Subsurface Temperature Monitoring: Present-Day Temperature Change and Its Variability

4.1 Geothermal Observatories and Subsurface Temperature Monitoring

In the previous chapters we have described the reconstruction of the past ground surface temperature (GST) changes from the temperature-depth profiles measured in boreholes. Such profiles actually represent an important part of borehole geophysics, the science that records and analyzes measurements of various physical properties in wells or test holes. The geophysical logging system consists of probes, cable and draw works, and power and processing modules as well as data recording units. Modern logging systems are controlled by a computer. Probes (thermometers) that measure temperature-depth distribution are lowered into the borehole to collect continuous or point-by-point data, so-called temperature log, with one pass of the probe (for details see Section 2.1 and Figure 18). These records may be used for various environmental investigations including paleoclimate reconstructions and assist in better understanding of the subsurface conditions. Because of reduction of the resolving power of the "geothermal" method in the past, the GST reconstructions inferred from borehole temperature logs capture only the general course of the climate variations and not the precise variance or periodic signals clearly presented in the time series of meteorological records.

Temperature monitoring represents another data collection method. This measurement scheme applies several temperature sensors (thermistors) fixed at various positions along a cable that is then placed into a borehole for long-term recording of temperatures at selected depths. An interval between neighboring measurements may be from minutes to hours. The sampling design depends on the objectives of the research program. Temperature monitoring is frequently accompanied by additional instruments that determine air temperature and other meteorological variables and/or subsurface parameters such as soil moisture, water level change in borehole, etc. This procedure provides fine-scale and accurate temperature time series over multiyear time intervals. While temperature logs from deeper holes (>200-300m) are used for the GST history reconstructions, the monitoring experiments are generally performed in the shallow (< 100m depth) holes.

A special kind of underground measurements represents the soil temperature monitoring. Soil temperature monitoring is the recording of the temperature of soil at specific levels just below the surface. For the measurements at or very near the surface, thermometers can be buried directly into the soil. The main processes affecting soil temperature in the upper meters are solar radiation and heat exchange at the surface. They depend on seasons, physical properties of the soil such as soil type, compaction and moisture content, and vegetation cover. Because atmospheric processes are strongly reflected in soil temperatures, soil temperature monitoring can provide valuable data for the climate change analysis, especially for the detection of the recent climate change magnitude and capturing of the high-frequency climate variability. Soil temperature can also be used to give background data for other environmental monitoring programs such as plant phenology,1 soil decay rate, species diversity, invertebrate2 studies, etc. Recent perspective utility of the soil temperature monitoring is the tracking changes in temperature to determine the effectiveness of greenhouse gas emission reduction measures (see below).

It should be mentioned that the borehole and/or soil temperature monitoring represents only a part of the general climate monitoring efforts that include an observation, measurement, and analysis of the past and the present states of climate from systematic networks all over the world. Sure conclusions about an extent of the present climate change and the role of the human activities can be achieved only through an understanding the past climate change and its natural variability. To obtain this information, scientists monitor five components of the climate system: atmosphere, oceans hydrology, land surface, and cryosphere. The land temperature monitoring represents an essential part of the net land monitoring. It can be regarded as an indispensable prerequisite of regional and global environmental studies and management activities.

Systematic climate monitoring provides valuable data that can assist in developing climate models for prediction of future trends. Tracking changes in temperature can also help to determine the effectiveness of greenhouse gas emission reduction measures. An establishment of the joint air quality and climate monitoring system in the Black Triangle Region represents a typical example of such efforts. The Polish, Czech, and German border area (so-called Black Triangle) has been recognized as the most heavily industrialized and simultaneously the most environmentally degraded region of Europe. It covers an area of 32400km2, and has a population of more than 6million. The region is one of the largest basins of lignite coal in Europe. Significant amount of sulfur dioxide (SO2) is emitted by area power stations, district heating plants, and other industries. This region accounts for about 30% of Europe's total SO2 emissions. The member states as well as the EU's actions have been made to reverse the Black Triangle's air pollution legacy. The latest data provided by the trans-boundary monitoring system have detected that promising results of this efforts are already in evidence (see, e.g.,

'Phenology is the study of cyclic events of nature - usually the life cycles of plants and animals - in response to seasonal and climatic changes to the environment.

2Invertebrate is a term for any animal lacking a backbone. The group includes 97% of all animal species.

Concerning borehole climatology, subsurface temperature monitoring in boreholes is generally performed in three main related and/or complementary investigation directions:

(1) Empirical site-specific observations of the GST-SAT coupling at single sites using monitoring of the air/subsurface temperatures and other meteorological variables. A comparison of soil and air temperatures provides a direct test of details of their coupling at shorter timescales (from daily/annual to decadal) and accounts for how air temperature and other meteorological conditions influence the downward propagation of the surface temperature signal. It is expected that continuous monitoring of the ground temperatures and related meteorological variables that is being carried out at numerous locations in the frames of various international programs will significantly extend available climatologic database and improve our present understanding of the GST-SAT linkage. Examples of such monitoring have been described in Section 2.6 of Chapter 2.

(2) Monitoring of the ground temperatures at shallow depths where seasonal/annual temperature variations vanish can serve as an alternative useful tool for a direct quantitative assessment of the global warming rate. For the data collected from climate monitoring to be useful, measurements have to be taken at least over a decade or longer. Any gaps in information make it harder to capture trends and changes in climate. Both above-mentioned kinds of research can assist in resolving the differences between the influence of the past climatic effects and the effects of the present-day air-ground temperature coupling and of how this coupling may change through time. Methods for analysis depend on the specific research questions being asked.

(3) An investigation of the shallow subsurface temperature time series can significantly advance our knowledge of the temporal and spatial patterns of the recent changes in the climate variability. A special kind of such research represents permafrost monitoring (see, e.g., web site; and Section 2.8, Chapter 2).

An analysis of the microtemperature time series monitored at depth in boreholes can also be used successfully in other fields of the geophysical research. Thus, they can help to quantify the stochastic heterogeneity of the temperature signal and provide valuable information on the fine scale features of the heat transfer process in different geological environments (see, e.g., Bodri and Cermak, 2005b). However, these applications lie beyond the scope of our book; thus, next sections will be devoted to the above-enumerated three directions of the climatic research using temperature monitoring data.

4.2 Detection of the Present-Day Warming by Temperature Monitoring in Shallow Boreholes

As known, temperature changes at the Earth's surface occur at various temporal scales. The oscillations are more regular on diurnal, seasonal, and annual scales. Interannual and long-term temperature change patterns are generally irregular. As was demonstrated in

Sections 1.3 (Chapter 1) and 2.1 (Chapter 2), as the surface temperature signal propagates downward, its amplitude decreases exponentially with depth due to the diffusive nature of the heat conduction process. Each variation vanishes over a vertical distance related to the period of change and to the thermal diffusivity of the ground. Shorter period fluctuations attenuate more rapidly. Thus, the Earth selectively filters out high-frequency component of the surface temperature oscillations, and the deeper we go, the more distant past is archived there (unfortunately also more diffused and less credible).

As a part of the UNESCO International Geological Correlation "Borehole and Climate" Program of the IGCP 428 project (for details see Section 3.1, Chapter 3), two experimental shallow boreholes were drilled in two different environments to monitor the depth response of the underground temperature field to changes on the ground surface. Both holes were equipped with a measuring chain of temperature sensor elements at a number of selected depths covering the whole 0-40 m interval. It was expected that several years' temperature records would provide direct evidence of the decade-scale GST warming. This warming that can be related to the present-day global change was already detected in the territory of the Czech Republic by the more traditional inversion of the borehole temperature logs. Figure 14 (Chapter 1) illustrates the amplitude attenuation of the temperature signal when propagating downwards and the delay of its phase by showing the results of the 12-year temperature monitoring at several shallow depths in the experimental borehole Sporilov (Prague, the Czech Republic) (Cermak et al., 2000). The daily temperature wave is practically not observable below 1m depth. Similarly, annual GST fluctuations vanish near approximately 10-15 m depth and are not measurable below this depth. The temperature from the 20-30 m depth level is free of any response to the annual and/or shorter temperature variations and contains exclusively the fingerprints of the longer scale climatic trends with characteristic time of at least several years. Such signal may characterize well the pattern of the long-term climate change.

Figure 117 illustrates the amplitude decrement and phase shift of the annual temperature wave with depth in more details. It shows the 2003-year segment of the long-term temperature time series from Sporilov presented in Figure 14 (Chapter 1). Temperature was monitored at several shallow depths from 2.5 to 38.3 m. As the surface temperature signal propagates downward, it is delayed in time and its amplitude decreases exponentially with depth (see also Figure 15, Chapter 1). Each variation vanishes over a vertical distance related to the period of change and to the thermal diffusivity of the ground. Thus, the amplitude of the annual wave decreases to 50% of its surface value at ~2m depth with time delay of about 40 days. It already decreases to ~15% of its surface value at 5 m depth where it arrives with approximately three months' delay. Higher frequency oscillations vanish more rapidly. Similar attenuation is observable also in statistical characteristics of the records, e.g., in standard deviations of measured temperatures, the parameters of the linear trends, etc. Monitoring results from the depth interval 25-38.3 m contain fingerprints of the long-term linear warming trend only.

Figure 118 compares 10 years' long monitoring time series measured in Sporilov hole at the surface and at 38.3 m depth. The regular almost linear warming trend of 0.029 K/year is clearly visible at 38.3 m depth. It is not difficult to identify trend parameters in the time series where the trend is monotonous (consistently increasing or decreasing). If the time series contains significant variations over observational period,

Fig. 117. Results of one-year temperature monitoring at several shallow depths in Sporilov hole (Prague, the Czech Republic). Profiles illustrate the amplitude decrement and phase delay of the temperature change versus depth.

the trend identification is more problematic. Because of the strong and irregular oscillations of the surface temperature, detected at 38.3m depth, warming tendency is practically not visible in the surface data series. Faulty trend estimates can be obtained by simple linear regression procedure. Even the use of more complex techniques (e.g., different kinds of smoothing and data decomposition into significant components, e.g., Grieser et al., 2002) reveals the warming trend with lower reliability. The situation is similar to that described in Section 3.1.2. An inversion of numerous temperature-depth profiles in North America has revealed the presence of unambiguous ground surface warming during the past 100-150 years with the amplitude varying in the order of

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