No mutta sen ilmastonmuutoksen aiheutti mannerlaattojen erkaneminen toisistaan, ja se oli vaan paikallinen kun tämä tuleva on maailmanlaajuinen.
???
niinkö
tää oli kyllä ihan globaali ilmiö
anteeksi tää hirveen pitkä quote
"Plio-Pleistocene Climate Deterioration
Since the mid-Miocene, about 14 million years ago, the Earth's temperature has dropped several degrees. The causes of the drop are not well understood, but are probably the result of basic geological processes (Partridge, et al., 1995). The arrangement of the continents is probably important. Continental drift has produced a situation with Antarctica in the South Polar region, insulated from warm ocean currents by the circum-Antarctic currents. Similarly, the Arctic Ocean is closely surrounded by land masses, insulating it from the penetration of warm ocean currents to the north polar region. Ice cover at high latitudes reflects much sunlight back to space, significantly lowering the Earth's total heat income. Slowing sea-floor spreading rates over the whole Tertiary period may have lowered the output of CO2 from the Earth's interior. A lower concentration of this important greenhouse gas in the atmosphere win result in a lower global temperature.
The amplitude of fluctuations in rainfall and temperature increased as mean temperature dropped. As time series analysts say, the climate record of the past few million years is highly non-stationary. Not only does the climate vary, but the statistics that describe the variation-the variance and patterns of autocorrelation--change with time.
The pattern of fluctuation in climate is very complex. Much of the variation seems to arise from an enhanced sensitivity to radiation changes caused by periodic variations in the Earth's orbit. The radiation income in high northern latitudes has a 20% range of variation due to these effects. Milankovitch (1941) developed this hypothesis in its modem form. Broecker and Denton (1990) give a good introductory discussion to this and other aspects of the physics of climate variation. The eccentricity of the Earth's orbit varies on a 95,800 year time scale, the inclination of its axis with a periodicity of 41,000 years, and the precession of the equinoxes with a periodicity of 21,700 years. The magnitude of the direct forcing of climate by these cycles is out of phase in the Northern and Southern Hemispheres, yet the cycles of ice growth and decay are in phase. The Milankovitch cycles reach far back into the earth's history, but the ice age began in earnest only two million years ago. As the deterioration has proceeded, different cycles have dominated the pattern. The 21,700 year cycle dominated the early part of the period, the 41,000 year cycle between about 3 and 1 million years ago, and the 95,800 year cycle from 1 million years ago to the present (de Menocal and Bloemendal, 1995). Thus, complexities of the response of the atmosphere-ocean-ice sheet system must somehow amplify and coordinate the effects of the orbital periodicities. Alternatively, the Milankovitch theory may be incorrect (Broecker, 1992; Brownlee, 1995). Most likely, the present disposition of the continents and oceans affects ocean currents and wind patterns in such a way as to make the global climate very sensitive to small fluctuations in insolation. For example, the Arctic Ocean is sufficiently isolated from the warm Atlantic and Pacific oceans that it is frozen. Its ice cover reflects the sunlight falling on it, substantially preventing it from storing heat in summer. This in turn means that the high Northern latitudes can build ice sheets, which reflect still more sunlight, leading to other chilling. The small forcing from the Milankovitch cycles in high northern latitudes can thus be amplified by the growth and wasting of continental ice in the north.
The exact driving mechanisms of the late Cenozoic climate system are still unknown. The most influential hypothesis is that of Broecker et al. (1985). The ocean-atmosphere coupling includes elements that are affected by the deep circulation of the ocean. Under the current climatic regime, the North Atlantic near Greenland is the source of much of the bottom water for the World Ocean. A subsurface current of warm, rather saline, water moves north in the Atlantic and upwells during the winter near Greenland. This water then loses an immense amount of heat to the atmosphere, becomes heavier, and sinks to become the North Atlantic Deep Water. NADW eventually flows out the South Atlantic to the Indian and Pacific Oceans where it reaches the surface in upwelling zones. Thus, there is a great ocean conveyor concentrating immense amounts of heat in the North Atlantic and moderating the climate of the surrounding land masses, especially NW Eurasia. (When the poles were not occupied by or surrounded by continents, similar conveyors presumably kept the poles ice free.) Among other things, the conveyor circulation efficiently draws greenhouse gases, especially CO2, into the ocean abyss. This circulation apparently shuts down during glacial conditions, probably because glacial melt water flowing from the Eastern North American ice sheet makes the ocean surface water too fresh and hence too light to sink even when chilled to maximum density. Once in a glacial mode, the growth of reflective ice sheets and the dust from the dry climates of the glacial period further chill the earth.
For the last 120,000 years, data is available from ice cores taken from the deep ice sheets of Greenland and Antarctica with time resolution as high as few decades. During the last glacial (65,000-12,000 years before present, Emiliani oxygen isotope stages 2-4), the climate was highly variable on time scales of centuries to millennia (GRIP, 1993; Lehman, 1993; Ditlevsen, et al., 1996), as can be seen in figure 2. Even when the climate is in the grip of the ice, there were brief excursions of about a thousand years duration, called Dansgaard-Oeschger events, in which the climate briefly reached interglacial or near interglacial warmth. The Dansgaard-Oeschger events themselves come in clumps in which the most extreme warm spikes are just preceded by the appearance of ice-rafted debris in North Atlantic sediments, called Heinrich events. The most highly touted hypothesis to explain the Dansgaard-Oeschger-Heinrich excursions is the cyclic production of glacial melt waters that affect the NADW conveyor, wind patterns over the North Atlantic, and other major regulators of global climate (Lehman, 1993).
The last Interglacial (65,000-130,000 years before present, oxygen isotope stage 5) may also have been highly variable on the millennial time scale. Interpretation of the deeper portions of the Greenland ice cores is controversial because of the possibility that ice from colder and warmer periods has been folded by ice movement to create false fluctuations (Grootes, et al., 1993). Nevertheless, many lower-resolution records of the last interglacial also suggest that it was frequently punctuated by episodes of near-glacial cold (e.g. Lamb 1977: 333). As figure 2 shows, the Holocene interglacial period of the last I 0,000 years (oxygen isotope stage 1) has been extraordinarily tranquil. The resolution of the ice core data is extraordinary. Near the top of the GRIP core the resolution to seasons is possible, and at 0 1,000 years before present resolution is estimated to by 1O years (Ditlevsen, et al. 1996). High resolution data do not yet exist prior to the last Interglacial to indicate just how unusual the Holocene is compared to the whole Pleistocene record. The possibility that Holocene climate stability might be easily tipped into a regime of much greater variability by relatively weak forcing is cause for considerable worry regarding human caused increases in carbon dioxide and other greenhouse gasses (Broecker, 1997).
High latitudes chilled much more than the equator as the climate cooled during the latter part of the Cenozoic. New habitats arose, such as periglacial tundras. Other habitats, especially arid and semiarid deserts and grasslands replaced forests over large areas from equatorial to temperate latitudes. The spatial as well as the temporal variability of the earth thus increased considerably as the Plio-Pleistocene deterioration advanced."
Peter J. Richerson
Department of Environmental Science and Policy
University of California
Davis, California USA 95616
pjricherson@ucdavis.edu
Robert Boyd
Department of Anthropology
University of California
Los Angeles, California USA 90024
rboyd@thro.ucla.edu
