Carbon Dioxide and Climate A Second Assessment (1982) / Chapter Skim
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PRINCIPAL SCIENTIFIC ISSUES IN MODELING STUDIES
PRINCIPAL SCIENTIFIC ISSUES IN MODELING STUDIES
Pages 15-47
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From page 15... ...
from empirical observations keyed to the surface energy balance alone. Simplified climate models, ranging from zero-dimensional empirical relationships to highly parameterized multidimensional models, can be useful for inexpensive studies of climate change and climate mechanisms over a wide range of time scales, if used with appropriate caution.
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From page 16... ...
when CO2 is doubled; this is the flux after the atmosphere has adjusted to the radiative perturbation within the model constraints indicated but before the surface temperature has increased. TRH, fixed relative humidity: FAH, fixed absolute humidity; 6.SLR, 6.5°C km"1 limiting lapse rate; MALR, moist adiabatic limiting lapse rate; FCA, fixed cloud altitude; FCT, fixed cloud temperature; SAF, snow-ice albedo feedback; VAF, vegetation albedo feedback.
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From page 17... ...
. Models of the Earth's Complete Energy Balance Numerical climate models treat the energy balance of the complete surfaceatmosphere system.
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From page 18... ...
if the amount of water vapor in the atmosphere is held fixed. This value increases to about 2 K as a result of the increased water vapor abundances expected to accompany increasing atmospheric temperatures, a positive feedback included in most climate models.
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From page 19... ...
Heating of the surface by additional radiation from a warmer troposphere; 4. Increases in sensible and latent heat fluxes from the surface to the atmosphere, thus moderating the surface-temperature increase, heating the atmosphere, and increasing atmospheric moisture; 5.
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From page 20... ...
compares the temperature of an airless Earth heated by solar radiation alone with that of today's Earth, which receives energy from both the sun and the atmosphere. Here, however, the true initial forcing would be the hypothetical effect of imposing on rather cool, airless Earth a correspondingly cool but radiatively active atmosphere.
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From page 21... ...
They estimate the change in the surface energy budget produced by the increased atmospheric moisture (resulting from the assumed enhanced evaporation) to be about as large as the initial change due to CO2, doubling the effect of the CO2 itself.
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From page 22... ...
Agung observations cannot be taken as definitive indications of climatic sensitivity. Thus, the results of Newell and Dopplick on the tropical surface energy balance do not refute the inferences of a global climate sensitivity obtained from comprehensive models of the complete global climate system.
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From page 23... ...
and 3-D climate models suggests that the overall magnitude of the CO2 warming does not depend greatly on the details of the convective parameterization employed, although that component of the models is one of many that warrant more careful study. The preceding discussion clearly illustrates the complex nature of the surface energy budget and the dangers involved in inferring global climate sensitivity from local surface observations.
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From page 24... ...
The heat capacity of the upper ocean is potentially great enough to delay for several decades the establishment of new equilibrium temperatures associated with increased atmospheric CO2, with consequent impact both on social implications and on verification strategies. The dominant effect of a sudden doubling of atmospheric CO2, in the absence of ocean warming, would be a net downward flux of heat at the ocean surface of about 4 W m ~2 and an almost imperceptible change in atmospheric temperature over most of the globe (Ramanathan, 1981)
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From page 25... ...
In general, this dynamical assertion is certainly false, but it is relatively plausible in the upper ocean, where vertical mixing is driven predominantly by the wind and seasonal overturning and where the horizontal circulation is due mainly to wind stress rather than gradients in surface temperature. The nature of these exchange processes may be inferred from observations of other tracers in the ocean.
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From page 26... ...
26 ll - 2 o a o 0 = clir BO uj § O O -- g E>-, M -- o SO> i> o .£*
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From page 27... ...
The atmospheric effects of CO2 would be perceived by the ocean as a sudden change in downward heat flux at the ocean surface, as a change in wind stress, and as a change in net evaporation minus precipitation. Although variations in wind stress can substantially alter the circulation in the ocean, and hence the surface temperature, the changes calculated from atmospheric models appear to be small.
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From page 28... ...
fresh. However, since the present rate of renewal of abyssal water is relatively slow, believed to be once per 1000 years, salinity is probably not of dominant importance for the heat budget of the surface layers, except perhaps indirectly through its influence on the extent of sea ice.
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From page 29... ...
with the instantaneous equilibrium temperature increase (roughly proportional to the logarithm of the atmospheric CO2 divided by the base value)
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From page 30... ...
Future modeling studies should stress the regional nature of oceanic thermal inertia, and atmospheric energy transfer mechanisms, taking into account that the local response time is proportional to the ocean's thermal inertia and the rate at which energy is exchanged with the atmosphere. Reliable quantitative estimates of the role of the ocean in the climate system will require a better understanding of the processes that give rise to the entire general circulation in the ocean and substantial improvements in our ability to model it.
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From page 31... ...
But if cloud amounts, types (e.g., cumiliform versus stratiform) , heights, optical properties, and structure are influenced by climatic change, then both the solar and the infrared component of the radiation budget will be altered; it is the relative role of these probably small changes and their regional distributions that constitutes the second uncertain aspect of the problem.
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From page 32... ...
have, for doubling and quadrupling of atmospheric CO2, suggested that equatorward of 50° latitude, net cloud amount and effective cloud height are reduced by CO2-induced warming, with both effects acting to increase the outgoing infrared radiation. However, this is nearly compensated within their model by the corresponding increase in absorbed solar radiation due to reduced cloud amount.
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From page 33... ...
Stratus-Sea-Ice Interactions In simple climate models, ice-snow albedo feedback contributes substantially to the CO2 warming at high latitudes. However, it seems likely that in regions where sea ice is reduced, evaporation will increase and possibly lead to increased low-level stratus cloud cover, which would reflect solar radiation and at least partially reduce the albedo feedback.
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From page 34... ...
CO, 300 ppm -- » 600 ppm 2.9 N,0 0.28 ppm -> 0.56 ppm 0.6 CH4 1.6 ppm -- > 3.2 ppm 0.3 CC12F2 + CC1,F 0 -- > 2 ppb each 0.6 0, 25% decrease -0.5 TRACE GASES OTHER THAN CO, Most modeling endeavors concerning the CO2-climate problem address only the question of the climatic response to increasing atmospheric CO2, while the amounts of other atmospheric gases remain fixed. But associated changes, either climatologically or anthropogenically induced, of minor atmospheric constituents can also be of significance.
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From page 35... ...
The resulting negative feedback would, however, be quite minor, reducing the CO2-induced global warming by about 10 percent. Such decreases in tropospheric CH4 and O3 do not account for increasing anthropogenic emissions of CO, CH4, and NOV resulting from the fossil-fuel burning that produced the increased atmospheric CO2.
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From page 36... ...
a global warming of 2.0°C, but when the other trace-gas changes are included, global warming is increased to 3.6°C. Put another way, the model scenario suggests that roughly 40 percent of the global warming would be due to changes in trace gases other than CO2.
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From page 37... ...
ATMOSPHERIC AEROSOLS In addition to changes in atmospheric CO2 and other trace gases, atmospheric aerosols provide another potentially significant source of climate variability. But this problem is far more complex than that involving trace-gas changes, since the radiation effects of the aerosols depend on their composition, size, and vertical and global distributions.
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From page 38... ...
, and aerosol-climate models that employ this value (Ohring, 1979; Coakley et al., 1982) suggest that increased windblown dust would also lead to cooling on a global scale, but perhaps with important regional exceptions.
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From page 39... ...
Our confidence in climate models comes from a combination of tests of the correctness of the models' parameterizations of individual processes and comparisons of the models' sensitivity to observed seasonal variations. All models, however, require the parameterization of a number of subgrid-scale processes important to climate, such as cloudiness, precipitation, and the radiative and turbulent heat fluxes in the planetary boundary layer.
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From page 40... ...
Present State of Model Validation The primary method for validating a climate model is to determine how well the model-simulated climate compares with observations. Usually some verification against observed data is performed during the preliminary testing of a model, especially in those models with a large number of parameterized processes, and here it is important to distinguish between model calibration or tuning and model validation.
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From page 41... ...
An example is the surfaceair temperature simulated over the oceans in a GCM in which the sea-surface temperature has been prescribed. Most climate models have been given at least preliminary validation in terms of a comparison of climatological averages with the simulated time means of selected climatic variables.
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From page 42... ...
Such "process" validation is the aim of several large-scale observational programs either now under way or in the planning stages. The behavior of climate models is also influenced to some extent by the treatment of land-surface processes such as albedo and evapotranspiration.
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From page 43... ...
In view of the importance of the ocean in the response of climate to increasing CO2, it is recognized that the development of ocean models to a level comparable with that of atmospheric models is a matter of urgency. It is recommended that the requirements of ocean models be given high priority in the planning of oceanographic field programs planned by the World Climate Research Program.
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From page 45... ...
The lapse rate is near the dry adiabatic value for the predominantly CO2 atmosphere (~7°C km^1) , because of the absence of large latent heat effects and the small effect of large-scale dynamics on the vertical temperature gradient.
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From page 46... ...
Improvement of Model Validation In order to improve our knowledge of the performance of climate models and to increase their ability to project the climate changes likely to result from increased atmospheric CO2 in particular, we recommend that a climatemodel validation methodology be developed and that it be vigorously pursued for as many documented models as possible. As key elements of such a validation methodology, we recommend: • The systematic determination of the statistical properties of the performance of a hierarchy of climate models, including the geographical and seasonal distribution of the simulated means and higher-order statistics of modeled variables and processes.
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From page 47... ...
• The systematic assembly of climate information from paleoclimates and other sources such as planetary atmospheres, which are helpful in the validation of climate models.
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