Non-Frozen Ground

About this map

Across the Northern Hemisphere, much of the land surface freezes and thaws every year, driven by changes in temperature and season. This interactive map is derived from measurements of how many days per year have near-surface temperatures of soil have been above 0° Celsius (or unfrozen). Each year’s total of unfrozen-soil days is compared to the long-term average from 1979 to 2012. The corresponding bar graph shows how much the overall number of unfrozen days departs from the long-term average. 

Color key and bar graph

When you select a year on the lower-left slider, the map will show unfrozen ground data.

• Areas with higher-than-average unfrozen soil days will appear in shades of red, and areas with lower-than-average unfrozen soil days will appear in shades of blue.
• Areas with unfrozen soil days at or near the long-term annual average are white or nearly white.
• The greater the departure from average, the darker the color.

The bar graph indicates the unfrozen soil days anomaly (departure from the long-term average) for the region from 50°N of the equator to the North Pole.  Blue bars indicate negative anomalies (years in which the number of unfrozen soil days is less than the long-term average) and red bars show positive anomalies (years in which the number of unfrozen soil days is greater than the long-term average). The bars show every year in the time series, and the bar that correlates with the map on display is highlighted in light gray.

How to change the display

• To change the year displayed, move the slider in the year box (lower left).
• To animate the time series, click the play arrow (lower left). The animation will display maps for all years in the time series.

What is frozen ground?

Frozen ground is soil that has a surface temperature of 0° Celsius (32° Fahrenheit) or lower. More than half of all the land in the Northern Hemisphere freezes and thaws each year, but not all of that land stays frozen for the same length of time.

• Intermittently frozen ground is near-surface soil that freezes from one to 15 days per year.
• Seasonally frozen ground is near-surface soil that freezes for more than 15 days per year. 
• Some soil remains frozen for a year or longer.
• Permafrost is any kind of ground—soil, sand, sediment, or rock—that remains frozen for at least two years. Some permafrost has been frozen for thousands of years. Permafrost is often beneath an “active layer” of soil that is thawed at least part of every year. 

These maps show the frozen or unfrozen status of the soil surface, including areas with and areas without permafrost.

Why frozen ground matters

Whether ground freezes and for how long, influences vegetation, wildlife, and human activity.

Soil temperatures, combined with changing seasonal amounts of solar radiation, determine how long plants can grow each year, and when they are dormant. Wildlife adapted to living with seasonally frozen ground endure lean winter seasons by migrating to warmer climates, or by hibernating. Soils themselves comprise rich ecosystems incorporating arthropods, worms, fungi, and microbes, and these subterranean ecosystems are strongly influenced by soil temperature. Soil’s frozen or thawed status can also affect people who must farm, manage livestock, or maintain infrastructure.

What the data show

Continuous observations of unfrozen ground dates back to the start of the satellite record in 1979. Soil temperatures are derived from passive microwave satellite observations.

Over the course of the time series, the number of unfrozen days per year increases, although with considerable year-to-year variation. From 1979 through 1987, every year has a negative anomaly compared to the 1979 to 2012 average. From 2005 onward, the number of unfrozen days has a positive anomaly every single year.

By viewing different years, you can use this map to examine changes in the number of annual unfrozen soil days over time. Try using these maps to answer questions such as:

• In years that display large anomalies in the bar graph, are the anomalies driven primarily by widespread above- or below-average unfrozen days, or driven by intense departures from average in select regions?
• Do the regions with above- or below-average unfrozen days remain consistent over time?

By comparing frozen ground maps with other maps in Satellite Observations of Arctic Change, you may see correlations with other changes in other parts of the Arctic ecosystem, such as near-surface air temperatures, vegetation greenness, or snow cover duration.

The duration of the non-frozen period in each year is the total number of days a given pixel (the smallest area of land that a satellite sensor can observe) 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 states and transitions.

Data source(s)

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.

Explore the source data for this map:

Kim, Y., J. S. Kimball, J. Glassy, and K. C. McDonald. 2017. MEaSUREs Global Record of Daily Landscape Freeze/Thaw Status, Version 4. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: 10.5067/MEASURES/CRYOSPHERE/nsidc-0477.004. 

Data processing steps

To create this map, NSIDC took the following steps:

• Acquire source data directly from NSIDC. Use the SMMR_SSMI HDF5 subset (Northern Hemisphere EASE-Grid 25km) of the MEaSUREs Global Record of Daily Landscape Freeze/Thaw Status, Version 4 data set.
• [a] Create annual ‘thawed days’ dataset, i.e., for each year in the timeseries:
  • Accumulate the number of days marked as thawed in the dataset
  • Mask out missing data
  • Mask out latitudes south of 50 degrees north.
• [b] Generate climatology gridded dataset
  • From the ‘thawed days’ dataset [a], calculate the mean grid for 1979-2012
• [c] Generate yearly gridded anomaly datasets
  • For each year in the timeseries, subtract the climatology [b] from that year’s ‘thawed days’ grid [a]
• Generate yearly anomaly images
  • From the anomaly dataset [c], create an anomaly image
  • Images are reprojected to EPSG:3413
• Generate yearly anomaly CSV from [c].

Quick links

Learn about Frozen Ground