mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Changing Paleoclimates and Mass Extinctions The Climatic Models by Donald L. Blanchard *Current Conditions:* Climatic conditions such as we enjoy today occur during periods of low to moderate obliquity: estimated at between 12.2° and 37.6°. This represents the familiar six Hadley Cell atmospheric circulation system described earlier (in the chapter "The Mechanism Behind Modern Climate Zones "). It should be noted that polar ice caps only occur in this model under two conditions, both of which prevail today. Continental ice sheets can form over continental land masses that straddle or sit very close to a pole, as in Antarctica and Greenland. An oceanic ice sheet such as occurs over the North Pole can only form when the polar ocean is land-locked, and cut off from the circulation of warmer waters from lower latitudes. This produces very cold but relatively dry conditions over the adjacent continental margins. For continental glaciers to form over those continental margins, a substantial increase in precipitation is required. This probably requires *considerable global warming*, in order to increase evaporation of ocean water to furnish the necessary precipitation. (The northern margins of northern continents today are quite cold enough for glaciers to form, *provided* that winter precipitation exceeds the melting and evaporation of snowfall during the brief arctic summers.) When warm ocean currents circulate through polar regions, a situation much more common in the geologic past than the current two polar ice caps condition, subpolar continents enjoy a mild but very humid climate. Low annual average temperatures prevail, but with little or no sub-freezing weather and substantial rainfall all year around. These conditions favor the formation of temperate rain forests, such as are found today on the Olympia Peninsula of the northwestern United States and in the coastal regions of southern Chile. ------------------------------------------------------------------------ *The Low Obliquity Condition:* During times of zero to very low obliquity (less than 12.2°, using the numbers suggested in this paper), it is predicted that the atmosphere will settle into a 10 Hadley Cell circulation pattern. The reasoning behind this prediction stems from the fact that as the Sun's path shifts north and south with the seasons, the boundaries between Hadley Cells tend to trail along behind, migrating at the present time by approximately the same 23½° that the Sun shifts. Turbulences and perturbations along the cell boundaries brings the Temperate and Inter-Tropical Convergent Zones considerably closer together during the summer months, apparently coming dangerously close to intersecting. With reduced or no obliquity, the seasonal shifting of cell boundaries, and probably the turbulence along those boundaries, should be small enough to allow more circulation cells to exist. Under 10 Hadley Cell circulation, a greater amount of Solar heat should be retained in the tropical zones, which would be substantially reduced in area. Polar regions should be considerably colder than under 6 Hadley cell conditions, and probably more arid as well, due to reduced evaporation of ocean water at lower temperatures. Continental glaciers are probably less likely to form, also due to the reduced precipitation. The middle Temperate zone would probably be colder as well, from less summer insolation and greater decoupling of tropical heat. Bands of aridity, producing cold desert conditions, should exist between the Sub-Polar and Temperate zones, while the hot desert bands between Tropical and Sub-Tropical cells should draw closer to the Equator. The Sub-Tropical zone should enjoy a climate more reminiscent of Temperate climates today, only with less severe winters. Periods during which either six or ten Hadley Cells occur are referred to in this article as 'DRY' climate periods, as distinguished from the 'WET' climate periods discussed below. The terms 'Icehouse' and 'Greenhouse', favored by paleoclimatologists, have come to be associated with glaciation or no glaciation, respectively, a distinction avoided in this article. (Conditions under which glaciers can form are discussed elsewhere in the article.) ------------------------------------------------------------------------ *The High Obliquity Condition:* During periods of higher obliquity (greater than 37.6°, using the current numbers), atmospheric circulation is expected to settle into a pattern of only two Hadley Cells, with one rising boundary between them and the return air downwelling at both poles. Such a condition should produce some startling and unexpected climatic effects. As this mode of circulation is predicted for 111½ MYr out of every 191½, or 58% of the time, this could be considered the 'normal' climatic regimen for our planet. With two Hadley Cells, surface air movement will flow from the poles equatorward and to the west, much like the Trade Winds of today. Travelling over the entire surface of the globe, these winds arrive at the convergence zone (i.e. the rising cell boundary) carrying a heavy load of moisture. As this humid air rises along the converging boundary, large quantities of water will be precipitated as the air cools adiabatically (i.e. cooling from the reduction in air pressure as the elevation increases). Heavy cloud cover and monsoon-like rains can be expected to follow behind the Sun as it tracks north and south with the seasons. As obliquity increases, the Sun's path, and the monsoon rain, will extend farther and farther towards and ultimately into the polar regions during the summer months. An average /global/ rainfall in excess of 200" [500 cm] per year is expected, hence their designation as 'WET' climate cycles. At the beginning and end of 'WET' cycles, arid polar regions are expected, as the monsoon rains, which essentially follow the Sun's migration north and south with the seasons, won't reach as far poleward when obliquity is less extreme. During the middle of 'WET' cycles, when obliquity is close to 90°, monsoonal rains should sweep across the entire surface of the planet from pole to pole. At the convergence zone, the heat of condensation of all the moisture will be added back into the rising air, even as it is being cooled adiabatically. Thus the rising air will reach the upper atmosphere cold, but not nearly as cold as it would be had the air been dry to begin with. Because of the opaque cloud cover, heat loss by radiation from the surface will be minimal, while radiant heat loss at the cloud tops will be small because of the drastically lowered temperatures. (Heat loss by radiation is proportional to the fourth power of temperature differential. In this case, cloud top temperature is only slightly higher than the radiant sky temperature, thus minimizing heat losses from radiation to the sky.) Therefore, when the upper atmosphere air reaches the polar regions and is reheated by compression, it reaches the surface at very nearly the same temperature as it was on the surface at the convergence zone. A two Hadley Cell circulation system should therefore be expected to thoroughly mix the air, producing nearly uniform temperatures over the entire surface of the Earth. The present average surface temperature of the Earth is estimated (by some highly scientific wild guesswork) to be around 55°F [15°C]. The increased cloud cover predicted for 'WET' cycles is expected to increase the Earth's albedo somewhat. Therefore, a temperature of around 50°F [12°-13°C.] is predicted as the average temperature, with absolute minimum and maximum temperatures at sea level only varying by a few degrees from that average. Even at the winter pole, where the Sun doesn't shine for months at a time, warm downwelling air would maintain essentially the same temperatures as the Summer pole. The planet Uranus presently has an axial tilt of ~82°, and its dark pole was found to be 2-3°C. /warmer/ than its sunlit pole. (Hanel, et al, 1986 ). The absence of significant differences in temperature translates into an absence of pressure differences as well, meaning very little wind. Rather, a steady 2-3 MPH [3-5 KPH] breeze flowing westward and towards the convergence zone is predicted. Gale force winds, by 'WET' climate standards, are not expected to exceed 5 MPH [8 KPH]. ------------------------------------------------------------------------ *The Oceans during 'WET' Cycles:* A two Hadley Cell atmospheric circulation also causes profound changes in the ocean environment. During 'DRY' climate cycles, frigid temperatures in the polar regions chill the surface ocean water. Cold water is more dense than warm water, and sinks to the bottom. The presence of polar ice caps insures a steady supply of cold bottom ocean water at 39°F. [4°C.] This cold bottom water flows equatorward, ultimately circulating throughout all the oceans of the world. Thus a vertical thermal gradient is established; the surface water may be quite warm, but temperature decreases rapidly with depth. (Even in tropical waters, divers need to wear protective gear, such as wet suits, when working in as little as a few hundred feet of water.) During 'WET' cycles, the source of cold bottom water is cut off, and the abyssal plains gradually warm to near surface temperatures. Thus, in the absence of glaciation, little or no vertical thermal gradient exists. There are no indications that any glaciation occurred during the Late Jurassic or Cretaceous Periods, which comprise the most recent 'WET' cycle. However, there are numerous reports of glaciation for the Late Carboniferous (Pennsylvanian) and Early Permian periods, during the middle of the next previous 'WET' cycle. With warm, dry air descending over the poles, it is difficult to imagine polar ice of any kind forming. However, with a sea level temperature of only 50°F. and monsoon-like rainfall, it is easy to imagine that the snow line might not be very high, and that quite heavy snowfall could occur in mountainous regions. With the high amount of precipitation predicted, very fast-moving mountain glaciers should be expected at relatively low elevations. It is quite reasonable to expect that such glaciers could reach low elevations and even extend out into the sea, thereby supplying cold ocean bottom water, even during 'WET' cycles. (Thus the avoidance in this article of the 'Icehouse' and 'Greenhouse' terminology.) ------------------------------------------------------------------------ *Rising Sea Levels:* Another curious phenomenon that occurs during 'WET' cycles is the gradual rise in eustatic sea levels. A gradual inundation of continents characterizes most, if not all, of the Carboniferous period, but seas abruptly receded in the Permian. Again in the Late Jurassic and Cretaceous periods, the seas rose, culminating at the close of the Cretaceous with around 90% of all continental surfaces under water. Another abrupt drop in eustatic sea level marked the start of the Cenozoic era, and sea levels have remained generally low ever since. In the present model, this rise is attributed to a significant increase in the rate of erosion during 'WET' cycles, stemming from the significant increase in rainfall. In the geological record, there are indications that the relative proportion of land and ocean, and the average depth of the world's oceans, has remained relatively constant for at least the last one billion years of the Earth's history. This is inferred from the fact that the maximum thickness of undisturbed marine sedimentary deposits has remained essentially the same, around 50,000' [~15 km], for that span of time. (The correlation between sedimentary deposits and ocean depth is discussed in the lesson on buoyancy in the companion article: "The ABC's of Plate Tectonics <../tectonic/ptABCs.php>", elsewhere on this Web site.) Plate Tectonics is now generally recognized as the ultimate cause of orogeny (mountain building). Orogeny, by whichever mechanism, represents a process of thickening continents vertically. If the volume of continental material is to remain constant, vertical thickening must represent a reduction in their surface area. If the surface area of continents is reduced, then the area covered by oceans must increase, which, if ocean volume is also constant, represents a lowering of sea levels. Over the long span of geological time, this shift in area from land to ocean is balanced by the forces of erosion. Erosion, whether by wind, water, or glaciers, ultimately results in sediments being removed from the above water surfaces of continents and transported into the oceans. Sedimentary material deposited in the ocean displaces sea water, leading to a rise in sea level. It is suggested that the drastic increase in average rainfall during 'WET' climate cycles favors the forces of erosion over those of tectonics, causing sea levels to gradually rise. During 'DRY' climate cycles, which represent a significant reduction in rainfall and rainfall-caused erosion, tectonic forces prevail, and sea levels quickly fall. Glaciation (even mountain glaciation) also causes a significant drop in eustatic sea levels, which may account for high and dry conditions throughout much of the Permian period, which in this model is depicted as the last 1/3 of the Carboniferous/Permian 'WET' cycle. (This explanation also suggests that ice buildup might have been fairly extensive during the Early Permian, despite rather spotty and difficult to interpret geological evidence. Of course, mountain glaciers shouldn't be expected to leave as much geological evidence as continental ice sheets.) The Geological Time Chart shows the relationships proposed between geological periods and climatic cycles. (info at webspinners.com ) WebMaster: Donald L. Blanchard