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INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 17, 155–162 (1997)
POLAR SNOW COVER CHANGES AND GLOBAL WARMING
HENGCHUN YE and JOHN R. MATHER
Center for Climatic Research, Department of Geography, University of Delaware, Newark, DE 19716, USA
Received 19 August 1995
Revised 1 March 1996
Accepted 20 May 1996
ABSTRACT
Many general circulation models suggest that current precipitation amounts in polar latitudes will increase under double CO2
scenarios. Even though temperatures in such high-latitude regions should also increase under a doubling of CO2, as long as
those temperatures remain below freezing, the increased precipitation should accumulate as snow. A study of both current and
double CO2 temperature and precipitation data for all land areas poleward of 60 latitude using three different general
circulation models suggests possible changes in snow accumulation due to increasing CO2. Increased snow accumulation will
occur in the Antarctic whereas a small decrease in snow depth is to be expected in the Northern Hemisphere. Total snow
accumulation for all land areas poleward of latitude 60 is found to increase under a double CO2 scenario.
KEY WORDS:
global climate change; snow cover; general circulation models.
INTRODUCTION
Many of the general circulation models (GCMs) that have been used to estimate climatic responses to increasing
concentrations of CO2 and other trace gases in the atmosphere suggest a marked warming in polar latitudes.
Although the GCMs do not provide direct information on snow cover or even snowfall, available polar warming
scenarios have been cited as evidence to suggest increased melting of polar ice masses. The present study
investigates temperature and precipitation conditions in the polar regions under conditions of a doubling of CO2
using three readily available GCMs to provide some understanding of the possible range of future estimates of
snow accumulation. Evaluating snowfall amounts under current and GCM simulated conditions permits an
estimate of snow depth change under possible global warming scenarios.
In 1934, Sir George Simpson postulated a unique theory for the cause of ice ages in the Pleistocene that
required two fairly uniform increases in solar radiation with a period of decreased radiation between the radiation
maxima. He theorized that a gradual rise in solar radiation would be accompanied by a gradual rise in air
temperature. This, in turn, would result in an increase in precipitation, which would fall as snow in the polar
regions. The increased precipitation would result from increased evaporation (due to the warmer water
temperatures) and the increased water vapour capacity of the warmer air. He reasoned that as long as air
temperatures in polar regions remained below freezing the increased precipitation would fall as snow and
accumulate as permanent snow fields on the ground. This would lead to advancing ice sheets and a glacial period.
As temperatures finally increased too much, more precipitation would fall as rain rather than snow and, at the
same time, there would be increased melting of continental ice sheets, leading to their retreat and ultimate
disappearance.
Although Simpson’s theory linking ice-age occurrence with an increase in solar radiation and air temperature
has not survived the test of time, his recognition that an increase in air temperature might lead to increased
snowfall in polar regions deserves further evaluation.
Thomas (1986) in identifying three major processes relating global warming to average sea-level change,
suggested that although two of the processes would lead to sea-level rise (increased ice melting and thermal
expansion of sea water), one process might lead to a sea-level fall. That process was increased snowfall on polar
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CCC 0899-8418/97/020155-08 $17.50
1997 by the Royal Meteorological Society
156
H. YE AND J. R. MATHER
ice sheets. Clearly, it is a physically sound process and needs to be evaluated in more detail to determine whether,
under global warming, increased evaporation of water from the oceans of the world and increased moisture
content in the warmer polar atmosphere, might produce a build-up of snow and ice in polar regions.
DATA SOURCES
The number of weather observing stations within the polar regions of the globe is quite limited. Willmott et al.
(1981) in their world-wide collection of all available temperature and precipitation records identified 805 landbased stations in the region north of 60 and 31 stations on the Antarctic continent. Location of these observing
stations are shown in Figures 1 and 2. Temperature and precipitation data from these stations have been
interpolated to a 015 015 latitude and longitude grid using a spherically based interpolation procedure (Legates
and Willmott, 1990a,b; Willmott et al., 1985). A total of 15 158 land-based grid-points exist poleward of 60 N,
and 26 915 land-based grid-points occur poleward of 60 S. The temperature and precipitation data extrapolated to
these grid-points are considered to be representative of current conditions.
Simulations of monthly temperature and precipitation under a doubling of CO2 and other trace gases have been
obtained from three available general circulation models. Rather than just accepting the temperature and
precipitation values produced by the GCMs, the temperature change from a single to a double CO2 condition was
added to the current climate temperature data and the percentage change in precipitation from a single to a double
CO2 condition was used as a multiplier of the current climate precipitation value to obtain double CO2 estimates.
This technique has been accepted by many investigators who feel that GCMs do not do a good job in estimating
either single or double CO2 climate distributions but believe that the magnitude of the changes noted between
single and double CO2 conditions may be portrayed more accurately by the GCMs (Mather and Feddema, 1986;
Gleick, 1987; McCabe and Ayers, 1989; Hodny and Mather, 1995). Applying the temperature and precipitation
6
Figure 1. Weather station locations for north polar area.
POLAR SNOW COVER AND GLOBAL WARMING
157
Figure 2. Weather station locations for Antarctic area.
change simulations to the current climate distribution therefore allows one to start with the most accurate
representation of the current climate distribution and to apply modelled change simulations in order to obtain an
estimate of future distributions.
The three GCMs evaluated are those developed by the NOAA Geophysical Fluid Dynamics Laboratory
(GFDL), the NASA Goddard Institute for Space Studies (GISS), and the United Kingdom Meteorological Office
(UKMO). These three models have been widely used as representative of available general circulation models.
Although their output varies, it may be reasonable to assume that the range of conditions simulated by these
models includes those future conditions that will develop under a doubling of CO2.
Table I lists some of the general characteristics of the three different GCMs used in this study. Although later
versions are now available in some cases, it was necessary to establish a fixed cut-off point in time and to use
those versions that were available to us at the beginning of the study. It is felt that later versions do not change the
overall conclusions of this study significantly. The GFDL model (Manabe and Wetherald, 1987) was constructed
on a 4144 latitude by 7150 longitude grid. It consists of an atmospheric circulation model coupled to a static
mixed-layer ocean model without ocean currents. Data are evaluated at nine unequally spaced atmospheric levels.
The model incorporates seasonally varying insolation, land and sea contrasts, and a variable cloud cover
(whenever the relative humidity exceeds 99 per cent). The concentration of CO2 is assumed to be constant
everywhere. Whereas the continental geography is more realistic than earlier versions, it is recognized that
topography is only poorly handled in this model.
The current GISS model is an extension of earlier models developed by the Goddard Institute for Space Studies
(Hansen et al., 1988). The model utilizes nine atmospheric layers and a fairly coarse horizontal grid of 7183
latitude by 10 longitude. Diurnal and seasonal cycles are included and the radiative calculations include the
radiatively significant atmospheric gases, aerosols, and cloud particles. Cloud cover and height are computed, and
the opacity of clouds is specified as a function of cloud type, thickness and height. The model incorporates a
158
H. YE AND J. R. MATHER
Table I. Characteristics of general circulation models utilized in study.
GISS
6
7183 10
9
yes
125a
Model resolution
Atmospheric levels
Diurnal cycle
Depth mixed
ocean layer (m)
Initial
1 CO2 (ppm)
Global T( C)
(2 CO2)
Global P (per cent)
(2 CO2)
6
D
6
D
6
GFDL
6
‡ 1
11
‡ 1
‡
6
4144 715
9
no
68
315
42
‡ 1
UKMO
5 715
11
yes
50
300
40
323
52
‡ 1
87
‡
15
Source: Wigley et al 1989. a From Hansen et al. (1988).
mixed-layer ocean, the layer having a seasonally varying depth specified from observations—the annual
maximum depth being approximately 125 m.
The UKMO model was developed in 1985–1986 (Wilson and Mitchell, 1987). The model couples an
atmospheric GCM to a 50-m-thick oceanic mixed layer and an energy balance sea-ice model. A regular
horizontal grid spaced at 5 of latitude by 715 of longitude has a vertical dimension composed of 11 layers
irregularly spaced, but concentrated near the boundary layer and the tropopause. Incoming solar radiation is
treated on both a diurnal and a seasonal cycle, and the radiative fluxes are dependent on temperature, water
vapour, carbon dioxide, ozone, and clouds. The model is designed to include a cloud-prediction scheme. Layered
clouds occupy one layer (low, medium, or high) whereas convective clouds can traverse more than one layer.
Stratiform cloud formations and amounts are based on relative humidities in the layer and can occur in any layer
except the top layer (Wilson and Mitchell, 1987). Cloud reflectivity is prescribed as 012 for high clouds and 016
for others. For longwave radiation calculations, high clouds have an emissivity of 015 and all others are assumed
to be black bodies (Wilson and Mitchell, 1987).
The actual monthly change in temperature calculated by each GCM as well as the percentage change in
monthly precipitation in going from a single to a double CO2 condition were determined for each GCM gridsquare. These change simulations were then applied to the current temperature and precipitation data determined
previously for each 015 latitude by 015 longitude grid-point. Thus, the same numerical change was applied to all
current grid-point data falling within each of the GCM grid-boxes.
Many attempts have been made to relate the type of precipitation (rain or snow) to different atmospheric
variables without convincing success. Most studies suggest that air temperature near the surface is as reliable as
any of the other variables tested for differentiating rain and snow. Because air temperature data are widely
available, they were used in this study to identify when the monthly precipitation fell as rain or snow. Analysing
some 2400 daily precipitation occurrences at Donner Summit, California from 1946 through to 1951, the U.S.
Army Corps of Engineers (1956) found that at 31 F, 97 per cent of the occurrences were snow but at 33 F only 74
per cent were snow, and at 35 F, rain occurred 31 per cent of the time. There seemed to be a significant change in
occurrences of rain and snow at or slightly below freezing air temperatures. A monthly temperature value of
ÿ1 C was selected to separate rain occurrences from snow occurrences. This same temperature value was
selected by Thornthwaite and Mather (1957) for distinguishing between rain or snow using monthly temperature
data.
The water equivalent of the annual snowfall was estimated by totalling all monthly precipitation whenever the
monthly temperature was equal to or below ÿ1 C. These calculations were performed on both the current and
the double CO2 precipitation simulations in order to determine quantitatively the change in the amount of water
held in the snow cover from current to double CO2 conditions at each 015 015 grid-point. It was assumed that
there was negligible evaporation loss as long as the temperature of the month was below ÿ1 C.
6
159
POLAR SNOW COVER AND GLOBAL WARMING
RESULTS
7
Using the assumption that all precipitation falling at monthly temperatures equal to or below 1 C will be snow,
the current as well as the double CO2 water equivalent of the snow accumulation has been calculated for all land
areas poleward 60 latitude. Values of the changes in the water equivalent of the snow from current to double
CO2 conditions based on the GFDL, GISS, and UKMO GCMs have been summed for all land grid-points by 1
latitude belts (Table II (a and b)). The table includes information on the total volume of water by latitude belt
involved in the change in water equivalent of snow (in lit). This has been obtained by multiplying the amount of
land area in each latitudinal belt by the change in the depth of water equivalent of snowfall in the belt. Two of the
GCMs indicate an overall decrease in snow accumulation in the Northern Hemisphere polar region, whereas the
third (the UKMO model) shows a small increase in snow cover poleward of 60 N. The total volume of water
involved in the changes is relatively small because of the small amount of land surface actually receiving
snowfall. In the Southern Hemisphere, some melting of the annual snow cover will occur around the periphery of
Antarctica, but the increase in water equivalent of the snow cover for the great interior of the continent more than
balances the losses found in latitudes from 63 to 69 S. Table III summarizes the total changes in water
equivalent of snowfall by hemisphere and for the polar regions in both hemispheres together. The combined total
of water equivalent change reveals an increase in storage of water in snow cover in the world’s polar regions of
from 422 1012 to 1791 1012 litres depending on the GCM being evaluated.
To put these results into another context, in the early 1950s (personal communication) Dr. Walter Munk at the
Scripps Oceanographic Institution suggested that some 510 1018 g of water were removed from the oceans of the
world between autumn (maximum sea-levels) and spring (minimum sea-levels). This water was returned to the
6
6
6
Table II. (a) Change of annual water equivalent of snowfall by
latitude belt in the south polar region.
Changes in water equivalent (1012L)
Latitude
GFDL
63–64
64–65
65–66
66–67
67–68
68–69
69–70
70–71
71–72
72–73
73–74
74–75
75–76
76–77
77–78
78–79
79–80
80–81
81–82
82–83
83–84
84–85
85–86
86–87
87–88
88–89
89–90
2 24
7 41
ÿ23153
ÿ86140
117100
ÿ79181
ÿ19199
33122
95162
129172
112115
101140
85149
60140
52149
37148
29134
27152
22142
18140
14137
8132
4115
2141
1116
0136
0104
ÿ 1
ÿ 1
7
GISS
UKMO
2 24
5 53
ÿ12118
ÿ15146
1133
28149
37141
75141
91198
89117
73159
71147
67121
64177
59168
66105
59130
46104
38157
32177
26129
17140
6197
4128
2118
0177
0111
2 24
7 41
ÿ19163
ÿ78186
ÿ102112
ÿ29120
48146
158121
211191
210117
188130
176164
186190
174123
149169
117178
88142
81102
70193
57172
44113
29100
14139
8192
4119
1137
0116
ÿ 1
ÿ 1
ÿ 1
ÿ 1
160
H. YE AND J. R. MATHER
Table II(b). Change of annual water equivalent of snowfall
by latitude belt in the north polar region
Latitude
Changes in water equivalent (1012 L)
GFDL
60–61
61–62
62–63
63–64
64–65
65–66
66–67
67–68
68–69
69–70
70–71
71–72
72–73
73–74
74–75
75–76
76–77
77–78
78–79
79–80
80–81
81–82
82–83
83–84
GISS
5 27
4 78
ÿ5194
ÿ5195
ÿ4164
ÿ4128
ÿ4134
ÿ3151
ÿ2120
ÿ0191
0114
0172
0156
0166
0165
0142
0101
0103
ÿ0101
0108
0116
0116
0108
0102
UKMO
2 72
2 22
ÿ1156
ÿ1122
ÿ0143
ÿ0177
ÿ0182
ÿ0128
0124
0175
0199
0190
0142
0139
0144
0143
0127
0123
0129
0140
0146
0135
0119
0104
5 88
4 80
ÿ3177
ÿ2160
0121
1135
0153
1107
1194
3108
3142
2174
1170
1122
0194
1120
0191
0164
0153
0168
1108
0187
0149
0111
ÿ 1
ÿ 1
ÿ 1
ÿ 1
ÿ 1
ÿ 1
ocean between spring and autumn. This transfer of water volume resulted in an annual variation in mean sea-level
of about 114 cm when spread evenly over all the oceans of the world. Van Hylckama (1956) attempted to locate
this water volume by computing the monthly change in water storage in the root zone of the soil for all land areas
of the earth. Summing his water budget computations by latitudinal belts for the whole Earth, he found that land
storage of water was 7167 1018 g greater in March than in September, a remarkably close agreement with the
oceanographers figures based on sea-level change. Van Hylckama’s results indicate an annual transfer of more
than 700 1013 litres of water from the ocean to the land, just one order of magnitude greater than the value
suggested in this study of water equivalent of snow storage in polar regions under double CO2 conditions.
Accumulation of this much water as snow for a 10-year period would equal the annual water movement from
ocean to land during the winter and spring seasons, an appreciable water volume.
The data in Table II (a and b) reveal a somewhat similar pattern in both hemispheres. The increased
temperatures predicted by the three GCMs will result in a decrease in water equivalent of snowfall up to 70 N for
the GFDL model, 68 N for the GISS model, and only to 64 N in the case of the UKMO model. The decrease in
water equivalent of snow cover extends to 67 S for the GISS model, 69 S for the UKMO model and 70 S for the
GFDL model. In the Southern Hemisphere there is a significant increase in the water equivalent of the snow cover
6
6
Table III. Total changes in water equivalent of snow (1012)
North polar region
South polar region
Total north and south
GFDL
GISS
UKMO
38114
500108
461194
ÿ3127
925120
921193
7155
1783108
1790163
ÿ
161
POLAR SNOW COVER AND GLOBAL WARMING
poleward of these particular latitudes, whereas in the Northern Hemisphere there are relatively minor increases in
snow cover and even a slight decrease between latitudes 78 and 79 for the GFDL model.
The only appreciable area of increased snow accumulation in the Northern Hemisphere occurs in central and
northern Greenland. In Antarctica, almost all of the continent will experience increased snow accumulation with
a doubling of CO2. Only on the peninsula in West Antarctica and along some coastal margins from about 90 E to
170 E are there regions of decreasing snow accumulation. The majority of the interior of the continent
experiences an increase in water stored as snow of more than 50 mm depth, and many near-coastal areas of the
continent experience increases of between 100 and 300 mm depth of water equivalent.
It must be remembered that the data in Tables II and III just express the changes in water equivalent of snow
accumulation for an average year in going from current average conditions to simulated average annual
conditions under a doubled CO2 scenario. Thus, they do not represent total increases in water accumulation in the
snow under a doubled CO2 atmosphere but rather an individual annual value of change. Because the snow does
not melt off each year, the tabular data represent an annual removal of water from the ocean area of the globe and
an annual growth in the depth of snow and ice over the polar regions. Thus, it is possible to think of these
particular annual changes in snow depth continuing year after year as long as doubled CO2 conditions persist.
CONCLUSIONS
Use of three current GCM models to provide simulated values of temperature and precipitation amounts in the
polar regions of the globe under double CO2 conditions have shown that there could easily be a significant
increase in snow accumulation as a result of global warming. This accumulation would be most noticeable in the
south polar region and total accumulation of water in the snow cover could result in a removal of about
900 1012 litres of water from the ocean yearly until the temperatures reach a high enough level to melt
additional areas of permanent snow cover. Provided that temperatures increase slowly, such significant losses in
water from the oceans could continue for a number of years and result in thicker ice caps, especially in Antarctica
and central to northern Greenland.
The use of GCM results in the foregoing paper should not imply acceptance by the authors of current GCM
results without question. However, because GCM results have been used extensively to estimate various future
conditions, it is important to be aware of what these models actually suggest about the possibility of snow
accumulation due to increased global warming.
6
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