Paleoclimatology

Climate is the average of weather elements over long periods of time and over large areas. The study of ancient climates, termed paleoclimatology, utilizes sedimentologic, paleontologic, and geochemical data to reconstruct ancient temperature, wind patterns, precipitation, and evaporation.

Climatic Processes

The basic elements of climate include radiation, temperature, atmospheric moisture and precipitation, evaporation, and wind. The features of ancient climates that have been most studied are temperature, wind patterns, amount of precipitation, and evaporation. To understand ancient climates, it is necessary to understand the fundamentals of modern climatic processes.

Solar radiation is the electromagnetic radiation emitted by the sun's surface. This radiation is primarily within the infrared, visible, and ultraviolet ranges. The incoming solar radiation is termed global insolation. The amount of energy the Earth receives at any one time is essentially constant, although paleoclimatic evidence indicates that it has not always been so. The amount of global insolation received depends on the output energy of the sun, the distance between the Earth and the sun, the angle at which the sun's rays strike the Earth's surface, the duration of daylight, and atmospheric composition. Therefore, global insolation depends primarily on the latitude and the seasons: Regions within the higher latitudes receive the least amount of insolation, primarily because of the angle at which the sun's rays strike the Earth, and they have the greatest seasonal variation. The result is an energy deficit poleward of 40 degrees north and 40 degrees south latitude, respectively; the equator has the least variation, so that between these degrees of latitude, there is an energy surplus. Therefore, a continuous horizontal exchange of energy between these regions occurs, which is the cause of atmospheric circulation and weather patterns.

The unequal heating of the Earth's surface creates pressure differences. Since gases move from areas of high pressure to areas of low pressure, this differential heating and pressure variation causes the wind to blow. If the Earth were not rotating, its general circulation pattern would be one of ascending air at the equator and descending air at the poles, with continuous horizontal flow between. The Coriolis effect, however, changes this ideal pattern, and free-moving objects are deflected from a straight-line path in response to the Earth's rotation. In the Northern Hemisphere, the deflection is to the right, and in the Southern Hemisphere, it is to the left. Ocean surface circulation patterns are, therefore, typically clockwise in the Northern Hemisphere and counterclockwise in the Southern, yet atmospheric patterns are, in fact, more complex because air parcels are ascending and descending as well as moving horizontally. General atmospheric circulation patterns from the equator poleward include the doldrums, the trade winds, the horse latitudes, the westerlies, and the easterlies. In the geologic record, general atmospheric circulation patterns are difficult to determine as a result of the scarcity of data points necessary for accurate calculation of such large-scale phenomena. Wind patterns, however, may be calculated for some areas on the basis of sedimentary structures, and general atmospheric circulation patterns may be inferred from models based on modern atmospheric circulation and the distribution of the Earth's land and water areas through time.

For precipitation to occur, there must be atmospheric instability. An air parcel will rise until it reaches an altitude where the surrounding air is of equal temperature; this process may be enhanced where intense solar heating creates lower pressures, where the air mass is heated by a warm surface below it, where sloping terrain such as mountains forces air to ascend, or where cool air acts as a barrier over which warmer, lighter air rises. As the unstable air parcel moves vertically, it cools as a result of the expansion of gases (the adiabatic process). Condensation will occur when the dew point temperature is reached, which varies according to the initial relative humidity of the air parcel. Provided that there is sufficient moisture and that nuclei are present around which the moisture may accumulate, precipitation may occur. The major types of precipitation include rain, freezing rain, sleet, snow, and hail. The type of precipitation that occurred in ancient times is difficult to determine, although occasional examples of raindrop prints are found as casts in sediments, and ancient glacial deposits indicate the accumulation of frozen precipitation. In some situations, the amount of precipitation may be inferred, typically through the utilization of fossil plants. In other cases, the amount of precipitation versus evaporation may be indicated, primarily through the utilization of sedimentary mineral types such as evaporites.

Climate Changes

The result of the interaction of these climatic processes is the formation of regional climates. On the modern Earth, a wet equatorial belt lies within about 20 degrees of the equator. The adiabatic cooling of very moist and warm air results in heavy precipitation within these regions, yielding a wet tropical climate. These air parcels then begin descending, they adiabatically heat up through compression, and the now-dry air encounters the Earth's surface. The result is the formation of tropical deserts, centered at approximately 25 to 30 degrees north and south latitude. At the middle latitudes (approximately 35 to 65 degrees), the interaction of air masses between the polar and tropical regions and the variation in global insolation throughout the year result in large variations in temperature and seasonality. Polar regions are characterized by cold and dry conditions because the sun's rays are typically at low angles and cold air parcels in those regions are unable to accumulate large amounts of moisture.

One phenomenon that seems to affect climate, at least for relatively short cycles of Earth's history, is sunspot activity. Although the data are incomplete, the increase and decrease in these solar disturbances seem to suggest that they create variations in global insolation and subsequent warming and cooling trends. The cooling of the Earth's climate during the early-to-middle portion of the second millennium has been linked to variations in sunspot activity. Another explanation of causes of Earth-climate cyclicity was put forth in the twentieth century by Milutin Milankovitch. Milankovitch suggested that the Earth's orbit around the sun may vary from more elliptical (with more pronounced differences in global insolation during the summer versus the winter) to more circular (with less seasonality). Such a cycle would take about 100,000 years to complete. Other Milankovitch cycles proposed include changes in the tilt of the Earth's axis in relationship to the sun (a 40,000-year cycle) and the wobble of the Earth upon its axis (a cycle of 21,000 years). Studies on ocean-floor sediments for the Pleistocene (in the past two million years of Earth's history) and in lake basin sediments of the Triassic and Jurassic (approximately 200 to 180 million years before the present) of the East Coast of the United States indicate that such cycles may indeed exist.

Another possible cause of short-term changes in climate may be asteroid impacts. Scientists have theorized that such impacts would increase the particulate levels in the atmosphere to such an extent that a phenomenon similar to “nuclear winter” would occur. Global insolation would decrease dramatically, and temperatures would plummet. Suggestions have been made that this phenomenon could account for the periodic extinctions that occur on Earth.

Another possible cause of changes in the Earth's atmosphere involves changes in carbon dioxide levelsa heating trend caused by increasing levels (the greenhouse effect) or a cooling trend caused by decreasing levels. Such changes may result from changes in the overall metabolism of organisms or by decreases and increases in volcanic activity. Increases in volcanic activity could also cause cooling trends because of a decrease in global insolation as a result of volcanic particles suspended in the atmosphere.

Ocean current patterns tremendously influence weather patterns and regional climates, and it has been suggested that the atmosphere and oceans can be viewed as a single, interacting unit. Changes in oceanic circulation would certainly modify climates, although the geologic evidence of a definite link between the two is equivocal. Stronger evidence for causes of variation may be related to plate tectonics. The movement of continental plates over polar areas may certainly account for the buildup of ice during certain periods of Earth's history, like those that occurred on the southern continents during the Permian period (about 290 to 240 million years before the present). In part, such shifting of plates may also account for the last ice ages in Europe and North America, although the presence of several periods of glaciation and interglacial episodes indicates that each episode of glaciation during the Pleistocene cannot be explained this way.

Principal Terms

atmospheric circulation: the movement of air as a result of regional pressure differentials

climate: the accumulative effects of weather; its basic elements include radiation, temperature, atmospheric moisture and precipitation, evaporation, and wind

insolation: incoming solar radiation; differences in global insolation at various places on the Earth's surface create weather and climate patterns

weather: the condition of the atmosphere at a given moment and place

Bibliography

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