Major Forcing Factor Research Articles

Diverse effects of climate changes and anthropogenic activities on sedimentary nitrogen isotope (δ15Nsed.) in lakes across different regions of China since the late Holocene

Nitrogen plays a vital role in the Earth's systems. Nitrogen isotopes have been widely used in environmental and climatic research. Various biogeochemical processes and nitrogen sources contribute to the sedimentary organic matter. Therefore, environmental implications of sedimentary nitrogen isotope (δ15Nsed.) require further investigation. In this study, we report and compile the δ15Nsed. records from eight lakes in China influenced by either climate changes or anthropogenic activities since the late Holocene to determine the major forcing factors of δ15Nsed.. Our results showed that: (1) During the late Holocene, lacustrine δ15Nsed. was mainly regulated by natural climate changes, such as temperature, precipitation, or both as anthropogenic activities were weak. (2) The impact of gradually increasing anthropogenic disturbances on δ15Nsed. in the different lakes varied. Globally, the declining lacustrine δ15Nsed. values could be attributed to atmospheric reactive nitrogen deposition. However, on the regional catchment scale, the increased δ15Nsed. values in lakes may be related to increased nutrient input and higher primary production caused by increased anthropogenic activities, whereas the rapid decline of δ15Nsed. values in lakes is possibly related to more nutrient input and 15N-depleted organic matter input resulting excessive anthropogenic activities. Our study suggests that the impact of anthropogenic activities on lake ecosystems gradually increased in recent decades, and the amplitudes in variations of δ15Nsed. values in some lakes exceeded those during periods of weak anthropogenic disturbances. If not properly managed, anthropogenic disturbances may outweigh nature (i.e., temperature, precipitation) as the leading cause of lake ecosystem changes in the future.

Improvement and Uncertainties of Global Simulation of Sulfate Concentration and Radiative Forcing in CESM2

AbstractSulfate is a major atmospheric pollutant and radiative forcing (RF) factor that influences air quality, cloud microphysics, and climate. Therefore, a better evaluation of sulfate concentrations and RF patterns is essential for policy‐making and the management of air pollution and climate change. This study comprehensively estimates the global distribution of sulfate concentrations and RFs and analyzes the sources of uncertainty in the Community Earth System Model version 2 and the Parallel Offline Radiative Transfer model. Compared with the observations, the incorporation of detailed in‐cloud aqueous‐phase chemistry and the enhanced wet deposition (WD) flux of sulfate significantly improved the simulations of sulfur species both near the ground and at high altitudes, which is beneficial for a more accurate estimation of the global RF of sulfate. The improved simulated RF of sulfate from 1850 to 2015 is −0.502 Wˑm−2. This study finds that WD is the key process governing both the horizontal and vertical distributions of sulfate concentrations. The overestimation of surface sulfate and the underestimation of high‐altitude sulfate made by the model are essential uncertainty factors of the sulfate RF estimation. This study emphasizes the importance of improving the simulation of global sulfate distribution as well as its RF, which may strongly pressure the near‐future warming potential when witnessing a rapid transition to a carbon neutral world that is phasing out fossil fuel. A more accurate assessment of sulfate levels and radiation effects will play a remarkable guiding role in the formulation of global emission reduction‐related policies in the future.

Can Ammonium Records in Antarctic Ice Cores be a Proxy for Sea Ice Fluctuations?

Abstract Ammonium (NH4+) accumulation and variability in Antarctic ice cores possess signals of various biological, anthropogenic, and atmospheric processes. However, the use of NH4+ variability in Antarctic ice cores for paleoclimate reconstructions is not well established. Addressing this, we have utilized, published NH4+ ice core records from the Dronning Maud Land (DML), Antarctica to assess the sources of NH4+, its transport, major forcing factors, and their mechanism in controlling the variability of ammonium signals in coastal ice cores. The first principle component (PC1) derived from PC analysis of NH4+ data comprises ~43% of total variability and also depicts a significant spatial correlation with the NSIDC Sea ice concentration (SIC) variability in the Weddell Sea region of the Southern Ocean. In addition, the PC1 shows strong 2-9 year periodicities related to El Niño Southern Oscillation along with two major decadal to centennial periodicities of ~40 and 100 years. The control of sea ice fluctuations and related oceanic processes (ENSO) provide evidence of the marine sources and influence of ocean-related processes as a major controlling factor for the NH4+ variability in DML ice cores. The cumulative accumulation of the NH4+ produced from enhanced biological decomposition, and unutilized ammonium during nitrification under high SIC conditions are possible mechanisms for the increased supply. The correlation of ERA5 surface air temperatures with Nino3.4 suggests that during El Niño conditions, the air temperatures significantly dropped in the Weddell Sea sector, fostering the high SIC conditions. The findings of this study regarding the role of the SIC in the ammonium variability in Antarctic ice cores would help us better interpret the paleo NH4+ records and their applications to deduce the paleoclimate conditions in coastal Antarctic regions.

Occurrence and Discrepancy of Surface and Column Mole Fractions of CO2 and CH4 at a Desert Site in Dunhuang, Western China

Carbon dioxide (CO2) and methane (CH4) are the two major radiative forcing factors of greenhouse gases. In this study, surface and column mole fractions of CO2 and CH4 were first measured at a desert site in Dunhuang, west China. The average column mole fractions of CO2 (XCO2) and CH4 (XCH4) were 413.00 ± 1.09 ppm and 1876 ± 6 ppb, respectively, which were 0.90 ppm and 72 ppb lower than their surface values. Diurnal XCO2 showed a sinusoidal mode, while XCH4 appeared as a unimodal distribution. Ground observed XCO2 and XCH4 were compared with international satellites, such as GOSAT, GOSAT-2, OCO-2, OCO-3, and Sentinel-5P. The differences between satellites and EM27/SUN observations were 0.26% for XCO2 and −0.38% for XCH4, suggesting a good consistency between different satellites and ground observations in desert regions in China. Hourly XCO2 was close to surface CO2 mole fractions, but XCH4 appeared to have a large gap with CH4, probably because of the additional chemical removals of CH4 in the upper atmosphere. It is necessary to carry out a long-term observation of column mole fractions of greenhouse gases in the future to obtain their temporal distributions as well as the differences between satellites and ground observations.

Abrupt Indian summer monsoon shifts aligned with Heinrich events and D-O cycles since MIS 3

The tropics experienced a unique monsoon climate that drove important changes in socio-economic conditions and shaped different phases of human history during the late Quaternary across the Indian subcontinent. This study presents a new high-resolution speleothem-record of Indian summer monsoon (ISM) variability ranging from ~45,000 to 34,000 yr BP combined with a published record up to 5500 yr BP from the Mawmluh cave, northeastern India. We document significant changes in solar insolation and precession harmonics, implicating solar activity as one of the major forcing factors for rapid monsoonal shifts. Our new δ 18 O time series shows high-amplitude shifts in the ISM during the marine isotope stage 3 (MIS 3), similar to Dansgaard-Oeschger (D-O) oscillations recorded at high northern latitudes, which mimic the Greenland ice sheet record. The sun driven Northern Hemisphere high latitude climate anomalies played an active role in driving ISM variability and changes in the regional hydrological cycle on centennial to millennial time scales across the Indian subcontinent. • Longest isotope record of speleothem from the South Asian region • Saw-tooth like isotope variability during MIS 3 • Changes in the NH temperatures impacted the hydroclimatic conditions in the Indian subcontinent. • Source region and transport pathways controlled the speleothem isotopic composition.

An assessment of coastal vulnerability using geospatial techniques

The Sundarbans, a UNESCO world heritage site, is of immense importance for its ecological and economic utilities. This paper attempts to identify, assess and classify the degree of vulnerability along the 269 km coastal stretches of Indian Sundarbans using GIS-based composite index. This study implies five physical and one social variable (geomorphology, regional elevation, absolute sea level rise, erosion-accretion, tidal range and population density) for calculating Coastal Vulnerability Index (CVI). The results indicate that nearly half of the coastlines (114 km) are under moderate to very high vulnerability regime. Inhabited mouzas like Beguakhali and Sibpur are found to be highly vulnerable on account of its high population densities, reclaimed coastline, moderate elevation, moderate erosion and medium tidal range. Lower elevation and higher rate of sea level rise were responsible for high vulnerability in uninhabited islands like Bhangadauni, Gosaba-E, Island-2 and Harin-W. Among the geomorphic categories, supra-tidal areas mostly belong to the high vulnerability category. This study provides valuable information to the decision makers and planners for understanding the problems and formulating suitable management strategies for this area. It also clarifies the major forcing factors and concludes that prioritization should be done based on relative importance, even if complete management is not feasible.

Transverse fluvial sedimentation in forelands: a numerical modelling approach for characterising petroleum reservoirs

Valley fills are important fluvial reservoir elements in producing foreland basins. The architecture, sizes and orientations of these fluvial sandbodies, however, present strong subsurface uncertainties to reservoir geologists. The well-exposed thick-bedded sandy channels and shale wedges of the Pennsylvanian Upper Breathitt Group, Central Appalachian Basin, Kentucky, provides an opportunity to construct a 3D model of the forcing factors for valley fill sedimentation styles in foreland basins. A numerical code and a geophysical software, Petrel, were used in a series of experiments to simulate the major forcing factors (sediment supply, rate of sea-level changes and rate of subsidence) of fluvial sedimentation in forelands. The subsidence rate was empirically calculated from a single-layer decompaction procedure at 40% pre-compaction porosity and 23% average subsurface porosity. Results showed that thicker sediment accumulated toward the orogen, and that sediment bodies thin down progressively away from it as a result of progradation. Varying the subsidence rates requires a simultaneous change in sediment influx from the hinterland for sedimentation style to have any significant preserved impact. Subsidence effectively forced an asymmetrical sedimentation that is tilted toward the orogen. Changes in eustasy revealed a complex environmental conditions where fluvial streams were restricted in advances and in competences to reveal any impact on sedimentation, while an increase in sediment influx resulted in aggradation and progradation farther into the basin. Overall, the impacts of sediment supply and subsidence rates show predictable fluvial styles whereas varying sea-level changes is less-predictable. These findings provide useful insights into the management of subsurface uncertainties in foreland reservoirs.

Stocking rate impacts on tallgrass prairie landscape carbon fluxes

AbstractCarbon sequestration in ecosystems reduces atmospheric CO2, a major forcing factor in global warming. We aimed to determine the ecosystem C balance under heavy and moderate stocking rates or ungrazed (UG) pastures during the first 2 yr of a 3‐yr period and to determine if changing to moderate stocking on a previously heavily stocked pasture would result in compensatory ecosystem C retention. During 2003 and 2004, net ecosystem C exchange (NEEc) was measured with eddy correlation towers on pastures that were UG, stocked at 2 acres per steer until the middle of the grazing season [intensive early stocking (IES)], and season‐long stocking at the high rate of 2 acres per steer (SLS‐H). In 2005, the SLS‐H pasture changed to a season‐long moderate stocking rate of 4 acres per steer (SLS‐H‐M). Net ecosystem C exchange was summed over an entire year from the burn date to the burn date the following year minus the C lost from burning and livestock export to estimate C balance. During the growing season, SLS‐H reduced ecosystem C retention compared with UG and IES in 2003 and 2004 (+223, +212, and –669 lb acre–1 for UG, IES and SLS‐H). In 2005, when the stocking rate on the SLS‐H pasture was changed to SLS‐H‐M, C retention was greater than under UG and IES (+107, +89, and +205 lb acre–1 for UG, IES and SLS‐H‐M). We conclude that following stocking at an abusive rate, there is compensatory C retention resulting from greater nutrient and water availability.

Spatiotemporal Variability of Remote Sensing Ocean Net Primary Production and Major Forcing Factors in the Tropical Eastern Indian and Western Pacific Ocean

Based on widely used remote sensing ocean net primary production (NPP) datasets, the spatiotemporal variability of NPP is first analyzed over the tropical eastern Indian and western Pacific Ocean for the period 1998–2016 using the conventional empirical orthogonal function (EOF), the lead–lag correlation and the ensemble empirical mode decomposition (EEMD) technique. Barnett and Preisendorfer’s improved Canonical Correlation Analysis (BPCCA) is also applied to derive covariability patterns of NPP with major forcing factors of the chlorophyll a concentration (Chla), sea surface temperature (SST), sea level anomaly (SLA), ocean rainfall (Rain), sea surface wind (Wind), and current (CUR) under climate changes of El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). We find that: (1) The first two seasonal EOF modes capture significant temporal and meridional NPP variability differences, as NPP reaches peaks approximately three months later in the western Pacific Ocean than that of in the eastern Indian Ocean. (2) The second and third interannual EOF modes are closely related with ENSO with a two-month lag and synchronous with IOD, respectively, characterized by southwesterly positive anomaly centers during positive IOD years. (3) NPP presents different varying tendencies and similar multiscale oscillation patterns with interannual and interdecadal cycles of 2~3 years, 5~8 years, and 9~19 years in subregions of the Bay of Bengal, the South China Sea, the southeastern Indian Ocean, and the northwestern Pacific Ocean. (4) The NPP variability is strongly coupled with negative SST, SLA, and Rain anomalies, as well as positive Chla, Wind and CUR anomalies in general during El Niño/positive IOD years. The results reveal the diversity and complexity of large-scale biophysical interactions in the key Indo-Pacific Warm Pool region, which improves our understanding of ocean productivity, ecosystems, and carbon budgets.

Fish thermal habitat current use and simulation of thermal habitat availability in lakes of the Argentine Patagonian Andes under climate change scenarios RCP 4.5 and RCP 8.5

Habitat use in relation to the thermal habitat availability and food source as a forcing factor on habitat selection and use of Percichthys trucha (Creole perch), Oncorhynchus mykiss (rainbow trout), Salmo trutta (brown trout) and Salvelinus fontinalis (brook trout) were determined as well as future potential thermal habitat availability for these species under climate change scenarios Representative Concentration Pathways 4.5 and 8.5. This study was conducted in three interconnected lakes of Northern Patagonia (Moreno Lake system). Data on fish abundance was obtained through gill netting and hydroacoustics, and thermal profiles and fish thermal habitat suitability index curves were used to identify current species-specific thermal habitat use. Surface air temperatures from the (NEX GDDP) database for RCP scenarios 4.5 and 8.5 were used to model monthly average temperatures of the water column up to the year 2099 for all three lakes, and to determine potential future habitat availability. In addition, data on fish diet were used to determine whether food could act as a forcing factor in current habitat selection. The four species examined do not use all the thermally suitable habitats currently available to them in the three lakes, and higher fish densities are not necessarily constrained to their “fundamental thermal niches” sensu Magnuson et al. (1979), as extensive use is made of less suitable habitats. This is apparently brought about by food availability acting as a major forcing factor in habitat selection and use. Uncertainties related to the multidimensionality inherent to habitat selection and climate change imply that fish resource management in Patagonia will not be feasible through traditional incremental policies and strategic adjustments based on short-term predictions, but will have to become highly opportunistic and adaptive.

Assessing the natural and anthropogenic influences on basin-wide fish species richness

Theory predicts that the number of fish species increases with river size in natural free-flowing rivers, but the relationship is lost under intensive exploitation of water resources associated with dams and/or landscape developments. In this paper, we aim to identify orthomorphic issues that disrupt theoretical species patterns based on a multi-year, basin-wide assessment in the Danshuei River Watershed of Taiwan. We hypothesize that multiple human-induced modifications fragment habitat areas leading to decreases of local fish species richness. We integrally relate natural and anthropogenic influences on fish species richness by a multiple linear regression model that is driven by a combination of factors including river network structure controls, water quality alterations of habitat, and disruption of channel connectivity with major discontinuities in habitat caused by dams.We found that stream order is a major forcing factor representing natural influence on fish species richness. In addition to stream order, we identified dams, dissolved oxygen deficiency (DO), and excessive total phosphorus (TP) as major anthropogenic influences on the richness of fish species. Our results showed that anthropogenic influences were operating at various spatial scales that inherently regulate the physical, chemical, and biological condition of fish habitats. Moreover, our probability-based risk assessment revealed causes of species richness reduction and opportunities for mitigation. Risks of species richness reduction caused by dams were determined by the position of dams and the contribution of tributaries in the drainage network. Risks associated with TP and DO were higher in human-activity-intensified downstream reaches. Our methodology provides a structural framework for assessing changes in basin-wide fish species richness under the mixed natural and human-modified river network and habitat conditions. Based on our analysis results, we recommend that a focus on landscape and riverine habitats and maintaining long-term monitoring programs are crucial for effective watershed management and river conservation plans.

Analytical model of sea level elevation during a storm: Support for coastal flood risk assessment associated with cyclone passage

Sea level oscillations are a result of continuous astronomic, oceanographic, and atmospheric interactions on different time and intensity scales. Thus, the collective action of forcing factors such as tide, wind, atmospheric pressure, and wave action may lead to elevated sea levels during cyclone events over the continental shelf, abruptly impacting adjacent coasts. The objective of this study is to evaluate the potential risks of sea level rise and coastal flooding associated with the passage of cyclones in southern Brazil. An analytical model was developed based on extreme storm events from 1997 to 2008. The model identifies the impact of each forcing factor during temporary sea level rise. Through the development of a digital terrain model, it was possible to identify the areas most vulnerable to flooding by superimposing the terrain model onto calculated sea levels. During storm events, sea level elevations ranged from 2 to 5m and show wind as the major forcing factor, followed by swells waves, astronomical tide and finally atmospheric pressure.

Millennial-scale variability in biomass burning covering the interval ~ 41,000–7050 cal BP in the tropical Leizhou Peninsula (south China)

Abstract Here, we present a well-dated terrestrial charcoal chronology covering the interval ~ 41,000–7050 cal BP and identifying the millennial-scale variability in biomass burning for the tropical Leizhou Peninsula, south China. Our results show that changes in biomass burning closely followed regional temperature and precipitation variations on orbital timescales, i.e., more biomass burning occurred during the warm/wet Marine Isotope Stage (MIS) 3, and less biomass burning occurred during the cold/dry MIS 2. Superimposed on this general trend, our charcoal record shows millennial-scale variability in biomass burning corresponding to the rapid climate changes associated with Dansgaard–Oeschger (D–O) cycles, i.e., biomass burning increased during D–O warming events, whereas biomass burning decreased during intervals of rapid cooling terminated by Heinrich events. During the cooler/drier periods, low biomass burning can be explained by the decline of vegetation productivity, whereas the warmer/wetter periods are characterized by higher biomass burning related to increasing fuel availability. Therefore, climate control can be considered as the major forcing factor of biomass burning from 41,000–7050 cal BP on both orbital and millennial timescales in the northern Leizhou Peninsula.

The Influence of Climatic Variations on River Delta Hydrodynamics and Morphodynamics in the Parnaíba Delta, Brazil

ABSTRACT Aquino da Silva, A.G.; Stattegger, K.; Schwarzer, K.; Vital, H., and Heise, B., 2015. The influence of climatic variations on river delta hydrodynamics and morphodynamics in the Parnaiba Delta, Brazil. In this work, we present a baseline study on the geo-environmental, morphodynamical, and hydrodynamical conditions of the Parnaiba River and its mouth, the Parnaiba River Delta (PRD). The outline of the PRD indicates that waves are the major forcing factor for delta development. Furthermore, the shape of the delta suggests that there is a considerable degree of asymmetry in its formation. The asymmetry is the result of coastal current influence on the reworking of the sediments that are discharged by the river. To investigate the hydrodynamic conditions of the area, field measurements were conducted on the river, offshore of the PRD, and on the tidal channels of the west part of the delta. At these locations, the currents, salinity, temperature, and tidal dynamics were measured. In addition, a time...

Response of OceanSat II scatterometer winds in the Bay of Bengal circulation

Surface wind is one of the major forcing factors in any ocean circulation model. The response of satellite-derived OceanSat II scatterometer (OSCAT) winds during spring (February and March) 2010 in the Bay of Bengal (BOB) surface circulation is described in this study. Wind stress is calculated from wind speed derived from OSCAT by the bulk-aerodynamic formula. The Regional Ocean Modeling System is used in this study because it is a free-surface, terrain-following, primitive-equations ocean model widely used by the scientific community for a diverse range of applications. The model is used after a climatological simulation with Comprehensive Ocean–Atmosphere Data Set (COADS) forcing when the model simulation reached the annual cycle. The paper also carried out a comparative study of National Centers for Environmental Prediction (NCEP) forcing over the same time period. The comparison of model-simulated surface temperature to National Oceanic and Atmospheric Administration (NOAA) sea surface temperature (SST) indicates that meso-scale features in the BOB are resolved due to the finer resolution of this model. Comparisons of water mass characteristics to the available ARGO floats show good agreement in different locations within the BOB. This study confirms the usefulness of OSCAT winds in simulating the meso-scale feature in the BOB.

Environmental and climatic changes in central Chilean Patagonia since the Late Glacial (Mallín El Embudo, 44° S)

Abstract. Multi-millennial environmental and climatic changes in central Chilean Patagonia (44–49° S) during the Last Glacial–Interglacial cycle have been of particular interest as changes in the position and strength of the southern westerlies are the major forcing factor conditioning the environmental dynamics. Recent attempts to reconstruct regional environmental and climatic signals from central Chilean Patagonia reveal some discrepancies and unclear issues among the records. This paper presents the 13 ka pollen and charcoal records from Mallín El Embudo (44° 40' S, 71° 42' W) located in the deciduous Nothofagus forest in the middle Río Cisnes valley. The paper aims to (1) establish the timing and magnitude of local vegetation changes and fire activity since the Late Glacial and (2) integrate these results at the regional scale in order to discuss the discrepancies and depict the environmental and climatic dynamics in central Chilean Patagonia since the Late Glacial. Open landscapes dominated by grasses associated with scattered Nothofagus forest patches dominated the middle Río Cisnes valley between 13 and 11.2 ka suggesting low effective moisture but also indicating that landscape configuration after glacial retreat was still ongoing. At 11.2 ka, the sudden development of an open and quite dynamic Nothofagus forest probably associated with the synchronous high fire activity occurred, suggesting a rise in effective moisture associated with dry summers. Since 9.5 ka, the record reflects the presence of a closed Nothofagus forest related to higher effective moisture conditions than before combined with moderate dry summers that may have triggered a high frequency of low-magnitude crown fires that did not severely affect the forest. The forest experienced a slight canopy opening after 5.7 ka, probably due to slightly drier conditions than before followed by a sudden change to open forest conditions around 4.2 ka associated with fire and volcanic disturbances. Around 2 ka, the recovery of a closed Nothofagus forest related to slightly wetter conditions (similar to present) occurred and persisted under highly variable climatic conditions up to 0.1 ka when massive forest burning and logging due to European settlements occurred. Central Chilean Patagonian climatic and environmental changes at millennial–centennial timescales since the Late Glacial were driven by changes in the southern westerlies latitudinal position and/or intensity, but during the late Holocene fire, volcanism and humans arose as forces contributing to environmental dynamics.

Residual currents and transport pathways in the Tagus estuary, Portugal: the role of freshwater discharge and wind

ABSTRACT Vaz, N.,Dias, J.M., 2014. Residual currents and transport pathways in the Tagus estuary, Portugal: the role of freshwater discharge and wind. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 610–615, ISSN 0749-0208. Estuaries are interface areas between rivers and the coastal sea. They also receive point discharges of contaminants which can affect estuarine biogeochemistry. The study of residual circulation in estuaries is a key issue since it is related to the transport and dispersal of dissolved and non-dissolved substances. This study investigates the residual flows in a coastal plain estuary (the Tagus estuary, Portugal) taking into account the major forcing factors: tide, river discharge and wind stress. The methodology used is the implementation of a high resolution estuarine model in a 2D mode to simulate the tidal dynamics of the estuary. Tidal flows were calculated by...

∼4 Ma coarsening of sediments from Baikal, Chinese Loess Plateau and South China Sea and implications for the onset of NH glaciation

Abstract Intense tectonic movement occurred along the northeastern margin of the Tibetan Plateau 3–4 Ma. During the same time period, global climate changed from a period of stability to a period of frequent and abrupt changes, which prevented fluvial and glacial systems from establishing a state of equilibrium. Thus, it is difficult to directly attribute the 3–4 Ma coarse-grained sediment accumulation in East Asia to climate change or tectonic activity. This study compares Late Cenozoic multi-proxy records from the Chinese Loess Plateau (CLP) to the low-latitude grain size, sea level and benthic δ 18 O records from the South China Sea (SCS) and the high latitude grain-size record from Lake Baikal. All records suggest a steady and persistent cooling since ~ 4 Ma, which correlates well with those cooling records from other regions around the world. This coincided in timing with the modeling results that the closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation between 4.8 and 4.0 Ma which led to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH). Cooling of the SH would induce a marked development of the Antarctic ice sheets at ~ 4 Ma, pushing the Intertropical Convergence Zone (ITCZ) northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained sediments in the Sikouzi section from the western CLP since 4.2 Ma. Notably, the absence of coarse-grained sedimentation in the Sikouzi section during 3.0-2.1 Ma and the gradual decrease in coarse-grained peaks since 2.1 Ma reflected by the Sikouzi grain-size record probably resulted from stepwise increase in the NH ice volume pushing the ITCZ southward. On the other hand, development of the Antarctic ice sheets would induce global cooling and enhancement of physical weathering, initiating increases in sedimentation rates as well as increases in grain size from Lake Baikal to the CLP to the SCS. Therefore the closure of the CAS during 4.8–4.0 Ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 Ma. A persistent and steady cooling during 4–3 Ma probably made a significant contribution to the establishment of the NH ice sheets at 2.75 Ma ago.

Short term climate variability during “Roman Classical Period” in the eastern Mediterranean

To obtain insight into character and potential forcing of short-term climatic and oceanographic variability in the southern Italian region during the “Roman Classical Period” (60 BC–AD 200), climatic and environmental reconstructions based on a dinoflagelate cyst record from a well dated site in the Gulf of Taranto located at the distal end of the Po-river discharge plume have been established with high temporal resolution. Short-term fluctuations in accumulation rates of the Adriatic Surface Water species Lingulodinium machaerophorum, the freshwater algae Concentricystes and species resistant to aerobic degradation indicate that fluctuations in the trophic state of the upper waters are related to river discharge of northern and eastern Italian rivers which in turn are strongly related to precipitation in Italy. The dinoflagellate cyst association indicates that local sea surface temperatures which in this region are strongly linked to local air temperatures were slightly higher than today. We reconstruct that sea surface temperatures have been relatively high and stable between 60 BC–AD 90 and show a decreasing trend after AD 90. Fluctuations in temperature and river discharge rates have a strong cyclic character with main cyclicities of 7–8 and 11 years. We argue that these cycles are related to variations of the North Atlantic Oscillation climate mode. A strong correlation is observed with global variation in Δ 14C anomalies suggesting that solar variability might be one of the major forcings of the regional climate. Apart from cyclic climate variability we observed a good correlation between non-cyclic temperature drops and global volcanic activity indicating that the latter forms an additional major forcing factor of the southern Italian climate during the Roman Classical Period.

Evaluating the Impacts of Global Warming on Geomorphological Systems

Ongoing climate change (global warming) is a major forcing factor on Earth’s ecological services including agricultural production, biodiversity, and carbon cycle. Climatic regime and climate change is also a major driver of the dynamics of Earth’s geomorphological systems, including its glaciers, rivers, mountains and coasts, especially over longer (103–105 year) time scales that correspond to climate forcing by orbital cycles. Many studies have considered how geomorphological systems have responded to climate forcing over long time scales, where system responses are approximately in phase with forcing (Lal 2004; Lowe et al. 2008). Over shorter time scales, however, geomorphological systems do not respond in phase with climate forcing, are affected by human (anthropogenic) activity, and yield nonlinear responses that cannot be fully predicted based on their previous behaviour (Perry 2002; Murray et al. 2009). The response of geomorphological systems to climate forcing can be examined by monitoring changes to their morphology and geomorphological processes during the recent past (last <150 years) for which instrumental climate data are available. This comparison allows for a more complete understanding of the relationship between climate forcing and geomorphological response. Global warming has given a new impetus to studies of climate forcing of geomorphological systems. This is because, despite the ability of global climate models (GCMs) to predict future temperature, precipitation and sea level, they do not consider likely responses of geomorphological (land surface) systems despite there being important feedbacks between surface processes and climate. These feedbacks include variations in snow/ice cover that result in changes to albedo and energy balance; changes in terrestrial ecosystems and land surface erosion that result in changes in carbon storage; and variations in continental weathering that lead to variations in cation flux to the ocean that influences its capacity for carbon dioxide (CO2) downdraw. Despite feedbacks being a significant source of uncertainty in GCMs (Boer and Yu 2003), the land surface conditions and geomorphological processes that give rise to these feedbacks are generally poorly known. GCMs do not consider the nature of the land surface, except at very broad scales, and cannot incorporate these feedbacks. Studies of geomorphological processes are usually based on small geographic areas, over short time scales, and are not closely linked to inputs or outputs from GCMs. Identifying the relationships between climate forcing and geomorphological response can yield a better understanding of the sensitivity of geomorphological systems to such forcing. Ongoing global warming in combination with landuse change, urbanisation and geoengineering, is now making geomorphological systems work at rates and within structural limitations unprecedented throughout human history. It is therefore critical to know how, where and at what rates these geomorphological systems will respond to climate and anthropogenic forcing. In this article, we describe how and why ongoing global warming is causing changes to the workings of geomorphological systems. We argue that the concept of geomorphological sensitivity is a useful means by which to understand why these changes occur, and provide a context for monitoring and modelling these changes into the future. This article first explains climate sensitivity from which we derive the concept of geomorphological sensitivity. We then apply this concept to consider how sediment yield changes as a result of climate forcing. A useful analogue for geomorphological system behaviour under present global warming is that of paraglacial processes under conditions of ice retreat. We argue that understanding the response of geomorphological systems to global warming has major implications for climate policy and adaption strategies.