Description: A new global climate model setup using FESOM2.0 for the sea ice‐ocean component and ECHAM6.3 for the atmosphere and land surface has been developed. Replacing FESOM1.4 by FESOM2.0 promises a higher efficiency of the new climate setup compared to its predecessor. The new setup allows for long‐term climate integrations using a locally eddy‐resolving ocean. Here it is evaluated in terms of (1) the mean state and long‐term drift under preindustrial climate conditions, (2) the fidelity in simulating the historical warming, and (3) differences between coarse and eddy‐resolving ocean configurations. The results show that the realism of the new climate setup is overall within the range of existing models. In terms of oceanic temperatures, the historical warming signal is of smaller amplitude than the model drift in case of a relatively short spin‐up. However, it is argued that the strategy of “de‐drifting” climate runs after the short spin‐up, proposed by the HighResMIP protocol, allows one to isolate the warming signal. Moreover, the eddy‐permitting/resolving ocean setup shows notable improvements regarding the simulation of oceanic surface temperatures, in particular in the Southern Ocean.

Description: There is an increasing need to understand the pre-Quaternary warm climates, how climate–vegetation interactions functioned in the past, and how we can use this information to understand the present. Here we report vegetation modelling results for the Late Miocene (11–7 Ma) to study the mechanisms of vegetation dynamics and the role of different forcing factors that influence the spatial patterns of vegetation coverage. One of the key uncertainties is the atmospheric concentration of CO2 during past climates. Estimates for the last 20 million years range from 280 to 500 ppm. We simulated Late Miocene vegetation using two plausible CO2 concentrations, 280 ppm CO2 and 450 ppm CO2, with a dynamic global vegetation model (LPJ-GUESS) driven by climate input from a coupled AOGCM (Atmosphere-Ocean General Circulation Model). The simulated vegetation was compared to existing plant fossil data for the whole Northern Hemisphere. For the comparison we developed a novel approach that uses information of the relative dominance of different plant functional types (PFTs) in the palaeobotanical data to provide a quantitative estimate of the agreement between the simulated and reconstructed vegetation. Based on this quantitative assessment we find that pre-industrial CO2 levels are largely consistent with the presence of seasonal temperate forests in Europe (suggested by fossil data) and open vegetation in North America (suggested by multiple lines of evidence). This suggests that during the Late Miocene the CO2 levels have been relatively low, or that other factors that are not included in the models maintained the seasonal temperate forests and open vegetation.

Description: The Middle Miocene (~17‒14 Myrs ago) climate is characterized by much warmer temperatures and an amplified hydrological cycle with respect to present-day climate. Though it is well known that a change in the global freshwater distribution via the hydrological cycle can impact climate and large scale ocean circulation, most modelling studies encompassing tectonic time scales rather focus on the sensitivity of the climate to orography, ocean gateways, land surface, ice-sheets, and CO2 changes. Alternatively, here we study the effect of two different kinds of hydrological discharge models and their respective boundary conditions, that are fully implemented in the earth system model COSMOS, on the climate of the Middle Miocene. The standard hydrological discharge model (HD) of COSMOS requires the information of high spatial resolution orography for the Middle Miocene that is conventionally tuned to present-day conditions, to calculate river routing following the steepest slope of the terrain. Instead, the flexible hydrological discharge model (FHD) calculates river routing by using Middle Miocene orography at identical grid resolution as the atmosphere model and additionally taking the dynamic topography of continental water levels into account. We find that the anomaly between a climate simulation of the Middle Miocene with COSMOS and HD versus a comparable simulation based on COSMOS and FHD reveals strong differences in the redistribution of freshwater in form of continental discharge from land to the ocean. As a consequence, deep water formation and large scale ocean circulation significantly differ between both model versions, emphasizing the importance of representing a realistic freshwater redistribution from land towards the ocean. We therefore conclude that a more realistic representation of climate states at tectonic time scales necessitates geological constraints on the freshwater redistribution, and changes in the freshwater redistribution may have had a profound impact on the history of global ocean circulation patterns over geological time scales.

Description: Central Asia is one of the largest arid regions in the world, however, multiple lakes have existed here since the Neogene. These lakes were able to sustain themselves despite the aridification trend in Asia through the PlioPleistocene. For example, long-term geological multiproxy records, including carbonate δ18O, from lake sediments of the Qaidam, Gaxun Nur, and Orog Nuur Basins indicate multiple changes in the hydrological cycle of the region with alternate phases of prevailing evaporation and precipitation. These changes are attributed either to Neogene global climate change or regional tectonic events. In this study, we use the isotope-equipped atmospheric general circulation model ECHAM5-wiso for modeling of Asia climate evolution and associated changes in precipitation δ18O during key periods of the Neogene. High-resolution simulations (T159L31, ca. 0.8°x0.8° and 31 vertical levels, 6 hour output frequency) with Mid-Holocene, Pleistocene, Pliocene and Miocene boundary conditions allow us to estimate the contributions of global climate change into the hydrological budget over the Central Asia. We complement this work with a Lagrangian Trajectory analysis (wind back-trajectories) applied to the ECHAM5-wiso outputs to trace changes in the origin of precipitation-producing air masses. We show that in addition to precipitation amount variations associated with changes in large-scale atmosphere dynamics, considerable changes in moisture sources between the time slices considered contribute to the isotopic signature of precipitation within the Qaidam, Gaxun Nur, and Orog Nuur Basins. Finally, comparison of simulated δ18O results to wind backtrajectory analysis suggests that local process, such as moisture recycling, exert an increasing control for more recent time periods.

Description: Understanding the dynamics of warm climate states has gained increasing importance in the face of anthropogenic climate change. During the Last Interglacial (LIG, ∼128 to 116 ka), greenhouse gas concentrations and high latitude insolation were higher than pre-industrial levels, causing a high-latitude warming (Turney and Jones, 2010; Pfeiffer and Lohmann, 2016). We present a suite of climate model results (COSMOS, MPI-ESM, AWI-CM, EC-Earth) to evaluate the patterns and compare the simulations with the above-mentioned surface temperature reconstructions, seasonal archives (Felis et al., 2015; Brocas et al., 2017), and sea ice reconstructions (Stein et al., 2017). As a result of this modestly warmer climate, polar ice sheets were smaller and estimates report that the global mean sea level was 6-9 meters higher than today (Dutton et al., 2015). The sensitivity of the Antarctic Ice sheet is related to the local temperature around the West Antarctic Ice Sheet (WAIS) (Sutter et al., 2016). Our ice sheet model experiments indicate that a 2-3°C local warming causes already a partially collapsed, irreversible WAIS. A pronounced subsurface oceanic warming can destabilize the WAIS, resulting in an oceanic gateway between the Ross and Weddell Seas. A sensitivity study using the new oceanic gateway between the Atlantic and Pacific Oceans as a bathymetrical boundary condition indicates that this region would be covered by sea ice. Mixing due to sea-ice formation prevents a pronounced warming around the WAIS and would stabilize the WAIS. Thus, the disintegration of the WAIS is probably related to non-local influences like in Hellmer et al. (2017) where the shelves of West Antarctica are warmed from below by Circumpolar Deep Water.

Description: The denudation history of active orogens is often interpreted in the context of modern climate gradients. Here we address the validity of this approach and ask what are the spatial and temporal variations in palaeoclimate for a latitudinally diverse range of active orogens? We do this using high-resolution (T159, ca. 80 × 80 km at the Equator) palaeoclimate simulations from the ECHAM5 global atmospheric general circulation model and a statistical cluster analysis of climate over different orogens (Andes, Himalayas, SE Alaska, Pacific NW USA). Time periods and boundary conditions considered include the Pliocene (PLIO, ∼3Ma), the Last Glacial Maximum (LGM, ∼21ka), mid-Holocene (MH, ∼6ka), and pre-industrial (PI, reference year 1850). The regional simulated climates of each orogen are described by means of cluster analyses based on the variability in precipitation, 2 m air temperature, the intra-annual amplitude of these values, and monsoonal wind speeds where appropriate. Results indicate the largest differences in the PI climate existed for the LGM and PLIO climates in the form of widespread cooling and reduced precipitation in the LGM and warming and enhanced precipitation during the PLIO. The LGM climate shows the largest deviation in annual precipitation from the PI climate and shows enhanced precipitation in the temperate Andes and coastal regions for both SE Alaska and the US Pacific Northwest. Furthermore, LGM precipitation is reduced in the western Himalayas and enhanced in the eastern Himalayas, resulting in a shift of the wettest regional climates eastward along the orogen. The cluster-analysis results also suggest more climatic variability across latitudes east of the Andes in the PLIO climate than in other time slice experiments conducted here. Taken together, these results highlight significant changes in late Cenozoic regional climatology over the last ∼3Myr. Comparison of simulated climate with proxy-based reconstructions for the MH and LGM reveal satisfactory to good performance of the model in reproducing precipitation changes, although in some cases discrepancies between neighbouring proxy observations highlight contradictions between proxy observations themselves. Finally, we document regions where the largest magnitudes of late Cenozoic changes in precipitation and temperature occur and offer the highest potential for future observational studies that quantify the impact of climate change on denudation and weathering rates.

Description: While current data on the state of the climate system is very detailed and vast, the understanding of a climate in equilibrium is hampered by the anthropogenic imprint on current climate conditions. Furthermore, preparation of future climate scenarios with numerical models - that by definition represent an approximation of reality, that have been created with the current climate in mind, and that hence cannot be granted to be equally suitable for applications in the framework of other, e.g. warmer, climate states - is our only means to understand the climatic conditions that humankind may have to adapt to in the future. Yet, in order to gain trust in our models also for the future, we can employ them to study past climate states, for which climate-model-independent information is available. Furthermore, Paleoclimatology, and in particular the marriage of a) climate models and b) reconstructions of past climate states, that are based on geologic and glaciologic archives, as it is done in the section Paleoclimate Dynamics at the Alfred Wegener Institute, provides a method of understanding both the mechanisms behind, and the large scale patterns of, warmer than modern climates - climates, that may reoccur in the not too distant future due to the alteration of the natural (background) climate forcing by humankind. This talk will highlight some aspects of these topics. I will start by giving an overview on some methods of studying modern climate as it is done at the Alfred Wegener Institute. An introduction on the relations between weather and climate follows, based on which I will illustrate how climate models are being developed. It is outlined how climate models may be employed to understand the evolution of current and future climate. Based on this information, I illustrate how proxy data and models may be combined in order to study the climate of the past. In this endeavor, the climate of the Pliocene is taken as an example of a past, warmer-than-present, climate state. Rationale for focusing on the Pliocene in the context of future climate is that the time period has been suggested as a potential analogue for future climate. This suggestion is motivated by similarities of land-sea-distribution, orography, and climate forcing between the Pliocene and our modern world.

Description: Eight general circulation models have simulated the mid-Pliocene warm period (mid-Pliocene, 3.264 to 3.025 Ma) as part of the Pliocene Modelling Intercomparison Project (PlioMIP). Here, we analyse and compare their simulation of Arctic sea ice for both the pre-industrial period and the mid-Pliocene. Mid-Pliocene sea ice thickness and extent is reduced, and the model spread of extent is more than twice the pre-industrial spread in some summer months. Half of the PlioMIP models simulate ice-free conditions in the mid-Pliocene. This spread amongst the ensemble is in line with the uncertainties amongst proxy reconstructions for mid-Pliocene sea ice extent. Correlations between mid-Pliocene Arctic temperatures and sea ice extents are almost twice as strong as the equivalent correlations for the pre-industrial simulations. The need for more comprehensive sea ice proxy data is highlighted, in order to better compare model performances.