Modeling the role of the last ice age for the present and future sea-level contribution from Antarctica

“模拟南极对当前和未来海平面贡献的最后一个冰期的作用”项目将包括使用平行冰盖模式（PISM）对南极冰盖-陆架系统的至少四个冰期循环进行数值模拟。这项研究计划的目的是确定南极冰盖过去的发展对过去和未来一个世纪的海平面贡献的作用。为此，我们研究了过去的气候演变，特别是末次冰消期对其当前动态状态的影响：在不考虑人为气候变化的情况下，20世纪和21世纪在物质平衡方面的持续变化受到末次冰旋回期间冰的进退历史的影响。我们的目标不是进行最佳猜测模拟，而是提供一系列模型模拟，其中包括气候边界条件、内部过程建模和冰参数选择的不确定性。除了回答上述关于冰盖历史对其当前动态的影响的研究问题外，我们还将朝着对未来南极冰流量的新一代预测迈出重要的一步：重要的是要知道有多少海平面贡献（如果有的话）不是由人为气候变化造成的。该计划包括对模型进行物理改进的开发和短期测试，以便进行四个冰期旋回（4GC）模拟并提供一个全面的集合。目前实施的气候边界条件，包括冰盖的上表面和与海洋接触的冰架的下表面，将被检查和扩展，以适合4gc模拟。基于过程的模式组件，关于冰原和冰架之间过渡带的数值表示将被评估和改进。将为PISM开发高分辨率嵌套模拟方法，以便更好地解决这些关键区域，以便进一步缩小有限差分模型（如使用应力平衡的浅近似的PISM）与高阶模型之间的差距。4gc模拟中的敏感性测试将揭示上述新方法、一般气候边界条件和内部模式参数如何影响4gc模拟以及最终模拟的当前状态。将进行一个整体选择过程，排除那些与过去的现有地质数据和对南极冰盖现状的观测不相符的参数和气候边界组合。这可以被认为是对现今冰盖动态状态的“盲目选择”。所谓“动态状态”，我们指的是模拟冰盖对外部强迫的响应，在一组垂直和水平冰范围非常相似的模拟冰盖之间，这种响应可能会发生巨大变化。因此，我们将为冰盖历史造成的南极在20世纪和21世纪对海平面的贡献提供一个不确定范围。

The project “Modeling the role of the last ice age for the present and future sea-level contribution from Antarctica” will encompass the numerical simulation of at least four glacial cycles of the Antarctic sheet-shelf system using the Parallel Ice Sheet Model (PISM). The objective of the research plan is to determine the role of the past development of the Antarctic Ice Sheet for its sea-level contribution of the past and future century. To this end we investigate the influence of past climate evolution, especially the last deglaciation, on its present dynamic state: The ongoing changes in terms of mass balance, disregarding anthropogenic climate change, during the 20th and 21st centuries are influenced by the history of the advance and retreat of the ice during the last glacial cycles. Instead of aiming at a best-guess simulation, we will work on providing an ensemble of model simulations that incorporates uncertainties from climate boundary conditions and internal process-modeling and ice parameter choices. Apart from answering the above mentioned research question concerning the influence of the history of the ice sheet on its present day dynamics, we will also take an important step towards a new generation of projections of future ice discharge from Antarctica: It is important to know how much sea-level contribution, if any, is not caused by anthropogenic climate change.The program encompasses the development and short-term testing of physical improvements to the model that are needed in order to perform four glacial cycles (4GC) simulations and to provide a comprehensive ensemble. The currently implemented climate boundary conditions, both for the upper surface of the ice sheet and the underside of the ice shelves in contact with the ocean, will be examined and expanded to be suitable for 4GC-simulations. Process-based model components, concerning the numerical representation of the transition zone between ice sheet and ice shelf will be evaluated and improved. High-resolution nested simulation approaches will be developed for PISM in order to better resolve these crucial zones in order the further close the gap between finite differences models like PISM using shallow approximations of the stress balance and higher-order models. Sensitivity tests within 4GC-simulations will shed light on how the above mentioned new methods, climate boundary conditions in general and internal model parameters, influence the 4GC-simulation and ultimately the modeled present day state. An ensemble selection process will take place, excluding those parameter and climate-boundary combinations that are not conform to available geologic data for the past and observations of the present day state of the Antarctic ice sheet. This can be thought of as a “blind selection” of the dynamic present-day state of the ice sheet. By that “dynamic state” we mean the responsiveness of the modeled ice sheet to external forcing, which can vary drastically among a set of modeled ice sheets that are quite similar with respect to vertical and horizontal ice extent.We will therefore provide an uncertainty range for Antarctica´s sea-level contribution in the 20th and 21th century caused by the ice-sheets history.