SEA-LEVEL CHANGE - A PHILOSOPHICAL APPROACH

The present Cenozoic era is an 'icehouse' episode characterized by a l ow sea level. Since the beginning of the industrial revolution, the hu man race has been emitting greenhouse gases, increasing the global atm ospheric temperature, and causing a rise in sea level. If emissions co ntinue to increase at the present rate, average global temperatures ma y rise by 1.5-degrees-C by the year 2050, accompanied by a rise of abo ut 30 cm in sea level. However, the prediction of future climatic cond itions and sea level is hampered by the difficulty in modelling the in teractions between the lithosphere, kryosphere, biosphere and atmosphe re; in addition, the buffering capacity of our planet is still poorly understood. As scientists cannot offer unambiguous answers to simple q uestions, sorcerer's apprentices fill in the gaps, presenting plans to save planet without inconveniencing us. The geological record can hel p us to learn about the regulation mechanisms of our planet, many of w hich are connected with or expressed as sea level changes. Global chan ges in sea level are either tectono-eustatic or glacio-eustatic. Plate tectonic processes strongly control sea levels and climate in the lon g term. There is a strong feed-back mechanism between sea level and cl imate; both can influence and determine each other. Although high sea levels are a powerful climatic buffer, falling sea levels accelerate c limatic accentuation, the growth of the polar ice caps and will hence amplify the drop in sea level. Important sources of fossil greenhouse gases are botanic CO2 production, CO2 released by volcanic activity, a nd water vapour. The latter is particularly important when the surface area of the sea increases during a rise in sea level ('maritime green house effect'). A 'volcanogenic greenhouse effect' (release of volcano genic CO2) is possibly not equally important, as intense volcanic acti vity may take place both during icehouse episodes as well as during gr eenhouse episodes. The hydrosphere, land vegetation and carbonate plat forms are major CO2 buffers which may both take up and release CO2. CO 2 can be released from the ocean due to changes in the pCO2 caused by growth of coral reefs and by uptake of CO2-rich freshwater from karst provinces. Efficient sinks of CO2 are the weathering products of silic ate rocks; long-term sinks are organic deposits caused by regional ano xic events which preferrably develop during sea level rises and highst ands; and coal-bearing strata. Deposition of limestone also removes CO 2 from the atmospheric-hydrospheric cycle at a long term. Biotic crise s are often related to either sea-level lows or sea-level highs. Long- term sea-level lows, characteristic of glacial periods, indicate cooli ng as major cause of extinction. During verly long-lasting greenhouse episodes the sea level is very high, climate and circulation systems a re stable and biotic crises often develop as a consequence of oxygen d epletion. On land, niche-splitting, complex food web structures and ge neral overspecialization of biota will occur. Whether the crisis is ca used by a single anoxic event (e.g. in the Late Devonian) or a disturb ance by an asteroid impact (e.g. the Cretaceous/Tertiary boundary), it will only trigger total collapse of an ecosystem if a large part of i t was already in decline. The regulatory mechanisms and buffers are th ermodynamically extremely efficient if they are given sufficient time in which to deploy their power. However, after major catastrophes the re-establishment of successful ecosystems will take millions of years. The present rate of sea level and associated temperature rise is much too fast to be compensated and buffered by the network of natural con trols. It is likely that the transitional time towards a new steady st ate will be an extremely variable and chaotic episode of unpredictable duration.