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NAME: Subsurface temperature regime
BRIEF DESCRIPTION: Temperatures in boreholes a few hundred metres deep can be an important source of information on recent climatic changes, because the normal upward heat flow from the Earth's crust and interior is perturbed by the downward propagation of heat from the surface. As temperature fluctuations are transmitted downward, they become progressively smaller, with shorter-period variations attenuating more rapidly than longer ones. Although seasonal oscillations may be undetectable below about 15 m, century-long temperature records may be observed to depths of 150 m or so. Bedrocks thus selectively retain the long-term trends required for reconstructing climate change.
The surface temperature is strongly affected by local factors such as thickness and duration of snow cover, type of vegetation, properties of organic soil layers, depth to the water table, and topography. It influences, in turn, a wide range of ground and surface processes, particularly in the near-surface portions of permafrost [see frozen ground activity]. Below the active layer, where ground temperature fluctuates seasonally as thawing and freezing take place, long-term temperature variations may be recorded. Here, repeated measurements of soil temperature at fixed locations can reveal both the long-term dynamics of seasonally frozen ground and long-term climatic fluctuations, though the conversion of ground temperature to climate history is a complex matter. In the northern Canadian prairies ground temperatures have risen by 2° C and permafrost has retreated northwards by 100 km in the past 50 years. In contrast, permafrost temperatures have fallen in northern Quebec in recent years.
SIGNIFICANCE: The thermal regime of soils and bedrocks exercises an important control on the soil ecosystem, on near-surface chemical reactions (e.g. involving groundwater), and on the ability of these materials to sequester or release greenhouse gases. It may affect the type, productivity and decay of plants, the availability and retention of water, the rate of nutrient cycling, and the activities of soil microfauna. It is also of major importance as an archive of climate change, indicating changes in surface temperature over periods of up to 2-3 centuries, for example in regions without a record of past surface temperatures. In permafrost, the ground temperature controls the mechanical properties of the soils, especially during the freeze-thaw transition in the active layer.
HUMAN OR NATURAL CAUSE: The subsurface temperature regime reflects both the natural geothermal flux from the Earth's interior and the surface temperature. The latter can be modified by human actions, such as land clearing, wetland destruction, agriculture, deforestation, flooding of land for reservoirs, or development of large settlements that give rise to a 'heat island' effect.
ENVIRONMENT WHERE APPLICABLE: Any terrestrial area, but particularly in permafrost regions.
TYPES OF MONITORING SITES: Remote sites no more than 500-1000 km apart and away from obvious human disturbances, bodies of surface water, or areas of high geothermal flow where the ground cover is left undisturbed. The best results are obtained from measurements in relatively impermeable bedrocks or where there has been minimal groundwater movement. To ensure a good representation of climate-induced change, measurements should be made in clusters of boreholes drilled specifically for this purpose.
SPATIAL SCALE: patch to landscape / mesoscale to continental
METHOD OF MEASUREMENT: Many parameters must be measured, and many factors need to be considered when converting the signal to changes in surface temperature. Temperatures must be accurately measured (+/-millidegrees) in boreholes, using thermocouples, thermistors, thermoresistors and other measuring devices. Automated data loggers are most convenient for repeated measurements.
FREQUENCY OF MEASUREMENT: At least once every 5 years for deep boreholes, more frequently (as often as twice daily) for near-surface temperatures in permafrost.
LIMITATIONS OF DATA AND MONITORING: The thermal coupling of the Earth's surface to the atmosphere is complex, and the temperature signal recorded in the near surface is a filtered version of changes in surface climate. Physical movements in the active layer of permafrost regions complicate the picture [see frozen ground activity], as do the effects of snow cover and vegetation in temperate and tropical areas, and human activities such as urbanization, agriculture or deforestation. Moreover local topography, precipitation, hydrology and vegetation can mask the downward propagation of atmospheric temperatures. The installation of bore-holes disturbs the natural temperature regime, which should be allowed to recover before monitoring begins.
APPLICATIONS TO PAST AND FUTURE: Inversions of subsurface temperature profiles can provide a record of surface temperature, especially over the last 200-300 years, and a reliable indication of average surface temperatures of the past. Deep borehole records can yield records back 10,000 years or more.
POSSIBLE THRESHOLDS: In near-surface permafrost, the freeze-thaw threshold, which may vary in temperature according to soil and water salinity, controls a wide range of surficial (periglacial) processes [see frozen ground activity].
Lachenbruch, A.H. & B.V.Marshall 1986. Changing climate: geothermal evidence from permafrost in the Alaskan Arctic. Science 234: 689-696.
Lewis,T. (ed.), 1992. Climatic change inferred from underground temperatures. Global and Planetary Change 6:71-281.
Williams, P.J. & M.W.Smith 1989. The frozen Earth - fundamentals of geocryology. Cambridge: Cambridge University Press.
OTHER SOURCES OF INFORMATION: Geological surveys, International Heat Flow Commission (IASPEI), World Data Center-A for Heat Flow, IPA.
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Climate change, groundwater flow. Changes in near-surface ground temperature may affect soil fauna and sensitive surface vegetation.
OVERALL ASSESSMENT: The subsurface temperature regime is a direct measure of ground temperature history. It constitutes a very important indicator of thermal change in the periglacial environment, in soils (e.g. due to past deforestation, draining of wetlands), and in climate.
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