Лёд и Снег • 2012 • № 2 (118)
Ледники и ледниковые покровы
УДК 551.324.82
The integrity of the ice record of greenhouse gases with a special focus on atmospheric CO2
© 2012 г. Dominique Raynaud
Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS/UJF Saint-Martin-d'Hères, France
raynaud@lgge.obs.ujf-grenoble.fr
Принята к печати 30 января 2012 г.
Антарктида, диоксид углерода, ледниковая скважина, ледяной керн, метан, станция Восток.
Antarctica, carbon dioxide, glacier borehole, ice core, methane, Vostok station.
Over the last 25 years, the ice core record has provided a unique and precious archive of past changes in three important greenhouse gases: carbon dioxide CO2, methane CH4 and nitrous oxide N2O. Recovering the Vostok ice core has played a major role, being the first ice record showing the variations of CO2 and CH4 during a full glacial-interglacial cycle, and a few years later being extended to three more cycles. This information, by revealing the tight coupling between climate and carbon cycle during the last glacial-interglacial cycles, has become a benchmark against which climate and carbon cycle models can be tested. The purpose of the present work is to discuss the degree of integrity of the ice core record of greenhouse gases and to assess to which degree it provides an accurate reconstruction of the past atmospheric changes. The various processes potentially affecting the integrity of the record are discussed. They include the interactions of trace gases with precipitation or firn grains, the effect of summer-melting at the surface of the ice sheet, the diffusion and the gravitational setting of gases in the open spaces of the firn, the physical, chemical and biological interactions between the air trapped and the ice matrix, the role of the transformation of air bubbles into air hydrates with depth in the ice column. Providing to select an appropriate sampling site, to take specific precautions during storage and transportation of the ice cores, and to select ice of good quality, the ice core record of initial atmospheric gases is hardly affected by the processes listed above. Such conclusion is strongly supported by the remarkable agreement of global signals like CO2 or CH4 measured in different cores taken at different locations. Finally, I bring back here the history of how the ice core record of atmospheric CO2 has been obtained, from the pioneering times to today, and summarize the main conclusions reached in terms of climate - carbon cycle interactions.
1. Introduction
Concerning the present and future evolution of the climate, the lesson of past changes, from the multi-decadal to the orbital time scales, can be essential. Indeed, we need to disentangle in the current changes of the climate, the part due to «natural» forcing from the one due to anthropogenic activities. Also, by recording accurately the past we get precious information related to the mechanisms of our climate machine. This information is needed for improving the hierarchy of climatic models used at the international level to simulate the future climate. It is not by chance that the huge assessment effort made by the International Panel for Climatic Change (IPCC) henceforth devotes a full chapter to paleoclimatology [19].
Paleo-archives are found in various environments: marine sediments, corals, lake sediments, soils, trees, fossils of fauna and vegetation, carbonate concretions and ice cores. All have kept climatic imprints of the past. In
this rich context of indicators, providing past climatic information from the different oceanic basins and continents both in the Northern and in Southern hemispheres we may question the interest to look at ice cores situated in the very remote Polar Regions. In fact, the possibility to observe the evolution of climatic or atmospheric properties, like temperature or dust, with good resolution, even year by year over the last 10000 years in some Greenland sites, is unequalled and climatic changes are expected to be amplified at high latitudes. Also, the archives preserved in Antarctic or Greenland ice provide a wealth of other information (Fig. 1) due to the ability of the ice to trap the past atmosphere and to its very high-purity level allowing to keep traces of tiny fallouts from continental, oceanic, volcanic, extra-terrestrial, or anthropogenic origin. Unique, is the capacity of the ice to continuously sample over time a parcel of atmospheric air. This occurs during the transformation of the snow accumulat-
Fig. 1. The wealth of information provided by ice cores about the evolution of the Earth's atmosphere and climate. The snow falling down at the surface of the ice sheet washes out the atmosphere and samples dust and aerosols. The isotopic composition (D/H or 18O/16O) of the snow H2O molecules reflects the temperature of the snow formation and then provides precious indications about the past evolution of the climate at the surface of the ice sheets. The analyses of the air entrapped as bubbles in ice have led over the last 40 years to a set of discoveries. Part of the information is of regional or hemispheric scale, some like CO2, or CH4 are even of global significance. The figure shows a picture of a thin layer of ice under polarized light. The ice crystals appear with different colours, depending on their orientations and the black spots are air bubbles. The bubbles at trapping time occupy roughly 10% of the total volume Рис. 1. Богатство информации об эволюции земной атмосферы и климата, получаемое из ледяных кернов. Снег, выпадающий на поверхность ледникового щита, вымывает из атмосферы и включает в себя пыль и аэрозоли. Изотопное содержание (D/H или 18O/16O) молекул воды в снеге отражает температуру воздуха в момент формирования снежных кристаллов и таким образом даёт драгоценную информацию о прошлой эволюции климата на поверхности ледниковых покровов. Анализ включённого в пузырьки воздуха приносит всё новые открытия на протяжении уже 40 лет. Часть информации отражает масштаб регионов или отдельного полушария, но сведения о CO2 или CH4 имеют глобальное значение. Рисунок показывает тонкий шлиф льда в поляризованном свете. Кристаллы льда имеют различные радужные цвета в зависимости от ориентации их оптических осей, а чёрные точки представляют собой пузырьки воздуха. Захваченные пузырьки занимают около 10% общего объёма льда
ed at the surface of the ice sheet into airtight bubbly ice, usually in the first 100 meters or so below the surface. Thus, the ice records of trapped air are unique and precious archives of our atmosphere. Nevertheless, because of the important implications of the paleo-record of greenhouse gases in terms of climate sensitivity to their changes and of understanding their cycles in the absence of anthropogenic perturbation or under different climatic conditions, it is essential to understand how close is the composition of the air extracted from the ice in the laboratory to the atmospheric composition prevailing at the time the air is trapped in ice.
In 1993, Raynaud et al. [30] discussed the reliability of the ice record of greenhouse gases. Since then, more ice core data have been obtained and more process studies have been carried out. The aim of this paper is to provide an updated and wider assessment of the various processes, which can alter the initial atmospheric composition, and to address the uncertainties limiting the climatic interpretation of the ice core record of greenhouse gases. I also will summarize some of the main lessons we got in terms of climate and carbon cycle from looking at the past record of CO2.
2. Processes involved in the trapping of air by ice and sources of uncertainties in reconstructing the original atmospheric signal
As illustrated in Fig. 2, various physical, chemical and biological processes can take place during air trapping in the snow and firn layers and later, in situ in ice. How far can they deviate the gas composition of the air enclosed in ice from its initial atmospheric composition? How accurately can we reconstruct the changes in atmospheric CO2 and other greenhouse trace gases from their ice core records?
2.1. Snow and its transformation into ice. Freshly fallen snow at the surface of the ice sheet can incorporate a small amount of air enriched in CO2 content [38]. If the air has been enclosed in the snowflakes as micro-bubbles, most of them should be lost during recristallization in the upper layers near the surface [32]. Also, ice formed at or near the surface of the ice sheet by refreezing of summer melting layers may enclose air whose initial atmospheric composition has been changed due to the different solubility coefficients of its various gaseous components (especially for the most soluble components like CO2 and N2O, which become enriched) or due to fractionation processes occurring during refreezing. After deposition, the snow grains
are progressively overlaid by new precipitations and the polar ice results from the densification of the snow deposited at the surface. The transformation of snow into ice (this stage is called firn, Fig. 3), which generally occurs in the first 50 to 120 meters, takes from decades to millennia, depending on temperature and accumulation rate. During the first stage of densification, rearrangement of the snow grains occurs until the closest dense packing stage is reached at relative densities of about 0.55-0.6, which corresponds to the snow-firn transition; the pores between the grains are still communicating with the atmosphere and convection can takes place under the influence of the wind. Then plastic deformation becomes the dominant process and the pores progressively become isolated from the atmosphere. The end product of this huge - probably the worldwide largest - natural sintering experiment is ice, an airtight material, which encloses air bubbles.
As illustrated in Fig. 3, the air is well mixed by convection in the upper layers
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