Abrupt Events Research Articles

Multiproxy analysis of a Lateglacial-Holocene sedimentary section in the Fuegian steppe (northern Tierra del Fuego, Argentina): Implications for coastal landscape evolution in relation to climatic variability and sea-level fluctuations

We report the sedimentology, palynology, geochemistry, and diatom assemblages of a well-dated succession at Laguna Las Vueltas (LV) on the Atlantic coast of the Fuegian steppe in southern Argentina. The multiproxy analysis offers a unique opportunity to study the evolution of this coastal landscape in relation to past sea level and climate changes in the southern high latitudes. After ca. 19,000 cal yr BP, the sediments were subjected to subaerial exposure and oxidation linked to a low sea level. The Sr/Ba ratio (a paleosalinity geochemical parameter) indicates variations in the water body level, and diatoms imply the existence of an ephemeral and brackish-freshwater body subject to desiccation under arid conditions. Between ca. 11,600 and 8500 cal yr BP, a shrubby steppe vegetation, mainly composed of Asteraceae subf. Asteroideae, spread over the coastal area associated with a shallow freshwater body and the deposition of fine-grained sediments in low-energy environments, all indicative of low effective moisture and warm climate conditions. Between ca. 8500 and 8100 cal yr BP, salt carpet vegetation, dominated by Chenopodiaceae, developed around a coastal lagoon in response to the Holocene marine transgression. The record of dinocysts and geochemical data indicate marine influence at this time. Between ca. 8100 and 7000 cal yr BP, salt marshes colonized the coastal area. A coastal lagoon with a more restricted connection to the sea and minor freshwater input are inferred, linked to the beginning of a regressive phase. After ca. 7000 cal yr BP, grassland vegetation and an open shrub stratum developed along with a shallow lake (pan) that was disconnected from the sea, following a fall in the sea level and regression. Fluctuations in the water level of the pan and changes in the physical-chemical conditions, as indicated by fluctuations in Mn/Fe, Ti/Ca, Rb/Sr, Ca, TIC and TOC, suggest abrupt events of desiccation and flooding. Between ca. 4500 and 2000 cal yr BP a drier phase is indicated by changes in vegetation, algal content and geochemistry of the pan. A grassland vegetation and a shallow lake with seasonal water level variations characterized the last 2000 years. Our results from LLV area match well with Andean forest fluctuations and regional climatic conditions recorded from Tierra del Fuego and Chilean southwest.

Past perspectives on the present era of abrupt Arctic climate change

Abrupt climate change is a striking feature of many climate records, particularly the warming events in Greenland ice cores. These abrupt and high-amplitude events were tightly coupled to rapid sea-ice retreat in the North Atlantic and Nordic Seas, and observational evidence shows they had global repercussions. In the present-day Arctic, sea-ice loss is also key to ongoing warming. This Perspective uses observations and climate models to place contemporary Arctic change into the context of past abrupt Greenland warmings. We find that warming rates similar to or higher than modern trends have only occurred during past abrupt glacial episodes. We argue that the Arctic is currently experiencing an abrupt climate change event, and that climate models underestimate this ongoing warming. In recent decades, the Arctic has warmed at over twice the global rate. This Perspective places these trends into the context of abrupt Dansgaard–Oeschger warming events in the palaeoclimate record, arguing that the contemporary Arctic is undergoing comparably abrupt climate change.

Glacial fluctuations in tropical Africa during the last glacial termination and implications for tropical climate following the Last Glacial Maximum

The tropics are a dynamic component of the global climate system and exert a profound influence on modern climate variability. Yet the role of the tropics in past climate changes, for example during the last glacial termination, is uncertain. This uncertainty is due in part to the relative paucity of terrestrial temperature records from the tropics, which hinders efforts to understand the mechanisms that influenced deglacial warming and abrupt climate events. Tropical glaciers are sensitive to changes in climate, and mapping and dating past tropical glacial fluctuations provides a valuable record of terrestrial climate changes in the low latitudes. We report a chronology of past glacial extents in the Rwenzori Mountains of Uganda (0.3°N, 29.9°E) based on 51 beryllium-10 (10Be) surface-exposure ages from three separate glacial catchments. Results indicate that Rwenzori glaciers retreated considerably during Heinrich Stadial 1. The rate of net glacial recession slowed between ∼15 and 11 ka, although glaciers continued to retreat during this period. Rwenzori glaciers then retreated more rapidly during the early Holocene. The Rwenzori glacial chronology is broadly similar to glacial chronologies reconstructed elsewhere in East Africa and in tropical South America. We suggest that this similarity may reflect coherent, tropics-wide temperature fluctuations during the last deglaciation. The Rwenzori glacial chronology provides crucial information on the global footprint of deglacial warming and abrupt climate events and, thus, the potential mechanisms that influenced tropical climate during the last glacial termination.

Urban water security assessment in the context of sustainability and urban water management transitions: An empirical study in Shanghai

Despite the relevance of urban water security in protecting water resources and supporting sustainable urban development, the literature seldom addresses sustainability and the features of urban water management practices in the assessment. Hence, based on the four sustainability dimensions (environment, society, economy and institution) and the cumulative objectives of urban water management transitions (from facilitating water supply, sewage, drainage, pollution treatment, water utilization, to resilience), this study analyzed the water security condition in Shanghai during 2011–2017 and examined the progress of the sequence of the Three-Year Action Plan on Environmental Protection and Construction implemented in Shanghai. Besides, the Criteria Importance Through Intercriteria Correlation (CRITIC) method was performed to determine indicator weights without integration of value choices. The results showed that the water security condition in Shanghai has gently variated during 2011–2013 and distinctly improved from 2013 (the single score as 0.270) to 2017 (the single score as 0.859). The condition of abrupt environmental events, water use, river quality, and pollution discharge has significantly fluctuated during the investigated period. Policy analysis pointed out that river quality improvement, pollutant control, and wastewater treatment advancement have been the focus throughout the urban water policy measures in Shanghai. However, unstable water resource availability leads to a risk of water security deterioration in Shanghai in the long term and sewage infrastructure planning shall be refined to facilitate sufficient sewage collection. Furthermore, Shanghai shall notice incorporating water use and disaster management into water policy planning to better support water utilization efficiency and urban resilience. This study contributes to the framework development and comparative assessment practices in urban water security studies. The application of the suggested framework and assessment approach can allow policymakers to analyze the improvement needs of urban water security and favor policy design.

Analysis of the Evolution of Drought, Flood, and Drought-Flood Abrupt Alternation Events under Climate Change Using the Daily SWAP Index

With the increase of drought and flood frequency, the drought-flood abrupt alternation events occur frequently. Due to the coexistence and rapid transformation of drought and flood, the drought-flood abrupt alternation events is often more harmful and threatening than the single drought or flood event to the security of the society. This study is to synthetically evaluate the evolving characteristics of drought, flood, and drought-flood abrupt alternation events under climate change, which are identified by using the Standard Weighted Average Precipitation (SWAP) index. The variability of drought, flood, and drought-flood abrupt alternation events in the future is predicted by using GCM projections, whose outputs are corrected by using a daily bias correction method. The results show that: (1) The SWAP index has the capability to judge reliably the onset, duration, and intensity over the study areas, and can be used to monitor drought-flood abrupt alternation events efficiently; (2) In the reference period (1961–2005), for the drought-flood abrupt alternation events, the frequency has a downward trend in the upper reaches and an upward trend in the lower reaches, and the spatial distribution of intensity shows a contrary law to that of frequency; (3) The frequency and intensity of drought-flood abrupt alternation events show an upward trend in the whole basin in the future period (2021–2095), under the RCP4.5 and RCP8.5 scenarios. These results indicate that drought-flood abrupt alternation events can be more frequent, and the intensity will significantly increase in the 21st century, which may likely pose a serious impact on this basin.

Contrasting impacts of the 8.2‐ and 4.2‐ka abrupt climatic events on the regional vegetation of the Hulun Lake region in north‐eastern China

ABSTRACTStudies of the vegetation response to abrupt climatic events may provide valuable insights into assessing the impacts of such events under future global warming scenarios. Here we present a detailed record of regional vegetation change based on high‐resolution pollen records for the intervals 9000–7000 and 5000–3000 cal a bp from the Hulun Lake region of north‐eastern China. Our aims were to determine the regional expression of the 8.2‐ and 4.2‐ka events in north‐eastern China, especially their effects on vegetation composition. The results show that the response of the vegetation of the Hulun Lake region to the climatic cooling of the 8.2‐ka event was relatively minor; however, the 4.2‐ka event, which resulted in climatic aridification, resulted in pronounced vegetation degradation. We suggest that the specific characteristics of the 8.2‐ and 4.2‐ka events, together with their duration and the regional climatic background on an orbital time scale, were the main factors responsible for the contrasting vegetation responses. The strengthening and northward shift of the Intertropical Convergence Zone (ITCZ) tended to offset the cooling effect of the 8.2‐ka event on regional vegetation in the mid–high latitudes of East Asia; in contrast, the decrease in summer insolation in the Northern Hemisphere and the southward shift of the ITCZ may have intensified the drought effect of the 4.2‐ka event in East Asia and the associated vegetation degradation. Under future global warming scenarios, both extreme cooling events and extreme drought events may cause a deterioration of the ecological environment; however, the resulting regional vegetation responses may be substantially different.

Socio-Ecological Contingencies with Climate Changes over the Prehistory in the Mediterranean Iberia

We conducted palynological, sedimentological, and chronological analyses of a coastal sediment sequence to investigate landscape evolution and agropastoral practices in the Nao Cap region (Spain, Western Mediterranean) since the Holocene. The results allowed for a reconstruction of vegetation, fire, and erosion dynamics in the area, implicating the role of fire in vegetation turnover at 5300 (mesophilous forests replaced by sclerophyllous scrubs) and at 3200 calibrated before present (cal. BP) (more xerophytics). Cereal cultivation was apparent from the beginning of the record, during the Mid-Neolithic period. From 5300 to 3800 cal. BP, long-lasting soil erosion was associated with the presence of cereals, indicating intense land-use during the Chalcolithic and Bronze Age periods. The decline of the agriculture signal and vegetal recolonization is likely explained by land abandonment during the Final Bronze Age. Anthropogenic markers reappeared during the Iberian period when more settlements were present. A contingency of human and environmental agencies was found at 5900, 4200, and 2800 cal. BP, coinciding with abrupt climate events, that have manifested locally in reduced spring discharge, an absence of agropastoral evidence, and a marked decline in settlement densities. This case study, covering five millennia and three climate events, highlights how past climate changes have affected human activities, and also shows that people repeatedly reoccupied the coast once the perturbation was gone. The littoral zone remained attractive for prehistoric communities despite the costs of living in an area exposed to climatic hazards, such as droughts.

Edge Detection Reveals Abrupt and Extreme Climate Events

AbstractThe most discernible and devastating impacts of climate change are caused by events with temporary extreme conditions (“extreme events”) or abrupt shifts to a new persistent climate state (“tipping points”). The rapidly growing amount of data from models and observations poses the challenge to reliably detect where, when, why, and how these events occur. This situation calls for data-mining approaches that can detect and diagnose events in an automatic and reproducible way. Here, we apply a new strategy to this task by generalizing the classical machine-vision problem of detecting edges in 2D images to many dimensions (including time). Our edge detector identifies abrupt or extreme climate events in spatiotemporal data, quantifies their abruptness (or extremeness), and provides diagnostics that help one to understand the causes of these shifts. We also publish a comprehensive toolset of code that is documented and free to use. We document the performance of the new edge detector by analyzing several datasets of observations and models. In particular, we apply it to all monthly 2D variables of the RCP8.5 scenario of the Coupled Model Intercomparison Project (CMIP5). More than half of all simulations show abrupt shifts of more than 4 standard deviations on a time scale of 10 years. These shifts are mostly related to the loss of sea ice and permafrost in the Arctic. Our results demonstrate that the edge detector is particularly useful to scan large datasets in an efficient way, for example multimodel or perturbed-physics ensembles. It can thus help to reveal hidden “climate surprises” and to assess the uncertainties of dangerous climate events.

RESPONSE OF CALCAREOUS NANNOPLANKTON TO PALEOCENE–EOCENE THERMAL MAXIMUM IN NORTHERN IRAQ

The Paleocene–Eocene Thermal Maximum (PETM) was an abrupt global warming event in the geological record. Based on calcareous nannofossils from thirteen samples of Aaliji Formation from K-116 well, Northern Iraq, sixty species are identified and can be used to divide the studied section into five biozones. Especially, the transition in the Discoaster multiraditus Biozone (CP8) occurred which is marked by higher speciation for calcareous nannofossils and the occurrences of Discoaster Tan. Such atransition is closely related to global warming during the transition from Paleocene to Eocene.

Carbon isotopes and Pa∕Th response to forced circulation changes: a model perspective

Abstract. Understanding the ocean circulation changes associated with abrupt climate events is key to better assessing climate variability and understanding its different natural modes. Sedimentary Pa∕Th, benthic δ13C and Δ14C are common proxies used to reconstruct past circulation flow rate and ventilation. To overcome the limitations of each proxy taken separately, a better approach is to produce multiproxy measurements on a single sediment core. Yet, different proxies can provide conflicting information about past ocean circulation. Thus, modelling them in a consistent physical framework has become necessary to assess the geographical pattern and the timing and sequence of the multiproxy response to abrupt circulation changes. We have implemented a representation of the 231Pa and 230Th tracers into the model of intermediate complexity iLOVECLIM, which already included δ13C and Δ14C. We have further evaluated the response of these three ocean circulation proxies to a classical abrupt circulation reduction obtained by freshwater addition in the Nordic Seas under preindustrial boundary conditions. The proxy response is shown to cluster in modes that resemble the modern Atlantic water masses. The clearest and most coherent response is obtained in the deep (> 2000 m) northwest Atlantic, where δ13C and Δ14C significantly decrease, while Pa∕Th increases. This is consistent with observational data across millennial-scale events of the last glacial. Interestingly, while in marine records, except in rare instances, the phase relationship between these proxies remains unclear due to large dating uncertainties, in the model the bottom water carbon isotope (δ13C and Δ14C) response lags behind the sedimentary Pa∕Th response by a few hundred years.

Evidence for climate instability during the Younger Dryas interval in the Hengduan Mountains, Yunnan, southwestern China

The Younger Dryas (YD) is the most recent abrupt climatic event recognized during the transition from the last glaciation to the Holocene. Growing evidence from the middle and high latitudes of Europe and North America shows that the climate within the YD event was highly unstable. Until recently, however, climate instability during this event has received little attention elsewhere in the world. Here we present evidence of a climatically unstable YD based on pollen-climate transfer function and REDFIT spectral analysis of pollen data from Haligu in the Hengduan Mountains of southwestern China, a low latitude temperate biodiversity hotspot. Our data show that the climate fluctuated significantly during the YD, with three warm peaks recognized at 11.5, 11.7 and 12.3 cal kyr BP. In particular, we estimate a gradual rise in mean annual temperature by 3.7 °C between the end of the YD and the early Holocene (11.2–10.9 cal kyr BP). In response to this, the subalpine conifer forest dominated by Picea and Abies may have migrated about 610 m upwards. We also detected that the climate fluctuations during the YD-Holocene transition show significant centennial periodicities of 76 yr, 84 yr, 91 yr, 95 yr, and 638 yr. The former four cycles are possibly related to the Gleissberg (around 80 yr) solar cycle, which was also detected in the Hongyuan peat, the PC-6 core of the East China Sea and the GISP2 ice core. These new findings enhance our understanding of millennial-scale abrupt climate events which may be of great significance in predicting the impact of future climate change on mountainous ecosystems in southwestern China. The warming at the end of the YD may provide a potential analog for global warming in the near future, during which the upward migration of vegetation zones will likely lead to the loss of habitat for threatened species. We therefore should take action to restore habitats and provide protected areas for species at risk.

A Gas-Emission Crater in the Erkuta River Valley, Yamal Peninsula: Characteristics and Potential Formation Model

Methane is a powerful greenhouse gas, and the abrupt degassing events that recently have formed large craters on the Russian Arctic Yamal and Gydan Peninsulas have caused major concern. Here we present field data on cover sediments and evolution of a gas-emission crater discovered in the Erkuta–Yakha River valley in the southern Yamal Peninsula in June 2017. The crater is located south of other similar craters discovered over the past decade in northern West Siberia. Data were collected during a field trip to the Erkuta crater in December 2017 which included field observations and sampling of permafrost soil and ground ice from the rim of the crater. All soil and ice samples were measured for contents of methane and its homologs (ethane and propane) and carbon dioxide. The contents of carbon dioxide in some samples are notably higher than methane. The strongly negative δ13С of methane from ground ice samples (−72‰) is typical of biogenic hydrocarbons. The ratio of methane to the total amount of its homologs indicate a component of gases that have migrated from a deeper, thermogenic source. Based on obtained results, a potential formation model for Erkuta gas-emission crater is proposed, which considers the combined effect of deep-seated (deep gas migration) and shallow (oxbow lake evolution and closed talik freezing) causes. This model includes several stages from geological prerequisites to the lake formation.

Asymmetric response of the subtropical western South Atlantic thermocline to the Dansgaard-Oeschger events of Marine Isotope Stages 5 and 3

The last glacial period (116–11.7 ka BP) was an interval characterized by a sequence of abrupt millennial-scale events well documented in Greenland and Antarctica ice-cores. Throughout this period, Greenland cold stadials were accompanied by warm conditions in the thermocline to intermediate waters of the Atlantic Ocean that may have played a role in both the basal melting of ice shelves and the rapid atmospheric warming during the onset of warm interstadials. Climate model simulations indicated an accentuated response of the subtropical western South Atlantic thermocline to the disturbances in the Atlantic circulation. Such works encourage investigations upon thermocline/deep-dwelling planktic foraminifera in this region; however, a study with this aim was not performed. Here we present a paleoceanographic reconstruction from the subtropical western South Atlantic based on the thermocline planktic foraminifera Globorotalia inflata. Our high-resolution δ18O record for the last glacial period presents a millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles during Marine Isotope Stage (MIS) 5. During MIS 3, this millennial-scale variability is absent or considerably dampened. Mg/Ca-derived temperature and seawater δ18O corrected for ice-volume for the MIS 5 interval demonstrate that the region was warmer and saltier (colder and fresher) during early-glacial stadials (interstadials). Our data suggest a reorganization of the northward heat transport throughout the last glacial, in which regions as far south as 24 °S acted as heat reservoirs in periods of weakened Atlantic Meridional Overturning Circulation during MIS 5 but not necessarily (or only marginally) during MIS 3.

The projected timing of abrupt ecological disruption from climate change.

As anthropogenic climate change continues the risks to biodiversity will increase over time, with future projections indicating that a potentially catastrophic loss of global biodiversity is on the horizon1-3. However, our understanding of when and how abruptly this climate-driven disruption of biodiversity will occur is limited because biodiversity forecasts typically focus on individual snapshots of the future. Here we use annual projections (from 1850 to 2100) of temperature and precipitation across the ranges of more than 30,000 marine and terrestrial species to estimate the timing of their exposure to potentially dangerous climate conditions. We project that future disruption of ecological assemblages as a result of climate change will be abrupt, because within any given ecological assemblage the exposure of most species to climate conditions beyond their realized niche limits occurs almost simultaneously. Under a high-emissions scenario (representative concentration pathway (RCP)8.5), such abrupt exposure events begin before 2030 in tropical oceans and spread to tropical forests and higher latitudes by 2050. If global warming is kept below 2 °C, less than 2% of assemblages globally are projected to undergo abrupt exposure events of more than 20% of their constituent species; however, the risk accelerates with the magnitude of warming, threatening 15% of assemblages at 4 °C, with similar levels of risk in protected and unprotected areas. These results highlight the impending risk of sudden and severe biodiversity losses from climate change and provide a framework for predicting both when and where these events may occur.

Combined unsteady denudation and climatic gradient factors constrain carbonate landscape evolution: New insights from in situ cosmogenic 36Cl

Abstract In situ cosmogenic nuclides are widely employed to quantify topographic and geomorphic landscape evolution. However, few studies are devoted to investigating possibly unsteady denudation. In this work the denudation rates of five surface bedrock and one depth profile are measured using in situ cosmogenic 36Cl in different climate zones in China; cosmogenic 36Cl is a powerful proxy for the surface denudation processes of carbonate landscapes in different climatic gradients since carbonates are more sensitive to climate change than silicates. Compiling previous worldwide surface denudation studies in different gradients, we find that: (I) denudation rate increases systematically with mean annual precipitation (MAP) in arid climatic zones (MAP lower than ~700 mm yr−1) and is dominated by long-term climatic gradients; (II) humid climatic zone (MAP greater than ~700 mm yr−1) denudation rates are about constrained to 34.1 ± 11.7 mm kyr−1 by carbonate dissolution kinetics; and (III) denudation rate is also influenced by short-term physical erosion or abrupt erosive events. Accordingly, the reconstructed evolutionary history of the Guizhou depth profile results from either a relatively high denudation rate (55.5–4.4 +5.6 mm kyr−1) and long duration exposure (274–147 +120 ka), or an alternative a low denudation rate (0.8–0.8 +13.3 mm kyr−1) and a recent abrupt erosive event (13.0–1.6 +1.9 ka). These two possible solutions suggest that assumed long-term averaged denudation rate can conceal episodic erosion events. Given tectonic quiescence we propose that climate change might mainly control an erosive event during the last glacial termination (~14 ka), which also implies that caution should be taken when determining the denudation rate of a landscape where climate change and/or tectonic activity plays an important role.

Characterizing over Four Decades of Forest Disturbance in Minnesota, USA

Spatial information about disturbance driven patterns of forest structure and ages across landscapes provide a valuable resource for all land management efforts including cross-ownership collaborative forest treatments and restoration. While disturbance events in general are known to impact stand characteristics, the agent of change may also influence recovery and the supply of ecosystem services. Our study utilizes the full extent of the Landsat archive to identify the timing, extent, magnitude, and agent, of the most recent fast disturbance event for all forested lands within Minnesota, USA. To account for the differences in the Landsat sensors through time, specifically the coarser spatial, spectral, and radiometric resolutions of the early MSS sensors, we employed a two-step approach, first harmonizing spectral indices across the Landsat sensors, then applying a segmentation algorithm to fit temporal trends to the time series to identify abrupt forest disturbance events. We further incorporated spectral, topographic, and land protection information in our classification of the agent of change for all disturbance patches. After allowing two years for the time series to stabilize, we were able to identify the most recent fast disturbance events across Minnesota from 1974–2018 with a change versus no-change validation accuracy of 97.2% ± 1.9%, and higher omission (14.9% ± 9.3%) than commission errors (1.6% ± 1.9%) for the identification of change patches. Our classification of the agent of change exhibited an overall accuracy of 96.5% ± 1.9% with classes including non-disturbed forest, land conversion, fire, flooding, harvest, wind/weather, and other rare natural events. Individual class errors varied, but all class user and producer accuracies were above 78%. The unmatched nature of the Landsat archive for providing comparable forest attribute and change information across more than four decades highlights the value of the totality of the Landsat program to the larger geospatial, ecological research, and forest management communities.

Variation of summer monsoon intensity in the North China Plain and its response to abrupt climatic events during the early-middle Holocene

Ningjinpo paleolake was the largest lake in the North China Plain during the Holocene, and its continuous sedimentary record has the potential to reconstruct the summer monsoon intensity and determine the limnological response to abrupt climatic events at a high resolution. Here we use AMS 14C dating and analyses of grain-size and major element contents, and the results show that: (1) During 11.5–10.0 cal kyr BP, the geochemical climate change index (C) (1.16), sediment accumulation rate (SAR) (0.007 cm/yr) and median particle diameter (Md) (20.6 μm) are the lowest within the section, while the geochemical drought index (0.0323), Fe/Mn ratio (0.0088) and clay content (17.3%) are the highest, which indicate that the North China Plain received less precipitation, and chemical weathering and erosional intensity in the Ningjinpo paleolake catchment were weak. (2) During 10.0–7.7 cal kyr BP, the C index (1.36), content of sand (19.1%) and silt (68.0%), Md (27.2 μm) and SAR (0.019 cm/yr) increase significantly. These characteristics indicate that the climate gradually became wetter, chemical weathering and erosion intensity increased, and there was increased runoff to the lake, which raised the lake level. (3) During 7.7–5.5 cal kyr BP, the C index (1.56), silt content (71.8%) and SAR (0.030 cm/yr) increase to the highest, while the Fe/Mn ratio (0.0068), sand content (14.4%) and Md (21.4 μm) decrease to their lowest levels. These suggest the climate was warm and humid, the runoff to the lake was large, chemical weathering in the catchment was strong, and the water level of the lake reached the maximum for the Holocene. (4) During 5.5–3.3 cal kyr BP, the C index (1.26) and SAR (0.011 cm/yr) decrease substantially, whereas the sand content (28.2%) and Md (18.3 μm) increase to the highest levels. These characteristics demonstrate that the climate gradually became arid, the runoff to Ningjinpo paleolake was reduced, the intensity of chemical weathering and erosion in the catchment weakened, and the lake level decreased, although the high frequency of floods in this period. (5) In addition to the globally-recorded abrupt climatic events at ~8.2 cal kyr BP, ~5.3 cal kyr BP and ~4.2 cal kyr BP, which were recorded in the northern monsoon region of China, an abrupt climatic event in the middle Holocene, at ~7.2 cal kyr BP, was also recorded. In the northern monsoon region of China, these events may have been amplified by unusually active ENSO since the middle Holocene.

Controls on the Tropical Response to Abrupt Climate Changes

AbstractAbrupt climate change events during glacial times have distinct tropical imprints, despite their cause being at high latitudes. The mechanisms by which high latitudes affect low latitudes are currently unclear. We present climate model simulations of a set of different abrupt events and find changes in tropical rainfall can not be comprehensively explained by either changes in sea ice extent or ocean circulation. Changes in tropical meridional temperature gradients are the clearest way to explain tropical rainfall changes across all events. We find some tropical regions are unlikely to record Heinrich events because the rainfall is moved far enough away from them during stadial periods that they are insensitive to further change. Greenland temperature responds linearly to Atlantic sea ice extent, suggesting that the absence of change in Greenland temperature during Heinrich events implies no sea ice expansion, despite major changes in the climate system elsewhere.

Research on political instability, uncertainty and risk during 1953–2019: a scientometric review

We are now living in an integrated world full of abrupt political events and socio-political uncertainty. A comprehensive scientometric review in this aspect is useful but lacked in the existing literature. This paper fills the void by using VOSviewer and CiteSpace to systematically analyze the 4200 papers on “political instability”, “political uncertainty” and “political risk” during 1953–2019 from Web of Science Core Collection. We present the annual distribution of papers, the related disciplines, the mainstream journals, hot topics, references with the strongest citation bursts, recent publications with transformative potentials, as well as the collaboration networks of authors, organizations and countries. The hot spots include (1) the causes and impact of political instability, (2) the relationship among political uncertainty, energy price and macroeconomic/financial factors, and (3) the determinants of foreign direct investments. Rangan Gupta from University of Pretoria in South Africa has the most publications in this field, while Witold J Henisz from University of Pennsylvania in the USA has the most citations. As the first endeavor to do comparative study of easily confused terminologies with bibliometric analysis, we find that though literature on “political instability” and “political risk” appeared earlier and have larger volume, research on “political uncertainty” is currently more active and catching up fast. Keywords with on-going citation bursts, two thirds of active co-citation clusters, and potentially transformative papers published recently are all related to political/policy uncertainty. The three strands of literature have obvious overlap and meanwhile their own special foci. Main areas of research on political instability, political uncertainty and political risk

Radiocarbon Dating of Small-sized Foraminifer Samples: Insights into Marine sediment Mixing

ABSTRACTRadiocarbon (14C) can be used to build absolute chronologies and reconstruct ocean ventilation over the last 40 ka. Sample size requirements have restricted14C measurements in marine cores with low foraminifer content, impeding14C-based studies focused on abrupt climate events. Recent developments have demonstrated that small-sized foraminifer samples can now be dated using a gas introduction system at the cost of a small decrease in precision. We explore the potential of gas measurements on benthic and planktonic foraminifers from core SU90-08 (43°03′1″N, 30°02′5″W, 3080 m). Gas measurements are accurate, reproducible within 2σ uncertainty and comparable to graphite measurements. Both techniques yield negative14C benthic-planktonic (B-P) age-offsets after Heinrich event 1. We argue that negative B-P ages result from bioturbation and changes in foraminifer abundances, with the chance of negative B-P especially increased when the14C age gradient between the deep and surface waters is decreased. Small-sized14C measurements seem to capture the variance of the foraminifera age distribution, revealing the active mixing in those archives. Sediment deposition and mixing effects possibly pose a greater obstacle for past14C-based dating and ocean ventilation reconstructions than the measurement precision itself, particularly in relatively low sedimentation rate settings.