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 # 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. 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