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Trends in Snow and Ice Cover


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In the Northern Hemisphere, the average snow extent has decreased by 3.75 million square Km in the last 39 years. The greatest loss of snow cover has occurred in the months of June and July.  In the Northern Hemisphere, the average extent of sea ice decreased by 1.5 million square Km during the period 1978-2008, from about 9.9 million square Km to about 8.4. In the Southern Hemisphere in this period, the extent of sea ice did not change significantly. Reduced snow and ice cover changes albedo, increasing the rate of warming.

Changes in Sea Ice Area and Extent, 1978-2008

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Data files tabulate extent and area (in millions of square kilometers) by year for a given month. In the analysis below, we averaged the monthly areas for the Northern and Southern Hemispheres. "Area" excludes those locations not imaged by the sensor near the pole. Such an area is assumed to be entirely ice covered, with at least 15% concentration. "Extent" includes locations not imaged near the pole.

Results depend on the areas measured.

Area: In the Northern Hemisphere, the average area of sea ice decreased during the period 1978-2008, while in the Southern Hemisphere, the average area increased in the same period.  In 2007 and 2008, the area of sea ice in the North was actually less than in the South. In the Northern Hemisphere, the extent of sea ice has been decreasing at a rate of -3.3% per decade since 1970.  In the Southern Hemisphere, the extent has increased at a rate of 1.1% per decade since 1970. Source:

Extent: In the Northern Hemisphere, the average extent of sea ice decreased by 1.5 million square Km during the period 1978-2008, from about 9.9 million square Km to about 8.4. In the Southern Hemisphere in this period, the extent of sea ice did not change significantly.

Source: and

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Changes in Snow Extent, Northern Hemisphere, 1967-2008

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The Rutgers University Global Snow Lab evaluates weekly maps of the northern hemisphere to determine the snow cover extent in square kilometers. The maps analyzed are produced by NOAA meterologists, who rely primarily on daily visible satellite imagery to construct the maps.

Annual snow cover extent (SCE) over Northern Hemisphere lands averaged 24.0 million square kilometers in 2007.  This is 1.5 million sq. km. less than the 38-year average and ranks 2007 as having the 3rd least extensive cover of record.

To assess changes in snow area, we found the mean measured snow area per year, and compared this value across years. Between 1967 and 2008, the Northern Hemisphere has lost about 3.75 million square kilometers of snow cover.

Source: Rutgers University Global Snow Lab. downloaded January 1, 2009.

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Changes in Snow Extent, Northern Hemisphere, 1967-2008, by

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While the mean snow extent has been decreasing in the last 40 years, this decrease has not been uniformly spread across the seasons.  To examine this, we found the mean snow extent for the four seasons. The graph below shows mean snowfall for the seasons over this period.  It is clear that from the linear regression lines that it is summer (June, July, and August) where the greatest decrease in extent has been occurring, with some additional loss in spring.

The summer slope of the linear least-squares fit to the data is -.106; spring slope is -.068, fall slope is -.020, and winter slope is -.002.

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Changes in Snow Extent, Northern Hemisphere, 1967 - 2008, by

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The steepest declines in snow extent for the period 1967 - 2008 occurred in the months of June and July.  That is, our summers in the Northern Hemisphere may be warming more than our winters are warming.

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Global Annual Freezing and Thawing Indices

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In permafrost regions, annual freezing and thawing indices have been used to predict permafrost distribution and active layer extent, providing researchers with important information on climate variability as well as offering engineers information specific to cold region structural design. These indices can also be used in more temperate, permafrost-free regions to classify snow-type, estimate depth of ground-frost penetration, and predict the maximum thickness of sea, river and lake ice.

The freezing and thawing indices are defined as the cumulative number of degree-days below and above 0 degrees Celsius for a given time period (Permafrost Subcommittee 1988). Generally, four main types of freezing and thawing indices have been used: (i) approximate freezing and thawing indices; (ii) total annual freezing and thawing indices; (iii) seasonal freezing and thawing indices; and (iv) design freezing and thawing indices.

Global annual freezing and thawing indices were calculated at NSIDC from air temperature data compiled by Legates and Willmott (1990).

Strictly speaking, the indices should be called the 'Global Approximate Annual Freezing and Thawing Indices'. The global annual freezing and thawing indices measure the magnitude of air temperatures below (e.g. Northern Hemisphere late fall, winter and early spring) and above (e.g. Northern Hemisphere late spring, summer and early fall) 0 degrees Celsius over the course of one year. Most of the global mean monthly air temperatures were compiled between 1920 and 1980 over the terrestrial surface and between 1950 and 1979 over the oceanic surface (Legates and Willmott 1990). The global annual freezing and thawing indices are generally representative of the freezing and thawing climatology during that period.

The source data compiled by Willmott and Legates is a global climatology of mean monthly surface air temperatures. Terrestrial station records and oceanic grid-point records were interpolated to a 0.5 degree latitude by 0.5 degree longitude lattice using a spherically-based interpolation algorithm (Willmott and Legates 1990).

Willmott and Legates' data were acquired from various sources including Wernstedt 1972, Willmott et al. 1981, the National Center for Atmospheric Research (Spangler and Jenne 1984), the Commonwealth Scientific and Industrial Research Organization (CSIRO 1962-1971), the Australian Department of National Development (ADND 1965), and individual research stations in Antarctica (van Rooy 1957; Schwerdtfeger 1984), China and the Far East (Nuttonson 1947; Terjung et al. 1985). Oceanic surface air temperature data were obtained from the Comprehensive Ocean-Atmospheric Data Set for the period 1950-1979 (Fletcher et al. 1983; Slutz et al. 1985; Woodruff 1985; Woodruff et al. 1987).

Potential errors in the freezing and thawing indices may include error from relocation of individual stations, exposure of instruments to the elements, human error in instrument readings, and instrument malfunction due to relocation. In addition errors may have resulted from the compilation of the station air temperatures into the 0.5 by 0.5 grids. A detailed discussion of these errors can be found in Legates and Willmott (1990). Errors may also result from using monthly mean air temperature to calculate the annual freezing and thawing indices without making corrections for positive (above 0 degree Celsius) and negative (below 0 degree Celsius) degree-days in spring and fall, respectively. In the high Arctic, errors in calculating the annual freezing and thawing indices using both monthly mean and daily mean air temperatures are generally less than five percent (Zhang et al. 1996). However, in temperate regions, the errors can potentially become larger.

Source: Data available at DICES/


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ADND. 1965. Fitzroy Region, Queensland - Climate. Canberra, Australian Department of National Development Resources Information and Development Branch.

CSIRO. 1962-71. Land Research Series Nos. 6-14, 17-22, 24-29. Melbourne, Commonwealth Scientific and Industrial Research Organization.

Fetterer, F., and K. Knowles. 2004. Sea ice index monitors polar ice extent. Eos: Transactions of the American Geophysical Society 85, 163.

Fletcher, J.O., R.J. Slutz, and S.D. Woodruff. 1983. Towards a comprehensive ocean-atmosphere data set. Tropical Oceanic and Atmospheric Newsletter. 20:13-14.

Legates, D. R. and C. J. Willmott. 1990. Mean Seasonal and Spatial Variability in Global Surface Air Temperature. Theoretical and Applied Climatology 41:11-21.

Meier, W., J. Stroeve, F. Fetterer, K. Knowles. 2005. Reductions in arctic sea ice cover no longer limited to summer. Eos: Transactions of the American Geophysical Society 86, 326.

Nuttonson, M.Y. 1947. Ecological Crop Geography of China and its Agro-climatic Analogues in North America. Washington, D.C. American Institute of Crop Ecology. 28 pages.

Permafrost Subcommittee. 1988. Glossary of Permafrost and Related Ground-ice Terms. Associate Committee on Geotechnical Research, National Research Council of Canada.

Schwerdtfeger, W. 1984. Weather and Climate of the Antarctic. The Netherlands, Elsevier. Developments in Atmospheric Science No. 15. 261 pages.

Slutz, R.J., S.J. Lubker, J.D. Hiscox, S.D. Woodruff, R.L. Jenne, D.H. Joseph, P.M. Steurer, and J.D. Elms. 1985. Comprehensive Ocean-atmospheric Data Set; Release 1. Boulder, CO. NOAA Environmental Research Laboratories, Climate Research Program. 268 pages.

Spangler, W.M.L., and R.L. Jenne. 1984. World Monthly Surface Station Climatology. Boulder, CO. National Center for Atmospheric Research, Scientific Computing Division. 14 pages.

Terjung, W.H., J.T. Hayes, H-Y. Ji, P.E. Todhunter, and P.A. O'Rourke. 1985. Potential Paddy Rice Yields for Rainfed and Irrigated Agriculture in China and Korea. Annual Association of American Geographers. 75:83-101.

van Rooy, M.P. 1957. Meteorology of the Antarctic. South Africa, Government Printer. 240 pages.

Wernstedt, F.L. 1972. World Climatic Data. Lemont, P.A., Climatic Data Press. 552 pages.

Willmott, C.J., J.R. Mather, and C.M. Rowe. 1981. Average Monthly and Annual Surface Air Temperature and Precipitation Data for the World. Part 1: the Eastern Hemisphere. Part 2. The Western Hemisphere.

Woodruff, S.D. 1985. The comprehensive ocean-atmospheric data set. Third Conference on Climate Variations Symposium on Contemporary Climate 1850-2100, American Meteorological Society, 14-15.

Woodruff, S.D., R.J. Slutz, R.L. Jenne, and P.M. Steurer. 1987. A comprehensive ocean-atmosphere data set. Bulletin of the American Meteorological Society, 68:1239-50.

Zhang, T. 1998. Global Annual Freezing and Thawing Indices. Boulder, CO, USA: National Snow and Ice Data Center. Digital media.

Zhang, T. T.E. Osterkamp, and K. Stamnes. 1996. Some characteristics of the climate in northern Alaska, USA. Arctic and Alpine Research 28(4):509-518.

Other Papers in this Series

© 2009 David Stang. All rights reserved.

Dr. Stang is available for presentations on this topic. Contact him by email.

Last Revised: Friday, January 16, 2009