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OCR for page 12
Carbon Dioxide and Climate: A Scientific Assessment
4
Models and Their Validity
The independent studies of the CO2/climate problem that we have examined range from calculations with simple radiative-convective models to zonally and vertically averaged heat-balance models with horizontally diffusive heat exchange and snow-ice albedo feedbacks to full-fledged three-dimensional general circulation models (GCM’s) involving most of the relevant physical processes. Our confidence in our conclusion that a doubling of CO2 will eventually result in significant temperature increases and other climate changes is based on the fact that the results of the radiative-convective and heat-balance model studies can be understood in purely physical terms and are verified by the more complex GCM’s. The last give more information on geographical variations in heating, precipitation, and snow and ice cover, but they agree reasonably well with the simpler models on the magnitudes of the overall heating effects.
The radiative-convective models have been reviewed by Ramanathan and Coakley (1978). The latitudinally varying energy-balance models were originally developed by Budyko (1969) and Sellers (1969) for studies of climatic change. More recently they have been employed by many authors, including Ramanathan et al. (1979) and MacDonald et al. (1979), for CO2/climate-change determinations. These models prescribe the infrared feedback but calculate the snow-ice albedo feedback by coupling to a simple parameterized horizontal heat transport; the snow and ice occur poleward of the latitude at which the temperature has an empirically prescribed value. The principal value of these models lies in their inclusion of the snow-ice albedo feedback. However, they do not deal with real geography or explicit dynamics and therefore can yield only crude approximations to the latitudinal variations of the CO2-induced temperature changes.
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Carbon Dioxide and Climate: A Scientific Assessment
4.1
THREE-DIMENSIONAL GENERAL CIRCULATION MODELS
We proceed now to a discussion of the three-dimensional model simulations on which our conclusions are primarily based. Some of the existing general circulation models have been used to predict the climate for doubled or quadrupled CO2 concentration. The results of several such predictions were available to us: three by S.Manabe and his colleagues at the NOAA Geophysical Fluid Dynamics Laboratory (hereafter identified as M1, M2, and M3) and two by J.Hansen and his colleagues at the NASA Goddard Institute for Space Studies (hereafter identified as H1 and H2). Some results obtained with the British Meteorological Office model (Mitchell, 1979) were also made available to us but will not be described here because both the sea-surface temperature and the sea-ice distribution were prescribed in this model, thus placing strong constraints on the surface ΔT, whereas it is just the surface ΔT that we wish to estimate.
The only one of the five predictions available in published form is M1. M2 is described in a prepublication manuscript, and H1 in a research proposal. We learned of M3 and H2 through personal communication.
The Geophysical Fluid Dynamics Laboratory and the Goddard Institute for Space Studies general circulation models, which are the basic models used in the M and H series, respectively, were independently constructed and differ from one another in a number of physical and mathematical aspects. They also differ in respect to their geographies, seasonal changes, cloud feedbacks, snow and ice properties, and horizontal and vertical grid resolutions. These differences are summarized in Table 1. In this table “swamp” means that the model ocean has no heat capacity though it provides a water surface for evaporation, and “mixed layer” means that the model ocean has a heat capacity corresponding to that of an oceanic mixed layer of constant depth. Heat transport by ocean currents is neglected in both model oceans. This is one of the weaknesses of all the predictions, as discussed in Section 3.3.
The horizontal resolution of the H series is rather coarse and perhaps only marginal for meaningful climate prediction. On the other hand, these models take into account more physical factors, such as ground heat storage, sea-ice leads, and dependence of snow-ice albedo on snow age, than do the models of the M series.
The models M1, H1, and H2 were run for doubled CO2 concentrations, M2 for both doubled and quadrupled concentrations, and M3 for quadrupled concentrations. The temperature changes for doubled CO2 in M2 were approximately half of those for quadrupled CO2. Since it can be expected that a similar result would have been obtained for M3, we have halved the M3 temperature changes.*
*
It should, however, be pointed out that the snow-ice albedo feedback may not be linear. For example, quadrupled CO2 in M3 melts the arctic ice altogether in summer.
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Carbon Dioxide and Climate: A Scientific Assessment
TABLE 1 Characteristics of General-Circulation Models Examined (λ, Longitude; , Latitude; T, Temperature)
Model Characteristics
Model Predictions
M1a
M2a
M3a
H1b
H2b
Domain
0°<λ<120°c
0°<λ<120°c
Global
Global
Global
Land-ocean distribution
Ocean for
Ocean for
60°<λ<120°
60°<λ<120°
Realistic
Realistic
Realistic
Ocean
Swamp
Swamp
Mixed layer
Mixed layer
Swamp
Seasonal change
No
No
Yes
Yes
No
Cloud feedbacks
No
Yes
No
Yes
Yes
Snow and ice albedo
When T<−25°C
When T<−10°C
Depends on depth and underlying surface albedo
For snow, depends on snow age, snow depth, underlying surface albedo, etc.
Same as H1
0.7
0.7
When T>−25°C
When T>−10°C
0.45 for snow
0.45 for snow
For deep snow, 0.8
0.35 for ice
0.35 for ice
For thick ice, 0.7
For ice, 0.45
Horizontal resolution
About 500 km on a mercator projection
5° in longitude
4.5° in latitude
Spectral model with the maximum zonal wave number 15
10° in longitude
8° in latitude
Same as H1
Vertical resolution
9 layers
9 layers
9 layers
7 layers
7 layers
aModels developed by S.Manabe and colleagues at the NOAA Geophysical Fluid Dynamics Laboratory, Princeton, N.J.
bModels developed by J.Hansen and colleagues at the NASA Goddard Institute for Space Studies, New York, N.Y.
cCyclic continuity assumed at boundaries.
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Carbon Dioxide and Climate: A Scientific Assessment
At low latitudes, the predicted values of the mean surface ΔT for doubled CO2 concentration were slightly more than 1.5°C in the M series, 2.5°C in H1, and 3.0°C in H2. Both series predict larger ΔT at upper levels, primarily because of added heating by cumulus convection. The discrepancy in the surface ΔT may well be due to differences in the respective parameterizations of cumulus convection.
The hemispheric mean surface ΔT is about 3°C in M1 and M2, and the global mean about 2°C in M3, 3.5°C in H1, and 3.9°C in H2. The 1°C difference between M3 and M1/M2 has been ascribed partly to the exclusion of seasonal changes and southern hemisphere effects in M1/M2 and their inclusion in M3; in the southern hemisphere the area covered by land, and therefore the snow-ice albedo feedback, is smaller than in the northern hemisphere, and there is no albedo feedback over Antarctica. The differences between the M series and the H series may be at least partially attributed to differences in the areas covered by snow and ice.
All the GCM’s predict larger surface ΔT at high latitudes. This is partly due to the snow-ice albedo feedbacks and also to the fact that the strong gravitational stability produced by cooling from below suppresses convective and radiative transfer of heat and thereby concentrates the CO2-enhanced heating in a thin layer near the ground. Although the magnitudes and locations of the temperature increases vary significantly, all the predictions give a maximum of between 4°C and 8°C in polar or subpolar regions for the annual mean surface ΔT. More detailed descriptions of the model predictions for high latitudes are given in the Appendix.
With regard to clouds, M2 gives a decrease of high clouds in low latitudes, whereas H1 and H2 give an increase. This discrepancy may well be due to differences in the parameterization of cumulus convection. The M series relies on an adjustment process for distributing heat and moisture by cumulus convection. This process takes place when and only when a layer of air is both saturated and moist-convectively unstable. In contrast, the H series permits cumulus convection to extend through nonsaturated and stable layers; but because it does not allow for entrainment of noncloud air, the penetrating cumuli extend higher than they otherwise would. At high latitudes, M2, which has no seasons, predicts an increase of both high and low clouds; in comparison, H1, which does have seasons, predicts an increase of high clouds throughout the year but an increase of zonally averaged low cloud amount only in spring. It may be shown from data presented by Manabe and Wetherald that the M2 cloud radiative feedback effects are relatively small intrinsically and are rendered even smaller by the tendency of their short and long wave components to compensate. This tendency is not apparent in H1, but there the negative and middle cloud feedback is on the average weak or nonexistent.
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Carbon Dioxide and Climate: A Scientific Assessment
For comparison purposes, the convective adjustment parameterization was introduced into an H model with fixed sea-surface temperatures and was found to reduce appreciably the penetration of water vapor and cloud to high levels (J.Hansen, NASA Goddard Institute for Space Studies, personal communication). Since the original penetration was probably too high because of lack of noncloud air entrainment, we conclude that the surface ΔT’s due to the upper water-vapor-cloud feedback may very well have been overestimated in the H series, whereas, because of insufficient penetration, they were probably underestimated in the M series. Since, moreover, the snow-ice boundary is too far equatorward in H1 and too far poleward in M1 and M2 (see Appendix), we believe that the snow-ice albedo feedback has been overestimated in the H series and underestimated in M1 and M2. For the above reasons, we take the global or hemispheric surface warmings to approximate an upper bound in the H series and a lower bound in the M series (with respect to positive water-vapor-cloud and snow-ice albedo feedback effects). These are at best informed guesses, but they do enable us to give rough estimates of the probable bounds for the global warming. Thus we obtain 2°C as the lower bound from the M series and 3.5°C as the upper bound from H1, the more realistic of the H series. As we have not been able to find evidence for an appreciable negative feedback due to changes in low- and middle-cloud albedos or other causes, we allow only 0.5°C as an additional margin for error on the low side, whereas, because of uncertainties in high-cloud effects, 1°C appears to be more reasonable on the high side. We believe, therefore, that the equilibrium surface global warming due to doubled CO2 will be in the range 1.5°C to 4.5°C, with the most probable value near 3°C. These estimates may be compared with those given in our discussion of feedback effects in one-dimensional, radiative-convective models. There the range was 1.6°C to 4.5°C, with 2.4°C estimated as a likely value.
We recall that the snow-ice albedo feedback is greater in the northern than in the southern hemisphere because of the greater land area and the lack of albedo change over Antarctica. Hence we estimate that the warming will be somewhat greater in the northern hemisphere and somewhat less in the southern hemisphere.
The existing general circulation models produce time-averaged mean values of the various meteorological parameters, such as wind, temperature, and rainfall, whose climate is reasonably accurate in global or zonal mean. Their inaccuracies are revealed much more in their regional climates. Here physical shortcomings in the treatments of cloud, precipitation, evaporation, ground hydrology, boundary-layer turbulent transport phenomena, orographic effects, wave-energy absorption and reflection in the high atmosphere, as well as truncation errors arising from lack of sufficient resolution combine to produce large inaccuracies. Two models may give rather similar zonal averages
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Carbon Dioxide and Climate: A Scientific Assessment
but, for example, very different monsoon circulations, positions, and intensities of the semipermanent centers of action and quite different rainfall patterns. It is for this reason that we do not consider the existing models to be at all reliable in their predictions of regional climatic changes due to changes in CO2 concentration.
We conclude that the predictions of CO2-induced climate changes made with the various models examined are basically consistent and mutually supporting. The differences in model results are relatively small and may be accounted for by differences in model characteristics and simplifying assumptions. Of course, we can never be sure that some badly estimated or totally overlooked effect may not vitiate our conclusions. We can only say that we have not been able to find such effects. If the CO2 concentration of the atmosphere is indeed doubled and remains so long enough for the atmosphere and the intermediate layers of the ocean to attain approximate thermal equilibrium, our best estimate is that changes in global temperature of the order of 3°C will occur and that these will be accompanied by significant changes in regional climatic patterns.