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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 12°C cooling (at 25 km) in the strato-
sphere, with moderate seasonal depenclency in the polar regions. The
troposphere warmed by 3 to 5°C, on average, but up to 13°C at the
North Pole in winter. The antarctic region troposphere, however,
warmed by 6 to 7°C, 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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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 4°C in tropical latitudes
at all times of the year. In contrast, the mode! gives a seasonal
variation from 1°C warming in summer to 12°C in winter in the
North Polar region and from 3°C in summer to 12°C in winter at
the margins of Antarctica (70°S 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 18°C 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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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.
Representative terms from entire chapter:
lapse rate
