Ward, P.J., Renssen, H., Aerts, J.C.J.H., van Balen, R., Vandenberghe, J.
Hydrology and Earth System Science, 12(1): 159-175, 2008
In recent years the frequency of high-flow events on the Meuse (northwest Europe) has been relatively great, and flooding has become a major research theme. To date, research has focused on observed discharge records of the last century and simulations of the coming century. However, it is difficult to delineate changes caused by human activities (land use change and greenhouse gas emissions) and natural fluctuations on these timescales. To address this problem we coupled a climate model (ECBilt-CLIO-VECODE) and a hydrological model (STREAM) to simulate daily Meuse discharge in two time-slices: 4000-3000 BP (natural situation), and 1000-2000 AD (includes anthropogenic influence). For 4000-3000 BP the basin is assumed to be almost fully forested; for 1000-2000 AD we reconstructed land use based on historical sources. For 1000-2000 AD the simulated mean annual discharge (260.9 m(3) s(-1)) is significantly higher than for 4000-3000 BP (244.8 m(3) s(-1)), and the frequency of large high-flow events (discharge >3000 m(3) s(-1)) is higher (recurrence time decreases from 77 to 65 years). On a millennial timescale almost all of this increase can be ascribed to land use changes (especially deforestation); the effects of climatic change are insignificant. For the 20th Century, the simulated mean discharge (270.0 m(3) s(-1)) is higher than in any other century studied, and is ca. 2.5% higher than in the 19th Century (despite an increase in evapotranspiration). Furthermore, the recurrence time of large high-flow events is almost twice as short as under natural conditions (recurrence time decreases from 77 to 40 years). On this timescale climate change (strong increase in annual and winter precipitation) overwhelmed land use change as the dominant forcing mechanism.
D. M. Roche, T. M. Dokken, H. Goosse, H. Renssen, and S. L.Weber
CLIMATE OF THE PAST, 3(2): 205-224, 2007
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The Last Glacial Maximum climate is one of the classical benchmarks used both to test the ability of coupled models to simulate climates different from that of the present-day and to better understand the possible range of mechanisms that could be involved in future climate change. It also bears the advantage of being one of the most well documented periods with respect to palaeoclimatic records, allowing a thorough data-model comparison. We present here an ensemble of Last Glacial Maximum climate simulations obtained with the Earth System model LOVECLIM, including coupled dynamic atmosphere, ocean and vegetation components. The climate obtained using standard parameter values is then compared to available proxy data for the surface ocean, vegetation, oceanic circulation and atmospheric conditions. Interestingly, the oceanic circulation obtained resembles that of the present-day, but with increased overturning rates. As this result is in contradiction with the current palaeoceanographic view, we ran a range of sensitivity experiments to explore the response of the model and the possibilities for other oceanic circulation states. After a critical review of our LGM state with respect to available proxy data, we conclude that the oceanic circulation obtained is not inconsistent with ocean circulation proxy data, although the water characteristics ( temperature, salinity) are not in full agreement with water mass proxy data. The consistency of the simulated state is further reinforced by the fact that the mean surface climate obtained is shown to be generally in agreement with the most recent reconstructions of vegetation and sea surface temperatures, even at regional scales.
Richard Gyllencreutz, Jan Mangerud, John-Inge Svendsen and Øystein Lohne
Applied Quaternary research in the central part of glaciated terrain, edited by Peter Johansson and Pertti Sarala
Geological Survey of Finland, Special Paper 46, 113-120, 2007
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The goal of the ongoing project DATED is to describe and document the ice growth towards the LGM and the deglaciation of the large ice sheets in northwest Eurasia. Digitized ice margins and other relevant published features are compiled in a geographical information system, which is coupled to a database with dates (14C, OSL, TL, cosmogenic exposure, clay-varve). The fi rst version of the database and the GIS will be available on the internet in 2007, and will be successively updated. The main purpose of DATED is to serve as a source for the deglaciation history of the Eurasian ice sheets to provide accurate digital maps with calendar-year isochrones to modelers and other researchers. It will also serve as an aid for identifying areas where data is lacking, and to facilitate future reinterpretation of the deglaciation pattern.
Weber, S.L. and S.S. Drijfhout
Geophys. Res. Letters, 34(22): L22706, 2007
The stability of the glacial Atlantic Meridional Overturning Circulation is examined using a coupled model of intermediate complexity. Two slightly different climatic states are generated. One has a southward overturning freshwater transport at the southern border of the Atlantic basin, the other a northward transport. Pulse experiments with varying magnitude always result in a collapsed circulation in case of a southward transport, while the overturning recovers in case of a northward transport. In the latter case recovery is due to a positive salinity-overturning feedback, which strengthens the remnant circulation cell that exists in the `collapsed' state. This is amplified by advection by wind-driven currents and a southward ITCZ shift. The glacial circulation is more easily perturbed than the modern and restoring timescales are considerably longer, matching the duration of Heinrich events.
Helmens, K., Bos, J.A.A., Engels, S., Van Meerbeeck, C., Bohncke, S.J.P., Renssen, H., Heiri, O., Brooks, S.J., Seppä, H., Birks, H.J.B., Wohlfarth, B
Geology, 35(11): 987-990, 2007
Scandinavia is generally considered to have been covered extensively with ice throughout marine isotope stages (MIS) 4-2 between 75 and 10 ka. Here we present evidence for ice-free, warm conditions in the central area of the Scandinavian glaciations during MIS 3. Our multiproxy data obtained from a lacustrine sequence in northern Finland reveal not only significant response in the northeastern sector of the Scandinavian Ice Sheet to warming during the early part of MIS 3, but also indicate rapid climate warming to present-day temperatures in this ice-free period. New climate-model simulations for interstadial conditions in MIS 3 confirm the high mean July temperatures northeast of the Scandinavian Ice Sheet in response to the high insolation values and the presence of the ice sheet during MIS 3.
Jongma, J.I., Prange, M., Renssen, H., and Schulz, M.
Geophysical Research Letters, 35(15): L15706, 2007
Using a three-dimensional global climate model, we show that mode-transitions in North Atlantic deep-water production can provide an amplifying mechanism of relatively weak climate perturbations during the Holocene. Under pre-industrial boundary conditions, a freshwater forcing in the Labrador Sea pushes the North Atlantic overturning circulation into a deterministically bistable regime, characterized by stochastic ``on'' and ``off'' switches in Labrador Sea convection. On a multicentennial time-scale these stochastic mode-transitions can be phase-locked by a small ( subthreshold) periodic freshwater forcing. The local small periodic forcing is effectively amplified with the assistance of noise, to have a large-scale impact on North Atlantic overturning circulation and climate. These results suggest a stochastic resonance mechanism that can operate under Holocene boundary conditions and indicate that changes in the three-dimensional configuration of North Atlantic deep-water formation can be an important component of multicentennial climate variability during interglacials.
Renssen, H., Goosse, H. and Fichefet, T.
Quaternary Science Reviews, 26(15-16): 2019-2029, 2007
Three potential mechanisms behind centennial-scale Holocene cooling events are studied in simulations performed with the coupled climate model ECBilt-CLIO: (1) internal variability, (2) solar forcing, and (3) freshwater forcing. In experiments with constant preindustrial forcings, three centennial-scale cooling events occur spontaneously in 15,000 years. These rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation, a more southward location of the main site of deep-water formation, expanded sea-ice cover and cooling of 10 degrees C over the Nordic Seas. This mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 Wm(-2). Prescribing a solar forcing of the same magnitude, but following a sinusoidal function with a period of 100 or 1000 years, does not result in any centennial-scale cooling events. The latter forcing does however result in more frequent individual cold years in the North Atlantic region that are related to local weakening of the deep convection and sea-ice expansion. Adding realistic freshwater pulses to the Labrador Sea is also able to trigger centennial-scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8.2 kyr BP, suggesting that freshwater forcing is a valid explanation for early Holocene cooling events.
Renssen, H., Kasse, C., Vandenberghe, J., and Lorenz, S.J.
Journal of Quaternary Science, 22(3): 281-293, 2007
Reconstructions of the Weichselian Late Pleniglacial wind direction in northwest and central Europe are reviewed and compared with palaeoclimate simulations performed with an atmospheric general circulation model. These reconstructions are based on proxy data containing information on former wind directions, such as relic dune forms, sediments and wind-polished rock surfaces. The objective is to investigate whether: (1) the proxy information is internally consistent; and (2) in agreement with the model simulations. We find a general consensus in the proxy-based reconstructions, indicating a dominant westerly to northwesterly wind in winter during the Late Pleniglacial. The model results indicate over the study area an atmospheric circulation in winter that is dominated by southwesterly to west-northwesterly winds, which are stronger than the southwesterly winds in the present-day climate. The main driving factors behind the anomalous atmospheric circulation in the Late Pleniglacial are the Laurentide Ice Sheet and a colder North Atlantic Ocean with a relatively extensive sea-ice cover, leading to an eastward relocation of the Icelandic Low and an enhanced pressure gradient over northwest Europe. The minor difference in Late Pleniglacial wind direction between the reconstructions and model can be explained by a combination of uncertainties in the proxy data and the relatively low spatial resolution of the applied climate model.
Renssen, H., Lougheed, B.C., Aerts. J.C.J.H., de Moel, H., Ward, P.J., Kwadijk, J.C.J.
Earth and Planetary Science Letters, 261(3-4): 685-693, 2007
To improve our understanding of the relationship between climate change and variations in Caspian Sea level (CSL), we performed simulations of annual CSL for the period 8 ka to 2100 CE using a coupled model setup representing climate, hydrology and sea level. We forced our climate model with long-term changes in orbital parameters and atmospheric greenhouse gas concentrations, using the IPCC A1b scenario for the 21st Century. Our simulations produce an orbitally forced, long-term decline in CSL of 5 in from 5.5 to 0 ka, caused by a decrease in river runoff and over-sea precipitation that is not fully compensated by a decrease in over-sea evaporation. Superimposed on this long-term downward CSL trend we simulated centennial-scale fluctuations of up to 4 in and decadal-scale variations of up to 2 in, caused by the internal variations of our modeled climate system, amplified by the sensitivity of CSL to small changes in river runoff and in the over-sea P-E budget. The A1b anthropogenic emission scenario causes a 4.5 in fall in CSL in the 21st Century, due to a pronounced increase in over-sea evaporation that is stronger than the enhanced river discharge. This decline in CSL is of the same order of magnitude as the orbitally-forced millennial-scale downward CSL trend simulated for the last 8000 years. Our results are generally consistent with CSL estimates based on geological, historical and measured data, as well as with most other model studies.
Roche, D.M., Renssen, H., Weber, S.L., Goosse, H.
Geophysical Research Letters, 34(24): L24708, 2007
The lack of climatic imprint left by the Meltwater Pulse-1A (similar or equal to 14.5 ka BP), equivalent to a sea-level rise of 14 to 20 meters, is puzzling. Recent studies suggest the event might have occurred as a hyperpycnal flow in the Gulf of Mexico, preventing its detection in oceanic records throughout the North Atlantic. We present a suite of simulations with the LOVECLIM climate model, which mimic the effect of hyperpycnal flow under LGM conditions, in a first attempt to constrain its climatic effects. Analysing the ocean dynamics associated with the anomalous freshwater input, we show that the proposed mechanism is capable of sneaking a significant proportion of the MWP into the ocean (similar or equal to 6 meters equivalent sea-level rise using our model under LGM boundary conditions). We also demonstrate that, in our model, the meridional circulation is more sensitive to such inputs in the Arctic Ocean than in the Gulf of Mexico.
Ward, P.J., Aerts, J.C.J.H., de Moel, H., Renssen, H.
Global and Planetary Change, 57(3-4): 283-300, 2007
We have coupled a climate model (ECBilt-CLIO-VECODE) and a hydrological model (STREAM) offline to simulate palaeodischarge of nineteen rivers (Amazon, Congo, Danube, Ganges, Krishna, Lena, Mackenzie, Mekong, Meuse, Mississippi, Murray-Darling, Nile, Oder, Rhine, Sacramento-San Joaquin, Syr Darya, Volga, Volta, Zambezi) for three time-slices: Early Holocene (9000-8650 BP), Mid-Holocene (6200-5850 BP) and Recent (1750-2000 AD). To evaluate the model's skill in retrodicting broad changes in mean palaeodischarge we have compared the model results with palaeodischarge estimates from multi-proxy records. We have compared the general trends inferred from the proxy data with statistical differences in modelled discharge between the three periods, thereby developing a technique to assess the level of agreement between the model and proxy data. The quality of the proxy data for each basin has been classed as good, reasonable or low. Of the model runs for which the proxy data were good or reasonable, 72% were in good agreement with the proxy data, and 92% were in at least reasonable agreement. We conclude that the coupled climate-hydrological model performs well in simulating mean discharge in the time-slices studied. The discharge trends inferred from the proxy and model data closely follow latitudinal and seasonal variations in insolation over the Holocene. For a number of basins for which agreement was not good we have identified specific mechanisms which could be responsible for the discrepancy, primarily the absence of the Laurentide ice sheet in our model. In order to use the model in an operational sense within water management studies it would be useful to use a higher spatial resolution and a daily time-step.
Wohlfarth, B., Helmens, K., Wastegard, S., Bohncke, S., Renssen, H., Sanchez-Goni, M.F., d'Errico, F., Rasmussen, T., Johnsen, S. and Spötl, C.
PAGES Newsletter, 15(1), 2007
Koren, J.H., Svendsen, J.I., Mangerud, J., Furnes, H
Quaternary Science Reviews, 27(1-2): 85-94, 2008
A new volcanic ash, named the Dimna Ash, geochemically similar to the rhyolitic component of the Vedde Ash, is described in a core from a palaeo-lake in Western Norway. The Dimna Ash occurs more than a metre below the Vedde Ash and radiocarbon datings indicate a minimum age of 15,100 calendar years BP (12,800 C-14 years BP). In comparison the age of the Vedde Ash is 12,120 calendar (NGRIP) years BP (10,300 C-14 years BP). The Dimna Ash is invisible to the naked eye in the core and was detected by applying a density separation technique. It comprises, however, up to 12,900 ash shards per cm, 3 sediment. We conclude that the Dimna Ash is wind-blown from Iceland and should therefore be found also in marine cores between Iceland and Norway and thus has the potential to become an important marker horizon. The core also contains scattered ash shards, geochemically similar to the Borrobol Ash, that are spread over a 50cm thick zone deposited between 13,400 and 12,700 calendar years BP. This is the first discovery of Borrobol-like ash in Norway.
Lohne, Ø., Bondevik, S., Mangerud, J., Svendsen, J.I.
Quaternary Science Reviews, 26(17-18): 2128-2151, 2007
After the first emergence following deglaciation, relative sea level rose by 10m in western Norway and culminated late in the Younger Dryas (YD). The relative sea-level history, reconstructed by dating deposits in isolation basins. shows a sea-level low-stand between similar to 13 640 and 13 080 cal yr BP, a 10 m sea-level rise between similar to 13 080 and 11 790 cal yr BP and a sea-level high-stand between similar to-11 790 and 11550 cal yr BP. Shortly after the YD/Holocene boundary, sea level fell abruptly by similar to 37 m. The shorelines formed during the sea-level low-stand in the mid-Allergd and during the sea-level high-stand in the YD have almost parallel tilts with a gradient of similar to 1.3 m km(-1), indicating that hardly any isostatic movement has taken place during this period of sea-level rise. We conclude that the transgression was caused by the major re-advance of the Scandinavian Ice Sheet that took place in western Norway during the Lateglacial. The extra ice load halted the isostatic uplift and elevated the geoid due to the increased gravitational attraction on the sea. Our results show that the crust responded to the increased load well before the YD (starting similar to 12 900 cal yr BP), with a sea-level low-stand at 13 640 cal yr BP and the subsequent YD transgression starting at 13 080 cal yr BP. Thus, we conclude that the so-called YD ice-sheet advance in western Norway started during the Allerod, possibly more than 600 years before the Allerod/YD transition.
Astakhov, V., Mangerud, J
Doklady Earth Sciences, 416(7): 1022-1026, 2007
Mangerud, J., Landvik, J.Y.
Boreas, 36(3): 278-285, 2007
The outermost moraines in front of the Scottbreen glacier in Spitsbergen date from c. AD 1900. These moraines rest on top of a marine shoreline radiocarbon-dated to about 11 200 C-14 yr BP and demonstrate that the AD-1900 moraines show the maximum glacier extent since late Allerod time. This means that Scottbreen was smaller during the Younger Dryas than at AD 1900, in contrast with glaciers on mainland western Europe, which were all much larger during the Younger Dryas. The explanation is probably starvation of precipitation on western Spitsbergen during the Younger Dryas. In contrast, ice sheets and glaciers in Spitsbergen reacted more or less in concert with glaciers in western Europe, during the global Last Glacial Maximum and the Little Ice Age.
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