ALBERT

Description: A prototype of a coupled climate-ice sheet model has been developed by the work package 1.1.3 "IskappeANT." The coupled system comprises the climate model EC-Earth and the Parallel Ice Sheet Model (PISM), representing Antarctica. Since the direct implementation of the involved processes, such as the implementation of ice shelf geometries, the ocean-ice shelf interaction, or the computation of the surface mass balance, would exceed the funding period of one year, we exploit state-of-the-art parameterizations. However, the robust system is open for enhancements in consecutive steps afterward and allows exploring scientific frontiers. The coupled system is one of the first state-of-the-art global climate models where the climate system interacts with the Antarctic ice sheet and its fringing ice shelves. This ambitious package includes these tasks: infrastructure to run the Parallel Ice Sheet Model (PISM) version 1.1.4 and version 1.2, setup and configuration of PISM to simulate Antarctica as a standalone model, coupling infrastructure, and first coupled simulations. This document describes the design decisions of the coupling. It presents the analysis of the preindustrial climate state in the Southern Ocean and across Antarctica. These states are subject to sufficiently large biases suggesting anomaly coupling between the climate model and the ice sheet model as an adequate coupling strategy.

Description: Deglacial transitions of the middle to late Pleistocene (terminations) are linked to gradual changes in insolation accompanied by abrupt shifts in ocean circulation. However, the reason these deglacial abrupt events are so special compared with their sub-glacial-maximum analogues, in particular with respect to the exaggerated warming observed across Antarctica, remains unclear. Here we show that an increase in the relative importance of salt versus temperature stratification in the glacial deep South Atlantic decreases the potential cooling effect of waters that may be upwelled in response to abrupt perturbations in ocean circulation, as compared with sub-glacial-maximum conditions. Using a comprehensive coupled atmosphere–ocean gen-eral circulation model, we then demonstrate that an increase in deep-ocean salinity stratification stabilizes relatively warm waters in the glacial deep ocean, which amplifies the high southern latitude surface ocean temperature response to an abrupt weakening of the Atlantic meridional overturning circulation during deglaciation. The mechanism can produce a doubling in the net rate of warming across Antarctica on a multicentennial timescale when starting from full glacial conditions (as compared with interglacial or subglacial conditions) and therefore helps to explain the large magnitude and rapidity of glacial termina-tions during the late Quaternary.

Description: Using transient climate forcing based on simulations from the Alfred Wegener Institute Earth System Model (AWI-ESM), we simulate the evolution of the Greenland Ice Sheet (GrIS) from the last interglacial (125 ka, kiloyear before present) to 2100 AD with the Parallel Ice Sheet Model (PISM). The impact of paleoclimate, especially Holocene climate, on the present and future evolution of the GrIS is explored. Our simulations of the past show close agreement with reconstructions with respect to the recent timing of the peaks in ice volume and the climate of Greenland. The maximum and minimum ice volume at around 18–17 ka and 6–5 ka lag the respective extremes in climate by several thousand years, implying that the ice volume response of the GrIS strongly lags climatic changes. Given that Greenland’s climate was getting colder from the Holocene Thermal Maximum (i.e., 8 ka) to the Pre-Industrial era, our simulation implies that the GrIS experienced growth from the mid-Holocene to the industrial era. Due to this background trend, the GrIS still gains mass until the second half of the 20th century, even though anthropogenic warming begins around 1850 AD. This is also in agreement with observational evidence showing mass loss of the GrIS does not begin earlier than the late 20th century. Our results highlight that the present evolution of the GrIS is not only controlled by the recent climate changes, but is also affected by paleoclimate, especially the relatively warm Holocene climate. We propose that the GrIS was not in equilibrium throughout the entire Holocene and that the slow response to Holocene climate needs to be represented in ice sheet simulations in order to predict ice mass loss, and therefore sea level rise, accurately.

Description: To assist researchers and modellers by reducing avoidable complexity, we developed the ESM-Tools software, which provides a standard way for downloading, configuring, compiling, running and monitoring different models on a variety of High Performance Computing (HPC) systems. It should be noted that ESM-Tools is not a coupling software itself, but a workflow and infrastructure management tool to provide access to increase usability of already existing components and coupled setups. As coupled ESMs are technically the more challenging tasks, we will focus on coupled setups, always implying that stand-alone models can benefit in the same way.

Description: The Eocene-Oligocene Transition (EOT; ~34.4–33.7 Ma) was a major shift in Earth’s long-term climatic evolution, marking the cooling from the early Paleogene greenhouse to the icehouse regime that has prevailed from the Oligocene until today. However, it remains uncertain which landmasses were already covered by ice sheets during the Early Oligocene Glacial Maximum (EOGM; ~33.7–33.2 Ma), an interval of peak glaciation immediately following the EOT. The scarcity of earliest Oligocene climate records in both Arctic and Antarctic regions hitherto prevented the reconstruction of environmental conditions and ice-sheet extent during the EOGM. Such constraints, however, are critical for assessing ice–ocean–atmosphere interactions during the early stages of the Cenozoic icehouse. Here, we present the first shallow-marine drill-core record of earliest Oligocene environmental conditions in West Antarctica’s Pacific sector. It comprises marine mudstones documenting the presence of a cool-temperate Nothofagus-dominated forest situated within a marine archipelago at 73.5°S palaeolatitude. Any evidence for marine-terminating glaciers is lacking, thus no land-based ice or only small ice caps existed in West Antarctica during the EOGM. Our new EOGM temperature and topographical constraints allow for more reliable verification of a fully-coupled Earth System Model. It simulates a large East Antarctic ice sheet already during the EOGM. However, West Antarctica does not glaciate until ~26 Ma, thereby illustrating the significance of asymmetric Antarctic ice sheet response during initial Cenozoic glaciation and highlighting the importance of differential regional response for future cryospheric change.

Description: The Eocene-Oligocene Transition (EOT; ~34.4–33.7 Ma) was a major shift in Earth’s long-term climatic evolution, marking the cooling from the early Paleogene greenhouse to the icehouse regime that has prevailed from the Oligocene until today. However, it remains uncertain which landmasses were already covered by ice sheets during the Early Oligocene Glacial Maximum (EOGM; ~33.7–33.2 Ma), an interval of peak glaciation immediately following the EOT. The scarcity of earliest Oligocene climate records in both Arctic and Antarctic regions hitherto prevented the reconstruction of environmental conditions and ice-sheet extent during the EOGM. Such constraints, however, are critical for assessing ice–ocean–atmosphere interactions during the early stages of the Cenozoic icehouse. Here, we present the first shallow-marine drill-core record of earliest Oligocene environmental conditions in West Antarctica’s Pacific sector. It comprises marine mudstones documenting the presence of a cool-temperate Nothofagus-dominated forest situated within a marine archipelago at 73.5°S palaeolatitude. Any evidence for marine-terminating glaciers is lacking, thus no land-based ice or only small ice caps existed in West Antarctica during the EOGM. Our new EOGM temperature and topographical constraints allow for more reliable verification of a fully-coupled Earth System Model. It simulates a large East Antarctic ice sheet already during the EOGM. However, West Antarctica does not glaciate until ~26 Ma, thereby illustrating the significance of asymmetric Antarctic ice sheet response during initial Cenozoic glaciation and highlighting the importance of differential regional response for future cryospheric change.

Description: We developed a new version of the Alfred Wegener Institute Climate Model (AWI-CM3), which has higher skills in representing the observed climatology and better computational efficiency than its predecessors. Its ocean component FESOM2 has the multi-resolution functionality typical for unstructured-mesh models while still featuring a scalability and efficiency similar to regular-grid models. The atmospheric component OpenIFS (CY43R3) enables the use of latest developments in the numerical weather prediction community in climate sciences. In this paper we describe the coupling of the model components and evaluate the model performance on a variable resolution (25–125 km) ocean mesh and a 61 km atmosphere grid, which serves as a reference and starting point for other on-going research activities with AWI-CM3. This includes the exploration of high and variable resolution, the development of a full Earth System Model as well as the creation of a new sea ice prediction system. At this early development stage and with the given coarse to medium resolutions, the model already features above CMIP6-average skills in representing the climatology and competitive model throughput. Finally we identify remaining biases and suggest further improvements to be made to the model.

Description: Transient simulations of the global fully coupled climate model COSMOS under realistic varying orbital and greenhouse gas forcings are systematically compared to diatom oxygen isotope (δ18O_diatom ) records from Russian lakes with focus on Eurasian Holocene climate trends. The measured δ18O_diatom decrease and other temperature proxies are interpreted as large‐scale cooling throughout the Holocene while the model simulations are biased too warm, likely through missing radiative forcings. This large‐scale warm bias also dictates the modeled δ18O_precipitation. Hence, at locations where the signs of model and proxy temperature/precipitation trends agree, measured δ18O_diatom and modeled δ18O_precipitation trends show notable accordance. An increased temporal variability of modeled δ18O_precipitation is linked to persistent atmospheric circulation patterns. Applying the transient forcings in an accelerated way (every 10th year only) yields a similar, yet weaker or delayed model response, especially in the ocean.