EFFECTS OF CLIMATE CHANGE ON THE FRESHWATERS OF ARCTIC AND SUBARCTIC NORTH AMERICA

Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO₂ scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring. © 1997 John Wiley & Sons, Ltd.     

河南省内黄县河流地质考古研究

河流地质考古学是基于地层研究河流和考古遗址之间关系的学科.近年来我们在河南省内黄县开展的河流地质考古研究揭示了黄河复杂的演化历史,在此基础上进一步探讨了古代人类活动与周围环境的相互作用.本文主要介绍了2010~2016年我们在河南省内黄县3个全新世遗址(岸上、三杨庄和大张龙村)的地质考古工作中所取得的成果.研究区域内遗址的地层记录表明,许多考古遗址被深埋于地下,并可能影响了3000 a B.P.以来的河流沉积过程.我们在岸上遗址发掘了A、B、C、D共4处青铜时代的沟渠遗迹,这些沟渠的堆筑可能影响了后期的沉积过程并导致了遗址周边微地貌的改变;在三杨庄遗址识别出了多层不同时期的人为古土壤,包括新石器晚期、战国时期、汉代和唐代;在大张龙村发现了北宋时期黄河泛滥沉积物,其沉积过程可能受周边村落遗址的影响.根据测得的14C年代和沉积层厚度,本研究进一步对这3处遗址的沉积速率进行了估算,并与前人对华北平原沉积速率的相关研究进行了对比.结果表明,这3处遗址所显示的沉积速率自3000 a B.P.开始显著增加,与对早期历史时期黄河河道沉积速率的估算结果相吻合.因此,基于遗址的地质考古研究能够为探索人与环境的互动关系提供大量信息.未来的工作中,我们需要开展更多基于考古遗址的河流地质考古研究,以深入探讨华北平原的自然沉积过程与文明演进过程之间的关系.     

轨道尺度亚洲气候演化机理的数值模拟:历史与展望

轨道尺度亚洲气候演化是古气候热点问题之一,其变化过程和机理对理解当前全球变暖下亚洲气候变化具有重要参考意义。最近几十年,基于黄土、石笋、湖泊等载体的轨道尺度亚洲气候重建研究获得显著进展,气候演化历史的基本框架已被构建,不同区域和指标记录之间的差异暗示了气候演化机理的复杂性。数值模拟作为研究气候动力学的重要工具之一,在轨道尺度亚洲气候变化中也得到广泛应用和快速发展。基于此,本文尝试对最近十数年轨道尺度亚洲气候演化机理的数值模拟研究做一简单总结和梳理。目前的数值模拟尚未对地质记录给出的各种变化特征、区域差异等现象,尤其是东亚夏季风的黄土和石笋差异、季风和干旱气候的耦合关系等,给出合理解释。因此,在未来工作中亟须涵盖多轨道旋回的高分辨率瞬变试验,结合良好定年的重建记录,以期对轨道尺度亚洲气候变化机理获得更深入完整的认识。     

Climatological Characteristics of the East Asian Winter Monsoon Simulated by CWRF Regional Climate Model

Based on the ERA-Interim atmospheric reanalysis data from the European Medium-Term Weather Forecast Center from 1979 to 2016 and the ERSSTv4 sea surface temperature data from the US National Oceanic and Atmospheric Administration, the regional climate model CWRF was used to simulate the climate characteristics in East Asia. The results show that the CWRF model can well reproduce the average characteristics of the East Asian winter monsoon circulation, including the location and intensity of the low-level continental cold high pressure and variation characteristics of wind field in high and low levels. The occurrence area and frequency of the north wind in the simulation and the reanalysis data were further calculated and compared. It is shown that they are basically consistent. The distribution of air temperature and precipitation over China are well represented by the model. The water vapor transport is also in good agreement with the reanalysis data. The water vapor from the Bay of Bengal plays a vital role in the precipitation over South China. The simulation results of apparent heat source and apparent moisture sink show that the model can well simulate the thermal difference between the East Asian continent and the adjacent sea area. The analysis results indicate that CWRF model has the ability to simulate the main characteristics of the East Asian winter monsoon.     

唐代治乱分期与气候变化的关系*

将唐代定量划分为治世与乱世5个时期,探讨了唐代治乱分期与气候变化的关系。治世、乱世的划分标准参考一套系统的战争数据集,并用单因素方差分析的方法验证各项数据指标在治世与乱世间的差异性,从而说明唐代治世、乱世分期的合理性。治世、乱世分期结果如下: 公元618—626年为乱世,公元627—742年为治世,公元743—784年为乱世,公元785—859年为治世,公元860—907年为乱世。方差分析的结果显示,除了总边境战争外,气候变化、总战争、反叛战争、进攻型边境战争、防御型边境战争、农业丰歉等级和人口增长率在治世与乱世之间均表现出显著差异。用战争定量体现唐代社会治世、乱世变化,探讨气候变化与社会治、乱的关系即是探讨气候变化与战争的关系,乱世的主要战争类型是反叛战争和防御型边境战争,治世的主要战争类型是进攻型边境战争。相关分析结果表明,唐代温度降低、降水减少的时期,农业收成减少导致资源供给不足,更容易发生反叛战争;温度暖期、降水充沛的时期,农业产量提高,刺激了快速增长的人口,在技术改善能力有限、土地资源一定的情况下,更易发生以对外扩张为目的的边境战争,因此气候变化是影响唐代社会治乱变化的重要原因之一。     

Coastal ridge constructive processes at a multi-decadal scale in Barreta Island (southern Portugal)

Multiple ridges across prograding coasts may display variable geometries, commonly expressed through varying elevations. Changes in ridge elevation have been traditionally related to the occurrence of fluctuating progradation rates, which might, in turn, be driven by shifting environmental conditions. Here, we explore the geometry and growth mechanisms of multiple ridges, generated at Barreta Island (Ria Formosa, southern Portugal), as a consequence of the rapid progradation of the island over the last 70 years, following the artificial fixation of the downdrift Faro-Olhão inlet with jetties in 1955. The variability in the morphology of these features was analysed in combination with available wind and wave data, in order to better distinguish growth mechanisms and understand the main parameters determining the final geometry of the observed ridges. The results suggest that (1) most of the identified ridges fall in the beach ridge classification, as they have been mostly built by marine processes, and (2) the parameters derived from, or closely related to wave climate variability (e.g. progradation rates, storm occurrence) can jointly explain most of the observed morphological changes, while aeolian processes played a secondary role. Indeed, ridge geometry appears mainly controlled by progradation rates, with higher ridges associated with lower progradation rates. Progradation rate, in turn, is mostly related to longshore wave power, storminess, and the occurrence storm groups. Yet, the final configuration of ridges can also be affected by runup levels and onshore winds. Therefore, establishing the relation between ridge geometry and wave climate is not a straightforward task, because of the complex processes and interactions that control coastal morphodynamics. © 2019 John Wiley & Sons, Ltd.     

Hillslopes in humid-tropical climates aren't always wet: Implications for hydrologic response and landslide initiation in Puerto Rico

The devastating impacts of the widespread flooding and landsliding in Puerto Rico following the September 2017 landfall of Hurricane Maria highlight the increasingly extreme atmospheric disturbances and enhanced hazard potential in mountainous humid-tropical climate zones. Long-standing conceptual models for hydrologically driven hazards in Puerto Rico posit that hillslope soils remain wet throughout the year, and therefore, that antecedent soil wetness imposes a negligible effect on hazard potential. Our post-Maria in situ hillslope hydrologic observations, however, indicate that while some slopes remain wet throughout the year, others exhibit appreciable seasonal and intra-storm subsurface drainage. Therefore, we evaluated the performance of hydro-meteorological (soil wetness and rainfall) versus intensity-duration (rainfall only) hillslope hydrologic response thresholds that identify the onset of positive pore-water pressure, a predisposing factor for widespread slope instability in this region. Our analyses also consider the role of soil-water storage and infiltration rates on runoff generation, which are relevant factors for flooding hazards. We found that the hydro-meteorological thresholds outperformed intensity-duration thresholds for a seasonally wet, coarse-grained soil, although they did not outperform intensity-duration thresholds for a perennially wet, fine-grained soil. These end-member soils types may also produce radically different stormflow responses, with subsurface flow being more common for the coarse-grained soils underlain by intrusive rocks versus infiltration excess and/or saturation excess for the fine-grained soils underlain by volcaniclastic rocks. We conclude that variability in soil-hydraulic properties, as opposed to climate zone, is the dominant factor that controls runoff generation mechanisms and modulates the relative importance of antecedent soil wetness for our hillslope hydrologic response thresholds.     

Proglacial groundwater storage dynamics under climate change and glacier retreat

Proglacial aquifers are an important water store in glacierised mountain catchments that supplement meltwater-fed river flows and support freshwater ecosystems. Climate change and glacier retreat will perturb water storage in these aquifers, yet the climate-glacier-groundwater response cascade has rarely been studied and remains poorly understood. This study implements an integrated modelling approach that combines distributed glacio-hydrological and groundwater models with climate change projections to evaluate the evolution of groundwater storage dynamics and surface-groundwater exchanges in a temperate, glacierised catchment in Iceland. Focused infiltration along the meltwater-fed Virkisá River channel is found to be an important source of groundwater recharge and is projected to provide 14%–20% of total groundwater recharge by the 2080s. The simulations highlight a mechanism by which glacier retreat could inhibit river recharge in the future due to the loss of diurnal melt cycling in the runoff hydrograph. However, the evolution of proglacial groundwater level dynamics show considerable resilience to changes in river recharge and, instead, are driven by changes in the magnitude and seasonal timing of diffuse recharge from year-round rainfall. The majority of scenarios simulate an overall reduction in groundwater levels with a maximum 30-day average groundwater level reduction of 1 m. The simulations replicate observational studies of baseflow to the river, where up to 15% of the 30-day average river flow comes from groundwater outside of the melt season. This is forecast to reduce to 3%–8% by the 2080s due to increased contributions from rainfall and meltwater runoff. During the melt season, groundwater will continue to contribute 1%–3% of river flow despite significant reductions in meltwater runoff inputs. Therefore it is concluded that, in the proglacial region, groundwater will continue to provide only limited buffering of river flows as the glacier retreats.     

How charophytes (Streptophyta,Charales) survive in severe conditions of the permafrost area in Far North-East Asia

Eight species of charophytes have been found in the vast area of the ubiquitous permafrost in Far North-East Asia (i.e. the Republic of Sakha (Yakutia), the Magadan Region, and the Chukotka Autonomous Area, Russia): Chara contraria, C. globularis, C. strigosa, C. virgata, Nitella flexilis, N. opaca, N. wahlbergiana and Tolypella canadensis.Charophytes are found in a variety of water bodies and watercourses in areas with various thicknesses of permafrost, within the boreal and subarctic climate zones. Charophyte habitats are mostly associated with the distribution of carbonate rocks. The number of species and their records decrease towards north and east mainly due to shortening of the vegetation period. The highest density of records of charophytes and their most northern records are in river valleys, where the topography and heating effect of the river mitigate the influence of climate and permafrost. This association can be used to model and predict charophyte distribution in periglacial areas during glaciation.Widespread eurytopic species with a flexible life cycle, Chara contraria and C. globularis, are found in valley water bodies due to their ability to withstand environmental variation associated with marked changes in water temperature and flood disturbance. Rare perennial species such as C. strigosa and Tolypella canadensis are represented in deep lakes with less variation in environmental conditions.This paper discusses the distribution, habitat and life history strategies of charophytes that exist in the severe conditions of the permafrost area in Far North-East Asia based on previously published and new data.     

Investigating potential future changes in surface water flooding hazard and impact

Surface water flooding (SWF) is a recurrent hazard that affects lives and livelihoods. Climate change is projected to change the frequency of extreme rainfall events that can lead to SWF. Increasingly, data from Regional Climate Models (RCMs) are being used to investigate the potential water-related impacts of climate change; such assessments often focus on broad-scale fluvial flooding and the use of coarse resolution (>12 km) RCMs. However, high-resolution (<4 km) convection-permitting RCMs are now becoming available that allow impact assessments of more localised SWF to be made. At the same time, there has been an increasing demand for more robust and timely real-time forecast and alert information on SWF. In the UK, a real-time SWF Hazard Impact Model framework has been developed. The system uses 1-km gridded surface runoff estimates from a hydrological model to simulate the SWF hazard. These are linked to detailed inundation model outputs through an Impact Library to assess impacts on property, people, transport, and infrastructure for four severity levels. Here, a set of high-resolution (1.5 km and 12 km) RCM data has been used as input to a grid-based hydrological model over southern Britain to simulate Current (1996–2009) and Future (~2100s; RCP8.5) surface runoff. Counts of threshold-exceedance for surface runoff and precipitation (at 1-, 3- and 6-hr durations) are analysed. Results show that the percentage increases in surface runoff extremes, are less than those of precipitation extremes. The higher-resolution RCM simulates the largest percentage increases, which occur in winter, and the winter exceedance counts are greater than summer exceedance counts. For property impacts, the largest percentage increases are also in winter; however, it is the 12-km RCM output that leads to the largest percentage increase in impacts. The added-value of high-resolution climate model data for hydrological modelling is from capturing the more intense convective storms in surface runoff estimates.