Soil Non-Frozen Period: More information

The map and bar graph show how the number of days in each year the soil surface is unfrozen compares to the average for the period 1981 to 2010. On the map, areas where soil surfaces are unfrozen for more days than normal are shown by reds (positive anomalies). Areas where soil surfaces are unfrozen for fewer days than normal are shown by blues (negative anomalies). The map of anomalies helps to show where changes the number of days with unfrozen soil are the strongest.

The bar graph shows the anomaly in the average number of non-frozen days for the region north of 50° N. The graph helps to illustrate Arctic-wide changes in the non-frozen period.

sample soil non frozen period imageThis sample image shows soil non-frozen period anomalies for the year 1979.

Much of the Arctic land surface is seasonally frozen. In late fall and winter, near-surface soil temperatures are less than 0 °C. In spring, near-surface soils thaw. This seasonal freezing and thawing, along with snow cover, and the annual cycle of solar radiation, exerts a strong control over vegetation.

Strong increases in the duration of the non-frozen period start to emerge around 2005. In the period before 2005, periods of positive anomalies are mixed in with periods of negative anomalies. The longer non-frozen periods in recent years are associated with earlier spring thaws.

Increases in the duration of the non-frozen period in recent years are wide spread north of 50° N but year to year variations in the patterns of these increases can be seen. Much of Alaska and eastern Siberia exhibit increases in the non-frozen period in most years since 2005. However, central Siberia and northern Canada show more variability.

The data shown here are from the MEaSUREs Global Record of Daily Landscape Freeze/Thaw Status Version 2 product. This product is a global record of the daily freeze/thaw (FT) status of the landscape derived from satellite observations of radiometric brightness temperatures. The FT record from the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) that spans 1979 to 2010 is used here.  Microwave radiation emitted from the earth surface is sensitive to changes in water in vegetation, snow and soil from frozen to thawed state. These changes in FT state are manifested in the radiometric brightness temperature; the temperature an idealized perfectly radiating surface would have to be to emit the same intensity of radiation at the wavelength of interest. Transitions from the frozen to non-frozen (thawed) state can be detected. The duration of the non-frozen period in each year is the total number of days a given pixel was classified as non-frozen. This satellite derived time series of freeze/thaw states has been evaluated against observations at a number of sites around the globe and has shown good correspondence with the timing of observed freeze/thaw transitions and states.

References

Kim, Y., J. S. Kimball, J. Glassy, and K. C. McDonald. 2013. MEaSUREs Global Record of Daily Landscape Freeze/Thaw Status. Version 2. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://dx.doi.org/10.5067/MEASURES/CRYOSPHERE/nsidc-0477.002.

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