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Carbon Dioxide and Climate: A Scientific Assessment
3
Physical Processes Important for Climate and Climate Modeling
In order to assess the climatic effects of increased atmospheric concentrations of CO2, we consider first the primary physical processes that influence the climatic system as a whole. These processes are best studied in simple models whose physical characteristics may readily be comprehended. The understanding derived from these studies enables one better to assess the performance of the three-dimensional circulation models on which accurate estimates must be based.
3.1
RADIATIVE HEATING
3.1.1.
Direct Radiative Effects
An increase of the CO2 concentration in the atmosphere increases its absorption and emission of infrared radiation and also increases slightly its absorption of solar radiation. For a doubling of atmospheric CO2, the resulting change in net heating of the troposphere, oceans, and land (which is equivalent to a change in the net radiative flux at the tropopause) would amount to a global average of about ΔQ=4 W m−2 if all other properties of the atmosphere remained unchanged. This quantity, ΔQ, has been obtained by several investigators, for example, by Ramanathan et al. (1979), who also compute its value as a function of latitude and season and give references to other CO2/climate calculations. The value 4W m−2 is obtained by several methods of calculating infrared radiative transfer. These methods have been directly tested against laboratory measurements and, indirectly, are found to
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Carbon Dioxide and Climate: A Scientific Assessment
be in agreement with observation when applied to the deduction of atmospheric temperature profiles from satellite infrared measurements. There is thus relatively high confidence that the direct net heating value ΔQ has been estimated correctly to within ±25 percent. However, it should be emphasized that the accurate calculation of this term has required a careful treatment of the thermal radiative fluxes with techniques that have been developed over the past two decades or more. Crude estimates may easily be in error by a large factor. Thus, in an interim report, MacDonald et al. (1979) obtain a ΔQ of 6 to 8 W m−2, a value about 1.5 to 2 times too large.
Greater uncertainties arise in estimates of the resulting change in global mean surface temperature, ΔT, for this quantity is influenced by various feedback processes that will increase or decrease the heating rate from its direct value. These processes will influence the feedback parameter λ in the expression ΔT=ΔQ/λ. For the simplest case in which only the temperature change is considered, and the earth is assumed to be effectively a blackbody, the value of λ=4σT3 is readily computed to be about 4 W m−2 K−1. For such a case, doubled CO2 produces a temperature increase of 1°C.
3.1.2
Feedback Effects
The most important and obvious of the feedback effects arises from the fact that a higher surface temperature produces a much higher value of the surface equilibrium water-vapor pressure through the highly nonlinear Clapeyron-Clausius relation. This, in turn, leads to increased water vapor in the atmosphere. A plausible assumption, borne out qualitatively by model studies, is that the relative humidity remains unchanged. The associated increase of absolute humidity increases the infrared absorptivity of the atmosphere over that of CO2 alone and provides a positive feedback. There is also increased absorption of solar radiation by the increased water vapor, which further increases the infrared feedback by about 10 percent. As with CO2, the radiative transfer calculation of water-vapor effects is relatively reliable, and the consequence is that λ is decreased and ΔT increased by about a factor of 2. For doubled CO2, the temperature increase would be 2°C.
One-dimensional radiative-convective models that assume fixed relative humidity, a fixed tropospheric lapse rate of 6.5 K km−1, and fixed cloud cover and height give λ=2.0 W m−2 K−1 (Ramanathan and Coakley, 1978). This value is uncertain by at least ±0.5 W m−2 K−1 because of uncertainties in the possible changes of relative humidity, temperature lapse rate, and cloud cover and cloud height.
Snow and ice albedo provide another widely discussed positive feedback mechanism (see, for example, Lian and Cess, 1977, and additional references therein). As the surface temperature increases, the area covered by snow or
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Carbon Dioxide and Climate: A Scientific Assessment
ice decreases; this lowers the mean global albedo and increases the fraction of solar radiation absorbed. Estimates of this effect lead to a further decrease of λ by between 0.1 and 0.9 W m−2 K−1 with 0.3 a likely value. Some uncertainty in albedo feedback also arises from cloud effects discussed in the next section. Taking into consideration all the above direct effects and feedbacks, we estimate λ to be 1.7±0.8 W m−2 K−1 and hence ΔT for doubled CO2 to lie in the range of 1.6 to 4.5 K, with 2.4 K a likely value.
3.2
CLOUD EFFECTS
Most clouds are efficient reflectors of solar radiation and at the same time efficient absorbers (and emitters) of terrestrial infrared radiation. Clouds thus produce two opposite effects: as cloud amount and hence reflection increase, the solar radiation available to heat the system decreases, but the decreased upward infrared radiation at the tropopause and downward radiation from the base of the clouds raises the temperature of the earth’s surface and troposphere.
Because the change of solar absorption dominates, the net result of increased low cloudiness, and very likely also middle cloudiness, is to lower the temperature of the system. The net effect of an increased amount of high cirrus clouds is less certain because their radiative characteristics are sensitive to height, thickness, and microphysical structure. Present estimates are that they raise the temperature of the earth’s surface and the troposphere.
It follows that if a rise in global temperature results in an increased amount of low or middle clouds, there is a negative feedback, and if a rise in global temperature results in an increased amount of high clouds, there is a positive feedback. The effect of cloud albedo by itself gives a negative feedback. Thus if clouds at all levels were increased by 1 percent, the atmosphere-earth system would absorb about 0.3 m−2 less solar radiation and lose about 0.5 W m−2 less thermal radiation. The net effect would be a cooling of about 0.4 W m−2, or, if this occurred together with a doubling of CO2, a decrease of ΔQ from 4.0 to 3.6 W m−2.
How important the overall cloud effects are is, however, an extremely difficult question to answer. The cloud distribution is a product of the entire climate system, in which many other feedbacks are involved. Trustworthy answers can be obtained only through comprehensive numerical modeling of the general circulations of the atmosphere and oceans together with validation by comparison of the observed with the model-produced cloud types and amounts. Unfortunately, cloud observations in sufficient detail for accurate validation of models are not available at present.
Since individual clouds are below the grid scale of the general circulation models, ways must be found to relate the total cloud amount in a grid box to
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Carbon Dioxide and Climate: A Scientific Assessment
the grid-point variables, Existing parameterizations of cloud amounts in general circulation models are physically very crude. When empirical adjustments of parameters are made to achieve verisimilitude, the model may appear to be validated against the present climate. But such tuning by itself does not guarantee that the response of clouds to a change in the CO2 concentration is also tuned. It must thus be emphasized that the modeling of clouds is one of the weakest links in the general circulation modeling efforts.
The above uncertainties, and others such as those connected with the modeling of ground hydrology and snow and ice formation, create uncertainties in the model results that will be described in Chapter 4.
3.3
OCEANS
Existing numerical models of the atmosphere, which treat the ocean as having no meridional heat transports of its own, may give somewhat improper accounts of the CO2 impact. It is currently estimated that at some latitudes the ocean transports as much as 50 percent of the poleward heat flux in the existing climatic system. A proper accounting for oceanic dynamics has several possible consequences as levels of CO2 continue to rise.
The role of the ocean as an active transporter of heat meridionally leads one to consider several possible feedback mechanisms. Atmospheric models suggest that the warming at high latitudes will be larger than at low latitudes. If this reduced atmospheric baroclinicity reduces the wind stress at the ocean surface (and there are not good estimates of the anticipated size of such a reduction), it is possible that oceanic meridional heat flux might be reduced, Because of the required overall radiative heat balance of the total system, the atmosphere would then be required to compensate for reduced oceanic heat transport by steepening the equator-to-pole temperature gradient, thus ameliorating somewhat the predicted polar warming, However, the total atmospheric warming would not likely be greatly affected, merely its distribution in latitude.
The only part of the ocean that has been included in the general circulation modeling of the CO2 effects is the mixed layer. The rationale for this simplification is that only the mixed layer needs to be modeled in order to deal with the annual cycle, while the heat capacity of the deeper ocean does not matter once thermal equilibrium has been reached.
On time scales of decades, however, the coupling between the mixed layer and the upper thermocline must be considered. The connections between upper and lower ocean are generally presumed to have response times of the order of 1000 years, the essential coupling being local vertical diffusion and formation of bottom water at high latitudes. This ignores the mechanism of Ekman convergence of the surface mixed layers in the large subtropical gyres,
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Carbon Dioxide and Climate: A Scientific Assessment
which pumps water down into the upper thermocline over more than half the ocean surface area, a reservoir much larger than that of the mixed layer alone, The connections between the upper-thermocline reservoir and the deep ocean may indeed require very long time constants, but the carbon and heat budgeting on the decadal time scale must account properly for the potentially large reservoir directly beneath the mixed layer.
Simple model calculations involving Ekman pumping from the mixed layer into the intermediate waters of the order of 10–20 cm/day−1 and estimates of mixing coefficients for the intermediate waters from tracer studies (Östlund et al., 1974; National Science Foundation, 1979) suggest that the upper-thermocline reservoir communicates effectively with the mixed layer on time scales of several decades. Therefore, the effective thermal capacity of the ocean for absorbing heat on these time scales is nearly an order of magnitude greater than that of the mixed layer alone.* If this reservoir is indeed involved, it could delay the attainment of ultimate global thermal equilibrium by the order of a few decades. It would also increase the rate at which the ocean can take up carbon from the air and might at least partially account for the current discrepancies between the observed rise in atmospheric CO2 and the estimated rise due to the anthropogenic input of CO2 into the air.
*
The existence of the Ekman pumping underlies all the generally accepted ideas about the physics of the general circulation of the oceans. The order of magnitude estimated above (10–20 cm/day) is consistent with a variety of oceanographic data, including wind stress, chemical tracers, and local heat-budget calculations.