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17
A Greenhouse Forcing and Temperature Rise Estimation Procedure
This chapter provides a compact estimation procedure for
projecting greenhouse warming. It can be applied to a broad but
simple family of emission scenarios and to either of two
postulated climate sensitivities. Using this procedure, one can
determine (1) the increment in the radiative forcing from 1990 to
2030 that would accompany the scenario under examination and (2)
the equilibrium global mean temperature increase that would be
consistent with that increment and the chosen forcing. Because of
limitations in our understanding of the oceans, it is not possible
to incorporate a simple estimation procedure for time-dependent
(transient) climatic changes. But since the focus here is on the
long-term implications of human actions, transient effects are less
important than equilibrium effects.
Although emissions will undoubtedly continue after the year 2030
and would contribute to further climate change, the procedure
presented in this chapter is limited to the next few decades. It is
limited in this way both because of the difficulty of producing
credible projections beyond that period and because of the panel's
emphasis on practical actions that can be undertaken now.
Certainly, actions or inaction during this period will have
important longer-term implications that deserve attention in
broader considerations of a sustainable environment. In addition,
this interval was selected because 2030 is roughly the time at
which the IPCC's high scenario (sometimes referred to as
"business-as-usual") suggests that an equivalent doubling of the
preindustrial CO2 concentration may
occur if minimal (or no) actions are taken to limit the recent,
precontrol rates of increase of greenhouse gas emissions
(Intergovernmental Panel on Climate Change, 1990). Beyond 2030,
various approximations that are made may also become less
valid.
The procedure takes into account the rate at which, following
its emission
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Representative terms from entire chapter:
climate sensitivity
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into the atmosphere, each greenhouse gas is removed from the
atmosphere (e.g., by transport to the oceans or by chemical
reaction), but no account is taken of the extent to which one
greenhouse gas (including ozone and water vapor in the
stratosphere) may be affected by or introduced as a by-product of a
chemical reaction that depletes another. In particular, in accord
with the uncertainties attending the fate of CO2 emissions (Emanuel et al., 1989), the
procedure is approximately consistent with the observations of the
past century; i.e., approximately 60 percent of the CO2 emissions introduced into the atmosphere
are removed promptly, and the remaining 40 percent contribute to
long-term (i.e., several century) enhancement of the CO2 concentration. The current
concentrations, current emission rates, and lifetimes of the most
important of the greenhouse gases that were considered are given in
Table 17.1, and projected concentrations are shown in Figure 17.1.
The radiative forcing associated with each of these gases is
depicted as a function of its concentration level in Figure
17.2.
As indicated in Chapter 18, the Effects Panel agrees that it is
plausible to expect that the increase in the equilibrium global
mean temperature of our climatic system that might be implied by an
equivalent CO2 doubling would
TABLE 17.1 1990 Atmospheric Concentrations, Emissions,
and Lifetimes of Key Greenhouse Gases
1990 Emissions
Species
1990 Atmospheric Concentration
Natural
Anthropogenica
Assumed Lifetime (years)
CO2
354 ppmv
—
6 Pg C/yr
—b
CH4
1.72 ppmv
200 Tg/yr
340 Tg/yr
10
N2O
310 ppbv
9.3 Tg N/yr
4 Tg N/yr
150
CCl4
146 pptv
—
119 Gg/yr
50
CH3CCl3
158 pptv
—
738 Gg/yr
7
CFC-11
280 pptv
—
361 Gg/yr
60
CFC-12
484 pptv
—
428 Gg/yr
130
CFC-113
60 pptv
—
202 Gg/yr
90
CFC-114
15 pptv
—
15.7 Gg/yr
200
CFC-115
5 pptv
—
6.9 Gg/yr
400
HCFC-22
122 pptv
—
179 Gg/yr
15
Halon-1301
2 pptv
—
7 Gg/yr
110
aOnly
anthropogenic emissions are assumed to increase or decrease because
of future policy and technological developments.
bThere is
no simple method for calculating CO2
lifetime.
SOURCE: Courtesy of Michael C. MacCracken.
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FIGURE 17.1 Projected concentrations of various
greenhouse gases in the year 2030
as a function of change in anthropogenic emissions of those gases
over the period 1990 to 2030.
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FIGURE 17.2 Increment in radiative forcing of
the surface-troposphere system
over the period 1990 to 2030 for concentrations of various
greenhouse gases that may occur over that period.
lie between 1° and 5°C (1.8° to 9°F).
Accordingly, the change from its 1990 value of the equilibrium
global mean temperature associated with the 2030 concentration of
each of the greenhouse gases is calculated for the 1°
sensitivity and for the 5° sensitivity.
The values of the forcing and of the change in equilibrium
global average temperature that have been compiled in accord with
the foregoing description can be extracted for any chosen scenario
from Figure 17.3.
Results
Future changes in concentration are based on changes in
emissions from their 1990 baseline values. Table 17.1 lists the
1990 baseline atmospheric concentrations and estimated emissions,
subdivided into natural and anthropogenic sources. These estimates
are quite close to the IPCC estimates, differing slightly because
of updated estimates of lifetimes and because of calibration of the
model used in representing the carbon budget.
Figure 17.1 shows the atmospheric concentrations in the year
2030 resulting from linear changes in 1990 emissions over the
period from 1990 to 2030. Generally, percentage changes from +100
percent to -100 percent encompass the plausible scenarios. However,
CO2 emissions could, under some
scenarios more than double over the next 40 years (e.g., see
Trabalka,
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FIGURE 17.3 The incremental change to the
radiative forcing of the surface-troposphere system (in watts per
square meter)
as a function of the percentage change in anthropogenic emissions
of various species from 1990 to 2030. The asterisk indicate
the projected emissions of the various species, assuming no
additional regulatory policies, based on IPCC estimates and
the
original restrictions agreed to under the Montreal Protocol.
Interactions among different species and the indirect
chemical
effects induced by these species (e.g., on stratospheric ozone) are
not included. The right-hand vertical scales show two
ranges of global average temperature responses. The first
corresponds to a climate whose temperature response to an
equivalent of doubling of the CO2
concentration is 1°C (1.8°F); the second corresponds to a
rise of 5°C (9°F) for an
equivalent doubling of CO2. These
scales give the incremental change in temperature that is protected
from the 1990 to
2030 emissions once climatic equilibrium is reestablished. To
obtain the projected equilibrium change in temperature from
preindustrial times through 2030, add to the value from the
right-hand scale an amount equal to 55 percent of the climate
sensitivity to doubling of the CO2
concentration. Of this total amount, a warming of 0.3° to
0.6°C (0.5° to 1.1°F) is estimated
to have already occurred from the mid-nineteenth century to 1990.
Assumptions are as in Figure 17.2.
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1985), and emissions of hydrochlorofluorocarbons (HCFCs) and
hydrofluorocarbons (HFCs) that are being introduced as replacement
compounds for those CFCs being controlled under the Montreal
Protocol could increase by 1,000 percent or more from present
emission levels.
The effects of the changes in emissions can then be converted
into changes in the radiative forcing as a function of
concentration. Figure 17.2 shows the relationships based on the
formulae for estimating changes in radiative forcing as selected by
IPCC, which are generally in reasonable agreement with the other
results (Intergovernmental Panel on Climate Change, 1990). Note
that, among the simplifications, no account is taken here of the
radiative effects of the CO2 and
stratospheric H2O that would result
from chemical destruction of CH4 and
no allowance is made for changes in stratospheric or tropospheric
ozone. In addition, the radiative forcings caused by the different
species are not completely equivalent (Wang et al., 1991), but the
approximations here are adequate for this comparative analysis.
Based on these relations, Figure 17.3 relates changes in
radiative forcing to possible changes in emissions over the period
1990 to 2030 for CO2, CH4, N2O,
CFCs controlled by the Montreal Protocol (CFCs 11, 12, 113, 114,
and 115), and halocarbons not controlled by the Montreal Protocol
(including HCFC-22, CH3CCl3, and CCl4). As indicated above, these estimates do
not account for the climate-chemistry couplings involving species
that are not directly emitted (e.g., ozone changes, stratospheric
water vapor, and CH4 conversion to
CO2), which will likely cause
noticeable changes, but not so large as to change the general
character of the curves.
Figure 17.3 also shows (with asterisks) the expected changes in
emissions for CO2, CH4, and N2O
assuming a scenario similar to the IPCC high scenario. The asterisk
on the CFC curve, for example, indicates a 50 percent reduction,
even though stricter controls have recently been agreed upon under
the Montreal and London Protocols.
To provide an indication of the potential climatic importance of
the change in radiative flux, temperature change multipliers have
been used to produce the vertical coordinates on the right-hand
side of the figure. The commitment to future warming (i.e., the
expected equilibrium temperature increase) that would occur as a
consequence of emissions from 1990 to 2030 can be derived by taking
the product of the multiplier and the climate sensitivity to a
CO2 doubling. This product, for two
different climate sensitivities, is displayed on the vertical
coordinate on the right-hand side of Figure 17.3. Thus, if the
sensitivity is 1°C (1.8°F), the CO2 contribution to future warming assuming
constant emissions (0 percent change) is about 0.24°C
(0.43°F). The estimates for climate sensitivity span a rather
wide range, indicated in the figure by including coefficients for
climate sensitivities of 1° and 5°C (1.8° to 9°F).
This wide range of climate sensitivity estimates creates a large
range in possible temperature changes from 1990 to 2030,
demonstrating
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the importance of uncertainties created by our limited
understanding of oceanic behavior and other phenomena.
Although the full warming due to emissions from 1990 to 2030
will not occur until a few decades thereafter, there will also be
warming during the period from emissions prior to 1990. In
addition, the continuing emissions beyond 2030 will lead to further
warming over the longer term.
Implications
The results of this analysis offer several points for
consideration in attempting to optimize a greenhouse gas limitation
policy.
1. The slope of the N2O
curve in Figure 17.3 is so flat that even large changes in
emissions would have a relatively minor effect over this period.
This assumes, however, that the ozone interactions with N2O are small. In any case, continued
emissions of N2O, over the long
term, will not lead to a significant increase in warming despite
its long lifetime.
2. Once CFC emissions are reduced by 50 percent, little
more is gained (with respect to their greenhouse warming effect) by
further reduction in the period to 2030, although a CFC buildup
would continue to occur at this level of emissions. (In addition,
the effects of CFCs on ozone need to be considered.) Clearly,
however, a failure to implement the Montreal Protocol would have a
substantial warming effect (as pointed out by Hansen et al., 1989).
Unless emissions of the uncontrolled CFCs increase substantially
(and they might), their greenhouse warming effect will be
relatively modest over this period, although continued emissions
would allow an additional concentration buildup and the associated
forcing. The uncontrolled CFCs do not generally have long
lifetimes.
3. Strong controls on CH4
emissions, though perhaps difficult to implement, would produce a
large effect. (Note that the potential for additional CH4 emissions from CH4 hydrates now tied up in permafrost has
not been included.)
4. Carbon dioxide is clearly the major factor and has the
steepest slope and the potential to lead to the largest temperature
changes. Note, however, that 25 to 50 percent reductions in CO2 emissions over the period 1990 to 2030
will still lead to rather substantial increases in the radiative
flux (and ultimately in temperature change).
Summing the radiative flux changes assuming no change in
emissions (already a rather stringent measure) produces a flux
increase of about 1.6 W/m2. This,
when added to the 2.45 W/m2
already experienced since 1765 (or the 1.95 W/m2 since about 1900), indicates that the
climate will have been committed to the radiative equivalent of a
CO2 doubling (4.4 W/m2) by about 2030 or a little later. Any
increases in emission rates will only
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make the changes greater. In addition, although beyond the time
horizon of this report, continued emissions beyond 2030 will
further increase the projected temperature change.
The extent to which changes in radiative forcing will be
significant to society depends on the climate sensitivity and the
consequent climatic impacts on human activities and natural
systems. Given that past climates have varied substantially as a
result of comparable forcings and that ecosystems under such
conditions were quite different than at present, however, this
schematic analysis suggests that significant climate change will be
very difficult to avoid, although its rate of onset may be
slowed.
References
Emanuel, W. R., G. G. Killough, W. M. Post, H. H. Shugart, and
M. P. Stevenson. 1989. Computer Implementation of a Globally
Averaged Model of the World Carbon Cycle. TR010. Washington, D.C.:
Carbon Dioxide Research Division, U.S. Department of Energy.
Hansen, J., A. Lacis, and M. Prather. 1989. Greenhouse effect of
chlorofluorocarbons and other trace gases. Journal of Geophysical
Research 94:16417–16421.
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.
Trabalka, J. R., ed. 1985. Atmospheric Carbon Dioxide and the
Global Carbon Cycle. DOE/ER-0239. Washington, D.C.: U.S. Department
of Energy.
Wang, W.-C., M. P. Dudek, X.-Z. Liang, and J. T. Kiehl. 1991.
Inadequacy of effective CO2 as a
proxy in simulating the greenhouse effect of other radiatively
active gases. Nature 350:573–577.
