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    (in TWTW Sep 18, 2010)

    Guest Editorial by Dr. Harrison "Jack" Schmitt


    Sep 18, 2010

    Analysis of ice cores from Antarctica [1] and Greenland [2] play an important role in understanding the history of global temperatures and atmospheric concentrations of carbon dioxide, methane, and other gases and aerosols. Through analysis of dust, they also provide up to 800,000-year chronologies of global scale volcanic eruptions and major trends toward desertification. Clearly, data from ice cores play a critical underlying role in the science of climate change.

    Unfortunately, ice cores do not always appear to be a reliable record of past carbon dioxide or methane concentrations in the atmosphere. Their information needs to be confirmed by consistency with data from other sources. Particular care must be taken in the interpretation of the carbon dioxide record in ice cores due to uncertainties in the mechanics of gas preservation over time.[4]

    In some cases, the trapped atmosphere in the ice sheets may not be part of a closed system. To be a closed system for carbon dioxide or methane, no gas components can escape or be added during the burial process; liquid water cannot have interacted with the gases; none of the trapped gas components can combine, separate, diffuse, or solidify; and all components must stay in the same proportions as pressure increases with time due to added ice above. The observational science of ice has demonstrated that for some cores all these conditions do not hold. Further, the process of core extraction from great depth to surface pressure may open and disturb the gas systems.

    For example, the Siple Antarctic ice core indicates that carbon dioxide reached a level of about 330ppm in about 1900. Comparison with the 1960 initial Mauna Loa measurement of 260ppm suggests that either (1) the Siple data is just wrong, or (2) there was a drop of about 60ppm in carbon dioxide level between 1900 and 1960, or (3) it takes 80-some years for the carbon dioxide gas system to close.[4] This discrepancy does not appear to have been resolved;[5] but the smooth shape of the Siple core carbon dioxide curve as a function of core depth (approaching a constant level with increasing core depth/age) suggests it might not ever have been a closed system. Over time, carbon dioxide in the sampled Siple ice may have gradually equilibrated to a constant carbon dioxide value of about 280ppm now indicated for the 1720-year old and older layers. Also, this core suffered some melting during transport and prior to analysis.[6]

    Not surprisingly, considering the known variability in ice preservation, measured carbon dioxide concentrations in the trapped gases of many cores older than about 300 years hold remarkably constant over the last 7-8000 years of ice accumulation.[7] This constancy is incompatible with other data, including that from other ice cores and from preserved Ginkgo leaf stomata, both indicating significant variation during that period. Stomata are pores through which a plant takes in carbon dioxide. They vary in size depending on the carbon dioxide concentration in the air, and preserved stomata suggest that carbon dioxide levels ranged between 270 and 326ppm over the last 7-8000 years.[8]

    Some Greenland ice cores do not show expected temperature-driven carbon dioxide increases during the Medieval Warm Period (~800-1300) or the expected decreases during the Little Ice Age (~1400-1900)[9], although these events show clearly in other cores[10]. This further indicates that some ice cores potentially give an unreliable history of atmospheric carbon dioxide, nitrogen, and methane concentrations.

    Analyses from the EPICA Dome C and Vostok cores of the Antarctic ice sheets, on the other hand, show plausible parameter variations. A strong correlation exists back to ~800,000 years ago between carbon dioxide and methane concentrations and deuterium and oxygen isotopic temperature determinations.[11] The five hundred year time resolution of these correlations, however, remains insufficient to determine if carbon dioxide and methane changes lead or lag temperature changes. Similarly, up to 123,000 years of climate temperature variations measured in three deep cores from the Greenland ice sheet (GRIP, GISP2, and NGRIP) appear to be consistent with other climate proxy data, such as North Atlantic sediment cores.[12]

    Although carbon dioxide measurements can be suspect in some ice cores, data from many others constitute extremely valuable records of additional parameters that exist within truly closed subsystems. For example, Greenland ice core data indicate that large climatic temperature shifts can occur over a very few years. Oxygen isotopes, deuterium, dust and calcium, sodium, and ice accumulation rates support data from cave deposits that indicate rapid cooling often follows periods of gradual natural warming.[13]

    A particularly prolonged warm period between 9000 and 6000 years ago, within the current interglacial, has been documented, most recently in oxygen isotopic analyses of Greenland ice cores.[14] That prolonged warm period resulted in significant thinning of Greenland's ice sheet to thicknesses within a 100m of those of today. Several other warm periods have occurred since, the most pronounced of which has been termed the Medieval Warm Period (500-1300)[15]. Warm periods of this nature were initially highly beneficial to fledgling human cultures. During the latter centuries of the Medieval Warm Period, however, severe weather and drought, overpopulation relative to available agricultural technology, and other factors forced migrations from many centers of civilization,[16] primarily to locations with more reliable water resources and better defensive positioning.

    Adverse effects of warming, however, stand in contrast to the general advancement of human civilization during the 10,000 years of warming since the last Ice Age. On the other hand, adaptation to the stresses of climate change, including cold periods, probably was a major factor in the evolution of modern humans.[17] The last Ice Age also permitted the advantageous migrations of modern humans from Asia into the Americas about 22,000 years ago. At that time, low sea levels created a land bridge between Asia and North America.[18] Adaptability has been the key for human survival and advancement.

    References can be found in the TWTW document for this Science Editorial.

    View The Week That Was in which this editorial appeared.

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