The extent, distribution, and topography of land areas also affect the Earth's climate. Land heats up and cools down more rapidly than the sea. The daily cycle of sea and land breezes in coastal areas is a well-known consequence of this. A similar phenomenon on a longer, seasonal timescale, and affecting a larger geographic area, is the monsoonal climate of India and the Arabian Sea. In the northern summer the large landmass of southern Asia heats up, and the air rising above this creates a low pressure area and draws in moisture laden air from the northwest Indian Ocean - the southwest monsoon. In the winter, cold, dense air over the cold land area creates high pressure, and gives rise to the dry, northeast monsoon, which blows from land to sea. Seasonal heating and cooling of the air over the Sahara produces a similar, but smaller scale, effect over central Africa and the equatorial Atlantic in the Gulf of Guinea, where a similar monsoon regime pertains. These two monsoonal areas account for the tropical rain forests of central Africa, and Burma, Sri Lanka, and parts of India.
The albedo of land areas is variable depending on the type, or lack, of vegetative cover, but it is typically higher than that of sea areas, which have a low albedo. The distribution of land and sea, and its affect on the Earth's albedo in the past, might be expected to have produced an appreciable effect on climate, but as yet this is poorly understood. Ice or snow covered land or sea has a high albedo, and clearly is significant, not least in that it provides a positive feedback mechanism: the greater the extent of the ice and/or snow, the greater the degree of cooling. Mountains, even in low latitudes, can be covered with permanent or seasonal snow, thereby increasing the Earth's
The carbon dioxide so released ultimately returns to the atmosphere. Thus the carbon fixed in the carbonates on land is redeposited on the sea floor, with no net change in the CO2 content of the atmosphere. The weathering of silicate rocks by carbonic acid, however, has important differences. A simplified weathering reaction may be expressed as:
Silicate mineral + 2CO2 + water ^ 2HCO3- + clay mineral + cation(s) (eq. 2)
In the ocean the HCO3- ions combine with Ca2+, as in the reverse of equation 1, to form calcium carbonate. In this case, two molecules of CO2 are removed from the atmosphere, for every one molecule returned to the atmosphere when CaCO3 is formed in the ocean. Increased weathering of silicate rocks could, therefore, draw down the CO2 content of the atmosphere, and be a possible cause of global cooling (Raymo & Ruddiman, 1992).
As a result of the most recent phase of continental drift, the Cenozoic was characterized by a major episode of mountain building, notably throughout the Alpine-Himalayan belt, and culminating in the uplift of the Tibetan Plateau in the Late Cenozoic. The elevation of mountains would have greatly increased physical and chemical weathering processes, particularly as they concentrate rainfall on their windward flanks. The elevation of the Tibetan Plateau, for example, is thought to have greatly intensified the southwest monsoon, bringing much heavier rainfall, and causing much more intense weathering, on the southern slopes of the Himalaya. The elevation of Tibet and surrounding areas is particularly important because, although this represents just 5% of the Earth's land area, 23% of the global flux of dissolved material in rivers is derived from rivers with a source in the Tibetan/Himalayan region. There is some indication from the fauna and flora, and the sedimentary record of northern India, that there was a major intensification of the southwest monsoon about 8 Ma ago (An et al., 2001). The question remains, however, whether this correlates with the uplift of the Tibetan Plateau (Section 10.4.3). Current models for this uplift, which may involve the convective removal of thickened lithosphere beneath Tibet (Section 10.4.6), imply that the final uplift phase may have been relatively sudden, in geologic terms. Attempts to obtain an independent estimate of the timing of this uplift, using paleobo-tanical evidence or the dating of fault systems, have proved to be inconclusive, with some results confirming the 8 Ma date, but others indicating a date of 14-15 Ma for the final uplift of Tibet (Spicer et al., 2003). Spicer et al. suggest that a possible explanation for this is that the uplift occurred progressively from south to north over a period of 6-7 million years.
Greatly enhanced weathering of silicate rocks in the late Miocene, would have removed CO2 from the Earth's atmosphere and might well account for the pronounced global cooling revealed by oxygen isotope studies at or near the Miocene-Pliocene boundary, i.e. at about 6 Ma ago (Fig. 13.8). As indicated above this may have produced effects that led, ultimately, to the initiation of the Ice Ages approximately 3 Ma ago.
Thus plate tectonic processes influence all the major factors that are currently thought to determine the Earth's long-term changes in climate. The concentration of CO2 in the atmosphere, at any particular point in time, is thought to be determined largely by the amount of volcanism at that time. Thus the exceptionally high levels of CO2 associated with the "Greenhouse Earth" of the Cretaceous period are related to superplume activity, and high rates of sea floor spreading and subduction, all three giving rise to enhanced volcanic activity. Conversely, systematic decreases in plume activity, and plate accretion and destruction, would cause global cooling. However, the periods of pronounced global cooling during the past 50 Ma are not associated with decreases in volcanism (Fig. 5.13). It seems probable therefore that one needs to invoke the other potential impacts of plate tectonic processes on the Earth's climate, notably changes in oceanic circulation and the consequences of mountain building, and enhanced weathering, to explain the mid-Cenozoic transition to an "Icehouse Earth," and eventually the triggering of the Ice Ages of the past 3 Ma.
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