National Academies Press: OpenBook

Understanding Climatic Change: A Program for Action (1975)

Chapter: PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES

« Previous: PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
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Page 36
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 37
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 38
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 39
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 40
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 41
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 42
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 43
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 44
Suggested Citation:"PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES." National Research Council. 1975. Understanding Climatic Change: A Program for Action. Washington, DC: The National Academies Press. doi: 10.17226/27501.
×
Page 45

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4 PAST CLIMATIC VARIATIONS AND THE PROJECTION OF FUTURE CLIMATES It is universally accepted that global climate has undergone significant variations on a wide variety of time scales, and we have every reason to expect that such variations will continue in the future. The development of an ability to forecast these future variations, even on time scales as short as one or two decades, is an important and challenging task. Study of the instrumental, historical, and paleoclimatic records not only offers a basis for projection into the future but furnishes insight into the regional effects of global climatic changes. This chapter attempts to summarize our knowledge of past climatic variations and to give some indication of the further research that must be carried out on critical aspects of this subject. Further details of the record of past climates are given in Appendix A. IMPORTANCE OF STUDIES OF PAST CLIMATES In order to understand fully the physical basis of climate and climatic variation, we must examine the earth’s atmosphere—ocean-ice system under as wide a range of conditions as possible. Most of our notions of how the climatic system works, and the tuning of our empirical and dynamical models, are based on observations of today’s climate. In order that these ideas and models may be useful in the projection of future climates, it is necessary that they be calibrated under as wide a range of conditions as possible. The only documented evidence we have of climates under boundary conditions significantly different from today’s 35

36 UNDERSTANDING CLIMATIC CHANGE comes from the paleoclimatic record. It is here that paleoclimatology, in conjunction with climatic modeling, can make an especially valuable contribution to the resolution of the problem of climatic variation. Modern instrumental data suggest that the atmosphere, at least, may be capable of assuming quite different circulation patterns even with relatively constant boundary conditions and that the resulting variability of climate is strongly dependent on geographical location. Although the data base is much less complete for the oceans, persistent anomalies of sea-surface temperature appear to be related to atmospheric circulation regimes over time scales of months and seasons, and the oceans may show other longer-period variations of which we are now unaware. In general, the record of past climate indicates that the longer the available record, the more extreme are the apparent climatic variations. An immediate consequence of this “red-noise” characteristic is that the largest climatic changes are not revealed by the relatively short record of instrumental observation but must instead be sought through paleo- climatic studies. The record of past climates also contains important information on the range of climatic variability, the mean recurrence interval of rare climatic events, and the tendency for systematic time- wise behavior or periodicity. Such climatic characteristics are in general shown poorly, if at all, by the available instrumental records. RECORD OF INSTRUMENTALLY OBSERVED CLIMATIC CHANGES Our knowledge of instrumentally recorded climatic variations is largely confined to the record of the past two centuries or so, and it is only in the last 100 years that synoptic coverage has permitted the analysis of the geographical patterns of climatic change over large portions of the globe. It is only during the past 25 years or so that systematic observa- tions of the free atmosphere (mainly in the northern hemisphere) have been made and that regular measurements of the ocean surface waters have been available in even limited regions. Enough data have been gathered, however, to permit the following summary. A striking feature of the instrumental record is the behavior of tem- perature worldwide. As shown by Mitchell (1970), the average surface air temperature in the northern hemisphere increased from the 1880's until about 1940 and has been decreasing thereafter (see Figure A.6, Appendix A). Starr and Oort (1973) have reported that, during the period 1958-1963, the hemisphere’s (mass-weighted) mean tempera- ture decreased by about 0.6°C. In that period the polar and subtropical arid regions experienced the greatest cooling. The cause of this variation is not known, although clearly this trend cannot continue indefinitely.

PAST CLIMATIC VARIATIONS—PROJECTION OF FUTURE CLIMATES 37 It may represent a portion of a longer-period climatic oscillation, al- though statistical analysis of available records has failed to establish any significant periodic variation between the quasi-biennial cycle and periods of the order of 100 years. The corresponding patterns of pre- cipitation, cloudiness, and snow cover have not been adequately deter- mined, and it would be of great interest to examine the simultaneous variations of oceanic heat storage and imbalances of the planetary radia- tion budget, once the necessary satellite observations become available. For the earlier instrumental period, there are scattered records of temperature, rainfall, and ice extent, which clearly show individual years and decades of anomalous character. The only apparent trend is a gradual warming in the European area since the so-called Little Ice Age of the sixteenth to nineteenth centuries. HISTORICAL AND PALEOCLIMATIC RECORD Two sources of data are available to extend the record of climate into the pre-instrumental era: historical sources, such as written records, and qualitative observations, which give rise to what may be called “historical” climatic data; and various natural paleoclimatic recorders, which give rise to what may be called “proxy” climatic data. Nature of the Evidence The historical record contains much information relating to climate and climatic variation over the past several hundred to several thousand years, and this information should be located, cataloged, and evaluated. Historical data on crop yields, droughts, and winter severity from manuscripts, explorations, and other sources sometimes provide the only available information on the general character of the climate of the historical past. Such information is especially useful in conjunction with selected tree-ring, ice core, and lake sediment data in diagnostic studies of the higher-frequency climatic variability on the time scales of years, decades, and centuries. For earlier periods, the paleoclimatic record becomes increasingly fragmentary and ultimately nil for the oldest geological periods. But for the past million years, and especially for the past 100,000 years, the paleoclimatic record is relatively continuous and can be made to yield quantitative estimates of the values of a number of significant climatic parameters. Each record, however, must first be calibrated or processed to provide an estimate of the climate. The elevation of an ancient coral reef, for example, is a record of a previous sea level, but before it can

38 UNDERSTANDING CLIMATIC CHANGE be used for paleoclimatic purposes the effect of local crustal movements must be removed. The taxonomic composition of fossil assemblages in marine sediments and the width of tree rings, for example, are known to reflect the joint influence of several ecological factors; here multi- variate statistical techniques can be used to obtain estimates of selected paleoclimatic parameters such as temperature and precipitation. In order to be useful, a proxy data source must also have a strati- graphic character; that is, the ambient values of a climatically sensitive parameter must be preserved within the layers of a slowly accumulating natural deposit or material. Such sources include the sediments left by melting glaciers on land; sediments in peat bogs, lakes, and on the ocean bottom; the layers in soil and polar ice caps; and the annual layers of wood formed in growing trees. Since no proxy source yields as long and continuous a record as would be desired, and the quality of data varies considerably from site to site, a coherent picture of past climate requires the assembly of data from different periods and with different sampling intervals. Such characteristics of the principal proxy data sources are summarized in Table A.1 of Appendix A. After proxy data have been processed and stratigraphically screened, an absolute chronology must be established in order to date specific features in the climatic record. The most accurate dating technique is that used in tree-ring analysis, where dates accurate to within a single year may be determined over the past several thousand years under favorable conditions. Annually layered lake sediments and the younger ice cores also have a potential dating accuracy to within several years over the past few millenia. For suitable materials, '‘*C-dating methods extend the absolute time scale to about 40,000 years, with an accuracy of about 5 percent of the material’s true age. Beyond the range of 7*C dating, the analysis of the daughter products of uranium decay make possible the reconstruction of the climatic chronology of the past million years. For even older records, our chronology is based primarily on potassium—argon radiometric dating as applied to terrestrial lava and ash beds. Stratigraphic levels dated by this method are then correlated with undated sedimentary sequences by the use of paleomagnetic reversals and characteristic floral and faunal boundaries. Summary of Paleoclimatic History From the overview of the geological time scale, we live in an unusual epoch: today the polar regions have large ice caps, whereas during most of the earth’s history the poles have been ice-free. As shown in Figure A.15 of Appendix A, only two other epochs of extensive continental

PAST CLIMATIC VARIATIONS—PROJECTION OF FUTURE CLIMATES 39 glaciation have been recorded, one during late Precambrian time (ap- proximately 600 million years ago) and one during Permo-Carbonif- erous time (approximately 300 million years ago). During the era that followed the Permo-Carboniferous ice age, the earth’s climates returned to a generally warmer, nonglacial regime. Before the end of the Mesozoic era (approximately 65 million years ago) climates were substantially warmer than today. At that time, the configuration of the continents and shallow ocean ridges served to block a circumpolar ocean current in the southern hemisphere. This barrier was formed by South America and Antarctica, which lay in approximately their present latitudinal positions, and by Australia, then a northeastward extension of Antarctica. About 50 million years ago, the Antarctic—Australian passage opened as Australia moved northeastward and as the Indian Ocean widened and deepened. By about 30 million years ago, the Antarctic circumpolar current system was established, an event that may have decisively influenced the sub- sequent climatic history of the earth. About 55 million years ago global climate began a long cooling trend known as the Cenozoic climate decline (see Figure A.15). Approxi- mately 35 million years ago there is evidence from the marine record that the waters around the Antarctic continent underwent substantial cooling, and there is further evidence that about 25 million years ago glacial ice occurred along the edge of the Antarctic continent in some locations. During early Miocene time (approximately 20 million years ago) there is evidence that the low and middle latitudes were somewhat warmer. There is widespread evidence of further cooling about 10 million years ago, including the growth of mountain glaciers in the northern hemisphere and substantial growth of the Antarctic ice sheet; this time may be taken as the beginning of the present glacial age. Evi- dence from marine sediments and from continental glacial features indicates that about 5 million years ago the already substantial ice sheets on Antarctica underwent rapid growth and even temporarily exceeded their present volume. Three million years ago continental ice sheets appeared for the first time in the northern hemisphere, occupying lands adjacent to the North Atlantic Ocean, and during at least the last 1 mil- lion years the ice cover on the Arctic Ocean was never significantly less than it is today. Once the polar ice caps formed, they began a long and complex series of fluctuations in size. Although the earlier record is still not clear, the last million years has witnessed fluctuations in the northern hemisphere ice sheets with a dominant period on the order of 100,000 years (see Figure A.2). These fluctuations may have occurred in parallel with

40 UNDERSTANDING CLIMATIC CHANGE substantial changes in the volume of the West Antarctic ice sheet. By comparison, however, changes in the volume of the ice sheet in East Antarctica were quite small and were probably not synchronous with glaciations in the northern hemisphere. The major climatic events during the past 150,000 years were the occurrence of two glacial maxima of roughly equal intensity, one about 135,000 years ago and the other between 14,000 and 22,000 years ago. Both were characterized by widespread glaciation and generally colder climates and were abruptly terminated by warm interglacial intervals that lasted on the order of 10,000 years. The penultimate interglacial reached its peak about 124,000 years ago, while the pres- ent interglacial (known as the Holocene) evidently had its thermal maximum about 6000 years ago. Between 22,000 and 14,000 years ago the northern hemisphere ice sheets attained their maximum extent (see Figure A.24). The eastern part of the Laurentide ice sheet (which covered portions of eastern North America) and the Scandinavian ice sheet (which covered parts of northern Europe) both attained their maxima between 22,000 and 18,000 years ago, several thousand years before the maximum of the Cordilleran ice sheet. About 14,000 years ago deglaciation began rather abruptly, and the Cordilleran sheet melted rapidly and was gone by 10,000 years ago. The interval of deglaciation (14,000 to 7000 years ago) was marked in many places by significant secondary fluctuations about every 2000 to 3000 years. In general, the period about 7000 to 5000 years ago was warmer than today, although the records of mountain glaciers, tree lines, and tree rings reveal that the past 7000 years was punctuated in many parts of the world by colder intervals about every 2500 years, with the most recent occurring about 300 years ago. For the last 1000 years, the proxy records generally confirm the scattered observations in historical records. The cold period identified above is seen to have consisted of two periods of maximum cold, one in the fifteenth century and another in the late seventeenth century. The entire interval, from about 1430 to 1850, has long been referred to as the Little Ice Age and was characterized in Europe and North America by markedly colder climates than today’s. INFERENCE OF FUTURE CLIMATES FROM PAST BEHAVIOR Notwithstanding the limitations of our present insight into the physical basis of climate, we are not altogether powerless to make certain in-

PAST CLIMATIC VARIATIONS—PROJECTION OF FUTURE CLIMATES 41 ferences about future climate. Beginning with the most conservative approach, we may use the climatic “normal” as a reference for future planning. In this approach, it is tacitly assumed that the future climate will mirror the recently observed past climate in terms of its statistical properties. Depending on the sensitivity of the climate-related applica- tion (and on the degree to which the climate is subject to change over a period of years following that for which the “normal” is defined), this kind of inference can be anything from highly useful to downright misleading. Of the various other approaches to the inference of future climate in which the attempt is made to capture more predictive information than is embodied in the “normal,” the most popular have been those based on the supposition that climate varies in cycles. Since the develop- ment of modern techniques of time-series analysis, in particular those involving the determination of the variance (or power) spectrum, it has become clear that almost all alleged climatic cycles are either (1) artifacts of statistical sampling, (2) associated with such small fractions of the total variance that they are virtually useless for prediction pur- poses, or (3) a combination of both. Other approaches, developed to a high degree of sophistication in recent years, include several kinds of nonlinear regression analysis (in which no assumption need be made about the periodic behavior of the climatic time series), which appro- priately degenerate to a prediction of the “normal” in cases where the series possess no systematic temporal behavior. The full potential of such approaches is not yet clear but appears promising, at least in certain situations. Natural Climatic Variations Regardless of the approach taken to infer future climates, the view that climatic variation is a strictly random process in time can no longer be supported. It has been well established, for example, that many atmo- spheric variables are serially correlated on time scales of weeks, months, and even years. For the most part such correlations derive from “per- sistence” and resemble the behavior of a low-order Markov process. Unfortunately, nonrandomness of this kind does not lend itself to long- range statistical prediction. In addition to persistence, long-term trends have a tendency to show up in great number and variety in climato- logical time series (see Appendix A). Many such trends are now understood to originate from what are called inhomogeneities in the series, as, for example, effects of station relocations, changes in observ-

42 UNDERSTANDING CLIMATIC CHANGE ing procedures, or local microclimatic disturbances irrelevant to large- scale climate. Even after statistical removal of such effects, many “real” trends nonetheless remain and may be recognized as part of a longer- term oscillation of climate. We must, moreover, recognize that the climatic record may also reflect various natural environmental disturb- ances, such as volcanic eruptions and perhaps changes of the sun’s energy output, which are themselves only poorly predictable, if at all. Clearly, a climatic prediction based on the linear extrapolation into the future of a record containing such effects would be highly unrealistic. The behavior of longer climatic series is seemingly periodic, or quasi- periodic, especially those series that extend into the geological past as reconstructed from various proxy data sources. It is a fundamental problem of paleoclimatology to determine whether this behavior is really what it seems or whether it is an illusion created by the character- istic loss of high-frequency information due to the limited resolving power of most proxy climatic indicators. Illumination of this question would be of great importance to the determination of the basic causes of the glacial-interglacial climatic succession and to the assessment of where the earth stands today in relation to this sequence. Spectrum analyses of the time series of a wide variety of climatic indices have consistently displayed a “red-noise” character (see, for example, Gilman et al., 1963). That is, the spectra show a gradual in- crease of variance per unit frequency as one proceeds from high fre- quencies toward low frequencies. The lack of spectral “gaps” provides empirical confirmation of the lack of any obvious optimal averaging in- terval for defining climatic statistics. Most spectra of climatic indices are also consistent in displaying some form of quasi-biennial oscillation (see, for example, Brier, 1968; Angell et al., 1969; or Wagner, 1971). This fluctuation is most obvious in the wind data of the tropical strato- sphere but also has been shown to be a real if minor feature of the climate at the earth’s surface as well. Time series of some of the longer instrumental records show some suggestion of very-low-frequency fluctuations (periods of about 80 years and longer), but the data sets are not long enough to establish the physical nature and historical continuity of such oscillations. While numerous investigators have reported spectral peaks corresponding to almost all intermediate periods, the lack of consistency between the various studies suggests that no example of quasi-cyclic climatic be- havior with wavelengths between those on the order of 100 years and the quasi-biennial oscillation have been unequivocally demonstrated on a global scale. Further discussion of these questions is given in Appendix A (p. 127 ff).

PAST CLIMATIC VARIATIONS—PROJECTION OF FUTURE CLIMATES 43 Man’s Impact on Climate While the natural variations of climate have been larger than those that may have been induced by human activities during the past century, the rapidity with which human impacts threaten to grow in the future, and increasingly to disturb the natural course of events, is a matter of concern. These impacts include man’s changes of the atmospheric com- position and his direct interference with factors controlling the all- important heat balance. Carbon Dioxide and Aerosols The relative roles of changing carbon dioxide and particle loading as factors in climatic change have been assessed by Mitchell (1973a, 1973b), who noted that these variable atmospheric constituents are not necessarily external parameters of the climatic system but may also be internal variables; for example, the changing capacity of the surface layers of the oceans to absorb CO., the variable atmospheric loading of wind-blown dust, and the interaction of CO, with the biosphere. The atmospheric CO, concentrations recorded at Mauna Loa, Hawaii (and other locations) show a steady increase in the annual average, amounting to about a 4 percent rise in total CO, between 1958 and 1972 (Keeling et al., 1974). The present-day CO, excess (relative to the year 1850) is estimated at 13 percent. A comparison with estimates of the fossil CO, input to the atmosphere from human activities indicates that between 50 and 75 percent of the latter has stayed in the atmosphere, with the remainder entering the ocean and the biosphere. The CO, excess is conservatively projected to increase to 15 percent by 1980, to 22 percent by 1990, and to 32 percent by 2000 a.p. The corresponding changes of mean atmospheric temperature due to CO, [as calculated by Manabe (1971) on the assumption of constant relative humidity and fixed cloudiness] are about 0.3°C per 10 percent change of CO, and appear capable of accounting for only a fraction of the observed warming of the earth between 1880 and 1940. They could, however, conceivably aggregate to a further warming of about 0.5°C between now and the end of the century. The total global atmospheric loading by small particles (those less than 5 um in diameter) is less well monitored than is CO, content but is estimated to be at present about 4x10" tons, of which perhaps as much as 1 x 10’ tons is derived both directly and indirectly from human activities. If the anthropogenic fraction should grow in the future at the not unrealistic rate of 4 percent per year, the total particulate loading

44 UNDERSTANDING CLIMATIC CHANGE of the atmosphere could increase about 60 percent above its present- day level by the end of this century. The present-day anthropogenic particulate loading is estimated to exceed the average stratospheric loading by volcanic dust during the past 120 years but to equal only perhaps one fifth of the stratospheric loading that followed the 1883 eruption of Krakatoa. The impact of such particle loading on the mean atmospheric tem- perature cannot be reliably determined from present information. Recent studies indicate that the role of atmospheric aerosols in the heat budget depends critically on the aerosols’ absorptivity, as well as on their scattering properties and vertical distribution. The net thermal impact of aerosols on the lower atmosphere (below cloud level) prob- ably depends on the evaporable water content of the surface in addition to the surface albedo. Aerosols may also affect the structure and distribution of clouds and thereby produce effects that are more im- portant than their direct radiative interaction (Hobbs et al., 1974; Mitchell, 1974). Of the two forms of pollution, the carbon dioxide increase is probably the more influential at the present time in changing temperatures near the earth’s surface (Mitchell, 1973a). If both the CO, and particulate inputs to the atmosphere grow at equal rates in the future, the widely differing atmospheric residence times of the two pollutants means that the particulate effect will grow in importance relative to that of COs. Thermal Pollution, Clouds, and Surface Changes There are other possible impacts of human activities that should be considered in projecting future climates. One of these is the thermal pollution resulting from man’s increasing use of energy and the inevitable discharge of waste heat into either the atmosphere or the ocean. Al- though it is not yet significant on the global scale, the projections of Budyko (1969) and others indicate that this heat source may become an appreciable fraction (1 percent or more) of the effective solar radiation absorbed at the earth’s surface by the middle of the next century. And if future energy generation is concentrated into large nuclear power parks, the natural heat balance over considerable areas may be upset long before that time. Recent estimates by Haefele (1974) indicate that by early in the next century, the total energy use over the continents will approach 10 percent of the natural heat density of about 50 W/m? and that in local industrial areas the man-made energy density may become several hundred times larger. There is also the possibility that widespread artificial creation of

PAST CLIMATIC VARIATIONS—PROJECTION OF FUTURE CLIMATES 45 clouds by aircraft exhaust and by other means may induce significant climatic variations, although there is no firm evidence that this has yet occurred. Such effects could serve to increase the already prominent role played by (natural) clouds in the earth’s heat balance (see Figure 3.2). Widespread changes of surface land character resulting from agri- cultural use and urbanization, and the introduction of man-made sources of evaporable water, may also have significant impacts on future climates. When the surface albedo and surface roughness are changed by the removal of vegetation, for example, the regional climatic anomalies introduced may have large-scale effects, depending on the location and scale of the changes. The creation of large lakes and reservoirs by the diversion of natural watercourses may also have widespread climatic consequences. The list of man’s possible future alterations of the earth’s surface can be considerably lengthened by the inclusion of more ambitious schemes, such as the removal of ice cover in the polar regions and the diversion of ocean currents. Again, how- ever, it is only through the use of adequately calibrated numerical models that we can hope to acquire the information necessary for a quantitative assessment of the climatic impacts.

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he increasing realization that man’s activities may be changing the climate, and mounting evidence that the earth’s climates have undergone a long series of complex natural changes in the past, have brought new interest and concern to the problem of climatic variation. The importance of the problem has also been underscored by new recognition of the continuing vulnerability of man’s economic and social structure to climatic variations. Our response to these concerns is the proposal of a major new program of research designed to increase our understanding of climatic change and to lay the foundation for its prediction.

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