The study of ancient climates. Climate is the long-term expression of weather; in the modern world, climate is most noticeably expressed in vegetation and soil types and characteristics of the land surface. To study ancient climates, paleoclimatologists must be familiar with various disciplines of geology, such as sedimentology and paleontology, and with climate dynamics, which includes aspects of geography and atmospheric and oceanic physics. Understanding the history of the Earth's climate system greatly enhances the ability to predict how it might behave in the future. Climatology Information about ancient climates comes principally from three sources: sedimentary deposits, including ancient soils; the past distribution of plants and animals; and the chemical composition of certain marine fossils. These are all known as proxy indicators of climate (as opposed to direct indicators, such as temperature, which cannot be measured in the past). In addition, paleoclimatologists use computer models of climate that have been modified for application to ancient conditions. Like modern climatologists, paleoclimatologists are concerned with boundary conditions, forcing, and response. Boundary conditions are the limits within which the climate system operates. The boundary conditions considered by paleoclimatologists depend on the part of Earth history that is being studied. For the recent past, that is, the last few million years, boundary conditions that can change on short time scales are considered, for example, atmospheric chemistry. For the more distant past, paleoclimatologists must also consider boundary conditions that change on long time scales. Geographic features—that is, the positions of the continents, the location and orientation of major mountain ranges, the positions of shorelines, and the presence or absence of epicontinental seaways—are important for understanding paleoclimatic patterns. Forcing is a change in boundary conditions, such as continental drift, and response is how forcing changes the climate system. Forcing and response are cause and effect in paleoclimatic change. Proxy indicators of paleoclimate are abundant in the geologic record. Important sedimentary indicators forming on land are coal, eolian sandstone (ancient sand dunes), evaporites (salt), tillites (ancient glacial deposits), and various types of paleosols (ancient soils), such as bauxite (aluminum ore) and laterite (some iron ores). Coals may form where conditions are favorable for growth of plants and accumulation and preservation of peat, conditions that are partly controlled by climate, especially seasonality of rainfall. Fossil indicators provide information about climate mostly by their distribution (paleobiogeography), although a few specific types of fossils may be indicative of certain climatic conditions. The latter are usually fossils from the younger part of the geologic record and are closely related to modern species that have narrow environmental tolerances. Another type of information available for documenting paleoclimatic patterns and change is stable isotope geochemistry of fossils and certain types of sedimentary rock. Many elements that are used by organisms to make shells, teeth, and stems occur naturally in several different forms, known as isotopes. The most climatically useful isotopes are those of oxygen (O). Although the effects of temperature change and ice volume change can be difficult to distinguish, the analysis of oxygen isotopes has provided a powerful quantitative tool for the study of both long-term temperature change and the history of the polar ice caps. A great deal of research in paleoclimatology has been devoted to understanding the causes of climatic change, and the overriding conclusion is that any given shift in the paleoclimatic history of the Earth was brought about by multiple factors operating in concert. The most important forcing factors for paleoclimatic variation are changes in paleogeography and atmospheric chemistry and variations in the Earth's orbital parameters. Atmospheric chemistry Biogeochemistry |
