Paleoclimatology (eBook)

Reconstructing Climates of the Quaternary

Raymond S. Bradley (Autor)

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2013 | 3. Auflage
696 Seiten
Elsevier Science (Verlag)
978-0-12-386995-1 (ISBN)

Infos

Paleoclimatology: Reconstructing Climates of the Quaternary, Third Edition-winner of a 2015 Textbook Excellence Award (Texty) from The Text and Academic Authors Association-provides a thorough overview of the methods of paleoclimatic reconstruction and of the historical changes in climate during the past three million years. This thoroughly updated and revised edition systematically examines each type of proxy and elucidates the major attributes and the limitations of each. Paleoclimatology, Third Edition provides necessary context for those interested in understanding climate changes at present and how current trends in climate compare with changes that have occurred in the past. The text is richly illustrated and includes an extensive bibliography for further research. - Winner of a 2015 Texty Award from the Text and Academic Authors Association - A comprehensive overview of the methods of paleoclimate reconstruction, and the record of past changes in climate during the last -3 million years - Addresses all the techniques used in paleoclimatic reconstruction from climate proxies - With full-color throughout, and thoroughly revised chapters on dating methods, climate forcing, ice cores, marine sediments, pollen analysis, dendroclimatology, and historical records - Includes new chapters on speleothems, loess, and lake sediments - More than 1,000 new references and 190 new figures - Essential reading for those interested in how present trends in climate compare with changes that have occurred in the past

Raymond S. Bradley has been involved in many national and international activities related to paleoclimatology, most notably as the current Chair of the Scientific Steering Committee for the International Geosphere-Biosphere Program on Past Global Changes (IGBP-PAGES). He has published dozens of articles in scientific journals, and has edited several important books in paleoclimatology. The first edition of Quaternary Paleoclimatology has been the definitive text in this field for over a decade. His research is in climatology, specifically in climatic change and the evidence for how the earth's climate has varied in the past. He has carried out research on climate variation, both on the long (glacial and interglacial) time-scale and on the short (historical and instrumental) time-scale, involving the analysis of data from all over the world. In recent years he has been involved in studies of natural climate variability, to provide a background for understanding potential anthropogenic changes in climate resulting from rapid increases in 'greenhouse gases' over the last century or so. R.S. Bradley has been a professor in the Department of Geosciences, University of Massachusetts, Amherst, USA, since 1984. He has been Head of the Department of Geosciences since 1993. Additionally, he is a member of Clare Hall at Cambridge.

Textauszug

Chapter 2

Climate and Climatic Variation

Abstract

Climate is the end-product of a multitude of interactions between several different subsystems—the atmosphere, oceans, biosphere, land surface, and cryosphere—which collectively make up the climate system. Each subsystem is coupled in some way to the others such that changes in one subsystem may give rise to changes elsewhere through positive or negative feedbacks. Climate is the statistical expression of daily weather events. The overall distribution of climatic parameters defines the climatic variability of the place. Climates may change on different scales of time and in different ways, due to forcing factors (orbital, solar, volcanic) and to internal variability within the climate system.

Keywords

Climate system; Paleoclimate; Climate variation; Climate sensitivity; Energy balance; Orbital forcing; Milankovitch; Volcanic eruptions; Solar variations

Outline

2.1 The Nature of Climate and Climatic Variation

2.2 The Climate System

2.3 Feedback Mechanisms

2.4 Energy Balance of the Earth and Its Atmosphere

2.5 Timescales of Climatic Variation

2.6 Variations of the Earth’s Orbital Parameters

2.7 Solar Forcing

2.8 Volcanic Forcing

2.1 The Nature of Climate and Climatic Variation

Climate is the statistical expression of daily weather events; more simply, climate is the expected weather. Naturally, for a particular location, certain weather events will be common (or highly probable); these will lie close to the central tendency or mean of the distribution of weather events. Other types of weather will be more extreme and less frequent; the more extreme the event, the lower the probability of recurrence. Such events would appear at the margins of a distribution of weather events characterizing a particular climate. The overall distribution of climatic parameters defines the climatic variability of the place. If we were to measure temperature in the same location for a finite period of time, the statistical distribution of measured values would reflect the geographical situation of the site (in relation to solar radiation receipts, degree of continentality, and elevation) as well as the relative frequency of synoptic weather patterns and the associated airflow over the region. Given a long enough period of observations, it would be possible to characterize the temperature of the site in terms of mean and variance. Similarly, observations of other meteorological parameters, such as precipitation, relative humidity, solar radiation, cloudiness, wind speed, and direction, would enable a more comprehensive understanding of the climate of the site to be obtained. However, implicit in such statistics is the element of time. For how long should observations be taken to obtain a reliable picture of the climate at a particular place? The World Meteorological Organization has recommended the adoption of standardized 30-year periods to characterize climate (World Meteorological Organisation, 2007; Arguez and Vose, 2011). Adoption of a standard reference period is necessary because the statistics that define climate in one area may vary over time so that climate, strictly speaking, should always be defined with reference to the period used in its calculation. Recent studies of global warming express global temperature changes relative to the 1951-1980 or 1961-1990 mean (e.g., Hansen et al., 2012), but many paleoclimate studies reference climatic data from earlier decades. This becomes important when attempting to compare the fairly subtle climatic variations of the recent past (or general circulation model simulations) with the climate of “today,” as in most areas the last 30 years has been significantly warmer than in previous decades; in fact, on a global scale, 1981-2010 was the warmest 30-year period of the last millennium (Mann et al., 1999). The problem is even more difficult in dealing with precipitation, where one 30-year climatic average may be quite different from another (Bradley, 1991). There is no simple solution, so changes in climate should always be expressed relative to some defined time interval, to allow different reconstructions to be appropriately compared.

Climate may vary in different ways. Some examples of climatic variation are shown in Figure 2.1. Variations may be periodic (and hence predictable), quasiperiodic (predictable only in the very broadest terms), or nonperiodic. Central tendencies (mean values) may remain more or less constant or exhibit trends or impulsive changes from one mean to another (Hare, 1979). Such occurrences may appear to be random in a time series, but this does not necessarily mean they are not predictable. For example, a number of studies have shown that abrupt changes in climate generally result from large explosive volcanic eruptions (e.g., Bradley, 1988). Consequently, the climatic effects of similar eruptions can be anticipated. Hansen et al. (1996), for example, used a general circulation model to estimate the changes in temperature expected from the 1991 eruption of Mount Pinatubo (Philippines). Their estimates tracked very closely observed temperature changes in the years following the eruption. Such studies indicate that in some circumstances, reliable climate predictions can be made, even though the eruptions themselves are nonperiodic.

Figure 2.1 Examples of climatic variation and variability. From Hare (1979).

A very important aspect of variability in the climate system involves nonlinear feedbacks, in which drastic changes may occur if some critical threshold is exceeded. One example of this is the oceanic thermohaline circulation that may cease to operate if the salinity-density balance in near-surface waters of the North Atlantic Ocean is disturbed beyond a certain point. The circulation would then cease, until salinity increased to the level where density-induced overturning of the water column could resume (see Chapter 6, Section 6.8).

Finally, climatic variation may be characterized by an increase in variability without a change in central tendency, though commonly a change in variability accompanies a change in overall mean. Climatic variability is an extremely important characteristic of climate in our increasingly overstressed world. Every year, unexpected weather events (extremes in the climate spectrum) result in hundreds of thousands of deaths and untold economic and social hardships. If climatic variability increases, the unexpected becomes more probable, and the strain on social and political systems increases. High-resolution paleoclimatic data can shed light on this important aspect of climatic variation.

In the light of these discussions, it is appropriate to consider the term climatic change. Clearly, climates may change on different scales of time and in different ways. In paleoclimatic studies, climatic changes are characterized by significant differences in the mean condition between one time period and another. Given enough detail and chronological control, the significance of the change may be calculated from statistics describing the time periods in question. Markedly different climatic conditions between two time periods imply an intervening period of climate characterized by an upward or downward trend or by an impulsive change in central tendency (Figure 2.1). Many paleoclimatic records appear to provide evidence for there being distinct modes of climate, within which short-term variations are essentially stochastic (random). Brief periods of rapid, steplike, climatic change appear to separate these seemingly stable interludes (Alley et al., 1997c). Analysis of several thousand 14C dates on stratigraphic discontinuities (primarily in pollen records from western Europe but including data from elsewhere) lends some support to this idea (Wendland and Bryson, 1974). Certain periods stand out as having been times of environmental change on a worldwide scale1 (Figure 2.2). Such widespread discontinuities imply abrupt, globally synchronous climatic changes, presumably brought about by some large-scale forcing. In particular, the period 2760-2510 years BP (the beginning of sub-Atlantic time) stands out in both palynological and archaeological data as a period of major environmental and cultural change, the cause of which is not known (though solar forcing is commonly cited). There is also mounting evidence for a change in the climate of many regions around 4200 years ago, which had significant societal impacts (Dalfes et al., 1997; Weiss and Bradley, 2001). Understanding the causes of such changes is increasingly important as world population continues to grow, increasing by a billion people every ~ 15 years, placing severe pressure on all environmental systems (Rockström et al., 2009). Abrupt disturbances of the climate system in the future will have major social, economic, and political consequences (Parry et al., 2007).

Figure 2.2 “Climatic discontinuities” revealed by analysis of over 800 14C dates on stratigraphic discontinuities in paleoenvironmental (primarily botanical) records (based on data in Wendland and Bryson, 1974). Major and minor discontinuities are shown by large and small jagged lines,...

Produkt-Details

• Erscheint lt. Verlag: 28.12.2013
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Meteorologie / Klimatologie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Mineralogie / Paläontologie
• Themenwelt: Technik ► Bauwesen
• ISBN-10: 0-12-386995-1 / 0123869951
• ISBN-13: 978-0-12-386995-1 / 9780123869951