12 13 14 15 16 17 18 19 Longitude (deg.E)
Fig. 83. Spatial correlation of warming patterns in Figure 82.
pronounced recent warming is observed in more populated and generally industrialized areas, while lower values occur in areas either agricultural or forested. On the other hand, at least a part of the observed warming can be explained simply as natural short-term oscillation of climate conditions. According to both GST reconstructions and meteorological measurements (see Figure 79), the second part of the nineteenth century can be characterized as one of the coldest periods of the entire Holocene. Part of the observed warming may thus bea natural return of climate from the previous colder conditions to the "normal" level.
Finland Examples of the estimates of SAT anomalies for some regions of Europe extended back to approximately 1650 are presented in Figure 8 (Chapter 1). Smoothed values were calculated from the data by Jones and Moberg (2003). Comparison reveals clear differences between illustrated regions. As shown, the range of variations of temperature anomalies is larger and the overall temperature trend is more pronounced in Fennoscandia rather than in Central Europe. On the other hand, the most dominant feature of the SAT at least in Finland is a strong interdecadal variability (Heino, 1994). While the nineteenth century appears as undoubtedly cold, the trends of the next century are less sure. The SAT record does not show any consistent warming or cooling.
All existing long-term paleoclimatic reconstructions in Fennoscandia are almost entirely based on the tree-ring proxies. Specific feature of the tree-ring method is that the low-frequency components may be damped because of the long response time of the tree to the weather variations and by the downward trends in growth associated with increasing tree age (see Section 1.2.3). Averaging the tree-width indices from many individual trees to gain continuous standardized record also effectively removes the long-term trends from the reconstructions (Siren, 1961). Figure 84 presents the longest and most informative tree-ring series for pines at sites distributed over northern Finland for the period 1181-1960 A.D. (Siren, 1961), i.e., from near the present northern forest limit. The record is highly variable even at the large scales of aggregation. The use of longer averaging
periods reveals better long-term trends. The difference of this record from 2000 years' long temperature anomalies time series reconstructed for the Northern Hemisphere (Figure 3, Chapter 1) is obvious. Despite the short 20-year cooling events around 1450 and 1600 A.D., the data do not reveal any pronounced cold period that could be interpreted as the Little Ice Age. Another 1400-year high-quality tree-ring record from Northern Sweden (Briffa et al., 1990) also indicates a relatively short Little Ice Age. Similarly, little evidence was obtained by Briffa et al. (1990) for the existence of the Medieval Warm Period. It is not clear, however, whether these findings can be attributed to the real climatic conditions or to the above-mentioned loss of long-term trends from the tree-ring record.
Considerable progress was achieved in the 1990s in borehole paleoclimatic reconstructions for this area, when the data measured for the geothermal heat flow investigation were applied for climate studies. Three research fields were of special interest: (1) modeling of the permafrost in bedrock in northern Fennoscandia and its dependence on climatic variations (see Section 2.8), (2) evaluation of the long-term GST changes in deep boreholes (Section 3.5), and (3) the GST conditions in the last two millennia or so. Modeling of permafrost variations in northern Fennoscandia suggested rapid variations in permafrost thickness during the Holocene depending on the present ground temperature and past climatic variations (Kukkonen and Safanda, 2001). Obtained results have been described in detail in Section 2.8. Observed vertical variation in the heat flow density in the Fennoscandian Shield and in the neighboring parts of the East European Platform was attributed to the major climate change at the Pleistocene-Holocene boundary, and the GST inversion results suggested an average warming of 8.0 ± 4.5 K from the Last Glacial Maximum time (Kukkonen and Joeleht, 2003, and the references therein). A significant vertical variation in heat flow density measured in a 12 km deep hole in the central part of the Kola peninsula was explained as arising due to the very low surface temperatures during the latest glaciation at times more than 10000 years B.P. (Kukkonen and Clauser, 1994). Attempts to interpret temperature anomalies observed in Finnish deep holes will be described in more detail in Section 3.5.
Climate changes of the last 2000 years were inferred from three borehole temperature logs. Boreholes were situated within the narrow (25-27°E) longitude strip in the northern (67.7°N), central (64.6°N), and southern (60.6°N) parts of the Finnish part of the Baltic shield. They were logged in the years 1988-1994 at least one year after the drilling had ceased. The measurements were performed with frequent readings (measured points were separated by a 2.5 m depth interval), and each record contains from 220 to almost 400 individual reading points. Measured temperature-depth profiles and reduced temperatures are presented in Figure 85. The bulk of the observed temperatures represent a quasi-steady-state geothermal field. As previously, to visualize temperature perturbations in measured profiles that might have been caused by climate change we used reduced temperature representation. As shown in Figure 85, departures from the steady-state conditions are significant only in the upper part of each hole up to the depth of 100-150 m. Below this depth reduced temperatures slightly oscillate around zero line. All reduced temperature profiles are curved and systematically positive above 250 m depth with amplitude of 0.5-1K indicating recent climatic warming.
All three temperature logs were inverted individually by SVD technique and for all of them the climatic episodes over the last 2000 years were identified. The method readily allows incorporation of additional information on both the measured data and the climate change. As was shown in Section 2.4.3 (Chapter 2), this procedure increases the number of parameters (individual intervals of constant temperature) that may be estimated. In addition, the information on decorrelation of the measured data and on the persistence of the climate changes has also been described (for details see Section 2.3.4). For all three investigated boreholes, a short-range dependence between data is characterized by a correlation that decreases exponentially fast. An example of
Fig. 86. Autocorrelation of measured temperatures at borehole DH MHA-4 in the interval of 34-162m.
Fig. 86. Autocorrelation of measured temperatures at borehole DH MHA-4 in the interval of 34-162m.
the exponential decay of the autocorrelation function of the reduced temperatures is shown in Figure 86. The decorrelation distance, D, corresponds to the depth lag at which the autocorrelation decreases to (1/e), and for all Finnish boreholes this was about 50 m. In other words, the individual measurements separated by this distance can be considered as statistically independent. Investigated boreholes represent relatively rare field example of boreholes with fast decorrelation (for comparison see Figure 35, Chapter 2; for Canadian borehole Hearst decorrelation distance equals 174 m; for the above-described Czech borehole HO-1 D = 246 m) and high number of measured points. Very effective technique of data thinning ("scarcing") can be used for inversion of such T-z profiles. In this case the original dataset of measured temperatures can be divided into subsets, and different parameters of the time discretization of the GST history can be estimated from different sections of temperature log. This procedure significantly enhances the number and reliability of estimated parameters but requires that the datasets used be statistically independent (Twomey, 1977).
The reconstructed GST histories (Figure 87) appear to be coherent for DH679 and DH VE-KR-1 boreholes situated at more northern latitudes; however, some incoherence arises in the last 300-year part of the GST history obtained for DH MHA-4 hole, which cannot be regarded simply as an artifact of the solution. The GST history reconstructed for another Finnish borehole, Outokumpu (62.72°N, 29.02°E) done by Kukkonen and Safanda (1996), yielded results coherent with the DH MHA-4 GST history presented in Figure 87. These authors revealed cold episode 1000-1200 A.D.,
then warming culminated near 1750 A.D., subsequent cooling with its minimum at the year 1900, and warming since then. Certain explanation of the incoherence of the GST histories shown in Figure 87 can be looked for in the microclimatic local variations. Processing of further borehole temperature-depth profiles from the area will increase our knowledge on the regional variations of the Finnish climate.
As about two rest boreholes, the early section of the reconstructed GST histories generally covers a cold interval between approximately 400 and 1000 A.D., followed by a long gradual warming up to 1500-1700 A.D. and a cold period around 1800 A.D. While in Europe cold periods before 1000 A.D. and ca. 1800 A.D. are documented by a variety of proxy records, the fifteenth to sixteenth century warming in Finland appears to be different from the general European trend (see, e.g., Section 1.1, Chapter 1). Period from fifteenth to seventeenth centuries in Europe corresponded to the well-known cold conditions of the Little Ice Age. It should be mentioned that tree-ring record in Figure 84 also shows the years 1500-1750 as a generally warm time. Provisional return of cold near 1600 A.D. was probably too short to be resolved in the GST reconstructions that integrated all this period as a single warming event. It is not quite clear whether the prolonged time interval with culminating temperatures around 1500-1700 A.D. in Figure 87 can be simply interpreted as the Medieval Warm Period and the Little Ice Age shifted by 100-200 years. Anyhow, some distinction of the Finnish climate compared with the general climate course in Europe and/or the Northern Hemisphere can be observed not only for the last two centuries, but also probably for the longer times. Since there is no long enough series of the instrumental observations of temperature in Finland and/or the proxy records are also geographically and temporally discontinuous, the GST histories extracted from geothermal data may serve as a useful independent dataset to complete significantly the climate history of the region. The amount of temperature logs measured in this area and their high quality provide the possibility to detect even more remote climate events and also their spatial variations.
Canada Remarkable wide massif of the GST reconstructions for the last millennium exists for the territory of Canada (Beltrami and Mareschal, 1992; Beltrami et al., 1992; Wang et al., 1992; Huang et al., 2000; Pollack and Huang, 2000; Beltrami, 2001; Majorowicz and Safanda, 2001, 2005; see also the references therein). The Canadian Geothermal Data Base consists of the temperature logs measured in more than 1000 boreholes (Wang et al., 1992). Although caution should be exercised to exclude temperature logs that come from boreholes clearly disturbed by terrain effects, this array of data represents possibly the largest regional temperature dataset.
The earlier GST histories were evaluated from the inversion of the temperature logs measured in a suite of boreholes from Ontario Province of Canada. Only deep boreholes were used for these GST inversions; thus, calculated GST histories were at least 1000 years long. The temperature-depth profile for borehole Hearst shown in Figure 17 (Chapter 1) is typical for this group. The curvature in the temperature profile is stationary in time and exhibits clear "U-shape", the signature of recent warming. The GST history inferred from the temperature log measured at Hearst site was discussed in detail in Section 2.4.3 of the Chapter 2 (see also Figures 34-37). It shows slightly warm conditions between 1200 and 1700 A.D., cold period around 1800, and the rapid warming since then. The estimated rise in the GST from 1880 to 1985 A.D. was about 2K. This trend coincides well with the longest annual mean SAT record in eastern Canada (Parry Sound, northern Ontario). The total SAT increase in this station was about 0.9K for the last 100 years and agrees well with approximately 1K average warming estimated for central and eastern Canada from the borehole temperature-depth profiles. Calculated for this area, GST histories are similar in that they all exhibit the common features of the Medieval Warm Period, the Little Ice Age, and a recent warming trend. However, individual reconstructions may differ significantly in terms of exact timing and amplitude of the temperature change. Revealed spatial and temporal variability of the GST histories in Canada is typical for the paleoclimate reconstructions over extensive territories.
Later on, the temperature-depth profiles measured in central and western Canada have also been drawn into the investigations. Recently three research groups have analyzed more than 100 borehole temperature profiles at 56 sites distributed from Newfoundland to Manitoba. An analysis of Canadian profiles revealed region-dependent ground surface warming of 1-2K. The works by Majorowicz and Safanda (2001, 2005) represent typical example of Canadian GST reconstructions. Fifty-one precise temperature logs were measured in the Western Canadian Plains, east of the Cordillera (Canadian Prairies). The majority of boreholes are from Alberta and Saskatchewan. Temperature logging was performed between 1992 and 1996. Figure 88 shows profiles of the transient component of measured temperature logs (reduced temperature). In their uppermost parts all profiles exhibit clear evidence of an extensive recent warming, while the negative temperatures in the 100-200m depth range signify the earlier cold conditions. Figure 89 shows examples of the GST reconstructions. The scatter of the obtained past climate histories is somewhat lower than that generally appearing in European GST collections. The majority of boreholes include evidence of the cold period between 1850 and 1950 and significant warming since then. To minimize the effects of noise in the individual temperature logs and to obtain a more reliable regional GST model the authors have calculated synthetic transient profiles using boxcar model of the pre-twentieth century climate and the SATs from the nearby weather station as a forcing function and have compared them with the
real transients. The boxcar models of the past temperature preceding recent warming have been considered as a possible first-order past climate approximation explaining well the transient features of the T-z profiles in the northern US plains and grasslands of southern Saskatchewan (Harris and Gosnold, 1999; Majorowicz et al., 1999). Majorowicz and Safanda (2001, 2005) have used the SAT data from Calmar meteorological station (Alberta, Canada; 53.27°N, 113.85°W, 720m a.s.l.) as a forcing function (see Section 2.5, Chapter 2). Temperature record exists there from 1915 (Figure 90). Mean temperature for the 1915-1993 observational period equals 2.2°C. According to general results of the GST reconstruction (Figure 89), the boxcar event was assumed as representing climatic conditions between 1790 and 1910, while pre-observational mean temperature (POM) is the long-term temperature average before 1790. It was also assumed that the difference between POM and boxcar temperatures is equal to 1 K. Thus, the resulting synthetic transients have depended on only one free parameter, namely the long-term average temperature before the year 1790 (POM). Comparison of the synthetic temperature-depth profiles with the real 51 Prairie transients has shown that satisfactory agreement between both data can be achieved within relatively narrow range of possible POM values (1.15-2.15 K), which indicates approximately 0-1K and 1-2 K warming in comparison with the 1915-1993 and 1983-1993 Calmar temperature means, respectively (Figure 90).
A composite ground-temperature history for southern Canada (Figure 91) was averaged from a number of GST reconstructions. This reconstruction exhibits high coherency with the above-described boxcar + POM model by Majorowicz and Safanda (2001, 2005). It shows pronounced cold period centered near 1800-1850 A.D. that may correspond to the Little Ice Age and warmer conditions at least since 1000 A.D. However, the most common feature of this long-term GST trend is the total approximately 2 K
temperature increase from the 1800-year minimum. This rapid warming slowed down and even has turned into —0.25 K cooling in the twentieth century. Even if temperature excursions in the second part of the twentieth century are smaller than the earlier GST changes, they deserve further attention, since such GST course hints that the continuous warming process has been highly colored by temperature fluctuations with frequencies of a decade or decades, and different regions may have experienced quite different conditions. Thus, e.g., the most of warming detected in southern Canada may simply represent recovery from the previous cold conditions. The most recent average GST reconstruction for Canadian territory was calculated by Beltrami (2001); the 112 temperature logs all across the country from Canada's geothermal database were used. All temperature logs were inverted individually. They were then averaged over the whole investigated region. This total GST history for the territory of Canada embracing the period 980-1930 A.D. is shown in Figure 91. As in the case of southern Canada, the GST history for the whole territory of the country exhibits a marked increase of temperature since about 1800. The parallel calculations by Beltrami (2001) detected a similar increase in the energy stored by the ground. However, both pre-1800 periods characteri-stic of the GST reconstruction for southern Canada are lost in the latter diagram. This implies that these climatic episodes were far not common for the whole extensive territory of Canada. Because the regional spatial and temporal variations of the GST in Canada are well documented, the average climatic trend shown in Figure 91 should be considered only as a highly simplified version of climatic history of the separate regions of the country.
Authors have interpreted detected warming in the context of increased levels of greenhouse gases (GHG) since the onset of the industrial revolution. On the other hand, at least part of inferred temperature increase could be simple recovery from the previous cold conditions to the level characteristic for the 1000-1400 A.D. period.
Examples of the GST reconstructions in permafrost environment of Canadian Arctic and the neighboring regions are presented in Section 2.8 (Chapter 2). The geothermal data were obtained from the oil exploration holes distributed all over the Alaskan Coastal Plain and Foothills. Configuration of measured temperature-depth profiles revealed clear curvature toward warmer temperatures (U-shape) in the uppermost 200 m (Figure 71, Chapter 2). An analysis of these T-z profiles has provided the first evidence that Alaskan Arctic has warmed by 2-4 K during the twentieth century prior to the mid-1980s (Lachenbruch and Marshall, 1986; Lachenbruch et al., 1988). Further GST estimates by Lachenbruch (1994) using the ramp/step approach have corroborated early results and have provided a value of 2.7 ± 1.0K for the last-century warming. Although the details of the climate warming that time could not be resolved, a recent warming of the permafrost was surely detected. Probably these results represent the most noticeable evidence of the recent climate warming extracted from boreholes.
The sizable volume of the boreholes was logged to a depth of 200-300 m. This depth interval preserves a robust signal for the surface temperature trends over the last century or so. Most of the nineteenth to twentieth centuries have actually been covered by instrumental observations (meteorological long-term series), and the geothermal reconstructions for the last 100-200 years could be better used as an addition to existing SAT records and/or for calibrating the method itself rather than independent source of information. The GST reconstructions from relatively shallow boreholes may serve as a valuable estimate of the recent climate changes only in the regions that appear as the "white spots" at the climatic map.
Since the beginning of the nineteenth century the global climate has generally been a one-way story, a trend of overall warming following the previous Little Ice Age with probably several shorter, generally not more than decade-long fluctuations of relatively colder and warmer conditions (Figure 4, Chapter 1). Observed global temperature increase during the last 100-150 years has become known as a global warming. Recent variations of the GST changes inferred from borehole T-z profiles have been extensively analyzed over the last 10-15 years in a series of papers. These investigations have shown that the traces of the recent warming (GST history of the past one to two centuries) are common in many borehole temperature records, indicating the temperature rise by 1-2K over the last century. As described above, most boreholes in the Czech Republic yield GST histories exhibiting significant warming starting not later than some decades ago. A certain geographical pattern exists of regions where the warming rate appears particularly intense and others where it is weaker (Figure 82). Their spatial distribution indicates a possible impact of human activities. Thus, the highest rate of 0.04 ± 0.01 K/year was obtained for the industrial regions of Sudety and in Ostrava coal basin; the lowest rates of less than 0.015 K/year correspond to the southwestern and southern slopes of the Bohemian Massif, areas generally forested. In most regions recently the rate of warming has been increasing. In area near Prague the rate of warming for the last 100 years amounted to 0.016 ± 0.006K/year, which doubled (0.033 ± 0.011) in the last 50 years.
The most convincing results showing GST changes for the past five centuries were reported from North America, while European data were more sparse and difficult to interpret. The likely explanation is that the past climate changes were more dramatic in mainly continental conditions of North America, while European climate was strongly affected by the Atlantic weather conditions. There may also be environmental reasons, namely massive total deforestation that occurred in Europe since the Middle Ages1 compared with the later and only partial deforestation in North America. The North American continent represents typical example of the ample twentieth-century warming trend reconstructions by the "geothermal" method, while European data are rather diffused and more difficult to interpret. The last 150-year GST history in Canada involved generally pronounced warming (Figure 91; see also Beltrami and Mareschal, 1991, 1995 and the references therein). The amount of surface temperature increase is 1.5-2°C. Majorowicz and Safanda (1998) and Majorowicz et al. (1999) have analyzed a number of temperature-depth records from 150 to 300 m depth boreholes in western Canada and obtained 300-year long GST histories for the period prior to the instrumental temperature recording. Their GST reconstructions indicates the presence of cold period in the eighteenth to nineteenth centuries with its minimum around 1820 ± 50K and subsequent warming with a magnitude of 1.9-2.5K starting in the mid-nineteenth century till present. Results of three-century long GST history reconstructions from the data of boreholes situated in the grasslands of southern Saskatchewan with prevailing semi-desert conditions showed that almost half of the detected warming occurred prior to 1900, thus before the dramatic buildup of the atmospheric GHG (see discussion in Section 3.4). The highest GST warming rates have been observed in the areas where there have been extensive land surface changes, such as forest clearing or forest fires together with conversion of prairie grassland to farming land that occurred prior to 1900.
Studies of geothermal data from Alaska provide evidence of unprecedented recent warming (Lachenbruch and Marshall, 1986; see Figure 71, Chapter 2). A series of boreholes distributed across 500km of the Alaskan Arctic indicate variable but widespread secular warming of 2-4K at the top of the permafrost near the ground surface. Temperature disturbances in the boreholes extend from the surface to approximately 100m depth that hints the onset of warming in the early part of the twentieth century. The widespread strong warming detected in Alaska is consistent with the simulations of the greenhouse warming by General Circulation Models (GCM) that predict polar regions to be particularly sensitive to the greenhouse effect (www.giss.nasa.gov/research/modeling/gcms.html).
Both regional and site-specific studies of the GST changes for the last 100-200 years exist in the territory of the USA. Investigations in the mid-continent region of North America (Deming and Borel, 1995; Harris and Gosnold, 1999) have shown that the warming characteristic of eastern Canada extends west to the front of the Cordillera. Chisholm and Chapman (1992) and Harris and Chapman (1995) have revealed that the warming in the western United States in and near the Great Basin was only approximately
'Middle Ages represent a period in the history of Western Europe that began in the fourth and fifth centuries after the disintegration of the West Roman Empire and lasted into the fifteenth century to the period of the Renaissance.
half of the value that was detected for eastern and central Canada. This fact was also supported by the GST reconstructions by Wang et al. (1994), who analyzed temperature logs from 85 boreholes in eastern and Cordilleran locations. Results have shown that the Cordillera of western Canada has warmed by only ~0.8K, while the warming in the area of the east Cordillera was by ~1.5K. To compare GST and SAT data and to test observed versus predicted climate change on a continental scale, Gosnold et al. (1997) has focused on a 500km X 1000km wide transect in the mid-continent of North America that extends from the Kansas-Nebraska border into southern Manitoba. The 300-year long GST histories, determined from a set of 29 borehole temperature profiles, showed a century-long warming trend that increases systematically with latitude, from +0.4 at 41.1°N to +2.0 K/100year at 49.6°N. The SAT warming also rises with latitude from +0.5 at 40°N to +1.6 K/100 year at 48.8°N. As described above in the case of the Alaskan Arctic (for details see Section 2.8), these warming trends agree with the regional warming pattern predicted by GCM simulations of global warming. While the GST and the SAT data coincide well in the regions where seasonal ground freezing does not occur, they differ significantly where seasonal ground freezing does occur (see also Section 2.6.2). As explained, the greater GST warming is due to a secular increase in soil moisture that corresponds to an increase in precipitation during the past 50 years.
Among the geothermal data from the territory of the USA, the Utah boreholes have a specific interest because high-quality temperature-depth logs are available in the sites with minimal terrain, hydrological and anthropogenic disturbances and where meteorological stations that have operated for about a century are geographically neighbors to borehole sites. Harris and Chapman (1997, 1998) analyzed a number of borehole temperature logs from SE and W Utah (see examples in Figure 16, Chapter 1), which provided generally consistent results suggesting that temperature in the mid-1800s was on average slightly cooler than in the previous centuries, followed by about 0.6K warming in the twentieth century. Comparison of the calculated transient temperature-depth profiles with the SAT records from nearby meteorological stations indicated that air and ground temperature are well correlated in this area; thus, the POM estimates (see Section 2.5, Chapter 2) yield reliable estimates of the long-term mean temperatures prior to the beginning of SAT records. These baseline temperatures are 0.6 ± 0.2K cooler than the 1951-1970 average SAT and provide further evidence that twentieth century warming represents a real and significant departure from the nineteenth century ground surface conditions.
Not all borehole studies revealed significant GST warming. Chisholm and Chapman (1992) investigated high-resolution T-z profiles in a suite of boreholes in western Utah. The amplitudes of the estimated recent warming ranged between -0.8 and 0.6K. Two of the boreholes exhibited small or no warming; one borehole even yielded a cooling. The temperature logging was repeated for "critical" boreholes in the years 1978 and 1990. Both logging results gave very similar GST histories. For example, inversion of the temperature-depth profile measured in borehole GC-1 in 1978 with ramp method indicated recent cooling of -0.8 ± 0.3 K over 18 ± 7 years preceding 1978, while the repeated temperature log measured in 1990 have shown -0.7 ± 0.1 K cooling over 47 ± 13 years preceding 1990. One of the weather stations in the area in fact exhibits a cooling for the twentieth century. Authors concluded that in the last century average GST has risen only by 0.3 K in western Utah.
In fact, the above-mentioned analyses of more than 100 temperature-depth profiles in North America have proved: (1) the presence of unambiguous ground surface warming during the past 100-150 years, and (2) that the amplitude of this warming varies in the wide range of 0.3-4 K, strongly depending on locality. The fact is that this warming was not derived from SAT records. For example, Karl et al. (1991) after analyzing the meteorological station records for the mid-continent concluded the absence of statistically significant climatic trends.
The first attempt of constructing a regional pattern of the recent warming for North America on the basis of estimated GST histories dates back to Deming (1995). He has summarized results of 10 independent investigations of the recent warming including approximately 370 GST reconstructions and has concluded that the revealed twentieth century temperature increase exhibits latitudinal amplification similar to that predicted by GCM. Table 7 represents an updated version of the estimated GST changes in North America embracing regions from Alaska to Texas. Data clearly demonstrate a significant latitudinal trend of the twentieth century warming early revealed by Deming (1995), when temperature increase at Alaskan Northern Slope amounts to 2-4K and equals to only 0.3 K at Texas. Warming is generally higher in the eastern part of the continent. Results by Deming (1995) (see also the summary in Table 7) show that while an average GST increase in the eastern and in the southeastern part of North America has reached 1-1.5K in the last 100 years, the characteristic warming rate in the western part of the USA is only half of this value (except of the high latitudes in Alaska). However, this latter trend is not such obvious as the latitudinal temperature increase.
Causes of climate change involve any process that is able to alter the balance between energy coming from the Sun and energy leaving the Earth. There are many natural causes of climate change, but recently researchers have become concerned with the anthropogenic influence, especially the effect of atmosphere pollution, on the global climate (for details see Section 1.2, Chapter 1). Thus, the reasons and/or explanations (and all their details) for the twentieth century warming in North America can be looked for in factors related to either climate and its natural variability or population dynamics connected with the transformations of the biosphere by various anthropogenic activities, e.g., deforestations, loss of terrestrial diversity, etc. The most evident influence may be global warming phenomenon. On the other hand, the magnitude of observed warming in North America is still within the range of the estimated natural climate variability for the Holocene. Anyhow, the studies of the borehole temperatures provide a relatively good constraint on the magnitude of warming, as well as the inferences concerning its timing and rate. Obtained evidences can serve as a database for the verification of the theoretical predictions of warming related to the accumulation of GHG in the Earth's atmosphere through anthropogenic activities. The separation of a likely anthropogenic contribution to the present warming due to industrialization and/or urbanization, or an apparent GST warming due to the change in the surface conditions caused by, e.g., extensive deforestation or land cultivation can be performed by the GST and SAT comparison. For example, change in the vegetation cover may seriously affect the reflective nature of surface; i.e., the soil energy budget, influence evaporation, and as a result increase GST, while the SATs are less affected (see also Section 2.6.4).
Data from North America can be completed by the GST reconstructions from Cuba. Very limited meteorological/climatic information exists for this island and surrounding
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