Climate in Earth History Studies in Geophysics (1982) / Chapter Skim
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Overview and Recommendations
Overview and Recommendations
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Climate in Earth History
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Much attention has already been devoted to the records of climatic change in the first two intervals; this study is devoted to the third, the vast (billions of years) pre-Pleistocene period of Earth history.
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Consequently, predictions or forecasts of short-term climate changes promise substantial benefits. Recognizing this potential, Congress recently enacted the National Climate Program Act, which calls for "a well-defined and coordinated program in climate-related research, monitoring, assessment of effects, and information utilization" (U.S.
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Coal deposits derive from swamps, generated in a humid climate. Much of our knowledge about past climates results from the search for such economic deposits, and our ability to locate additional resources will be improved by an increased understanding of climates throughout Earth history PAST CLIMATES The geologic record shows that surface temperatures on Earth have not been too different over most of the Precambrian and Phanerozoic times from those of today (Frakes, 1979)
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PRE-PLEISTOCENE CLIMATOLOGY: A MATTER OF SCALE The central task of paleoclimatology is to describe the climatic patterns throughout geologic time and to understand the trends, cycles, and discontinuities in these patterns—in short, to find out how the climate system works. Weather elements include temperature, precipitation (as either rain or snow)
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For the Paleozoic Era, fossils provide clues, and it is possible both to recognize climatic Donations and to estimate the rates of change of such Donations. Yet even within these remote times, there may be glimpses into climatic cycles, based on continuous sequences of finely layered rocks.
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SYNOPTIC STUDIES Paleoclimatic investigations can be convenient y characterized by three approaches: synoptic studies, time-series studies, and event and episode analysis, as summarized below. One way to study ancient climates is to select convenient intervals of past geologic time and to assemble all pertinent information bearing on climate within that interval, that is, to study synoptic intervals or time slices.
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For older periods within the Precambrian, the time slices become ever thicker and the identification of sites of sediment deposition become less precise. Investigations in the Precambrian must therefore focus increasingly on the statistical significance of climatic indicators.
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. The Paleozoic and Mesozoic Eras For each geologic epoch within the Paleozoic and Mesozoic Eras, several synoptic intervals hold promise for satisfactory worldwide climate correlation.
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Stratigraphic correlations are probably adequate within Miocene and Pliocene sequences, and data coverage is good because sediments of these ages are available over wide areas of the seafloor, as well as on continental shelves. TIME-SERIES STUDIES Thick continuous stratigraphic sections consisting of numerous thin beds may record climate variability if the bedding is rhythmic or repetitious and the thickness, or some other characteristic, is the response to a seasonal climate signal.
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, ancient seetions even as old as Precambrian warrant study. Times in which interesting climatic signals have been observed are the early Pliocene (preceding northern ice caps)
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The first class constitutes the "static setting." This is the arrangement of continents and ocean basins, which can be considered fixed on a short time scale. The second class of factors involves what may be called the "dynamic setting." This includes oceanic and atmospheric circulation, heat transfer, albedo distributions, biosphere patterns, and atmospheric concentration of infrared absorbing gases.
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Such studies yield insights about the dynamics and variability of climate systems at any one site that cannot he obtained from spatial reconstructions alone Analysis of Pleistocene climatic records inferred from deep-sea sediments has established relations between orbital parameters and glaciation by identifying astronomical cycles of 19,000 yr, 23,000 yr, 41,000 yr, and perhaps 100,000 yr within the geologic record. A search for these cycles in older records and for other cycles is now possible; they may be present in Mesozoic, Paleozoic, and perhaps even Precambrian sequences.
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On a long time scale, the average atmospheric carbon dioxide content may be fixed by weathering reactions and carbon storage in soil, rocks, and seawater. However, on a shorter time scale, the atmospheric reservoir is dominated by the much larger adjoining reservoirs of the ocean, the biosphere soil complex, and the reactive portion of marine organic and calcareous sediments.
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Although satisfactory computer models do not yet exist to explain all aspects of modern climates, such studies for ancient synoptic intervals can nevertheless serve as guides to explain climatic events. Therefore, models of several types should be applied to the paleogeographic reconstructions of selected times of climatic significance in the geologic past.
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National Climate Program Act, Public Law 9S 367, 95th Congress, September 17, 197B. Ziegler, A M, C
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