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Fig. 2. Climate of the last 50000 years: Temperature reported for central Greenland (adopted from Alley, 2000). Climate pattern in central Greenland is characterized by a pronounced rapid termination of the ice age about 15000 years ago, followed by an irregular transient cooling period known as the Younger Dryas, and by abrupt return to the warm interglacial conditions (warming of approximately 0.8 K/yr).

www.ncdc.noaa.gov/paleo/data.html). Temperature anomalies varied between +2 and -8°C reflecting the oscillations in global ice volume with a period of about 100000 years, although the time pattern is not perfectly regular. Multiple shorter time excursions were superimposed on the long-term cycles. It is obvious that the ice age conditions were characteristic for the most of the last 420000 years. The short warmer periods (the interglacials) typically continued not longer than few thousands to maximum 15-20 thousand years.

Figure 2 shows the Greenland ice core data for the last 50000 years (Alley, 2000). Temperature interpretation is based on stable isotope analysis and ice accumulation data from the GISP2 ice core (central Greenland). Table 1 summarizes the estimated durations of the main events shown in Figure 2. As seen, the peak of the last glacial period occurred 21000 years ago (the Last Glacial Maximum). That time the continental ice sheet reached to mid-latitudes of Europe and North America (Bradley, 1999; Ruddiman, 2001). This glacial period was somewhat abruptly transformed into the present interglacial not later than 12000-7000 years B.P. Until the 1990s, the general view of climate change was that the Earth's climate system changes gradually in response to the natural as well as human-induced forcing. However, recent evidences gained from various fields of climatology show that climate may change more rapidly, even abruptly. Greenland ice core record provides a clear picture of such abrupt climate change. One of the best-known and well-studied widespread abrupt temperature decreases is the Younger Dryas cold interval,

Table 1. Main paleoclimatic events of the last 125 000 years (guide to terminology)

Event

Last interglacial peak Last glacial maximum Last glacial Younger Dryas Last deglaciation Holocene

"Climatic Optimum" Holocene maximum warming

-124 -25 to 18 -74 to 14 -12.7 to 11.5 -18 to 10 -10 to present -4.5 to 6 (Europe) -10 to 6 (Southern hemisphere)

when the last warming trend was shortly interrupted by a sudden cooling at about 12700 years ago. This cooling event was ended even more suddenly about 11 500 years ago (Figure 2). Climate records proved that much of the Northern hemisphere was affected by extremely cold, dry, windy conditions. This event is important because it demonstrates that rapid temperature drops can still occur even during relatively stable and continuous interglacial conditions. The warming was restored at 11600-10500 B.P., and this most recent glacial retreat is still going on.

The Holocene is the name given to the last approximately 10000 years of the Earth history, the time since the end of the last major glacial epoch. The unusual, "flat" nature of the last 11000-12 000 years of the Greenland record represents striking contrast to the periods of cold that had preceded it. Temperature variations over the Holocene period (0.01 Ma to the present) show significantly smaller range in comparison with the early ice age oscillations. However, even such small variations might have significant impact on human civilizations. The Climatic Optimum was the most noticeable period of the mid Holocene. In Europe its maximum was centered around 6000-4500 years B.P., higher SAT by 1 to 2°C existed in some parts of the Earth (particularly in the extra-tropics of the Northern hemisphere). This period coincides with time when the great ancient civilizations were born and flourished.

Temperature records of the last two millennia for the Northern and Southern hemispheres and on the global scale are presented in Figure 3 (Mann and Jones, 2003). For more information see also the link of Goddard Institute for Space Studies, New York, (www.giss.nasa.gov/research/paleo). As seen, SATs have changed rather differently in the two hemispheres, and a sharp recent temperature increase in the Northern hemisphere does not bear a resemblance to more gradual increase in the Southern hemisphere. From the review of paleoclimatic data covering the last two millennia (late Holocene) Williams and Wigley (1983), Jones and Mann (2004; see also the references therein) have identified three main climatic excursions. As has been recently demonstrated, the timing of these cold and warm excursions of climate varies geographically over the globe (Crowley and Lowery, 2000). The comparison on the global scale has a trouble in doing because the direct evidence for temperature changes in past few centuries for the Southern hemisphere is sparse. Thus, the timing of the main climatic changes is generally tied on the conventionally-defined European region and/or the Northern hemisphere.

0 400 800 1200 1600 2000

Fig. 3. Last two millennia multiproxy temperature reconstructions for the Northern and Southern hemispheres and for the global scale (drawn from Mann and Jones, 2003). Temperature anomalies are based on 1961-1990 instrumental reference period. Smoothed course corresponds to 50-year running mean.

0 400 800 1200 1600 2000

Fig. 3. Last two millennia multiproxy temperature reconstructions for the Northern and Southern hemispheres and for the global scale (drawn from Mann and Jones, 2003). Temperature anomalies are based on 1961-1990 instrumental reference period. Smoothed course corresponds to 50-year running mean.

The first of the mentioned epochs was a cold period around eighth century, which caused, e.g. renewed ice growth in alpine glaciers and 1-2m sea level drop below present-day level. This period later changed back and restored warmer conditions between ninth to thirteenth century, the so-called Little Climatic Optimum or Medieval Warm Period (from eleventh to fourteenth centuries) that represented the warmest climate since the Climatic Optimum that occurred at 6000-5000 years B.P. At the Medieval Warm Period the warming, however, was not as intensive as under the earlier Climatic Optimum. During this period, global average annual temperature was approximately 1K (or less) warmer than in 1900. That time, for example, the Vikings established a colony in Greenland and the wheat was grown in Norway (64°N latitude). However, the regional evidence of this period is variable, sometimes even unclear. For example, Crowley and Lowery (2000) did not find evidence for warmth in the tropics. The twelfth and fourteenth centuries appeared mainly cold in China (Wang and Gong, 2000). The restricted reconstructions from the Southern hemisphere, such as tree-ring record from Tasmania (Cook et al., 2000), did not confirm any distinct warmer time during the Medieval Warm Period. Generally, this period appeared more evident in areas near and around the North Atlantic. Keigwin and Pickart (1999) hypothesized that the corresponding temperature changes were associated with changes in ocean currents in the North Atlantic, the fact that maintains the role of ocean circulation-related climate variability.

On the contrary to the Medieval Warm Period, the position of the Little Ice Age appears to have been much clearer. This time interval represents the greatest glacial advance of the Holocene that have continued from 1300-1450 until 1850-1900 A.D. While the geographic pattern of the Holocene climate fluctuations remains murky, the Little Ice Age and the subsequent warming were really global in their extent. The evidence from mountain glaciers suggests glacier advances in a number of widespread regions, in Europe prior to the twentieth century, as well as in Alaska, New Zealand, and Patagonia (Grove and Switsur, 1994). During the Little Ice Age, average annual air temperatures of the Northern hemisphere were about 1-2K lower than today, and unusually cold and dry winters prevailed in Europe. That time agricultural productivity dropped significantly, even farming became unmanageable in vast regions in northern Europe. The freezing of the canals in Holland for three months straight as recorded by famous Dutch and Flemish painters can be mentioned in this connection. The Little Ice Age cooling did not represent a one-way story and was sometimes interrupted by several provisional returns of warmth. The regional variability of cold conditions played a significant role. While the hemispherical averages of temperatures for the seventeenth century generally reflect the cold conditions in Eurasia, the nineteenth century cold is mainly associated with the cold climate in North America (Mann and Jones, 2003). Even the timing of peak coldness may depend on the particular season.

Since 1850 A.D. the climate is dominated by a clear steady warming trend, which has become known as global warming. Figure 4 shows that the twentieth century SAT has increased by 0.7K, with about half of that increase occurring since 1978. This warming is particularly noteworthy because the rate of temperature increase is enormously high. In addition, the recent 50-100 years have been the time of unprecedented growth of human activities, accompanied by industrialization, massive deforestation, and other human interferences with the nature with a thoughtful (harmful) effect on the environment. The natural agents, exerting their influence upon climate has been thus "recruiting" with a new powerful mean to produce sizeable changes in the climate. One of the essential problems of the present days is to answer the question to what degree the mankind may be responsible for the present-day climate warming. Is the observed global warming just of natural origin, or does it have certain anthropogenic component? Is the fact that the climate is getting warmer the result of human insensitive approach to its habitat? Is this warming to continue in the future and how serious are the potential environmental consequences? If so, the problem of the worldwide increasing air temperature comes to an end as the strictly scientific discipline, but became the uneasy task for everybody on this planet.

1860 1880 1900 1920 1940 1960 1980 2000 TIME, years

Fig. 4. Global warming of the twentieth century documented by the mean SAT anomaly (relative to the base period 1961-1990). Figure adopts data from www.cru.uea.ac.uk.

1860 1880 1900 1920 1940 1960 1980 2000 TIME, years

Fig. 4. Global warming of the twentieth century documented by the mean SAT anomaly (relative to the base period 1961-1990). Figure adopts data from www.cru.uea.ac.uk.

It is true, that some skeptical researchers have debated whether the observed temperature trend is reliable (see Chapter 3) and how the present knowledge is to be extrapolated into future. However, certain evidence of a sizeable warming was reported even after removing data from the urban areas where the "city heat-island effect" could have affected the long-term meteorological temperature data. In most cases, however, the SAT data are consistent with other evidence of warming, e.g. increase of ocean temperatures, shrinking mountain glaciers, decreasing polar ice cover, etc. During this period, the energy reaching the Earth's surface from the Sun had been measured precisely enough to confirm the conclusion that the reported recent warming has not been occurring just due to solar radiation changes. Although the reason for detected warming is not completely understood, the most of the climatologists interpret it as the result of the increasing concentrations of CO2, CH4, and other greenhouse gases into the atmosphere caused by anthropogenic activity. Greenhouse gases have increased significantly since the Industrial Revolution,2 mostly from burning fossil fuels for energy, industrial activities, and also by transportation (Figure 5, see also Figure 99, Chapter 3). Now the greenhouse gases are at their highest concentration levels in the last 400000 years and continue to rise. Even when the global

2Term Industrial Revolution implies a period of rapid industrial growth beginning in the second half of the eighteenth century. It originated in England when the steam engine was invented and later has spread over Europe and the world. It means the beginning of intense use of the fossil fuels with the corresponding emission of carbon dioxide and other numerous anthropogenic influences on the climate system.

http://cdiac.esd.ornl.gov) with the mean annual SAT (top panel)."/>
Fig. 5. The comparison of the increase of the greenhouse gases concentration in the atmosphere (bottom panel: CO2 and CH4 data by the Carbon Dioxide Information Analysis Center, CDIAC; http://cdiac.esd.ornl.gov) with the mean annual SAT (top panel).

warming is expedient in some parts of world bringing, e.g. milder winters and longer growing seasons, it may have fatal consequences in others, and globally the expected losses are to outweigh the potential benefits. The Intergovernmental Panel on Climate Change (IPCC; www.ipcc.ch/index.htm) that involves hundreds of scientists and was established to assess scientific, technical, and socio-economic information relevant for the understanding of climate change, predicted that by year 2100 the average global temperature will rise by 1.4 to 5.8 K above 1990 level. The uncertain broad range of possible temperature increase is due to different assumptions considered in the variety of model simulations. Lower boundary indicates that even low climate sensitivity and low economic growth will lead (if no measures are undertaken) to a mean global warming of above 1K, thus surmounting the warmest phase of the Holocene. The IPCC predicted that combined effects of melting ice and seawater expansion from ocean warming may cause the global average sea level rise of approximately 0.1 to 0.9 m between 1990 and 2100. Such rise may bring devastating consequences to coastal communities who will likely experience the loss of their land, increasing flooding due to sea level rise, and more severe storms and surges. Uncertainties remain only about the exact magnitude, rate, and impact of future changes as well as how climate change will afflict different regions. They are stipulated mainly by the lack of sufficient knowledge of how climate could be affected by so-called climate feedbacks (for details see Section 3.4, Chapter 3) and by the difficulty to predict future actions of the society, particularly in the countries of future economic growth and high-energy demands.

Another important question is how abrupt the future changes will be. Abrupt climate change generally refers to a large shift of climate that takes place so rapidly and unexpectedly (sometimes in the mere span of a decade) that human and/or natural ecosystems have difficulty to adapt. Further definition of "abrupt" or "rapid" climate change is subjective and depends on the long-term temporal pattern of the climate change within which the sudden shift is embedded and/or the sample interval used in a particular study. The shifts from dominantly glacial to interglacial conditions were the most distinct abrupt change over the past half million years. These sudden transitions support the hypothesis that the relatively minor changes in climatic forcing may lead to dramatic response of climate system (e.g. Mikolajewicz et al., 1990). Studying the climate evolution over the last 100000 years the researchers have discovered repeated examples of abrupt changes like, e.g. the Younger Dryas - the fast slide into and jump out of the last ice age. The termination of the Younger Dryas cold event, for example, is manifested in ice core records from Central Greenland as a near doubling of snow accumulation rate and a temperature shift of approximately 10K occurring within a decade (Alley, 2000). One of the more recent abrupt climate changes was the Dust Bowl drought, windblown dust, and agricultural decline of the 1930s that displaced hundreds of thousands of people in the American Great Plains. Numerous sudden changes over widespread areas are preserved in paleoclimatic archives and therefore could happen again in future.

The likely hypothesis to explain abrupt climatic transitions is that the ocean thermoha-line3 circulation switches between different stable modes. Warm climate intervals reflect, e.g. strong deep water formation in the northern North Atlantic and vice versa (Stocker, 2000). It has been suggested that oscillations on such timescale represent an intrinsic feature of the climate system and have persisted throughout the Holocene. If it proves to be the case, any prediction of future climate changes in the North Atlantic region would require accounting for this process. Other forcing can also join in the rapid climate changes. Some short-term, abrupt climate changes, for example, clearly reflect the impact of major volcanic eruptions (Briffa and Osborn, 2002). Growing attention is now to be paid to the possibility of anthropogenic influence on climate that may induce rapid climate changes that are far beyond the range of variability on which the social operating and planning schemes are based. One of the theories, for example, states that the global warming could trigger off the mechanism of abrupt cooling in northern Europe. It has been hypothesized that the melting ice caps will "freshen" the water in the North Atlantic, shutting down the natural ocean circulation that brings warmer Gulf Stream waters to the north. The actual regional drop in temperature may be as high as 6 to 8K. Such change in the ocean circulation could occur over relatively short period, perhaps within 50 to 100 years. As the present scientists do not know enough about exact mechanisms and details of abrupt climate changes to be able to accurately predict them. The larger and faster the climate change may be more difficult will be the human and natural systems adaptation and stronger expected adversity effects can be expected. Thus, more precise descriptions of the processes causing such changes should be developed. This is especially the case in relation to changes in the magnitude and frequency of extreme events (Knox, 2000).

3There are three basic processes that make the ocean water circulate, namely tidal forces, winds stress, and density differences. The latter occur due to the temperature (thermo-) and salinity (-haline) differences, thus, the density driven circulation is called the thermohaline circulation.

Comparing present and past climatic conditions one can conclude that we are indeed fortunate because we are living in one of the warmest and "quiet" periods of the past million years under the milder temperatures that the Earth could provide for human life to flourish. Thus, any significant change of existing climate should be met with an apprehension, and the investigation of past climatic variations holds not only academic interest. This is true especially for the last century when the natural variability of the climate is amplified by the anthropogenic disturbances arising from the drastic transformation of the planetary environment induced by an unimaginable explosion of human activity. Investigations of the past climate may be useful both to understand the present-day climate and its possible future changes, and to test the hypotheses about the causes of the climate change. More climate information from the distant past could be greatly valuable to strengthen our understanding of climate changes and to improve existing models of climate development. In particular, an enhanced effort is needed to expand the geographic coverage, temporal resolution, and variety of the paleoclimatic data.

The borehole climatology represents a useful addition to the available array of existing paleoclimate information. Because of numerous boreholes the method is applicable over most continents including polar ice caps. Subsurface temperature records measured in boreholes may represent a useful tool for the past temperature reconstructions in areas less covered by traditional climatic investigations. Borehole geothermometry could also provide data for other purposes, like the atmosphere and land "couplings".

1.2 Principal Sources of Data on the Earth's Climate System

1.2.1 Background

Climate is variable on all timescales. Its variations represent the complex product of the interaction of Sun and all components of the Earth including atmosphere, oceans, landmasses, snow and ice cover, life, and other structural elements. Geologically, short-term climate changes (<120 000 years) occur because of external forcings as well as due to internal factors, both natural and human-induced changes (www.ace.mmu.ac.uk). External causes of climatic changes include changes in the solar radiation and the Milankovitch cycles. Solar radiation is the radiation emitted by the Sun. Its spectral range is determined by the temperature of the Sun. About half of the radiation falls into visible short-wave part of the spectrum, while the other half is mostly in the near-infrared part with a small part in ultraviolet part. The output of energy from the Sun slightly varies over time, changing the total amount of energy absorbed by the Earth atmosphere and thus affecting the climate. The solar activity is linked to the sunspot cycle that occurs with a 22-year periodicity. Quasi-periodic oscillations of the sunspot number with the period of approximately180 years also appear to exist. Figure 6 illustrates the correlation of the global solar irradiance4 reconstructed by Bard et al. (2000) for the last 1200 years and the global temperature anomalies (Figure 3, bottom). The well-known solar minima are centered about 1900, 1810 (Dalton), and 1690 A.D. (Maunder) and correlate with the corresponding temperature falls.

4Irradiance is the term for the power of electromagnetic radiation that is incident on the surface per unit area. The SI unit for irradiance is W/m2.

Fig. 6. Correlation between global solar irradiance and global temperature anomalies for the last 1200 years (adopted from data by Bard et al., 2000 and Mann and Jones, 2003).

Appeared near 1200 A.D. maximum is characterized by the irradiance that is comparable or even slightly higher than the present-day level. It can be connected with the Medieval Warm Period, while the Little Ice Age can be attributed to a rather long period of low irradiance between 1450 and 1750 A.D. Notwithstanding that solar activity is recognized as undoubted cause of variations in the climate system (Blackford and Chambers, 1995; van Loon and Labitzke, 1998), its exact role in climate variations on the decadal/centennial timescale is a topic of continuing debate. Crowley and Kim (1996) investigated the correlation of several Northern hemisphere temperature proxy records with solar variability indices and concluded that solar forcing may explain as much as 30-55% of the climate variations on these timescales. The hypothesized source for the rest part of the climate change is internal climate dynamics. Recently van der Schrier and Versteegh (2001) applied new technique to separate solar activity and internal climate dynamics. Based on the 250 years long sunspot record and series of summer temperatures, these authors concluded that for low values of sunspot number the internal climate mechanics dominates, while at high sunspot number internal climate dynamics does not seem so important. Details on how sunspots affect the Earth climate and further references can be found on the web site <www.das.uwyo.edu/~geerts/cwx/notes/chap02/sunspots.htm>.

The Milankovitch theory relates climate variations to the changes of the parameters of the Earth orbit around the Sun, namely to the changes in eccentricity (the shape of the Earth's orbit), obliquity (the tilt of the Earth's axis), and in orbital precession (the shifting of the equinoxes). Each variation has its specific time period. For example, the orbit may be more elliptical and/or more circular completing period in about 110000 years, and the mean annual flux varies as a function of actual eccentricity. These three components of the orbital variations affect the total amount of energy received by the Earth, and its seasonal distribution at different latitudes. Fluctuations in solar energy input measured in tens of thousands of years are generally regarded as the cause of major climate fluctuations, and much evidence from paleoclimatic records has been found to support this theory. There is a good correlation between the glacials and periods of low eccentricity. The distribution of the interglacials also shows the evidence of the 41 000 (obliquity) and 21000 (orbital precession) years cycles. Now scientists have recognized, however, that such orbital variations alone are not enough to account for the whole oscillations in the global climate between ice ages and interglacials (Berger and Loutre, 2002). While external variations may indeed act as a pacemaker for glacial-interglacial transitions, additional climate forcing has been invoked to explain the significant changes in global average temperature up to several degrees.

The internal forcing factors include variability of the coupled ocean-atmosphere system, volcanism, producing large eruptions of particulates (dust) and gases into the atmosphere, cryosphere,5 and the land surface. The ocean-atmosphere system represents one of the main constituents of the climate. The atmosphere is involved in practically every physical process of potential importance for climate change. Atmospheric temperature, composition, humidity, cloudiness, and winds determine the global energy fluxes. The atmospheric circulation provides the possibility of rapid propagation of any climate forcing from one part of the Earth to another. The bulk of the energy absorbed by climatic system, much more than absorbed by the atmosphere, is stored at the ocean surface. Because of its huge thermal capacity as well as of its ability to circulate this energy over long timescales, the role of the ocean as the climate forcing factor is extremely important and complex. Warm water moves pole ward whilst cold water returns toward the equator. Energy is also transferred by moisture. The water evaporating from the ocean surface stores heat that is then released when the moisture condenses to clouds and rain. Heat moves also vertically within the oceans. Similarly to the currents in the atmosphere the surface and deep-water currents in the world's oceans are inter-linked forming the global ocean circulation. Changes in ocean circulation and especially the thermohaline circulation in the North Atlantic have been implicated in abrupt climate changes occurred in the past such as the Younger Dryas. Volcanism can increase the Earth's albedo (reflectivity) and induces cooling the climate. The cryosphere is the portion of the globe covered by ice and snow. It greatly affects temperature. The sea ice masses increase the reflective capacity of the Earth surface, thus, enhancing cooling. They also insulate the atmosphere from the relatively warm ocean causing steep decline of the winter air temperatures and reducing the supply of moisture to the atmosphere. The water frozen in the glaciers and snow cover on land can melt during warming events with consequent effects on sea level and atmospheric circulation patterns. Snow-covered lands promote cold conditions because of their high reflectivity and because land surface temperatures cannot rise above freezing until the snow melts. The reflectivity of the land surface strongly depends on its cover. While fresh snow reflects more than 90% of the sunlight, the dense forests similarly absorb more than 90% of striking energy. The land surface coverage can also affect cloud formation, precipitation, and the surface water flow, thus, feeding back on climate.

5Term cryosphere implies the component of the climate system including snow, ice, and permafrost both on and beneath the land and/or ocean surface.

The main anthropogenic causes of climate change include the emissions of greenhouse gases (carbon dioxide (CO2) and methane (CH4) production), changes in land-use, and the depletion of stratospheric ozone. Greenhouse gases such as carbon dioxide are accumulating in the atmosphere resulting in the increase of air and ocean temperatures. Increased concentration of greenhouse gases in the atmosphere is well documented and its climatic consequences are widely reported in climatic modeling literature (McGuffie and Henderson-Sellers, 2005; Figure 5). The anthropogenic land-use changes include re-and deforestation, urbanization, changes in the agricultural practice, desertification, major rivers, and other water masses engineering. The importance of these factors was captured by numerous climate modelers. And finally, the discovery of the Antarctic ozone hole6 in 1985, and more recently, less intensive, but observable ozone depletion over the Arctic (stratospheric ozone represents Earth's natural protection for all life forms, shielding our planet from harmful ultraviolet radiation) has focused the attention on including the twentieth century ozone destruction in the global climate models.

The striking complexity of the temperature records in Figures 1 and 2 probably reflects the complex interactions of all feedback mechanisms.7 Our ability to predict future climate strongly depends on the degree of understanding of the climate system operating mode. Such knowledge can be achieved from the study of the past climate variations and their modeling with appropriate forcing. Comparison of the climate models and simulations of their development with climatic reconstructions can provide constraints on the sensitivity of climate to different forcing (van der Schrier and Versteegh, 2001; Bauer et al., 2003). Complete spatial-temporal pattern of the past climate is thus the clue to successive climate modeling and prediction of possible future climatic changes. It is especially a case in the recent decades when the Earth's temperature has been increased. Is observed warming an ordinary climatic fluctuation or it is stimulated by intensified anthropogenic activity? How extraordinary is this warming relative to the variations occurred in the pre-industrial times? For understanding of the post-industrial impact to the climate and development of effective adaptation/mitigation strategies as precise as possible knowledge of the early climatic fluctuations is indispensable.

Generally, all methods for past climate reconstruction can be classified according to the timescale on which they consider climatic influence:

• long-term (periods of 100000 to 1-10 millions years).

Paleoclimatologists employ a wide variety of methodological approaches to reveal past climate changes. Except the direct measurements of climatic variables, there are three principal techniques to reconstruct past temperature variations, namely proxy methods,

6The Antarctic ozone hole is a part of the Antarctic stratosphere where ozone level has dropped to as low as 33% of their pre-1975 value. The ozone hole occurs during September to early December, when strong westerly winds start to circulate around Antarctica and create somewhat similar to an atmospheric container. Over 50% of the lower stratospheric ozone is destroyed in this container.

7Climate feedback implies such mode of interaction between climate forming processes, when the influence of an initial process triggers changes in other process that return back to the initial process and either intensify (positive feedback) or reduce it (negative feedback).

inversion of the temperature-depth profiles measured in boreholes, and modeling. Most traditional technique is based on the proxy sources that represent the fingerprint of climate changes on surrounding environment. The nature has provided a lot of indirect recording mechanisms. By analyzing records taken from numerous proxy sources, scientists can extend our understanding of climate far beyond of the approximately 200 years long instrumental recording. All methods possess their own timescale and temporal resolution, their summary is presented in Figure 7.

We have a general picture of how climate has changed over the last 150000 years (through the last glacial-interglacial cycle), but only in terms of very large-scale and of low-frequency changes. The knowledge of climatic changes at higher frequencies, say, variations on the decade to century scale, is very poor, while it is this timescale that is the most important to the current environmental concerns. Contemporary climatic variations must be viewed in the context of changes that have occurred before the global scale potential anthropogenic influence on the environment has started. Paleoclimatic reconstruction over the last millennium requires careful retrieval of all available climatic archives. Even when none of the available approaches to climate reconstruction is free from certain uncertainties, confidence of obtained results can only be provided by comparing several independent sources of information and thus support or verify the reconstruction model.

Fig. 7. Sources of paleoclimatic data and their timescales and temporal resolutions.

1.2.2 Short-term climate changes

Short-term temperature changes can be suitably detected on the base of the instrumental measurements and historical documents. Both sources possess high-resolution up to exact moment of occurrence. Instrumental surface temperature data sets are of primary interest for the recent global warming and for the detection of the 100 year long global temperature trends. The reconstruction is based on the compilation of SAT measured at land stations and ship-based marine sea surface temperature (SST) measurements. The overall characteristic of the instrumental sources is that they present vast volume of data rapidly decreasing in amount and geographical coverage when going back in time. Table 2 presents the evolution of the global instrumental observing system in time. It can suitably be divided into five periods. The first covers the time up to around the second half of the eighteenth century. This period, with a very few exceptions, is covered only by proxy data. The longest instrumental SAT series are available for some locations in Europe and North America back to the mid seventeenth century. Methodical thermometer-based records began at approximately 1850. The second period covers the times, say, from 1760 to 1880 and is characterized by a gradual built-up of surface synoptic network covering the inhabited parts of the globe as well as marine observations along the well-traveled routes. The reasonably coherent surface synoptic network was created in the third period from 1880 to the mid of the twentieth century. However, data sources over tropical regions and the oceans were still generally rare. During the fourth period (1946-1979) meteorologists have had a network of radiosonde stations reasonably covering most of the Northern hemisphere. Finally, the fifth period since 1979 is the only period where there has been a global observing network including the full depth of the atmosphere.

Obviously, the instrumental observation window is too short to provide real insight in the longer scale climate variability. Although meteorological records represent the principal and the most reliable data source for the climate change study, they possess numerous shortcomings. Because of the changes in the observing techniques and schedules over the years (e.g. different time of the day for measurements), changes in local exposure due to, e.g. urban development around the site and the re-location of meteorological stations, without overlapping of the record to calibrate new station, the integration of measured quantities into the homogeneous time series is not an easy task even for a single station. Because of significant spatial-temporal variability of the surface temperature, the compiling of the homogeneous records for more or less extensive regions represents a difficult problem and can significantly lower the accuracy of compound SAT series especially when measurements are performed over a century or longer period of time. The compilation of recently homogenized long European SAT series was presented by

Table 2. Development of meteorological observing systems

Time

Characteristic observing system

<1760

Essentially proxy data

Built-up of a surface synoptic system(mainly Europe and North America) Basic surface synoptic system

+Upper air radiosonde network(mainly Northern hemisphere) Comprehensive global observing system

1760-1880 1880-1946 1946-1979 >1979

Camuffo and Jones (2002). This massif is probably the most reliable volume of short-term climatologic data. However, even this careful reconstruction is a subject of some uncertainty in its earlier part. Examples of estimates of SAT anomalies extended back to approximately1650 are presented in Figure 8. Smoothed values were calculated from the data by Jones and Moberg (2003). Comparison shows clear differences between the illustrated regions.

Besides the limitations imposed by temporal inconsistencies in a single weather station record, the space averaging of single site observations over large territories for global and hemispheric analysis may present certain problem. The most straightforward way to obtain average global surface temperature is to calculate the weighted average of thermometer measurements from the weather stations distributed over the Earth. Weighting procedure is indispensable because the stations are not regularly and/or optimally arranged. Restriction of the observation sites to land and island stations, still large land areas without coverage, the varying number of stations and areas of coverage over the observational period, wide ocean spaces without fixed meteorological stations at all times put difficulties in the way of extracting large-area temperature changes from measurements (Jones et al., 2001). Figure 8 (bottom) shows the result of recent hemispheric averaging of combined land and marine temperature anomalies (model HadCRUT2v; data by Hadley Centre of the UK Meteorological Office, www.cru.uea.ac.uk). According this reconstruction the Northern hemispheric means vary by up to 1K for the recent 150 years. Data indicates colder temperatures in comparison with the base period 1961-1990 in the second half of the

Fig. 8. SAT (relative to base period 1961-90) and their 10-year running means for Central England, Central Europe, Fennoscandia (data by Jones and Moberg, 2003), and Northern hemisphere (see text).

nineteenth and in the beginning of twentieth century, warmed between 1910 and 1940. Slight oscillations around zero occurred between the years 1940-1975 and then climate has warmed again through 2000. Limited agreement between the Northern hemisphere and the European temperatures is obvious and proves that averages for such extensive units cannot be inferred from a single region series. Using these data, Jones et al. (2001) concluded that average temperature increased 0.6 ± 0.2K during twentieth century. The uncertainty given for this average reflects the statistical uncertainty in the meteorological station measurements and does not contain such systematic biases, e.g. ocean temperature measurements, urban heat-island effect, etc. The temperature increase is spatially unequal; Arctic regions show the greatest degree of warming, while a little or no warming corresponds to some low latitude areas.

Historical data are an important source of detailed information on the millennium scale, particularly for the period from about 1000 A.D. to the beginning of the era of instrumental meteorology. Of course, they are not equivalent in the reliability to the meteorological instrumental measurements. These sources contain generally written records of environmental indicators of climate (parameteorological phenomena) including myths and legends, annals, chronicles and scientific writings, records of social administration and government, commercial and private estate data (crop yields, harvests, and prices), maritime books, early journalism, private papers (diaries, correspondence), etc. Some pictorial documents also can be used as evidence of past climate, e.g. in the work by Camuffo et al. (2003) who studied the increase in the sea level and in flooding tide frequency at Venice on the base of early photographs and the "photographic" paintings by Canaletto and Bellotto. Sometimes climatic information can be extracted from unusual sources, e.g. the cherry tree blossom dates that were recorded at Kyoto, the old capital of Japan, since 812 A.D. (Lamb, 1977). In some cases documentary sources may also be completed by archeological evidence of climate change. Generally, the information fixed in documents does not represent systematic series nor can be readily expressed in terms of standard meteorological variables. Data vary widely in quality. While, for example, the note from the collection by Réthly (1962) on the winter of 1528/29 - "... Suleiman Turkish Emperor came near to occupy Vienna and only extremely cold winter drove his army away" can probably be accepted without limitations, the description of the Italian winter 1132/33 - "... the Po river was frozen to the bottom in its total length. The wine was benumbed with cold even in the deepest cellars" should be interpreted with caution. Historic information often contains exaggerations like "the coldest winter from the beginning of the mankind". Significant problem represents also that our knowledge of the intellectual and social parameters at which text was written is insufficient. Exact meaning of the words and forms of expression is interpreted in terms of the present scientific knowledge. Initial expressions may be distorted in translated, paraphrased, or summarized sources. Definite shortcoming represents also the fact that significant amounts of historical data can be found only in the regions with well-developed cultural tradition, thus, spatial distribution of this information is generally irregular, when vast areas or even continents can appear as "white spots". Another limitation of the documentary data is that they require independent calibration to the climatic variables and thus are not comparable in their reliability to instrumental meteorological measurements. However properly evaluated, historical data can yield both qualitative and quantitative information about past climate.

A wealth of historical data is available for Europe that represents one of the few regions of the world where it may be possible to reconstruct regional climatic variations for the last millennium in season-by-season scale. Outstanding examples of reasonably accurate climatic series obtained from documentary sources are the collection of climatic notes for Hungary and surrounding territories from second to eighteenth centuries by Rethly (1962, 1970), the reconstruction of temperatures prevailing in England and Wales since 800 A.D. (Lamb, 1977), the work by Le Roy Ladurie and Baulant (1980) and Chuine et al. (2004) based on wine-harvest dates in France from fourteenth to fifteenth centuries to the present. The most recent comprehensive review of the documentary data archive was published by Brazdil et al. (2005). Figure 9 demonstrates how historical grape-harvest dates in Burgundy (France) were used to reconstruct summer (April-August) temperature (Chuine et al., 2004). Results reveal generally warm conditions before up to the 1650s and somewhat colder climate since then. Temperatures as high as those reached in the 1990s have occurred several times during reconstructed period. China is another area rich in the documentary climate evidence. The regional instrumental temperature series in China have been extended back over much of the past millennium using documentary data combined with inferences from ice cores and tree-rings (Wang and Gong, 2000).

Fig. 9. Spring/summer temperature reconstruction based on grape-harvest data in Burgundy (France) from 1370 to 2003 (data by Chuine et al., 2004). Temperatures are given as anomalies with respect to the mean April-August temperature at Dijon for the base period 1960—1989, smoothed course corresponds to the 10-year running average.

Fig. 9. Spring/summer temperature reconstruction based on grape-harvest data in Burgundy (France) from 1370 to 2003 (data by Chuine et al., 2004). Temperatures are given as anomalies with respect to the mean April-August temperature at Dijon for the base period 1960—1989, smoothed course corresponds to the 10-year running average.

1.2.3 Medium- and long-term climate changes

The relatively short length of most instrumental records and historical sources restricts the study of climate variability. These data indicate warming trend occurred during twentieth century, however, cannot answer the question whether this warming was unusual and/or extraordinary in the whole Earth's history. The essential need to prolong directly measured climatic series back into the past inspired the development of various methods for the past climate reconstruction from the traces left by climatic changes in the world. The reconstruction of the past temperature variations on the global/hemispheric scales can be performed by three principal approaches: (1) proxy methods, (2) inversion of borehole temperature logs, and (3) modeling. Different proxy techniques are the oldest and traditional, while the "borehole" method and simulations of the past climate with the state-of-art General Circulation Models (GCM)8 represent recent developments. Three-dimensional climate models produce internally consistent simulations that in many features coincide with observed climate and are realistic enough to constitute a surrogate complex for the testing of different reconstruction methods and their basic assumptions (see Section 2.4.4, Chapter 2). The "Simulating the Planet Earth" represents probably the most well known such project. It is connected with the world's fastest supercomputer developed in Japan. Located at the Earth Simulator Center (ESC) at Yokohama (Japan) it can simulate the complex interactions between the Earth's atmosphere, ocean, and land for deeper understanding of our planet's climate, ocean currents, and earthquakes. All global environmental changes can be presented in a one thousand times more detailed grid pattern than that provided by previous supercomputers. This means more precise weather simulations as well as the ability to predict cyclone and typhoon paths. Current GCM simulations can be seen directly in the website www.nec.com/global/features/ index9/index.html. Measurements of borehole temperature profiles are the only direct measurements of the long-term past temperatures in contrast to the proxy indicators that must be interpreted in terms of climate changes using different transfer procedures. Anyhow, it is the proxy reconstructions that provided the most abundant paleoclimato-logic database.

The essence of the proxy method is the next. Temperature variations cause many changes in the biological and physical environment. Some of these changes are regular enough to be used as quantitative indicators of varying temperature. The "proxy" data is the term used to denote any material that contains indirect signatures of climate. A proxy climate indicator is a local record that is interpreted using physical or biophysical principles to represent some combination of climate-related variations back in time. Paleoclimate proxy indicators have the potential to provide evidence for long-term climatic variations prior to the period of existence of instrumental and documentary records. Generally, proxy methods are classified according the scientific branch that provides the data, e.g. biological, chemical, geological, and/or physical climate-related phenomena. For example, numerous evidence of past climate is interpreted by biological sciences: tree-rings, pollen remains,

8General Circulation is a term that denotes large-scale motions of the atmosphere and the ocean occurring in response to the differential heating of rotating Earth. The GC computer models are based on numerical solution of fundamental equations for the conservation of mass, momentum, and energy. They also consider the physical processes including sources and sinks of these quantities.

insect faunas, marine micro-fauna, etc. The family of the existing proxies is continuously growing. Except of well-known records some of them are still under development. For example, according to Weidman and Jones (1994) isotopes from mollusks can help to reconstruct bottom temperatures on the continental shelves of the North Atlantic. Daux et al. (2005) described the possibility to reveal past seasonal distribution of precipitation via oxygen isotope compositions of phosphate that were measured in human tooth enamel. Techniques for obtaining proxy temperature information from all the sources except of boreholes are described in the book by Bradley (1999; see also www.ncdc.noaa.gov/paleo/ proxies.html) and in the web site of the Johns Hopkins University (www.jhu.edu/~eps/ paleoguide/archive.html). The comprehensive review is presented in the work by Jones and Mann (2004; see also the references therein).

Proxy indicators possess different temporal coverage and resolution. Key aspects of a proxy data source are the minimum sampling interval and date resolution. These factors determine the degree of detail that can be extracted from the record. Some of them keep year-by-year patterns of the past climate, while others because of certain factors, e.g. uncertain radiometric dating, simplified "temporal model" assumptions, etc., cannot provide high-resolution data. Common property of the majority of proxy records is also the diminishing of the resolution into the past. For example, datable stratified systems (tree-rings, varves, ice cores, etc.) can provide time resolution of minimum one year. However, seasonal/annual layers in these records appear clearly within only recent periods and are biased to the past by numerous unrelated to climate factors. The properties of the most commonly used proxy measures are illustrated in Figure 7. High-resolution subgroup includes data resolved on the annual/seasonal or at least on decadal scales (tree-rings, corals, laminated ocean and lake sediment cores, high-resolution ice cores, speleothems,9 etc.).

Example of the high-resolution data is presented in Figure 10 that shows five regional reconstructions of summer half-year (April-September) mean temperature anomalies for western North America for the period 1600-1982 by Briffa et al. (1992). Tree-ring data are available from much of the continental land area, they can be accurately dated to an individual year, thus, represent primarily high-resolution source, and by reasonable cross-dating can provide continuous records of up to several thousand years in duration. The essence of the method is that in trees each year's growth creates a well definite ring. Because tree growth tends to hasten in warm conditions compared to cold weather, the width and density of tree-rings may serve as proxies for average temperature. Tree growth measurements can be made both by taking cores out of living trees and by investigation of cut, dead, and/or fossilized examples. On the other hand, Bradley (1999) has pointed out that the tree growth rarely owes to one climatic variable and generally embraces the full range of such factors as temperature, sunshine, precipitation, humidity, and wind intensity. Temperature and precipitation effects, for example, can be separated only if more than one measure of tree growth is available. Tree-ring characteristics also depend on the climate independent variables including the tree species and age (young trees

9A speleothem is a term denoting various cave deposits that occur as a complex interaction among rocks, water, and air during cave formation. Samples taken from speleothems can be used as a proxy record of past climate changes.

Fig. 10. Tree-ring reconstruction of summer temperatures across western North America for the period 1600-1982 and their 10-year running means (based on data by Briffa et al., 1992, 2001).

grow faster that older ones), nutrients in the soil, CO2 concentration, etc. The calibration of tree-ring measurements against climate variables represents heavy problem, since the biological response to climate forcing may change over time. The typical situation is that the calibration is performed using recent, in many cases less than century long meteorological data, and obtained information is then extrapolated on the remaining remote sections of measured tree-ring variables. Generally, average values from the multiple samples per tree and/or multiple trees in the study are calculated. Both procedures result in the damping or even loss of the long-term variability. As about tree-ring reconstructions presented in Figure 10, Briffa et al. (1992) have demonstrated that time series possess a good significance over the area 35-55°N, but have a large uncertainty north of the latitude 55°N, especially prior to 1750. All data series exhibit high short-term variability. Temperature anomalies averaged on decadal timescale revealed significant inter-decadal changes. However, centennial trends are expressed very weakly. The range of the temperature variations remains approximately the same during all reconstructed period. As was mentioned above, it is a specific feature of the tree-ring proxy. For these reasons, the tree-ring information appears to be more useful when it is supplemented by other types of proxy information in the "multiproxy" estimates of past climate change (see Figure 11).

"Coarser" group of proxies includes past pollen and spore records, earlier tree line position, lake level reconstructions, glacial moraine evidence, most sediment cores, and accumulation ice cores. They are useful to reveal the long-term climate variations on centennial and longer timescales. Typical example of such reconstruction was presented in Section 1.1 (Figure 2). The ice sheets that cover Antarctica, Greenland, the northern archipelagos of Canada and Russia, and the summits of some mountain systems reflect the accumulation of the long year snowfall and can provide several climate-related indicators. In cold dry regions, such as Antarctica and the interior of Greenland, because of

Fig. 11. "Multiproxy" temperature reconstructions for Northern hemisphere by Mann et al. (1998), Crowley and Lowery (2000), and Briffa et al. (2001). All series are anomalies for the 1961-1990 instrumental reference period, and smoothed with a 40-year low-pass filter (www.ncdc.noaa.gov/ paleo/recons.html). Multiproxy reconstruction by Huang (2004) is shown for comparison. As seen, temperature reconstructions based on the borehole data inversion suggest colder conditions in the past.

Fig. 11. "Multiproxy" temperature reconstructions for Northern hemisphere by Mann et al. (1998), Crowley and Lowery (2000), and Briffa et al. (2001). All series are anomalies for the 1961-1990 instrumental reference period, and smoothed with a 40-year low-pass filter (www.ncdc.noaa.gov/ paleo/recons.html). Multiproxy reconstruction by Huang (2004) is shown for comparison. As seen, temperature reconstructions based on the borehole data inversion suggest colder conditions in the past.

insignificant year-by-year evaporation and melt, snow compresses into annual layers of ice. The polar ice caps, for example, have 100000 of these layers or more. Ice core, cylinder of ice drilled out of glaciers, and polar ice sheets can provide several indicators of climate. Recorded there, stable isotopes of oxygen and deuterium are of primary use for the paleoclimatic reconstructions. The isotopic fractionation of oxygen in ice can then provide a proxy for temperature. The physical rationale for such reconstruction is the next. Almost all water is H216O, but two heavier forms, i.e. HDO (D = 2H is deuterium) and H218O present in the quantities sufficient to provide measurable basis for the proxy temperature record. In most respects these water molecules are the same as regular water except that because they are heavier, they do not evaporate as readily and condense a bit more easily than H216O water. Generally, the colder the air when the snow fell the richer the concentration of the 16O in the record. The isotope content in ice is determined primarily by the air temperature during snow storms. Warmer air contains a larger fraction of D and O18. When incorporated into a stratified deposit, the (18O/16O) ratio remains frozen. This ratio can be measured very accurately using a mass spectrometer. Over short timescales the change in temperature from summer to winter produces a clear oscillation in the (18O/16O) ratio. This oscillation is used to determine the age of the core at different depths, simply by counting the oscillations. Over longer time periods, this ratio indicates the average temperature in the investigated region between the evaporation site and the coring site. The ratio of 1H/2H (hydrogen to deuterium) can provide even finer details about source temperature and condensation history.

Generally, ice cores can store climate information over more than 105 years. However, significant shortcoming of this kind of proxies is that: (1) they are not good representation of average annual temperature conditions because snow accumulation is seasonal, (2) a change in storm tracks direction could change the isotope signature without temperature change at the given site, (3) the chronology of ice cores is disturbed in depth by horizontal and vertical sinking of the ice, which thins the deep layers to a small fraction of their original thickness, and also by the summer melting of the ice. The resolving power originated from counting of the annual layers, may appear two-three orders poorer in the lower sections of the ice cores (Johnsen et al., 1992). And (4) they are available only from a very small fraction of the Earth.

Important characteristic of various proxy data is the degree of their sensitivity to abrupt changes in climate. Pollen method, for example, allows to estimate the total amount of plant growth of given year by the pollen count, and thus, provides valuable information about dominant climatic conditions and their variations. However, plants have very long reaction to climatic "jumps", thus they are practically insensitive to abrupt changes. On the contrary, many insect populations are extremely temperature-sensitive.

Discussion of the details of the specific proxy sources is far over the purpose of this section. Independently of the kind of proxy data the common problems in their using are the dating, lag and response time, degree of stationarity in the nature of the proxy's response to the climate, and mainly their climatologic interpretation. Since the proxies are indirect traces of climate and only a part of measured variations can be attributed to the climatic changes, they need thorough calibration and validation against independent quantitative climatic information, e.g. with instrumental measurements in the vicinity of proxy site in the intervals of temporal overlapping. Because any single source of paleo-climate information has its limitations, sometimes it is more effective to reconstruct large-scale (regional, hemispheric) climate patterns applying multivariate statistical approaches to the combined proxy indicator network (Jones et al., 2001; Mann et al., 1998, 1999, 2003). Such approach is motivated by the fact that each proxy has its specific not overlapping strengths and limitations, and in principle they could assist each other. It is the reason for using a "multiproxy" reconstruction that summarizes advantages of different proxy information and reduces shortcomings of existing methods. Typical "two-proxy" example represents the merging of information from ice cores and deep-sea sediments. Deep-sea sediments accumulate very slowly relative to snow on the ice sheet. This results in much longer records from sediment cores, but with significantly reduced ability to resolve short-term changes. While the ice cores provide the annual and/or even seasonal resolution, the intervals of hundreds to thousands of years might be resolved in a sediment core. On the other hand, ice cores can provide only several hundred thousand years records compared with as long as several million years archived in the sediment cores. Because of this differences ice and sediment cores provide complimentary climate information.

In the recent decade, there have been several attempts to combine various types of proxy indicators to create long-scale paleoclimate series. Reliable regional proxy based temperature reconstructions of the past millennium and their comparison with GCM were performed by Crowley and Lowery (2000), Briffa et al. (2001), Mann et al. (1998, 1999), and Mann and Jones (2003), who presented reconstructions of Northern and Southern hemisphere as well as global mean surface temperatures over the past two millennia or so based on high-resolution proxy data, namely historical, tree-rings, ice cores, and sediment records (Figure 3). The latter reconstruction is possibly the most often cited one. It was performed using tree-ring,

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