Ground Surface Temperature History Research Articles

Ground surface temperature history from inversions of underground temperatures — a case study of the Western Canadian Sedimentary Basin

Analysis of 35 selected temperature well logs from the Western Canadian Sedimentary Basin shows evidence of an extensive warming of the recent ground surface temperature (GST) in most cases exceeding surface air temperature warming interpreted from the time series for this century. The functional space inversion (FSI) technique of Shen (Shen and Beck, 1991; Shen et al., 1995b) was applied to obtain GST histories. Due to a lack of rock samples from wells, no reliable thermal conductivity measurements were available. The temperature profiles were therefore subject to a ‘loose’ inversion procedure as described by Shen et al. (1995a). Three versions were considered, one with a-priori conductivity and temperature standard deviations of 0.5 W/mK and 0.05 K, respectively, the other two with 2 W/mK and 0.05 K, and 4 W/mK and 0.1 K. The a-priori conductivity of 1.6 W/mK is close to the average conductivity of the clastic rocks for the Western Canadian Sedimentary Basin (1.6 ± 0.4 W/mK). The other estimates of 1.2 W/mK and 1.4 W/mK for mostly clays and shales have been considered. In the majority of cases warming from the mid to late twentieth century is partially a recovery from a cold period with minimum surface temperatures occurring around the turn of the century. This result is similar to evidence of a cold period with its minimum GST in the 19th century found from wells of Quebec and Ontario (Wang and Lewis, 1992), but the western Canadian warming recovery was later in time than in the east. The difference in the onset time of anthropogenic changes to the land surface between eastern Canada (early 19th century) and western Canada (early 20th century) is interpreted to be partly responsible for the observed difference in recovery from the preceding colder period.

Ground temperature history in Romania inferred from borehole temperature data

Geothermal observations (temperature-depth data and thermal property measurements) from a suite of boreholes in the Romanian Carpathians area are used to infer ground surface temperature (GST) history. Temperature observations in boreholes are combined with meteorological data at nearby weather stations to test that temperature in the earth's subsurface contains a record of recent climate change. The subsurface temperature profiles are consistent with the air temperature records. The GST histories that can be extracted by inversion of perturbed temperature data from boreholes ranging in depth between 190 and 520 m cover the past 200–400 years. Results of inversion indicate for the last 150 years a cooler episode for the inner region of the Carpathian Mts. and a warmer time in the Carpathian foreland. The GST histories for this southeastern part of Europe are slightly different from those obtained in western and central Europe. These differences are consistent with the spatial variability of climatic trends.

Last 250 years climate reconstruction inferred from geothermal measurements in the Czech Republic

The subsurface of the Earth has a certain capability to remember what has happened on its surface tens to hundreds of years ago. Ground surface temperature (GST) history, reflecting past climate conditions, can be evaluated by analysing the excursions left on the present-day temperature-depth distribution measured by precise temperature loggings in the boreholes. Temperature logs from fifteen selected holes were inverted into GST histories to assess the corresponding climatic changes. These temperature records generally represent typical conditions existing on the territory of the Czech Republic. In combination with the GST reconstructions from our previous works, these data were used to assess the regional pattern of the last 250-year climatic change. The GSTs were obtained by applying the generalized least-squares inversion method [Jackson, D.D., 1972. Interpretation of inaccurate, insufficient and inconsistent data. Geophys. J. R. Astron. Soc. 28, 97–109], whereby the additional information on interdependence of both measured data and climate changes can be incorporated into the model in the form of covariance matrices. Three main episodes can be distinguished in the last 250 years: a warm period between 1790 and 1830, a slightly colder period in 1940–1960, and a general warming with the average rates of 0.01–0.03 K/year since then. The geographical distribution of the recent warming rate may indicate an impact of human activities. The verification of the GST assessment was accomplished by the independent past climate reconstructions gained by the use of historical sources and instrumental records.

Ground surface temperature history in central Canada inferred from 10 selected borehole temperature profiles

Among the 57 temperature‐depth profiles recently measured in Manitoba and Saskatchewan (Canada), only 10 are suitable for inferring recent changes in ground surface temperature. Many of the rejected temperature profiles show an apparent climatic signal but are affected by topography, changes in vegetation, or the proximity of lakes. At the northernmost site, near Lynn Lake, Manitoba, shallow horizontal variations in temperature have been identified that are related to intermittent permafrost. Such variations can induce an apparent “climatic” perturbation. In areas where lateral thermal conductivity contrasts have been measured, heat refraction effects can also incorrectly be interpreted as ground surface temperature histories. The analysis suggests that forest fires that occurred at some of the sites have had little influence on the temperature profiles. The 10 selected temperature profiles cover a wide area in northern Manitoba and Saskatchewan. They have been independently and jointly inverted by a singular value decomposition method. The ground surface temperature history shows two main episodes. A cold period, tentatively identified as Little Ice Age, with a minimum around 1820 A.D., was followed by marked warming after 1920. These trends are similar to those recognized in eastern Canada.

Geothermics and climate change: 1. Analysis of borehole temperatures with emphasis on resolving power

Temperature‐depth data from six boreholes in western Utah and nine boreholes in southeastern Utah are reanalyzed for evidence of ground surface temperature (GST) histories. We invert the temperature‐depth data using the functional space inverse algorithm of Shen and Beck [1991, 1992] which we prefer over previous inversions of these data because of its greater sophistication and flexibility in suppressing noise. GST histories for western and southeastern Utah are generally consistent and suggest that temperatures in the mid‐1800s are, on average, cooler than previous centuries, followed by about 0.6°C of warming in this century. Attention is given to the temporal resolution of our GST solutions showing the time‐smearing effects of heat conduction on the solutions. GST solutions represent an average ground temperature over a time window that expands as we look farther into the past. The size of the time window is a function of measurement and geologic noise and limits the ultimate resolution of GST reconstructions.

Climate change record in the Earth—Example of borehole data analysis in the Urals Region, Russia

Primary evidence of global warming, provided by surface air temperature data (SAT), is generally constrained to the last century. However, since the Earth continuously records ground surface temperature (GST) variations, information on earlier climate change is potentially contained in borehole temperatures and can be recovered with the use of appropriate inversion algorithms. Synthetic data were used to evaluate two inversion strategies: loose (robust, less sensitive to noise but less successful in identifying the climate signal) and tight inversion (able to recover the whole climate perturbation but sensitive to noise). The inversion was applied at 31 temperature logs from the Urals region, which forms a N-S oriented band 1000 km long. The regional GST history is characterised by a cold period of 0.5 – 1.0 K below the long term mean (LTM) which culminated in 1700–1750. GST then warmed to about 1 K above LTM by 1980 consistent with warming indicated by 160 year long SAT records.

The thermal regime of the crystalline continental crust: Implications from the KTB

An extensive geothermal research program within the German Continental Deep Drilling Program (KTB) covered an almost complete spectrum of experimental and theoretical aspects. The main results and conclusions are as follows: (1) Equilibrium temperature is 118.6°C at 4000 m in the KTB pilot borehole (KTB‐VB) and will be around 260°C at 9100 m in the KTB main borehole (KTB‐HB). Time required for thermal equilibration of the KTB‐HB to within 1% of the initial perturbation will be about 13–16 years. (2) The failure to predict temperature correctly for the KTB was mainly due to an unaccounted perturbation by a transient ground surface temperature history. (3) Pleistocene surface temperature variations affect the present‐day crustal temperature between 1.3 to 2.7 K up to a depth of 4000 m. Accordingly, present‐day heat flow density is systematically too low down to approximately 1500 m. Model simulations indicate that groundwater flow does not eliminate paleoclimatic signals, even though it may translate them to a depth incompatible with both their diffusive age and their amplitude. These results emphasize the importance of an adequate consideration of paleoclimatic effects for the interpretation of thermal data. (4) Lateral heat transport is significant when steep inclination and folding of the rock formations coincide with contrasts in thermal conductivity. This is indicated by typical variations in the vertical components of temperature gradient and heat flow density, such as in the KTB‐HB. In contrast, thermally relevant advection of heat is confined to the top 500–1000 m. In the vincinity of the KTB, this is about twice the maximum difference in elevation. (5) In the deeper crust, free convection systems require permeabilities greater than 10−17 m2 for large rock volumes, but simple numerical models indicate that the associated temperature regimes are imcompatible with KTB borehole data. (6) Heat production rate shows no systematic variation with depth and is related to lithology at the KTB as in other deep boreholes in crystalline rock. Numerical models, using heat production rate derived from seismic velocities, yield temperatures compatible with KTB borehole data.

Late Quaternary temperature changes seen in world‐wide continental heat flow measurements

Analysis of more than six thousand continental heat flow measurements as a function of depth has yielded a reconstruction of a global average ground surface temperature history over the last 20,000 years. The early to mid‐Holocene appears as a relatively long warm interval some 0.2–0.6 K above present‐day temperatures, the culmination of the warming that followed the end of the last glaciation. Temperatures were also warmer than present 500–1,000 years ago, but then cooled to a minimum some 0.2–0.7 K below present about 200 years ago. Although temperature variations in this type of reconstruction are highly smoothed, the results clearly resemble the broad outlines of late Quaternary climate changes suggested by proxies.

The borehole temperature record of climate warming in the mid-continent of North America

Ground-surface temperature (GST) histories, determined from a carefully selected set of twenty-nine borehole temperature profiles, show a warming trend over the last century that increases systematically with latitude in the mid-continent of North America. Except one site in north Texas, the borehole locations lie within a 500 × 1000 km transect that extends from the Kansas-Nebraska border into southern Manitoba. Ground-surface warming during the last century increases from +0.4°C at 41.1°N to + 2.0°C at 49.6°N. Surface air temperature (SAT) warming in the transect, determined from Historical Climatology Network stations, increases from + 0.5°C per century at 40°C per century at 48.8°N. These warming trends agree with the regional warming pattern predicted by GCM simulations of global warming. However, the magnitudes of warming determined from the GST and the SAT data agree in regions where seasonal ground freezing does not occur but differ significantly where seasonal ground freezing does occur. Analysis of ground and air temperature coupling suggests that the greater warming observed in the GST histories in seasonally frozen ground is due to a secular increase in soil moisture that corresponds with increased precipitation during the past 50 years.

Reconstruction of remote climate changes from borehole temperatures

Data from temperature measurements in drillholes are indicators of the temperature variations associated with past climate change. The aim of this paper is to investigate the maximum extent of ground temperature history which can be extracted from geothermal data. We discuss ground surface temperature histories reconstructed from borehole temperature measurements in twelve deep holes (∼ 700 to 6000 m) in the Czech Republic. The study has identified climatic episodes back to the last glaciation period. The reconstructed climatic histories exhibit features similar to those extracted from borehole temperature logs by other inversion techniques and also found by independent climate reconstruction methods. Results confirm the possibility to identify long-term climatic trends from the borehole temperature-depth profiles and to integrate it into the assemblage of traditional paleoclimatological records.

Simultaneous inversion of borehole temperature data for determination of ground surface temperature history

Recent variations of the ground surface temperature, recorded in the Earth's subsurface, can be inverted from borehole temperature data. The resolution of the inversion of borehole temperature logs is poor, even when the noise level is low. This report concerns the potential improvement in the resolution of ground surface temperature history when temperature logs from several boreholes are inverted simultaneously. For an inversion algorithm based on singular value decomposition, the singular values obtained for simultaneous inversion of several boreholes do not decrease as rapidly as the singular values for a single borehole. With the same value of the cut-off or of the damping parameter, more eigenvectors remain in the solution and give higher resolution for several boreholes than for one. Tests conducted with synthetic data for 15 boreholes show that the improvement in resolution is real but not spectacular. These tests indicate that borehole temperature logs should have approximately the same sampling interval. Temperature profiles over different depth ranges can be inverted if the reference heat flows and ground temperatures are included in the parameters determined by the inversion. Tests also show that no consistent ground-temperature history (GTH) is obtained when inverting data from boreholes that have experienced different surface-temperature variations. Simultaneous inversion can be applied to: (1) obtain a local GTH for a single site with several boreholes having identical surface conditions, or (2) obtain regional averages with data from different sites that have experienced the same variations in ground temperature. Temperature profiles measured in four boreholes near Belleterre, in eastern Canada, appear to have recorded the same perturbation. Inversion of the temperature data from individual boreholes yields similar, but not identical, GTHs. The GTH obtained by simultaneous inversion of these four boreholes indicates a cool period, with minimum temperatures at ca. 1800 AD, followed by warming above the reference level. It is consistent with other analyses indicating recent warming in eastern Canada.

Climate changes of the last two millennia inferred from borehole temperatures: results from the Czech Republic — Part II

Borehole temperature logs are indicators of the temperature variations associated with the past climate change. Temperature-depth records from 98 holes drilled on the territory of the Czech Republic were inverted to assess the ground surface temperature history. For most boreholes the climatic episodes over the two past millennia were identified, including warmer period around 400 A.D., followed by colder times between 700–1000 A.D., the Little Climatic Optimum with its culmination around 1250 ± 50 A.D., and the Little Ice Age with a temperature minimum at 1650 ± 30 A.D. Since the beginning of the 19th century a general warming has dominated the climate pattem interrupted by several short-term oscillations. The characteristic times of the minimum (min) and maximum (max) alternating extremes are: max 1730 ± 20, min 1780 ± 10, max 1820 ± 10, min 1880 ± 10, max 1935 ± 7, min 1943 ± 5, max 1976 ± 3 A.D. The results are in good agreement with the long-term meteorological observations and proxy climatic records. As to the recent general warming trend (last about 30 yr), there exists a certain geographical pattern of regions where the rate of warming was particularly intense or where it was relatively weaker possibly indicating a certain impact of human activities. The highest amount of warming of 2.6–2.8 K were obtained for the industrial regions of the Praha, northern Bohemia and Ostrava coal basins, while the lowest warming of 0.07–0.6 K corresponds to the southwestern and southern slopes of the Bohemian Massif, areas are generally forested.

Climate Change in the Urals, Russia, Inferred from Borehole Temperature Data

Borehole temperatures in the central and south Urals were analysed for the past ground surface temperature (GST) signal. 31 highquality temperature logs were selected for this purpose and inverted with algorithms based on the generalised least squares theory. The signal to noise ratio was improved by averaging the results of individual borehole inversions. No distinct regional trends were found in the studied region except for some indications of more pronounced warming in the south. The mean GST history (GSTH) was characterised by cooling down to −0.6 °C in the 18th century and subsequent warming to 0.5 °C above the longterm mean at the beginning of this century, and to 1 – 1.5 °C by 1980. The stability of the mean GSTH was tested in dependence on the number of holes used for the averaging. It showed that any subset of 15 holes yielded a GSTH similar to that obtained from the whole set. A surface air temperature (SAT) time series comprising the period 1832 – 1989 was combined from 17 meteorological records. Its least squares warming rate of 1.1 °C per 100 years is somewhat higher than that of the GST (0.7 – 0.8°C/100 years) in the same period.

Climate History Inferred from Borehole Temperatures, Data from the Czech Republic

The knowledge of the present-day underground temperatures may be important in the assessments of the past climate change. The method of inversion of the temperature-depth records into the ground surface temperature history is briefly introduced by showing an example of synthetic data and illustrated by a review of existing results obtained from the inversion of temperature logs measured in holes in the Czech Republic. Underground temperatures observed in holes of the depth of at least 1000–1500 m seem to confirm the preinstrumental climate pattern of the past several thousand years. Most of shallower temperature records (500–800 m) revealed general warming of climate followed the Little Ice Age of the 17–18th centuries and a pronounced increase of the soil temperatures by at least 1 K since the beginning of this century.

Inference of ground surface temperature history from subsurface temperature data: Interpreting ensembles of borehole logs

Ground surface temperature histories (GSTHs) inferred from borehole temperaturedepth (T-z) data are often degraded, to a various extent, by random or systematic noise in theT-z data and in the measurements of thermophysical properties of the earth. To minimize the effects of noise, and hence improve the fidelity of the inferred GSTH, a plausible approach is to perform a simultaneous inversion, of theT-z logs in a region, or alternatively, to invert the individualT-z logs and then average the resulting GSTHs. Averaging and simultaneous inversion are conceptually different: whereas an averaging can always be peformed, a simultaneous inversion is predicated on the assumption of a common transient component of the GSTH in all theT-z logs. In this work we examine and compare the two approaches, using a time domain inverse formulation based on the method of least squares. We consider a set of scenarios: (a) multipleT-z logs from a single borehole, (b) multiple boreholes from a single site, (c) multiple boreholes in similar climatological settings, and (d) multiple boreholes in different climatological settings. We show that for (a), (b) and (c), averaging and simultaneous inversion yield nearly identical results. For boreholes in different settings, the assumption of a common transient GSTH may be invalid and averaging and simultaneous inversion give divergent results.

Inference of ground surface temperature history from borehole temperature data: a comparison of two inverse methods

The problem of inferring ground surface temperature history (GSTH) from borehole temperature-depth data, like virtually every other geophysical inverse problem, is characterized by nonuniqueness because of an inadequate amount of data, and instability due to presence of noise. As such, the inverse solution for a given data set depends on model representation (parameterization), optimization method, and the subjective view of the researcher. In this work we attempt to identify the cause of differences that arise from the application of two inverse methods that are currently widely used in inferring a GSTH: a generalized non-linear Bayesian formulation based on the method of least squares called Functional Space Inversion (FSI), and a linear formulation solved by the method of singular value decomposition (SVD). We use the same forward solver and spatial and temporal discretizations in the two methods in order to eliminate possible differences arising from these sources. Experiments with synthetic noise-free data show that when noise is not a factor both methods are essentially equal in their ability to identify and represent both the pre-existing reference state and the transient excursion therefrom. However, in the presence of noise, differences arise between the different model representations, particularly when the borehole site has experienced a net warming or cooling in the recent millennium. For such a site, the two methods can give significantly different estimates for the prior baseline or reference surface temperature, and for the subsequent changes in surface temperature. SVD is less effective at recovering the steady-state reference temperature when the mean of the transient deviates significantly from the prior steady-state temperature. In FSI, the use of an a priori null hypothesis for the deviation from the steady-state, along with the imposition of smoothing constraints, yields a robust estimate of the prior steady-state, but a conservative estimate of the transient.

Climate changes of the last millennium inferred from borehole temperatures: Results from the Czech Republic—Part I

Ground surface temperature histories were reconstructed from the temperature logs of eleven boreholes drilled in the Czech Republic. The study has identified several climate episodes during the past millennium, when the ground surface temperature oscillated with an amplitude of at least 2–3 K. Pronounced warming by up to 2 K characterized the beginning of the 20th century. The results seem to correlate well with climate reconstructions based on proxy data, and also with the long-term meteorological records. The paper represents the first part of a more extensive study aimed to propose a regional pattern of the past climate change derived from geothermal data in Central Europe.

Records of climatic change in the Canadian Arctic: towards calibrating oxygen isotope data with geothermal data

Borehole temperature-depth data have been combined with oxygen isotope data from an ice core at a nearby location in the Canadian Arctic to calibrate the oxygen isotope data to ground temperature histories. The relationship of oxygen isotope data to ground surface temperature history is δ 18O = 0.42 ΔT g and this reproduces the main features of the observed subsurface temperature profile measured at the borehole site. These results suggests that oxygen isotope data are a good representation of the regional, long term climatic variations at these latitudes. Discrepancies between the observed subsurface profile and the theoretical profile calculated from the calibrated oxygen isotope time series may be explained in terms of the different character of the records, in particular snow cover, and active layer processes.

Ground surface temperature histories inverted from subsurface temperatures of two boreholes located in Panxi, SW China

Variation of the ground surface temperature in the past is recorded in the sursurface temperature field. We reconstruct the ground surface temperature history by analysis of temperature-depth ( T− z) profiles from two deep boreholes some 200 km apart in Panxi, southwest China. Consistent results have been derived using a variety of analysis methods: individual inversion of each T− z log, simultaneous inversion of the derived transient components of the two T− z logs, inversion of the average of the transients, and inversion of the residual T− z data after the steady state has been removed. Results show that a warming of about 1°C from 1600 to 1900 AD is evident in the borehole temperatures. The results also indicate that such a warming is at least in part a recovery of the climate from a preceding cold episode. The reconstructed ground surface temperature histories are in good agreement with the variation of the surface air temperature recorded in the Shanghai meteorological observatory for the period of their overlap.

Effects of subsurface heterogeneity on the inference of climate change from borehole temperature data: Model studies and field examples from Canada

All existing approaches to the analysis and interpretation of borehole temperatures in terms of a ground surface temperature history are based on the one‐dimensional theory of heat conduction. Deviations from this idealization are manifest as noise in the interpretation. We present numerical experiments that explore the effects of three‐dimensional subsurface heterogeneity on ground surface temperature histories inferred from borehole temperature profiles. Inversions of steady state temperature profiles containing such three‐dimensional “noise” reveal that in an inversion formulation incorporating a priori information, spurious temperature histories can emerge when the a priori constraints on subsurface temperatures and thermophysical properties are too tight, i.e., when arbitrarily small variations in subsurface temperatures and thermophysical properties are interpreted to have significance for the derived climate history. Relaxation of the a priori constraints enables an effective muting of the spurious histories. Similar experiments conducted on synthetic transient signals confirm the steady state results but also reveal that relaxation of the a priori constraints will also lead to a loss of signal if the level of relaxation is excessive. Our experiments suggest that relaxation of constraints on thermal conductivity is more efficient than relaxation of borehole temperatures in suppressing artifacts arising from the three‐dimensional effects. We identify a range of constraints in which reasonable noise suppression and signal recovery are both achieved and then reprocess data from 22 Canadian boreholes for which surface temperature histories had earlier been derived by ourselves and others. Our reprocessed results reveal a remarkably simple surface temperature history common to much of eastern Canada. This history comprises a recent warming interval commencing in the nineteenth century, in which the surface temperature increased by some 1–4°C. Approximately 0.5–1.0°C of this increase was a recovery from a preceding cooler interval in which the temperature was below the long‐term mean. The remainder of the increase represents warming that exceeds the long‐term mean.