US Global Ice Core Research Program
West Antarctica and Beyond

2. Global Environmental Change

Our natural environment has changed on a global scale in the past and will change in the future. Such environmental changes may negatively affect many areas of the world. Changes in the near future may result both from natural climate variability (which over the last few million years has been responsible for a sequence of cold and warm periods of variable intensity (glacials/interglacials; stadials/interstadials)) and from man-caused changes in atmospheric composition (especially of greenhouse gases like carbon dioxide (C02), methane (CH4) and nitrous oxide (N20)) and vegetation which may trigger climate changes of similar magnitude. If the adverse effects of such changes are to be reduced by long-range planning and policy, then the large-scale environmental changes that may be expected over the next several decades to centuries need to be predicted with considerably more accuracy and detail than is possible today. We therefore need to understand the complex interactions between insolation, atmosphere, ocean, biosphere, and cryosphere that determine climate.

Important new information on the dynamics of major climatic change has been obtained over the past years from ice cores and deep sea sediment cores. It is now well established that major changes in global climate occurred almost simultaneously in both hemispheres. Such changes were accompanied by large changes in the concentration of the radiatively active gases C02 and CH4 in the atmosphere. The concentration Of C02 in air trapped in ice from the last glacial maximum was 30% lower than in Holocene ice, while CH4 was 50% lower. During periods of maximum glaciation, more vigorous atmospheric circulation is indicated by deuterium excess and by increased concentrations of dust and ions in the ice.

Deep sea sediments record the major changes in global ice volume in the oxygen isotopic composition of benthic foraminifera. These sediments also show evidence of changes in ocean circulation and nutrient distribution. Somewhat more detailed information on global ice volume changes during the last 160,000 years is available from cores drilled into Holocene and Pleistocene coral reefs.

The new data, however, also bring out the limitations of our current understanding of global climate change.

Deep sea sediment records as well as the Vostok ice core record show clear periodicities at 20 ka, 40 ka, and 100 ka which correspond to those of variations in the earth's orbital parameters and provide support for the Milankovitch theory that variations in insolation caused by orbital changes are responsible for the glacial-interglacial/stadial-interstadial climate changes. Enigmatically, insolation forcing is out of phase between the two hemispheres while climate change is not. Moreover, most of the climatic variability over the last million years occurs with a 100,000 year period, but most of the insolation variability occurs with 20,000 and 40,000 year periods.

The role of greenhouse gases like C02 and CH4 in providing a positive feedback in climate change and synchronizing the response of both hemispheres has been widely discussed. Yet C02 and CH4 increase in phase with increasing temperature but show a different phase response to cooling; CH4 follows temperature directly (at the resolution of available data), but C02 lags. The exact phase relationship between temperature change and C02/CH4 change as well as the degree of amplification of temperature changes by positive feedbacks, e.g. from greenhouse gases and albedo, is still unknown. More precise data on past changes in temperature and atmospheric composition are needed.

Several ocean models show how increased productivity in high latitude southern oceans could have produced the observed C02 lowering during glacial times. Yet the Greenland Dye-3 core shows several episodes where climate and C02 concentration seem to have changed by more than half of the full glacial-interglacial difference over a very short time interval (Dansgaard/Oeschger events). If real (and not artifacts of ice deformation near the bed) such rapid changes cannot be produced by current ocean uptake models. Further questions arise from the fact that the rapid changes found in Greenland cores seem to be absent in the Antarctic ice, and from the fact that the increase in 813C in deep sea sediment predicted by the C02 uptake models has not been observed.

Carefully collected data from ice cores from both the Arctic and Antarctic and from lower latitudes as well as from strategically located ocean sediment cores and terrestrial records are needed to help answer these questions and to elucidate the ocean-atmosphere-cryosphere-biosphere interactions and to reconstruct global environmental change due to natural causes on time scales of decades to 105 years. This knowledge will improve the estimate of current man-made climate change and allow more accurate prediction of changes to be expected over the next decades to centuries.

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