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 1995Revised 1 March 1996Accepted 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 asthose temperatures remain below freezing, the increased precipitation should accumulate as snow. A study of both current anddouble CO2 temperature and precipitation data for all land areas poleward of 60� latitude using three different generalcirculation models suggests possible changes in snow accumulation due to increasing CO2. Increased snow accumulation willoccur in the Antarctic whereas a small decrease in snow depth is to be expected in the Northern Hemisphere. Total snowaccumulation 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 increasingconcentrations 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 warmingscenarios have been cited as evidence to suggest increased melting of polar ice masses. The present studyinvestigates 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 ofsnow accumulation. Evaluating snowfall amounts under current and GCM simulated conditions permits anestimate 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 thatrequired two fairly uniform increases in solar radiation with a period of decreased radiation between the radiationmaxima. He theorized that a gradual rise in solar radiation would be accompanied by a gradual rise in airtemperature. This, in turn, would result in an increase in precipitation, which would fall as snow in the polarregions. The increased precipitation would result from increased evaporation (due to the warmer watertemperatures) and the increased water vapour capacity of the warmer air. He reasoned that as long as airtemperatures in polar regions remained below freezing the increased precipitation would fall as snow andaccumulate 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 thesame time, there would be increased melting of continental ice sheets, leading to their retreat and ultimatedisappearance.

Although Simpson’s theory linking ice-age occurrence with an increase in solar radiation and air temperaturehas not survived the test of time, his recognition that an increase in air temperature might lead to increasedsnowfall 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 thermalexpansion of sea water), one process might lead to a sea-level fall. That process was increased snowfall on polar

CCC 0899-8418/97/020155-08 $17.50# 1997 by the Royal Meteorological Society

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 moisturecontent 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. Willmottet al.(1981) in their world-wide collection of all available temperature and precipitation records identified 805 land-based stations in the region north of 60� and 31 stations on the Antarctic continent. Location of these observingstations are shown in Figures 1 and 2. Temperature and precipitation data from these stations have beeninterpolated to a 0�5�60�5� latitude and longitude grid using a spherically based interpolation procedure (Legatesand Willmott, 1990a,b; Willmottet 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 tothese 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 beenobtained from three available general circulation models. Rather than just accepting the temperature andprecipitation values produced by the GCMs, the temperature change from a single to a double CO2 condition wasadded to the current climate temperature data and the percentage change in precipitation from a single to a doubleCO2 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 estimatingeither single or double CO2 climate distributions but believe that the magnitude of the changes noted betweensingle 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

Figure 1. Weather station locations for north polar area.

156 H. YE AND J. R. MATHER

change simulations to the current climate distribution therefore allows one to start with the most accuraterepresentation of the current climate distribution and to apply modelled change simulations in order to obtain anestimate 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 thesemodels 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 laterversions are now available in some cases, it was necessary to establish a fixed cut-off point in time and to usethose versions that were available to us at the beginning of the study. It is felt that later versions do not change theoverall conclusions of this study significantly. The GFDL model (Manabe and Wetherald, 1987) was constructedon a 4�44� latitude by 7�50� longitude grid. It consists of an atmospheric circulation model coupled to a staticmixed-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 constanteverywhere. Whereas the continental geography is more realistic than earlier versions, it is recognized thattopography 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(Hansenet al., 1988). The model utilizes nine atmospheric layers and a fairly coarse horizontal grid of 7�83�

latitude by 10� longitude. Diurnal and seasonal cycles are included and the radiative calculations include theradiatively significant atmospheric gases, aerosols, and cloud particles. Cloud cover and height are computed, andthe opacity of clouds is specified as a function of cloud type, thickness and height. The model incorporates a

Figure 2. Weather station locations for Antarctic area.

POLAR SNOW COVER AND GLOBAL WARMING 157

mixed-layer ocean, the layer having a seasonally varying depth specified from observations—the annualmaximum depth being approximately 125 m.

The UKMO model was developed in 1985–1986 (Wilson and Mitchell, 1987). The model couples anatmospheric GCM to a 50-m-thick oceanic mixed layer and an energy balance sea-ice model. A regularhorizontal grid spaced at 5� of latitude by 7�5� of longitude has a vertical dimension composed of 11 layersirregularly spaced, but concentrated near the boundary layer and the tropopause. Incoming solar radiation istreated on both a diurnal and a seasonal cycle, and the radiative fluxes are dependent on temperature, watervapour, carbon dioxide, ozone, and clouds. The model is designed to include a cloud-prediction scheme. Layeredclouds 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 layerexcept the top layer (Wilson and Mitchell, 1987). Cloud reflectivity is prescribed as 0�2 for high clouds and 0�6for others. For longwave radiation calculations, high clouds have an emissivity of 0�5 and all others are assumedto be black bodies (Wilson and Mitchell, 1987).

The actual monthly change in temperature calculated by each GCM as well as the percentage change inmonthly precipitation in going from a single to a double CO2 condition were determined for each GCM grid-square. These change simulations were then applied to the current temperature and precipitation data determinedpreviously for each 0�5� latitude by 0�5� longitude grid-point. Thus, the same numerical change was applied to allcurrent 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 atmosphericvariables without convincing success. Most studies suggest that air temperature near the surface is as reliable asany of the other variables tested for differentiating rain and snow. Because air temperature data are widelyavailable, they were used in this study to identify when the monthly precipitation fell as rain or snow. Analysingsome 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 74per cent were snow, and at 35�F, rain occurred 31 per cent of the time. There seemed to be a significant change inoccurrences of rain and snow at or slightly below freezing air temperatures. A monthly temperature value ofÿ1�C was selected to separate rain occurrences from snow occurrences. This same temperature value wasselected by Thornthwaite and Mather (1957) for distinguishing between rain or snow using monthly temperaturedata.

The water equivalent of the annual snowfall was estimated by totalling all monthly precipitation whenever themonthly temperature was equal to or belowÿ1�C. These calculations were performed on both the current andthe double CO2 precipitation simulations in order to determine quantitatively the change in the amount of waterheld in the snow cover from current to double CO2 conditions at each 0�560�5� grid-point. It was assumed thatthere was negligible evaporation loss as long as the temperature of the month was belowÿ1�C.

Table I. Characteristics of general circulation models utilized in study.

GISS GFDL UKMO

Model resolution 7�83610� 4�4467�5� 567�5�

Atmospheric levels 9 9 11Diurnal cycle yes no yesDepth mixedocean layer (m)

125a 68 50

Initial16CO2 (ppm) 315 300 323D Global T(�C)(26CO2)

�4�2 �4�0 �5�2

D Global P (per cent)(26CO2)

�11 �8�7 �15

Source: Wigleyet al 1989.a From Hansenet al. (1988).

158 H. YE AND J. R. MATHER

RESULTS

Using the assumption that all precipitation falling at monthly temperatures equal to or below71�C will be snow,the current as well as the double CO2 water equivalent of the snow accumulation has been calculated for all landareas poleward 60� latitude. Values of the changes in the water equivalent of the snow from current to doubleCO2 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 beltinvolved in the change in water equivalent of snow (in lit). This has been obtained by multiplying the amount ofland area in each latitudinal belt by the change in the depth of water equivalent of snowfall in the belt. Two of theGCMs indicate an overall decrease in snow accumulation in the Northern Hemisphere polar region, whereas thethird (the UKMO model) shows a small increase in snow cover poleward of 60�N. The total volume of waterinvolved in the changes is relatively small because of the small amount of land surface actually receivingsnowfall. In the Southern Hemisphere, some melting of the annual snow cover will occur around the periphery ofAntarctica, but the increase in water equivalent of the snow cover for the great interior of the continent more thanbalances the losses found in latitudes from 63� to 69�S. Table III summarizes the total changes in waterequivalent of snowfall by hemisphere and for the polar regions in both hemispheres together. The combined totalof water equivalent change reveals an increase in storage of water in snow cover in the world’s polar regions offrom 42261012 to 179161012 litres depending on the GCM being evaluated.

To put these results into another context, in the early 1950s (personal communication) Dr. Walter Munk at theScripps Oceanographic Institution suggested that some 5�061018 g of water were removed from the oceans of theworld between autumn (maximum sea-levels) and spring (minimum sea-levels). This water was returned to the

Table II. (a) Change of annual water equivalent of snowfall bylatitude belt in the south polar region.

Latitude Changes in water equivalent (1012L)

GFDL GISS UKMO

63–64 ÿ2�24 ÿ2�24 ÿ2�2464–65 ÿ7�41 ÿ5�53 ÿ7�4165–66 ÿ23�53 ÿ12�18 ÿ19�6366–67 ÿ86�40 ÿ15�46 ÿ78�8667–68 7 117�00 1�33 ÿ102�1268–69 ÿ79�81 28�49 ÿ29�2069–70 ÿ19�99 37�41 48�4670–71 33�22 75�41 158�2171–72 95�62 91�98 211�9172–73 129�72 89�17 210�1773–74 112�15 73�59 188�3074–75 101�40 71�47 176�6475–76 85�49 67�21 186�9076–77 60�40 64�77 174�2377–78 52�49 59�68 149�6978–79 37�48 66�05 117�7879–80 29�34 59�30 88�4280–81 27�52 46�04 81�0281–82 22�42 38�57 70�9382–83 18�40 32�77 57�7283–84 14�37 26�29 44�1384–85 8�32 17�40 29�0085–86 4�15 6�97 14�3986–87 2�41 4�28 8�9287–88 1�16 2�18 4�1988–89 0�36 0�77 1�3789–90 0�04 0�11 0�16

POLAR SNOW COVER AND GLOBAL WARMING 159

ocean between spring and autumn. This transfer of water volume resulted in an annual variation in mean sea-levelof about 1�4 cm when spread evenly over all the oceans of the world. Van Hylckama (1956) attempted to locatethis water volume by computing the monthly change in water storage in the root zone of the soil for all land areasof the earth. Summing his water budget computations by latitudinal belts for the whole Earth, he found that landstorage of water was 7�6761018 g greater in March than in September, a remarkably close agreement with theoceanographers figures based on sea-level change. Van Hylckama’s results indicate an annual transfer of morethan 70061013 litres of water from the ocean to the land, just one order of magnitude greater than the valuesuggested 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 fromocean 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 increasedtemperatures predicted by the three GCMs will result in a decrease in water equivalent of snowfall up to 70�N forthe GFDL model, 68�N for the GISS model, and only to 64�N in the case of the UKMO model. The decrease inwater equivalent of snow cover extends to 67�S for the GISS model, 69�S for the UKMO model and 70�S for theGFDL model. In the Southern Hemisphere there is a significant increase in the water equivalent of the snow cover

Table II(b). Change of annual water equivalent of snowfallby latitude belt in the north polar region

Latitude Changes in water equivalent (1012 L)

GFDL GISS UKMO

60–61 ÿ5�27 ÿ2�72 ÿ5�8861–62 ÿ4�78 ÿ2�22 ÿ4�8062–63 ÿ5�94 ÿ1�56 ÿ3�7763–64 ÿ5�95 ÿ1�22 ÿ2�6064–65 ÿ4�64 ÿ0�43 0�2165–66 ÿ4�28 ÿ0�77 1�3566–67 ÿ4�34 ÿ0�82 0�5367–68 ÿ3�51 ÿ0�28 1�0768–69 ÿ2�20 0�24 1�9469–70 ÿ0�91 0�75 3�0870–71 0�14 0�99 3�4271–72 0�72 0�90 2�7472–73 0�56 0�42 1�7073–74 0�66 0�39 1�2274–75 0�65 0�44 0�9475–76 0�42 0�43 1�2076–77 0�01 0�27 0�9177–78 0�03 0�23 0�6478–79 ÿ0�01 0�29 0�5379–80 0�08 0�40 0�6880–81 0�16 0�46 1�0881–82 0�16 0�35 0�8782–83 0�08 0�19 0�4983–84 0�02 0�04 0�11

Table III. Total changes in water equivalent of snow (1012)

GFDL GISS UKMO

North polar region ÿ38�14 ÿ3�27 7�55South polar region 500�08 925�20 1783�08Total north and south 461�94 921�93 1790�63

160 H. YE AND J. R. MATHER

poleward of these particular latitudes, whereas in the Northern Hemisphere there are relatively minor increases insnow 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 andnorthern Greenland. In Antarctica, almost all of the continent will experience increased snow accumulation witha doubling of CO2. Only on the peninsula in West Antarctica and along some coastal margins from about 90�E to170�E are there regions of decreasing snow accumulation. The majority of the interior of the continentexperiences an increase in water stored as snow of more than 50 mm depth, and many near-coastal areas of thecontinent 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 snowaccumulation for an average year in going from current average conditions to simulated average annualconditions under a doubled CO2 scenario. Thus, they do not represent total increases in water accumulation in thesnow under a doubled CO2 atmosphere but rather an individual annual value of change. Because the snow doesnot melt off each year, the tabular data represent an annual removal of water from the ocean area of the globe andan annual growth in the depth of snow and ice over the polar regions. Thus, it is possible to think of theseparticular 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 thepolar regions of the globe under double CO2 conditions have shown that there could easily be a significantincrease in snow accumulation as a result of global warming. This accumulation would be most noticeable in thesouth polar region and total accumulation of water in the snow cover could result in a removal of about90061012 litres of water from the ocean yearly until the temperatures reach a high enough level to meltadditional areas of permanent snow cover. Provided that temperatures increase slowly, such significant losses inwater from the oceans could continue for a number of years and result in thicker ice caps, especially in Antarcticaand central to northern Greenland.

The use of GCM results in the foregoing paper should not imply acceptance by the authors of current GCMresults without question. However, because GCM results have been used extensively to estimate various futureconditions, it is important to be aware of what these models actually suggest about the possibility of snowaccumulation due to increased global warming.

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