Fifty-year dynamics of the Lena River islands (Russia): Spatio-temporal pattern of large periglacial anabranching river and influence of climate change.

The Lena, a large river that drains the northern coldest region of the Northern Hemisphere, is deeply influenced by the continuous permafrost and degradation of the frozen ground has been shown to be the main cause of the marked increase in water discharge. The first objective of this study conducted on the middle Lena was to analyze the island dynamics for the last 50 years (1967 to 2017). Several morphological parameters were surveyed using a GIS on seven series of aerial photographs and satellite images of a 100 km-long reach: island size, eroded and deposited areas, position and morphology of the islands. This approach enabled the identification of evolutionary models. Our second objective was to evaluate the potential impact of ongoing climate change. We analyzed morphological parameters with respect to two main factors: efficient discharge (bar-full, bankfull and flood discharges) and water temperature. A potential erosion index (PEI) was calculated by coupling the duration of discharge exceeding the bar-full level and water temperature. The results identified several morphological changes that occurred at the end of the 20th century: an increase in the number of islands, greater eroded surfaces and accelerated migration of islands. Comparing the dynamics of islands with and without permafrost is a good indicator of their sensitivity to climate change. A major change was observed concerning the erosion and migration of islands with and without permafrost. This evolution seems to be linked both with the duration of the discharge that exceeds the bar-full level and with the number of flood peaks. The water temperature in May and August have a major influence on permafrost islands that become increasingly destabilized. Thus, as large rivers are assumed to slowly react to climate change, the recent changes in the Lena River prove that the global change deeply impacts periglacial rivers.

Lake Tauca highstand (Heinrich Stadial 1a) driven by a southward shift of the Bolivian High.

Heinrich events are characterized by worldwide climate modifications. Over the Altiplano endorheic basin (high tropical Andes), the second half of Heinrich Stadial 1 (HS1a) was coeval with the highstand of the giant paleolake Tauca. However, the atmospheric mechanisms underlying this wet event are still unknown at the regional to global scale. We use cosmic-ray exposure ages of glacial landforms to reconstruct the spatial variability in the equilibrium line altitude of the HS1a Altiplano glaciers. By combining glacier and lake modeling, we reconstruct a precipitation map for the HS1a period. Our results show that paleoprecipitation mainly increased along the Eastern Cordillera, whereas the southwestern region of the basin remained relatively dry. This pattern indicates a southward expansion of the easterlies, which is interpreted as being a consequence of a southward shift of the Bolivian High. The results provide a new understanding of atmospheric teleconnections during HS1 and of rainfall redistribution in a changing climate.

Paradoxical cold conditions during the medieval climate anomaly in the Western Arctic.

In the Northern Hemisphere, most mountain glaciers experienced their largest extent in the last millennium during the Little Ice Age (1450 to 1850 CE, LIA), a period marked by colder hemispheric temperatures than the Medieval Climate Anomaly (950 to 1250 CE, MCA), a period which coincided with glacier retreat. Here, we present a new moraine chronology based on (36)Cl surface exposure dating from Lyngmarksbræen glacier, West Greenland. Consistent with other glaciers in the western Arctic, Lyngmarksbræen glacier experienced several advances during the last millennium, the first one at the end of the MCA, in ~1200 CE, was of similar amplitude to two other advances during the LIA. In the absence of any significant changes in accumulation records from South Greenland ice cores, we attribute this expansion to multi-decadal summer cooling likely driven by volcanic and/or solar forcing, and associated regional sea-ice feedbacks. Such regional multi-decadal cold conditions at the end of the MCA are neither resolved in temperature reconstructions from other parts of the Northern Hemisphere, nor captured in last millennium climate simulations.

Irregular tropical glacier retreat over the Holocene epoch driven by progressive warming.

The causes and timing of tropical glacier fluctuations during the Holocene epoch (10,000 years ago to present) are poorly understood. Yet constraining their sensitivity to changes in climate is important, as these glaciers are both sensitive indicators of climate change and serve as water reservoirs for highland regions. Studies have so far documented extra-tropical glacier fluctuations, but in the tropics, glacier-climate relationships are insufficiently understood. Here we present a (10)Be chronology for the past 11,000 years (11 kyr), using 57 moraines from the Bolivian Telata glacier (in the Cordillera Real mountain range). This chronology indicates that Telata glacier retreated irregularly. A rapid and strong melting from the maximum extent occurred from 10.8 ± 0.9 to 8.5 ± 0.4 kyr ago, followed by a slower retreat until the Little Ice Age, about 200 years ago. A dramatic increase in the rate of retreat occurred over the twentieth century. A glacier-climate model indicates that, relative to modern climate, annual mean temperature for the Telata glacier region was -3.3 ± 0.8 °C cooler at 11 kyr ago and remained -2.1 ± 0.8 °C cooler until the end of the Little Ice Age. We suggest that long-term warming of the eastern tropical Pacific and increased atmospheric temperature in response to enhanced austral summer insolation were the main drivers for the long-term Holocene retreat of glaciers in the southern tropics.

SENEQUE: a multi-scaling GIS interface to the Riverstrahler model of the biogeochemical functioning of river systems.

The Riverstrahler model describes the biogeochemical functioning of an entire river system, from 100 to 100,000 km(2) or more, taking into account the constraints set by the morphology of the drainage network, the meteorological/hydrological conditions, and the inputs of material from point and non-point sources in the watershed. This tool has been applied for research purpose to several river systems differing in terms of hydrological regime and anthropogenic influences. In order to improve its capabilities and its generic dimension, as well as to develop a user-friendly interface allowing its transfer to non-specialist users including managers, the model has been coupled to a GIS interface. This gives the user the possibility to visualize the available geospatial database, to select the best geographical representation of the drainage network, to automatically prepare the corresponding input files required for the model, to pilot the model calculation and to visualize the results. The coupling with a GIS interface has considerably improved the capabilities of the Riverstrahler model. The code of the model is now entirely generic and can be run on any river system for which a suitable database is available. Its spatial resolution can be adapted to the requirement of the relevant problem, from the highest level, where each elementary watershed is individualized, to the lower level, where the whole basin is idealized as one basin with tributaries of each order having the same characteristics. As an illustration of the new potentialities offered by the coupling of Riverstrahler with a GIS through the SENEQUE interface, the results of a same modeling scenario are compared at different spatial resolutions. For the first time, with on-line coupling to a geodatabase, the effect of increasing the spatial resolution of the drainage network representation on the performance of the Riverstrahler model has been examined. At the outlet of the basin, the water quality results were found invariant to a large degree, whatever the details of its representation in the calculations. This result justifies the use of a low resolution representation of the upstream watershed when results are required only at the outlet of the basin.