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Representative terms from entire chapter:
vapor feedback
Page 100
12
Radiative Forcing and Feedback
Many facets of the earth's climatic system are poorly
understood. A significant uncertainty associated with the modeling
of future climatic changes is due to deficiencies in the
understanding of, and in the incorporation into the climate models
of, several interactive climate feedback mechanisms. In this
discussion of radiative forcing, the planet's heat balance, and
these feedbacks and their consequences, emphasis is given to global
mean quantities, since, conventionally, the concept of radiative
feedback mechanisms is applied to global mean quantities associated
with changes from one equilibrium climate to another. Many aspects
of these feedback mechanisms are controversial. The conventional
wisdom has been challenged on several points (see, for example, The
George C. Marshall Institute, 1989; Lindzen, 1990), and new
analyses of key issues are reported regularly (see, for example,
Jenkinson et al., 1991; Kirchman et al., 1991; Ramanathan and
Collins, 1991).
The Heat Balance
When the planet's climate is not undergoing change, the energy
flux to the earth from the sun is in balance with the sum of the
reflected portion of that solar flux and the outward infrared
radiation that emerges from the top of the atmosphere. Figure 12.1
gives a schematic view of these fluxes and also depicts several of
the components of the energy flow that occurs within the climatic
system.
In particular, Figure 12.1 indicates that, of the 340 watts per
square meter (W/m2) that are
incident on the atmospheric envelope from the sun, about 100
W/m2 are reflected (from clouds,
glaciated areas, and so on); 80 W/m2 are absorbed within the atmosphere,
and 160 W/m2 are absorbed into
Page 101
the oceans and the continental land masses. Heat is returned
from the oceans and the land by infrared radiation from the surface
(390 W/m2). Ninety W/m2 are returned by nonradiative processes
that lead to the upward transport of water vapor (with its latent
heat) and of sensible heat. All of the 90 W/m2 flux and a substantial part (call it
B) of the 390 W/m2
radiative flux are deposited in the atmosphere. The total energy
flux to the atmosphere (80 + 90 + B) is necessarily in
balance with the infrared emission from the atmosphere, 320
W/m2 of which is directed downward
into the ocean-land mass interface and the rest of which emerges
from the top of the atmosphere (along with that fraction of the
interface radiation (390 - B) that is not absorbed in the
atmosphere). One can see that these numbers are mutually consistent
by noting that the total nonreflected solar input is equal to the
total (infrared) output of the system, that the total energy flux
downward through the interface balances the upward flux from that
interface, and that the energy received by the atmosphere (80 + 90
+ B) plus the nonabsorbed upward flux from the interface
(390 - B) is in balance with the radiation from the
atmosphere that supplies the output of the system (at the top) and
the radiative flux downward into the interface.
The interplay among the transmission of infrared radiation
through the atmosphere, the absorption of infrared in the
atmosphere, and the emission of infrared from the atmosphere is
distributed throughout the height of the gaseous envelope. The
foregoing, highly oversimplified splitting of these aspects of the
infrared interchange into individual macroscopic items is needed
only for the schematic presentation within this chapter. One need
not choose a particular, necessarily artificial, value for
B, because here a chosen value for B would not affect
the conclusion drawn.
Note that, since each flux shown in Figure 12.1 is a globally
averaged quantity, there is no depiction of the horizontal heat
fluxes within the atmosphere and ocean that redistribute heat from
one part of the planet to another. Nevertheless, such internal
transfers play vital roles in the physical processes that determine
the climatic state.
There are two important points to note from Figure 12.1. The
first is that the 240 W/m2
emission at the top of the atmosphere (TOA) is 150 W/m2 less than the 390 W/m2 emission from the surface. This
radiative flux difference is the greenhouse effect of the earth's
present atmosphere, and it is caused by the absorption of infrared
radiation by greenhouse gases and clouds. The second important
point of Figure 12.1 is that the atmospheric greenhouse gases and
the clouds emit infrared radiation downward to the surface, and
this direct radiative heating of the surface by the atmosphere (320
W/m2) is twice the direct solar
heating (160 W/m2). By itself, the
additional 320 W/m2 provided by
infrared surface heating produces substantial warming of the
surface above the temperature that would otherwise prevail; thus it
is the greenhouse effect that makes our planet habitable.
Page 102
Radiative Forcing
Suppose that, at time t, the state of the climatic system
is that characterized by Figure 12.1. Suppose, too, that the
greenhouse gas content of the atmosphere at that time is equivalent
to a CO2 content of 300 ppmv.
Suppose (once more) that, just after time t, the greenhouse
gas levels are changed and the subsequent concentrations are
equivalent to a CO2 concentration of
600 ppmv. This scenario is adopted here, not because CO2 doubling has some special significance
but because it is an atmospheric modification that may occur within
the next century and particularly because the CO2 doubling scenario has become the
baseline case most commonly used by atmospheric scientists to
compare the performance of one model with that of another.
Immediately after this augmentation of greenhouse gas content, no
changes in interface temperature, atmospheric temperature
distribution, atmospheric moisture content, or any other component
can yet have occurred. Accordingly, the only heat flux changes at
that time would be those implied by an increased fractional
absorption of the 390 W/m2 of
infrared radiation up from the interface. That implication (for the
CO2 doubling scenario) is a
reduction of approximately 4.4 W/m2 in the upward infrared flux through
the tropopause (not shown in Figure 12.1), which, after a
radiatively controlled adjustment of the state of the stratosphere
that evolves on a time scale of about one month, leads to a
deficit, also of 4.4 W/m2, in the
240 W/m2 radiation out of the top
of the atmosphere. This 4.4 W/m2
imbalance in
FIGURE 12.1 Schematic illustration of the
radiative energy budget of the surface-atmosphere
system. Horizontal arrows denote heat fluxes taken up by the
atmosphere.
Numbers are in watts per square meter.
Page 103
the overall heat budget of the planet is the ''radiative
forcing" that accompanies the CO2
doubling scenario.
Comparisons of the radiative forcing associated with various
greenhouse gas emission scenarios provide quantified
characterizations of the effectiveness of such scenarios, and
Chapter 17 includes a more general quantitative relationship
between greenhouse gas emission scenarios and the radiative forcing
levels to which they lead.
Radiative Feedback Mechanisms
In order to demonstrate radiative feedback mechanisms, it is
convenient to assume initially that climate change is manifested
solely by temperature changes within the climatic system and that
all other climate parameters remain fixed at their unperturbed
values. In this framework, there is no change in the climatic
system's 240 W/m2 solar
absorption. Moreover, let G denote the 4.4 W/m2 radiative forcing, and let DF be the change in the TOA infrared
flux following the imposition of the forcing. Thus,
G = DF =
dF / dT · DTs,
where dF/dT is the black body rate of
change of the surface radiative flux per unit change of surface
temperature, DTs. For the surface temperature of this
calculation (288 K), dF/dT is 3.3 W/m2/°C, and therefore
DTs = 4.4 / 3.3 @
1.3°C.
If it were not for the fact that this warming introduces
numerous interactive feedback mechanisms, then
DTs = 1.3°C
(2.3°F) would be a robust estimate of that global mean
quantity. Unfortunately, such feedbacks introduce considerable
uncertainty in DTs estimates. Three of the more commonly
discussed radiative feedbacks are described in the following
sections. Although these phenomena are interrelated in the actual
climatic system, they are discussed separately here to clearly
identify their respective consequences. Additional feedback
mechanisms involving land-surface hydrology are described in
Chapter 15. The qualitative tools used to analyze energy transfers
in the climatic system are less well developed in these latter
mechanisms.
Water Vapor Feedback
The best understood, although still controversial, feedback
mechanism is water vapor feedback. This phenomenon is intuitively
easy to comprehend:
Page 104
a warmer atmosphere can contain more water vapor, which itself
is a greenhouse gas. Thus an increase in one greenhouse gas
(CO2) induces an increase in yet
another greenhouse gas (water vapor), resulting in a positive
(amplifying) feedback mechanism. Although it has been suggested
that the water vapor feedback might be negative (Lindzen, 1990),
recent combined observational and model results strongly support
the conventional interpretation that water vapor provides positive
feedback (Raval and Ramanathan, 1989; Rind et al., 1991).
Notwithstanding the connection between water vapor and clouds in
the climatic system, they are treated separately for analytic
purposes (see also the "Cloud Feedback" section, below).
To be specific on this point, Raval and Ramanathan (1989) have
employed satellite data to quantify the temperature dependence of
the water vapor greenhouse effect. From their results, it readily
follows (Cess, 1989) that water vapor feedback reduces dF/dT from the prior value of 3.3
W/m2/°C to 2.3 W/m2/°C. This in turn increases the
global warming, for a CO2 doubling,
from 1.3° to 1.9°C (2.3° to 3.4°F).
There is yet a further amplification. Because water vapor also
absorbs solar radiation, water vapor feedback leads to an
additional heating of the climatic system through enhanced
absorption of solar radiation. With Q denoting solar
absorption by the climatic system (240 W/m2 for the present climate), this effect
produces DQ/DTs =
0.2 W/m2/°C (Cess et al.,
1990). To incorporate this into a
DTs estimate,
extension of the previous analysis to include solar absorption
yields DTs = lG,
where l is the climate sensitivity
parameter defined by
It then follows that the inclusion of the solar component of
water vapor feedback results in
DTs = 2.1°C
(3.8°F), so that the net effect of water vapor feedback is to
amplify the initial DTs = 1.3°C(2.3°F) warming by the
factor of 1.6.
The progressive forcing and feedback amplifications are
summarized in Table 12.1. Here H denotes the greenhouse
effect (150 W/m2 for the present
climate). The radiative forcing (process 1) simultaneously
increases H and reduces F, so that the planet emits
4.4 W/m2 less energy than it
absorbs from the sun. It is this imbalance that causes greenhouse
warming and results in DTs = 1.3°C (2.3°F) (process 2).
Although the climatic system returns to its original radiation
balance, with 240 W/m2 both
absorbed and emitted, process 2 increases further the greenhouse
effect by 2.7 W/m2 (154.4 to 157.1
W/m2) because of enhanced surface
emission resulting from surface warming.
Page 105
TABLE 12.1 Forcing and Response of the Climatic System
Caused by a Doubling of Atmospheric CO2
dTs(°C)
H(W/m2)
F(W/m2)
Q (W/m2)
Present climate
0
150.0
240.0
240.0
Process
1. Radiative forcing
0
154.4
235.6
240.0
2. Temperature response without water vapor
feedback
1.3
157.1
240.0
240.0
3. Including infrared water vapor feedback
1.9
160.4
240.0
240.0
4. Including solar water vapor feedback
2.1
161.1
240.4
240.4
NOTE: See text for definition of symbols.
Process 3 incorporates the infrared consequences of water vapor
feedback, with a 3.3 W/m2 increase
in H (157.1 to 160.4 W/m2)
being simultaneously due to the increase in atmospheric water vapor
and to enhanced surface emission. The TOA radiation budget is only
slightly modified by process 4. An important point is that the
combined effects of water vapor feedback and surface warming have
amplified the 4.4 W/m2 greenhouse
forcing to 11.1 W/m2. As Raval and
Ramanathan (1989) have emphasized, this suggests that direct
monitoring, from satellites, could help identify future changes in
the greenhouse effect.
The most detailed climate models for the purpose of projecting
climate change are three-dimensional atmospheric general
circulation models (GCMs), and these models seem to depict properly
the infrared component of water vapor feedback. In a recent
intercomparison of atmospheric GCMs (Cess et al., 1990), it was
found that 19 GCMs collectively produced
DF /
DTs = 2.3±0.2 W
/ m2.
Page 106
Thus the models used by many investigators are mutually
consistent and in agreement with the observational result of Raval
and Ramanathan (1989). It must be noted, however, that a variety of
phenomena, such as the role of clouds, make such interpretation
difficult.
The above calculations are not dynamic and thus do not
completely characterize all relevant phenomena. A recent analysis
is more inclusive (Rind et al., 1991). First, a GCM predicts
increased water vapor in the middle and upper troposphere. This
prediction is compared to observed satellite-generated data on
seasonal water vapor content in the convective western Pacific and
the largely nonconvective eastern Pacific. These results suggest
that water vapor feedback is not overestimated in models and should
amplify the climatic response to increased concentrations of
greenhouse gases.
Snow-Ice Feedback
An additional feedback mechanism is snow-ice feedback, by which
a warmer earth has less snow and ice cover, resulting in a darker
planet that in turn absorbs more solar radiation. While this
conventional albedo feedback description is quite obvious, and by
itself constitutes a positive feedback, it now appears that the
retreat of snow and ice cover might activate other interactive
processes.
The same set of GCMs as used by Cess et al. (1990) to
investigate cloud feedback (see next section) has recently been
used to interpret and intercompare feedback associated solely with
a change in snow cover. What that study shows is that clouds can
significantly influence the snow-induced change in albedo and that
this effect is more subtle than a mere masking of the change in
surface albedo by the clouds. For example, in some models the
snow-induced planetary albedo change is greater when clouds
are present than when they are not. The reason for this is that the
cloudiness change induced by the snow retreat causes a shift in
clear-sky regions from snow-covered land to snow-free land, and
this by itself is a positive feedback. The snow retreat can also
induce an infrared feedback, which in some models is positive and
in others negative. Thus it is clear that snow-ice feedback is far
more complex than the conventional interpretation that it is a
direct albedo feedback.
Cloud Feedback
Feedback mechanisms related to clouds are extremely complex. To
demonstrate this, it is useful to first consider the impact of
clouds on the present climate. Summarized in Table 12.2 are the
radiative impacts of clouds on the global climatic system for
annual-mean conditions. These radiative impacts refer to the effect
of clouds relative to a "clear-sky" earth. The
Page 107
TABLE 12.2 Infrared, Solar, and Net Cloud Radiative
Forcing (CRF)
Component
CRF (W/m2)
Infrared
31
Solar
-44
Net
-13
NOTE: These are annual-mean values estimated from
data for January, April, July, and October (Ramanathan et al.,
1989).
presence of clouds heats the climatic system by 31 W/m2 through increasing the greenhouse
effect. Because of the similarity of this process to trace gas
radiative forcing, this impact is referred to as cloud radiative
forcing. Through reflection of solar radiation, clouds also result
in cooling of the system. As demonstrated in Table 12.2, the latter
effect dominates the former, and the net effect of clouds on the
annual climatic system is a 13 W/m2 radiative cooling.
Although clouds produce net cooling of the climatic system, this
does not mean that clouds will necessarily offset additional global
warming due to increasing greenhouse gases. As discussed in detail
by Cess et al. (1989, 1990), cloud feedback constitutes the
change in net cloud radiative forcing associated with a
particular change in climate. To emphasize the complexity of this
feedback mechanism, three contributory processes are
summarized:
• Cloud amount: If cloud amount decreases because of
global warming, as occurs in typical GCM simulations (e.g., Cess et
al., 1989), then this decrease reduces the greenhouse effect
attributed to clouds and so acts as a negative feedback mechanism.
But there is a related positive feedback; the solar radiation
absorbed by the climatic system increases because the diminished
cloud cover causes a reduction of reflected solar radiation by
clouds. There is no simple way of appraising the net sign of this
feedback component.
• Cloud altitude: A vertical redistribution of
clouds will also induce feedbacks. For example, if global warming
displaces a given cloud layer to a higher and colder region of the
atmosphere, this will produce a positive feedback because the
colder cloud will emit less radiation and will thus enhance the
greenhouse effect.
• Cloud water content: There has been considerable
recent speculation that global warming could increase cloud water
content, thereby resulting in brighter clouds and hence a negative
component of cloud feedback. This may oversimplify the situation.
Increases in cloud albedo can induce compensating positive infrared
feedback (Cess et al., 1990), and in some models the net effect may
be positive (Schlesinger, 1988; Cess et al., 1990; Rind et al.,
1991). Recent analysis using both satellite and model results
showed that highly reflective cirrus clouds are produced in
tropical regions when sea surface temperature increases
sufficiently (Ramanathan and Collins, 1991). The increased albedo
may be sufficient to counter further warming due to infrared
feedback.
Page 108
The above discussion illustrates some of the complexities
associated with cloud feedback; indeed, differences in models'
depictions of this feedback largely account for the significant
differences in climate sensitivity among the 19 GCMs (Cess et al.,
1990). This intercomparison employed a perpetual July simulation in
which the climate was changed by imposing a 4°C (7.2°F)
perturbation on the global sea surface temperature while holding
sea ice fixed. Since a perpetual July simulation with a GCM
produces little snow cover over land, this effectively eliminates
snow feedback. The details of this simulation are given elsewhere
(Cess et al., 1989, 1990). The approach was chosen to minimize
computer time and thus allow a large number of modeling groups to
participate.
Cess et al. (1990) have summarized climate sensitivity
parameters (l as defined in the "Water
Vapor Feedback" section above) for the 19 GCMs, and these results
are reproduced in Figure 12.2. The important point is that
FIGURE 12.2 Clear-sky and global sensitivity
parameters for 19 general circulation models.
SOURCE: Reprinted
courtesy of Robert D. Cess.
Page 109
cloud effects were isolated by separately averaging the models'
clear-sky TOA fluxes, so that in addition to evaluating the climate
sensitivity parameter for the globe as a whole (solid circles), it
was also possible to evaluate it for an equivalent "clear-sky"
earth (open circles). Note the remarkable agreement of the
clear-sky sensitivity parameters; this is due to the agreement of
water vapor feedback components, as discussed above. There is,
however, a nearly threefold variation of the global (clear plus
overcast) sensitivity parameter; clearly, given the clear-sky
agreement, most of the variation in the global sensitivity
parameters of current models can be attributed to cloud feedback.
Certainly, improvements in the treatment of cloud feedback are
needed if GCMs are ultimately to be used as reliable climate
predictors.
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