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51.
Dmytro Siergieiev Anders Widerlund Angela Lundberg Lisbeth Almqvist Magali Collomp Johan Ingri Björn Öhlander 《Aquatic Geochemistry》2014,20(1):59-80
Using hydrogeochemical analysis of two large boreal rivers (pristine Kalix and hydropower regulated Lule) discharging into the Gulf of Bothnia, the major impacts of regulation on water discharge, element transport and their seasonal redistribution have been assessed. The pre-regulation hydrogeochemical features were assumed to be similar for the two rivers. For the Lule River, the average maximum runoff was almost halved, while the average minimum was tripled as a result of the regulation. The fraction of winter transport of total organic carbon (TOC), Fe, Si, suspended Mn and P in the Lule River was, according to a conservative estimate, two to three times higher than in the pristine river. Longer residence time in the Lule River delayed arrival of the suspended Mn peak and dissolved Si decline to the river mouth. During summer, the suspended C/N ratio in the regulated river was 10–20 compared to <10 for the pristine, suggesting presence of predominantly old organic material. This was supported by a virtually constant suspended P/Fe ratio throughout the year in the Lule River, indicating low abundance of phytoplankton. TOC varied irregularly in the Lule River suggesting temporal disconnection between the river and the upper riparian zone. The disappearance of the spring flow maximum, a shift of element transport from spring to winter and supply of mainly old organic material during the vegetation growth season may have a pronounced impact on the ecosystem of the Gulf of Bothnia and the river itself. 相似文献
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Henning Haack John Schutt Anders Meibom Ralph Harvey 《Meteoritics & planetary science》2007,42(10):1727-1733
Abstract— Following discoveries of blue ice areas in Greenland resembling meteorite‐bearing blue ice fields in Antarctica, a surface search of several of the most promising sites was carried out in August 2003. The ice fields are located in Kong Christian X Land, in northeastern Greenland around 74°N at elevations between 2100 and 2400 m. No meteorites were found in any of the localities that were searched. Evidence of occasional significant melting (filled crevasses and melt sheets) suggest that summer temperatures are sometimes high enough that dark rocks, like meteorites, can melt through the upper layers of ice. Small terrestrial rocks and cryogenite were found down to 50 cm below the ice surface. Meter‐sized terrestrial rocks were found on top of the ice downstream from nunataks. These rocks shade the ice below, and since they were apparently too massive to warm up during warm days, they remained at the surface as the surrounding ice ablated away. Our findings strongly suggest that Greenland is currently unlikely to harbor significant meteorite concentrations on blue ice fields. 相似文献
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Regional climate modelling over complex terrain: an evaluation study of COSMO-CLM hindcast model runs for the Greater Alpine Region 总被引:1,自引:0,他引:1
In this study the results of the regional climate model COSMO-CLM (CCLM) covering the Greater Alpine Region (GAR, 4°–19°W and 43°–49°N) were evaluated against observational data. The simulation was carried out as a hindcast run driven by ERA-40 reanalysis data for the period 1961–2000. The spatial resolution of the model data presented is approx. 10 km per grid point. For the evaluation purposes a variety of observational datasets were used: CRU TS 2.1, E-OBS, GPCC4 and HISTALP. Simple statistics such as mean biases, correlations, trends and annual cycles of temperature and precipitation for different sub-regions were applied to verify the model performance. Furthermore, the altitude dependence of these statistical measures has been taken into account. Compared to the CRU and E-OBS datasets CCLM shows an annual mean cold bias of ?0.6 and ?0.7 °C, respectively. Seasonal precipitation sums are generally overestimated by +8 to +23 % depending on the observational dataset with large variations in space and season. Bias and correlation show a dependency on altitude especially in the winter and summer seasons. Temperature trends in CCLM contradict the signals from observations, showing negative trends in summer and autumn which are in contrast to CRU and E-OBS. 相似文献
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We propose linear response functions to separately estimate the sea-level contributions of thermal expansion and solid ice discharge from Greenland and Antarctica. The response function formalism introduces a time-dependence which allows for future rates of sea-level rise to be influenced by past climate variations. We find that this time-dependence is of the same functional type, R(t) ~ t α, for each of the three subsystems considered here. The validity of the approach is assessed by comparing the sea-level estimates obtained via the response functions to projections from comprehensive models. The pure vertical diffusion case in one dimension, corresponding to α = ?0.5, is a valid approximation for thermal expansion within the ocean up to the middle of the twenty first century for all Representative Concentration Pathways. The approximation is significantly improved for α = ? 0.7. For the solid ice discharge from Greenland we find an optimal value of α = ?0.7. Different from earlier studies we conclude that solid ice discharge from Greenland due to dynamic thinning is bounded by 0.42 m sea-level equivalent. Ice discharge induced by surface warming on Antarctica is best captured by a positive value of α = 0.1 which reflects the fact that ice loss increases with the cumulative amount of heat available for softening the ice in our model. 相似文献
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A set of global climate model simulations for the last thousand years developed by the Max Planck Institute is compared with paleoclimate proxy data and instrumental data, focusing on surface temperatures for land areas between 30° and 75°N. The proxy data are obtained from six previously published Northern Hemispheric-scale temperature reconstructions, here re-calibrated for consistency, which are compared with the simulations utilizing a newly developed statistical framework for ranking several competing simulations by means of their statistical distance against past climate variations. The climate model simulations are driven by either “low” or “high” solar forcing amplitudes (0.1 and 0.25 % smaller total solar irradiance in the Maunder Minimum period compared to the present) in addition to several other known climate forcings of importance. Our results indicate that the high solar forcing amplitude results in a poorer match with the hemispheric-scale temperature reconstructions and lends stronger statistical support for the low-amplitude solar forcing. However, results are likely conditional upon the sensitivity of the climate model used and strongly dependent on the choice of temperature reconstruction, hence a greater consensus is needed regarding the reconstruction of past temperatures as this currently provides a great source of uncertainty. 相似文献
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