sheet is quite unlikely, however, and virtually
impossible by the end of the next century. Estimates based on a
combined oceanic and atmospheric GCM suggest that several hundred
years would be required to achieve this amount of warming. The
principal effects on sea level of greenhouse warming over the time
period examined in this study will thus be due to thermal
expansion.
Page 26
Thermal expansion (and contraction) of the oceans, caused by a
combination of increasing (decreasing) temperature and salinity,
accounts for seasonal and interannual variations in sea level.
These changes are not large enough, however, to account for the
differences over tens of thousands of years. Warming the entire
ocean from 0°C (32°F) to the current global average ocean
temperature would result in a thermal expansion of about 10 m (33
feet).
In order to estimate oceanic thermal expansion from greenhouse
warming, changes in the temperature, salinity, and density of the
oceans have to be considered. Two types of models yield somewhat
different results, depending on the assumptions made concerning
transfer of heat into the deep ocean waters. The results are 20 to
110 cm (8 to 43 inches) when heat is carried downward by eddy
diffusion and 10 to 50 cm (4 to 20 inches) when some downward
diffusion is balanced by upwelling from the deep oceans. Both
estimates are for the year 2100 and an equivalent of doubling the
preindustrial atmospheric concentration of CO2. The panel used a range of sea level
rise from 0 to 60 cm (24 inches) for a doubling of CO2.
Possible Dramatic Changes
The behavior of complex and poorly understood systems can easily
surprise even the most careful observer. There are many aspects of
the climate system that we do not understand well and which could
provide such surprises. In particular, some radical changes that
could result from increases in global temperatures must be
considered plausible even though our understanding of them is not
sufficient to analyze their magnitudes or likelihoods:
1. CH4 could be released as
high-latitude tundra melts, providing a sudden increase of CH4, which would add to greenhouse
warming.
2. The combination of increased runoff of fresh water in
high latitudes and a reduced temperature differential from equator
to pole could result in radically changed major ocean currents
leading to altered weather patterns.
3. There could be a significant melting of the West
Antarctic Ice Sheet, resulting in a sea level several meters higher
than it is today.
Such major (and perhaps rapid) changes could be accompanied by
more dramatic warming of the atmospheric and oceanic systems than
is now apparent. No credible claim can be made that any of these
events is imminent: nonetheless, with continuing greenhouse gas
accumulations, none of them are precluded.
Conclusions
Neither the available climate record nor the limited
capabilities of the climate models permit a reliable forecast of
the implications of continued
Page 27
accumulations of greenhouse gases in the atmosphere. Neither do
they permit an assessment as to whether the increase from 1890 to
1990 in global average temperature can be attributed to greenhouse
gases. However, it is probable that some positive rate of warming
will accompany continued accumulation of greenhouse gases in the
atmosphere. An important question is: When will we have a more
definite fix on the rate at which warming will occur?
It is unlikely that our understanding of such basic phenomena as
the role of clouds and ocean dynamics will improve greatly over the
next few years. It is also unlikely that a useful level of
improvement in regional predictive capability will emerge in that
time. A few decades may be required before atmospheric scientists
produce the answers we seek. Some current limitations on our
knowledge could be reduced by better characterization of such
"subgrid" processes as precipitation and mechanical heat transfer,
better coupling of atmospheric, land surface, and oceanic models,
and better models of the role of ecosystems. Access to computers
with greater capacity and speed would accelerate these
improvements. All of these depend in large measure on progress in
the scientific understanding on which the models are based.
The overall magnitude of greenhouse warming and its rate of
emergence can only be inferred from several different kinds of
information. The pieces of the puzzle are currently understood with
varying degrees of uncertainty. Nevertheless, there is clear
evidence and wide agreement among atmospheric scientists about
several basic facts:
1. The atmospheric concentration of CO2 has increased 25 percent during the last
century and is currently increasing at about 0.5 percent per
year.
2. The atmospheric concentration of CH4 has doubled during that period and is
increasing at about 0.9 percent per year.
3. CFCs, which are man-made and have been released into
the atmosphere in quantity only since World War II, are currently
increasing at about 4 percent per year.
4. Items 1, 2, and 3 are primarily direct consequences of
human activities.
5. Current interpretations of temperature records reveal
that the global average temperature has increased between 0.3°
and 0.6°C (0.5° and 1.1°F) during the last century.
As a result, the panel concludes that there is a reasonable
chance of the following:
1. In the absence of greater human effort to the contrary,
greenhouse gas concentrations equivalent to a doubling of the
preindustrial level of CO2 will
occur by the middle of the next century.
2. The sensitivity of the climatic system to greenhouse
gases is such that the equivalent of doubling CO2 could ultimately increase the average
global temperature by somewhere between 1° and 5°C
(1.8° and 9°F).
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3. The transfer of heat to the deep oceans occurs more
slowly than within the atmosphere or the upper layers of the ocean.
The resulting transient period, or "lag," means that the global
average surface temperature at any time is lower than the
temperature that would prevail after all the redistribution had
been completed. At the time of equivalent CO2 doubling, for example, the global
average surface temperature may be as little as one-half the
ultimate equilibrium temperature associated with those
concentrations.
4. A rise of sea level may accompany global warming,
possibly in the range of 0 to 60 cm (0 to 24 inches) for the
temperature range listed above.
5. Several troublesome, possibly dramatic, repercussions
of continued increases in global temperature have been suggested.
No credible claim can be made that any of these events is imminent,
but none of them are precluded.
