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11 Use of Numerical Models to Project Greenhouse Gas-Induced Warming in Polar Regions (The Conceptual Basis Developed Over the Last Twenty Years) ROBERT E. DICKINSON National Center for Atmospheric Research The development of numerical models that will accurately pre- dict climatic change caused by greenhouse gases has been a long-term, continuing research effort (Bolin et al., 1986~. The focus of this talk will be the concepts involved and the results of that effort, especially as they apply to the polar regions. There are several major points to consider; ~ will list them briefly here and then return to each of them later in the talk. The major points are as follows: 1. Expected tropospheric temperature increases from green- house gases will be accompanied by temperature decreases, on a different time scale, in the stratosphere. 2. Classical (postulated before 1974) mechanisms of high-lati- tude amplification by means of positive feedback processes involving the temperature lapse rate and planetary albedo are important. 3. The projected magnitude of high-latitude warming is strong- ly dependent on the seasonal cycle. 4. Horizontal transport of heat by both the atmosphere and oceans has an important effect on high-latitude temperatures. 5. Vertical energy transfer, as modeled by convective parame- terization, also has a significant role for high-latitude temperatures (although not as large a role as in low latitudes). 6. High-latitude cloud cover has a key role through its effect on planetary albedo. 98

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NUAdERIaAL MODELS 99 7. Sea ice distribution has a critical feedback role and needs to be modeled correctly. 8. Permafrost and seasonal land ice have significant roles and also should be included in a comprehensive model. Some of these points are illustrated by Manabe and Stouffer's (1980) equilibrium mode! results for a fourfold increase in carbon dioxide. The mode} gave up to 12C cooling (at 25 km) in the strato- sphere, with moderate seasonal depenclency in the polar regions. The troposphere warmed by 3 to 5C, on average, but up to 13C at the North Pole in winter. The antarctic region troposphere, however, warmed by 6 to 7C, with little difference between winter and sum- mer. These results can be compared with the more recent results of Washington and Meeh] (1984) using the NCAR climate model. The results are generally very comparable when allowance is made for the fact that Washington and Meeh} assumed a doubling of carbon dioxide. However, the latter mode! predicts a much greater lower- tropospheric warming near the margin of Antarctica in winter than in summer and considerably more warming there in both seasons than that predicted by the Manabe and Stouter model. Returning to the difference between the stratosphere and tro- posphere, the troposphere and the surface are undergoing a coupled greenhouse warring process. Tropospheric carbon dioxide absorbs 15~ cron radiation from the surface and re-emits at a somewhat colder temperature to higher altitudes, with the differential heat en- ergy remaining below. To the "zeroth" order, the troposphere and surface respond as a coupled slab. The time scale of atmospheric response is primarily controlled by heat uptake in the world oceans and is of the order of decades. In the stratosphere, cooling occurs because the loss of photons to space dominates the radiative balance, with exchange of radiation between adjacent layers of the atmosphere essentially canceling out. Vertical convective coupling plays a secondary role in the strato- sphere. The carbon dioxide in the stratosphere is Optically thin" compared to carbon dioxide in the troposphere. As to positive feedback mechanisms, the lapse rate i8 of par- ticular importance in winter. Because there is strong wintertime vertical stratification of the atmosphere in high latitudes, the surface is only weakly coupled to the troposphere. Therefore, warming at the surface from greenhouse gases or any other source is largely con- fined to the surface. Greenhouse radiative warming is consequently concentrated near the surface. The resulting steeper thermal lapse

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100 ROBERT E. DICKINSON rate means that higher atmospheric layers will radiate away less heat energy, compared to the surface, thereby allowing the surface to heat up still more until a radiative balance is achieved. In this way, steepening of the thermal lapse rate acts as a positive feedback mechanism. The albedo of ice and snow also acts as a positive feedback. As surface temperatures rise, ice and snow cover diminish unless wintertime precipitation increases enough to maintain them. Less ice and snow cover results in less reflected solar radiation, thus amplifying the warming at the surface and quickening the ice and snow loss. The magnitude of this feedback depends on the product of surface albedo change and incident solar radiation at the surface. Therefore, seasonality plays a role, and the process is most sensitive to snow and ice that persist past the spring equinox. Since sea ice is much more persistent than land snow in spring and summer, the feedback process is mainly dependent on changes in sea ice extent. A climate modeling experiment by Hansen et al. (1984) for dou- bled carbon dioxide shows a warming of 3 to 4C in tropical latitudes at all times of the year. In contrast, the mode! gives a seasonal variation from 1C warming in summer to 12C in winter in the North Polar region and from 3C in summer to 12C in winter at the margins of Antarctica (70S latitude). A similar modeling ex- periment by Washington and Meeh! (1984) shows a generally similar depenclence of warming on the season for both polar regions, but with an especially pronounced wintertime warming of up to 18C at the margin of Antarctica. If one examines such differences in mode! results more closely, it turns out that most of the differences can be related to differences in the modeling of sea ice. Manabe and Stouffer (1980) in their modeling experiment in- terpreted the seasonal cycle in climate warming as follows: sea ice will disappear or puddles will form in the summer. This will allow more solar radiation to be absorbed in the ocean, but the ocean surface waters will warm only slightly because of large heat capac- ity and some mixing. In the milder winters associated with climate warming, sea ice will form more slowly than it does under present climate conditions, and the resultant sea ice will be thinner at the same time of year. As a result, the atmospheric surface layer will be warmed above present levels, triggering the lapse rate feedback process. However, the effect will be less pronounced in late winter as the ice thickens. Horizontal transport of atmospheric sensible and latent heat and

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NUMERICAL MODELS 101 of oceanic heat is also unportant. More accurate and detailed obser- vations are needed to better determine these kinds of transport. The total heat transport is known fairly well from the radiation balance at the top of the atmosphere. Atmospheric transport estimates are probably somewhat inaccurate, and oceanic transport, computed as a residual, is even more inaccurate. It may be better to estimate ocean transports by use of surface energy balance computed from a general circulation mode! (GCM) that uses observed ocean temperatures. There are two different ways to parameterize convection in the numerical models: (1) use of a moist convective adjustment process and (2) use of modeled penetrative convection. The choice of scheme can have a marked effect on the latitudinal and vertical distribution of warming due to greenhouse gases. High-latitude cloud cover is especially important because it po- tentially can mask much of the ice albedo feedback process mentioned previously. Unfortunately, observations of high-latitude clouds are very sparse for a number of reasons. The GCM models probably do a poor job of simulating the actual high-latitude cloud cover because of our inability to validate the models against sufficient observations. There has been a wide range of results for sea ice change in GCM simulations, with a consequent wale range of warming predicted for the high latitudes. It appears to be difficult to simulate present sea ice coverage correctly with GCMs, let alone have any confidence about future projections. Since sea ice coverage depends critically on ocean temperature and salinity, the latter must be accurately determined before there is any hope of obtaining the current sea ice coverage. With respect to the role of permafrost and sea ice modeling, it has been noted that conductive heat flux into frozen ground is large enough during spring and summer to significantly coo} the surface. Reduction of seasonal ice thickness and permanent ice cover should help amplify summertime greenhouse warming in the high latitudes of the Northern Hemisphere, but this hypothesis has not yet been quantitatively tested. In order to test the hypothesis, development of ground ice models and their inclusion in future GCM warming studies ~ needed. In conclusion, the basic mechanisms of the high-latitude am- plification of greenhouse warming are reasonably well understood. However, realistic simulations by GCMs at these latitudes are not yet available. The modeling groups that are looking at the climate change process are relatively small in size compared to the large task

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102 ROBERT E. DICKINSON they have undertaken, and as a result their focus is necessarily global, so that not enough attention has been given to the special problems of the high latitudes. There is a need for the larger community that works on modeling problems to focus attention on mode} physics at the high latitudes. (In response to a question): There has been little change over the last 20 years or so in the approaches of the various modeling groups that are looking at climate change. This appears to be due both to limited resources and to a tendency of the modelers to fixate on specific aspects of the total problem and not to explore other areas that need work. Although more and better observations, hopefully from satellite programs such as the earth observing system (EOS), should help, the modelers will need to be motivated to make full use of such observations. REFERENCES Bolin, B., B.R. Doos, J. Jager, and R.A. Warrick (eds.~. 1986. How will climate change? The climate system and modelling of future climate. Chapter 5 in The Greenhouse Effect, Climatic Change, and Ecosystems. John Wiley & Sons, Chichester, SCOPE 29, pp. 207-270. Hansen, J., A. Lacis, D. Rind, G. Russell, P. Stone, I. wing, R. Ruedy, and J. Lerner. 1984. Climate sensitivity: Analysis of feedback mechanisms. In Climate Processes and Climate Sensitivity, J.E. Hansen and T. Takahashi teds.), Maurice Ewing Series 5, American Geophysical Union, Washington, D.C., 368 pp. Manabe, S., and R.J. Stouffer. 1980. Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere. J. Geophys. Res. 85:5529-5554. Washington, W.M., and G.A. Meehl. 1984. Seasonal cycle experiment on the climate sensitivity due to a doubling of CO2 with an atmospheric general circulation model coupled to a simple mixed layer ocean model. J. Geophys. Res. 89:9475-9503.