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Soil Temperature: Overview
"The perpetually frozen soils of the Arctic and boreal regions
are thawing at an unprecedented rate. It's unclear what this bodes
for global warming."
- Science magazine, 11 June 2004
Soil temperature responds to changes in climate. Changes at great depth
(10 m to 200 m) reflect decade-to-century scale climatic variability,
because changes in surface conditions take years to propagate through
the soil. Soil temperatures closer to the surface, measured by thermistors
at about 1 meter depth, are reflective of changes on shorter time scales.
The presence or absence of snow and the type and density of vegetation,
as well as the composition of the soil itself, can affect the response
of soil temperature to changing air temperature. Deep snow and dense
vegetation, for example, insulate soil from changes in air temperature.
What happens when average soil temperatures change over time? Areas
where the soil remains frozen year to year are areas of permafrost. Permafrost
thaws when its temperature rises above zero degrees Celsius, resulting
in changes in soil moisture, vegetation, and the ability of the soil
to store greenhouse gasses (carbon dioxide and methane). A typical progression
would be thawing of permafrost underlying a spruce forest, resulting
in the creation of a bog, and loss of the spruce forest. Any man-made
structure built on the permafrost would undergo subsidence (Alaska and
Siberia are experiencing damage to infrastructure on a widespread area,
see Science, 30 Aug 2002, p. 1493). Subsequently the bog might dry out,
and be succeeded by woody shrubs.
Overall, soil temperatures are rising in much of the Arctic ("Permafrost
Temperature Records: Indicators of Climate Change," Eos 83
(50), 10 Dec 2002); however, these changes are manifest primarily in
seasonally frozen ground regions while areas underlain by permafrost
continue to be stable. How soil temperatures behave at any one location
depends on many site-specific factors. The soil temperature indicators
show examples of variability in soil temperature behavior.
The NOAA
Arctic Change Indicator Website has more information. See State
of the Cryosphere for a map showing areas of continuous and discontinuous
permafrost. The Frozen Ground Data Center provides
access to frozen ground data. |
