Assessment of Hydrologic Alterations in Elbe and Rhine Rivers, Germany

In light of recent anthropogenic-induced climate change, a burning question at present is how these changes influence the water regime of rivers, which are of vital importance for humans as well as for biota. In this study, we investigate the changes in the hydrologic regime of two major German rivers, Elbe and Rhine, after the middle of the 20th century. Here, we use the widely adopted Range of Variability Approach (RVA) method on daily streamflow data from five (Elbe) and seven (Rhine) hydrological stations to determine the variability and spatial pattern of hydrologic alterations. We discuss the potential effect of climate change on the water regime of these two rivers, as well as other potential causes. For both rivers, we find that some hydrologic parameters are highly altered, especially the number of reversals, indicating higher variability. The highest impact is found at Ems hydrological station on Rhine River. The order of affected hydrological stations follows mostly the downstream course of both rivers. Our study indicates that the hydrological behavior of Elbe and Rhine Rivers has altered since the middle of the 20th century, a probable consequence of climate change. These hydrologic alterations can lead to undesirable ecological effects on local biota

Greenland Ice Sheet influence on Last Interglacial climate: global sensitivity studies performed with an atmosphere–ocean general circulation model

During the Last Interglacial (LIG, ~130–115 kiloyears (kyr) before present (BP)), the northern high latitudes were characterized by higher temperatures than those of the late Holocene and a lower Greenland Ice Sheet (GIS). However, the impact of a reduced GIS on the global climate has not yet been well constrained. In this study, we quantify the contribution of the GIS to LIG warmth by performing various sensitivity studies based on equilibrium simulations, employing the Community Earth System Models (COSMOS), with a focus on height and extent of the GIS. We present the first study on the effects of a reduction in the GIS on the surface temperature (TS) on a global scale and separate the contribution of astronomical forcing and changes in GIS to LIG warmth. The strong Northern Hemisphere summer warming of approximately 2°C (with respect to preindustrial) is mainly caused by increased summer insolation. Reducing the height by ~1300m and the extent of the GIS does not have a strong influence during summer, leading to an additional global warming of only +0.24°C compared to the purely insolation-driven LIG. The effect of a reduction in the GIS is, however, strongest during local winter, with up to +5°C regional warming and with an increase in global average temperature of +0.48°C. In order to evaluate the performance of our LIG simulations, we additionally compare the simulated TS anomalies with marine and terrestrial proxy-based LIG temperature anomalies derived from three different proxy data compilations. Our model results are in good agreement with proxy records with respect to the warming pattern but underestimate the magnitude of temperature change when compared to reconstructions, suggesting a potential misinterpretation of the proxy records or deficits in our model. However, we are able to partly reduce the mismatch between model and data by additionally taking into account the potential seasonal bias of the proxy record and/or the uncertainties in the dating of the proxy records for the LIG thermal maximum. The seasonal bias and the uncertainty of the timing are estimated from new transient model simulations covering the whole LIG. The model–data comparison improves for proxies that represent annual mean temperatures when the GIS is reduced and when we take the local thermal maximum during the LIG (130–120 kyr BP) into account. For proxy data that represent summer temperatures, changes in the GIS are of minor importance for sea surface temperatures. However, the annual mean and summer temperature change over Greenland in the reduced GIS simulations seems to be overestimated as compared to the local ice core data, which could be related to the interpretation of the recorder system and/or the assumptions of GIS reduction. Thus, the question regarding the real size of the GIS during the LIG has yet to be answered

Future sea level contribution from Antarcticainferred from CMIP5 model forcing and itsdependence on precipitation ansatz

Various observational estimates indicate growing mass loss at Antarctica's margins but also heavier precipitation across the continent. In the future, heavier precipitation fallen on Antarctica will counteract any stronger iceberg discharge and increased basal melting of floating ice shelves driven by a warming ocean. Here, we use from nine CMIP5 models future projections, ranging from strong mitigation efforts to business-as-usual, to run an ensemble of ice-sheet simulations. We test, how the precipitation boundary condition determines Antarctica's sea-level contribution. The spatial and temporal varying climate forcings drive ice-sheet simulations. Hence, our ensemble inherits all spatial and temporal climate patterns, which is in contrast to a spatial mean forcing. Regardless of the applied boundary condition and forcing, some areas will lose ice in the future, such as the glaciers from the West Antarctic Ice Sheet draining into the Amundsen Sea. In general the simulated ice-sheet thickness grows in a broad marginal strip, where incoming storms deliver topographically controlled precipitation. This strip shows the largest ice thickness differences between the applied precipitation boundary conditions too. On average Antarctica's ice mass shrinks for all future scenarios if the precipitation is scaled by the spatial temperature anomalies coming from the CMIP5 models. In this approach, we use the relative precipitation increment per degree warming as invariant scaling constant. In contrast, Antarctica gains mass in our simulations if we apply the simulated precipitation anomalies of the CMIP5 models directly. Here, the scaling factors show a distinct spatial pattern across Antarctica. Furthermore, the diagnosed mean scaling across all considered climate forcings is larger than the values deduced from ice cores. In general, the scaling is higher across the East Antarctic Ice Sheet, lower across the West Antarctic Ice Sheet, and lowest around the Siple Coast. The latter is located on the east side of the Ross Ice Shelf

Long-term monitoring (1953–2019) of geomorphologically active sections of Little Ice Age lateral moraines in the context of changing meteorological conditions

We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the European Central–Eastern Alps, covering a total time period from 1953 to 2019 and including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM (digital elevation model) of Differences (DoDs) were calculated, based on multitemporal high-resolution digital elevation models from historical aerial images (generated by structure from motion photogrammetry with multi-view stereo) and light detection and ranging from airborne platforms. Two approaches were implemented to achieve the corresponding objectives. First, by calculating linear regression models using the accumulated sediment yield and the corresponding catchment area (on a log–log scale), the range of the variability in the spatial distribution of erosion values within the sites. Second, we use volume calculations to determine the total and the mean sediment yield (as well as erosion rates) of the entire sites. Subsequently, both the sites and the different time periods of both approaches are compared. Based on the slopes of the calculated regression lines, it can be shown that the highest variability in the sediment yield at the sites occurs in the first time period (mainly 1950s to 1970s). This can be attributed to the fact that within some sites the sediment yield per square metre increases clearly more strongly (regression lines with slopes up to 1.5). In contrast, in the later time periods (1970s to mid-2000s and mid-2000s to 2017/2019), there is generally a decrease in 10 out of 12 cases (regression lines with slopes around 1). However, even at sites with an increase in the variability in the sediment yield over time, the earlier high variabilities are no longer reached. This means that the spatial pattern of erosion in the gully heads changes over time as it becomes more uniform. Furthermore, using sediment volume calculations and corresponding erosion rates, we show a generally decreasing trend in geomorphic activity (amount of sediment yield) between the different time periods in 10 out of 12 sites, while 2 sites show an opposite trend, where morphodynamics increase and remain at the same level. Finally, we summarise the results of long-term changes in the morphodynamics of geomorphologically active areas on lateral moraines by presenting the “sediment activity concept”, which, in contrast to theoretical models, is based on actually calculated erosion. The level of geomorphic activity depends strongly on the characteristics of the sites, such as size, slope length, and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with influence of dead ice over decades in the lower slope area show high geomorphic activity. Furthermore, we show that system internal factors, as well as the general paraglacial adjustment process, have a greater influence on long-term morphodynamics than changing external weather and climate conditions, which, however, had a slight impact mainly in the last, i.e. most recent, time period (mid-2000s to 2017/2019) and may have led to an increase in erosion at the sites

Reaching the 1.5 degree limit: what does it mean for West Antarctica and the global mean sea level?

What are the benefits of limiting the global warming to 1.5 degree with respect to pre-industrial conditions for the vulnerable region of West Antarctica which might be prone to positive feedback mechanisms between ocean circulation, melting of shelf ice and instabilities of the ice sheet? There are indications that West Antarctic ice sheet instabilities have occurred in the Last Interglacial around 125.000 years ago. At that time the polar surface temperature was about 2K warmer than today. The question under which circumstances a tipping point may be reached and if this may happen again is therefore highly relevant, especially since a disintegration of the West Antarctic ice sheet could cause a global sea level rise between 3 and 5 m. Here we address this question with variable resolution, global coupled ice sheet - shelf ice - ocean - atmosphere multi-century simulations. With our innovative ocean modelling approach in the Finite Element Sea-ice Ocean Model FESOM it is possible to refine the ocean resolution to up to 3 km in the Amundsen Sea and 10 km around the whole Antarctica while keeping it relatively coarse in the order of a couple of hundred km in dynamically not very active regions such as the subtropical regions. This means that we can simulate the feedback between ocean and ice in the relevant regions highly resolved given that the ice sheet model runs at a resolution of 5 to 10 km. Three different emission scenarios are applied up to 2100, two of them limiting the global mean temperature increase to 1.5 ◦ C and 2 ◦ C respectively and one of them assuming business-as-usual conditions (IPCC SRES RCP8.5 scenario). The simulations are extended to 2400 with the greenhouse gas and aerosol concentrations kept constant at 2100 levels, respectively, to be able to simulate the long-term implications of different global warming levels

The greening of Arabia: multiple opportunities for human occupation of the Arabian peninsula during the Late Pleistocene inferred from an ensemble of climate model simulations

Climate models are potentially useful tools for addressing human dispersals and demographic change. The Arabian Peninsula is becoming increasingly significant in the story of human dispersals out of Africa during the Late Pleistocene. Although characterised largely by arid environments today, emerging climate records indicate that the peninsula was wetter many times in the past, suggesting that the region may have been inhabited considerably more than hitherto thought. Explaining the origins and spatial distribution of increased rainfall is challenging because palaeoenvironmental research in the region is in an early developmental stage. We address environmental oscillations by assembling and analysing an ensemble of five global climate models (CCSM3, COSMOS, HadCM3, KCM, and NorESM). We focus on precipitation, as the variable is key for the development of lakes, rivers and savannas. The climate models generated here were compared with published palaeoenvironmental data such as palaeolakes, speleothems and alluvial fan records as a means of validation. All five models showed, to varying degrees, that the Arabia Peninsula was significantly wetter than today during the Last Interglacial (130 ka and 126/125 ka timeslices), and that the main source of increased rainfall was from the North African summer monsoon rather than the Indian Ocean monsoon or from Mediterranean climate patterns. Where available, 104 ka (MIS 5c), 56 ka (early MIS 3) and 21 ka (LGM) timeslices showed rainfall was present but not as extensive as during the Last Interglacial. The results favour the hypothesis that humans potentially moved out of Africa and into Arabia on multiple occasions during pluvial phases of the Late Pleistocene

Holocene and Last Interglacial temperature trends and seasonality - Modelling and data analysis

The Last Interglacial (LIG, 130–115 kiloyear (kyr) before present (BP)) and the Holocene (10–0 kyr BP) provide a good test bed for climate models that are used for future climate projections, as the climatic forcings during these interglacial periods are well constrained. The LIG is characterized by a strong seasonal insolation forcing as compared to the present, that is driven by variations in the Earth's astronomical configuration. During the LIG, the northern high latitudes experienced higher temperatures than those of the late Holocene, as indicated by proxy records and modelling studies, and the Greenland Ice Sheet (GIS) was notably reduced. However, the impact of a reduced GIS on the global climate has not yet been well constrained. In this study, the contribution of the GIS to LIG warmth is quantified by performing various sensitivity studies, employing the Community Earth System Models (COSMOS). The focus is set on height and extent of the GIS. In order to assess the effects of insolation changes over time on temperature evolution and seasonality, and for a comparison of LIG climate with the current interglacial, transient simulations, covering the whole LIG and Holocene, are performed. The resulting surface temperature fields are analysed, and the contribution of different forcings to LIG warmth is separated. It is found that strong Northern Hemisphere warming is mainly caused by increased summer insolation. Reducing height and extent of the GIS leads to an additional warming of several degrees Celsius in the northern and southern high latitudes during local winter. In order to evaluate the performance of the COSMOS LIG simulations, the simulated surface temperature anomalies are compared to marine and terrestrial proxy-based LIG temperature anomalies. It is found that model results are in good agreement with proxy records with respect to the spatial pattern of the temperature change, but they underestimate the reconstructed temperatures. This mismatch between model and data is reduced by taking into account potential seasonal biases of the proxy records and by considering uncertainties in the dating of the proxy records for the LIG thermal maximum. Respective seasonal biases and uncertainty in the dating are estimated from the transient simulations performed with COSMOS. However, the COSMOS LIG simulations are not able to reproduce the full magnitude of temperature changes indicated by the proxies. A similar underestimation of proxybased temperatures by the COSMOS is found also for the Holocene period. In order to test whether the model-data mismatch is model-dependent, time slice simulations from the Paleoclimate Modelling Intercomparison Project (PMIP), representing LIG and Holocene temperatures, are as well compared to the proxy reconstructions. This comparison indicates an underestimation of the proxy reconstructions by all considered models. These results indicate that simulated and reconstructed temperature changes are, to a large degree, only qualitatively comparable, suggesting a potential misinterpretation of the proxy records, and/or deficits of the models, such as model sensitivity to orbital forcing

A model-data comparison of the Holocene global sea surface temperature evolution

We compare the ocean temperature evolution of the Holocene as simulated by climate models and reconstructed from marine temperature proxies. We use transient simulations from a coupled atmosphere–ocean general circulation model, as well as an ensemble of time slice simulations from the Paleoclimate Modelling Intercomparison Project. The general pattern of sea surface temperature (SST) in the models shows a high-latitude cooling and a low-latitude warming. The proxy dataset comprises a global compilation of marine alkenone- and Mg/Ca-derived SST estimates. Independently of the choice of the climate model, we observe significant mismatches between modelled and estimated SST amplitudes in the trends for the last 6000 yr. Alkenone-based SST records show a similar pattern as the simulated annual mean SSTs, but the simulated SST trends underestimate the alkenone-based SST trends by a factor of two to five. For Mg/Ca, no significant relationship between model simulations and proxy reconstructions can be detected. We test if such discrepancies can be caused by too simplistic interpretations of the proxy data. We explore whether consideration of different growing seasons and depth habitats of the planktonic organisms used for temperature reconstruction could lead to a better agreement of model results with proxy data on a regional scale. The extent to which temporal shifts in growing season or vertical shifts in depth habitat can reduce model–data misfits is determined. We find that invoking shifts in the living season and habitat depth can remove some of the model–data discrepancies in SST trends. Regardless whether such adjustments in the environmental parameters during the Holocene are realistic, they indicate that when modelled temperature trends are set up to allow drastic shifts in the ecological behaviour of planktonic organisms, they do not capture the full range of reconstructed SST trends. Results indicate that modelled and reconstructed temperature trends are to a large degree only qualitatively comparable, thus providing a challenge for the interpretation of proxy data as well as the model sensitivity to orbital forcing