Global and Planetary Change

Published by Elsevier
Online ISSN: 0921-8181
Publications
Article
The equatorial current system, by its response to global circulation changes, provides a unique recording mechanism for long range climatic oscillations. A permanent record of the changes in rate of upwelling and organic production is generated in the equatorial deep sea sediments, particularly by such biogenic components which are unaffected by secondary dissolution. In order to determine the rates of accumulation of various sedimentary components, a reliable differential measurement of age of the strata must be obtained. Various approaches to this problem are reviewed, and sources of error discussed. Secondary dissolution of calcium carbonate introduces a substantial and variable difference between the dissolution-modified, and hence a priori unknown, rate of deposition on one hand and the rate of accumulation, derivable from the observed concentration, on the other. The cause and magnitude of these variations are of importance, particularly since some current dating schemes are based on assumed constancy in the rate of accumulation of this and, in some cases, also all other sedimentary components. The concepts used in rate evaluation are discussed with emphasis on the difference between the state of dissolution, an observable property of the sediment, and the rate of dissolution, a parameter that requires deduction of the carbonate fraction dissolved, and of the time differential. As a most likely cause of the enhanced state of dissolution of the interglacial carbonate sediments is proposed the lowered rates of biogenic production and deposition, which cause longer exposure of the carbonate microfossils to corrosion in the bioturbated surface layer of the sediment. Historical perspective is included in the discussion in view of the dedication of the Symposium to Hans Pettersson, the leader of the Swedish Deep Sea Expedition 1947-1948, an undertaking that opened a new era in deep sea research and planetary dynamics.
 
Article
The Ecuadorian Amazon, one of the richest reserves of biodiversity in the world, has faced one of the highest rates of deforestation of any Amazonian nation. Most of this forest elimination has been caused by agricultural colonization that followed the discovery of oil fields in 1967. Since the 1990s, an increasing process of urbanization has also engendered new patterns of population mobility within the Amazon, along with traditional ways by which rural settlers make their living. However, while very significant in its effects on deforestation, urbanization and regional development, population mobility within the Amazon has hardly been studied at all, as well as the distinct migration patterns between men and women. This paper uses a longitudinal dataset of 250 farm households in the Northern Ecuadorian Amazon to understand differentials between men and women migrants to urban and rural destinations and between men and women non-migrants. First, we use hazard analysis based on the Kaplan-Meier (KM) estimator to obtain the cumulative probability that an individual living in the study area in 1990 or at time t, will out-migrated at some time, t+n, before 1999. Results indicate that out-migration to other rural areas in the Amazon, especially pristine areas is considerably greater than out-migration to the growing, but still incipient, Amazonian urban areas. Furthermore, men are more likely to out-migrate to rural areas than women, while the reverse occurs for urban areas. Difference-of-means tests were employed to examine potential factors accounting for differentials between male and female out-migration to urban and rural areas. Among the key results, relative to men younger women are more likely to out-migrate to urban areas; more difficult access from farms to towns and roads constrains women's migration; and access to new lands in the Amazon-an important cause of further deforestation-is more associated with male out-migration. Economic factors such as engagement in on-farm work, increasing resource scarcity-measured by higher population density at the farm and reduction in farm land on forest and crops-and increase in pasture land are more associated with male out-migration to rural areas. On the other hand, increasing resource scarcity, higher population density and weaker migration networks are more associated with female out-migration to urban areas. Thus, a "vicious cycle" is created: Pressure over land leads to deforestation in most or all farm forest areas and reduces the possibilities for further agricultural extensification (deforestation); out-migration, especially male out-migration, occurs to other rural or forest areas in the Amazon (with women being more likely to choose urban destinations); and, giving continuing population growth and pressures in the new settled areas, new pressures promote further out-migration to rural destinations and unabated deforestation.
 
Article
Microbial mats are stratified communities that develop within the environmental microgradients established at the interfaces of water and solid substrates (Cohen, 1989). Stromatolites, the lithified remains of layered accumulations of microbial mats, occur in rocks as old as 3.5 Ga (Lowe, 1980; Walter et al., 1980). These lithified microbial communities represent the most ancient, widespread ecosystems known, and it is useful to explore their role in the accumulation of free oxygen in the ancient atmosphere.
 
Article
A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen. In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O2, except possibly in localized regions of high productivity in the surface ocean. In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic. In Stage III, which commenced with the disappearance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O2. Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed. The rate of O2 increase during the Middle and Late Proterozoic is identified as another outstanding question.
 
Article
Data for the burial efficiency of organic carbon with marine sediments have been compiled for 69 locations. The burial efficiency as here defined is the ratio of the quantity of organic carbon which is ultimately buried to that which reaches the sediment-water interface. As noted previously, the sedimentation rate exerts a dominant influence on the burial efficiency. The logarithm of the burial efficiency is linearly related to the logarithm of the sedimentation rate at low sedimentation rates. At high sedimentation rates the burial efficiency can exceed 50% and becomes nearly independent of the sedimentation rate. The residual of the burial efficiency after the effect of the sedimentation rate has been subtracted is a weak function of the O2 concentration in bottom waters. The scatter is sufficiently large, so that the effect of the O2 concentration in bottom waters on the burial efficiency of organic matter could be either negligible or a minor but significant part of the mechanism that controls the level of O2 in the atmosphere.
 
Article
Earth's climate has remained reasonably temperate for at least the last 3.5 billion years, despite a large increase in solar luminosity with time. The increase in solar flux has probably been offset by a decrease in atmospheric CO2 concentration caused by a negative feedback in the carbonate-silicate geochemical cycle. The same feedback mechanism implies that an Earth-like planet could remain habitable (i.e. possess liquid water) out to a least the orbit of Mars. The initial atmospheric CO2 concentration may have been much higher than the amount required to offset the lower solar output, in which case the Earth may have originally been much hotter than it is today. However, once the initial accretion period was over, Earth should have been stable against either a runaway greenhouse, that is, complete evaporation of the oceans, or against rapid loss of water. Long-term climatic evolution has thus far been studied only with one-dimensional, globally-averaged climate models. Although such models can provide a qualitative understanding of climate history, they rely on a number of assumptions that may not have been valid in the past. Some problems that deserve to be investigated with more sophisticated climate models are discussed.
 
Article
Climate is an important environmental parameter of the early Earth, likely to have affected the origin and evolution of life, the composition and mineralogy of sedimentary rocks, and stable isotope ratios in sedimentary minerals. There is little observational evidence constraining Precambrian climates. Most of our knowledge is at present theoretical. Factors that must have affected the climate include reduced solar luminosity, enhanced rotation rate of the Earth, an area of land that probably increased with time, and biological evolution, particularly as it affected the composition of the atmosphere and the greenhouse effect. Cloud cover is a major uncertainty about the early Earth. Carbon dioxide and its greenhouse effect are the factors that have been most extensively studied. This paper presents a new examination of the biogeochemical cycles of carbon as they may have changed between an Archean Earth deficient in land, sedimentary rocks, and biological activity, and a Proterozoic Earth much like the modern Earth, but lacking terrestrial life and carbonate-secreting plankton. Results of a numerical simulation of this transition show how increasing biological activity could have drawn down atmospheric carbon dioxide by extracting sedimentary organic carbon from the system. Increasing area of continents could further have drawn down carbon dioxide by encouraging the accumulation of carbonate sediments. An attempt to develop a numerical simulation of the carbon cycles of the Precambrian raises questions about sources and sinks of marine carbon and alkalinity on a world without continents. More information is needed about sea-floor weathering processes.
 
Article
Recent biological studies demonstrating sensitivity to ultraviolet (UV) radiation coupled with research on the radiative impact of stratospheric ozone depletion pose a challenge to climate modelers and reconstructionists to determine past variations in UV radiation. In this study, solar radiative waveband totals are computed applying a numerical model linking the parameterization of the amount of radiation as a function of orbital parameters with the atmospheric solar scattering. Variations are computed for three continuous solar wavebands (0.225–0.285, 0.3–0.325 and 0.325–0.690 μm) and one non-continuous shortwave band (0.175–0.225/0.285–300 μm). With the assumption of invariant latitudinal ozone concentrations over the past 250,000 years, radiation in all wavebands displays significant variations from present levels due to changes produced by Milankovitch orbital mechanisms (obliquity, eccentricity and precession). Specifically: (a) orbital changes produced the following percent ranges from present-day values in the 0.3–0.325 μm waveband over the last 250,000 years: −19% to 30% (June Solstice), −26% to +32% (December Solstice) and −3% to +3% (equinoxes); (b) precessional influences control variations in the total daily radiation; and (c) orbital changes produce a decrease in the amount of visible and UV radiation over the next 10 kyr in the Northern Hemisphere for the spring and summer while increasing visible and UV for the fall and winter. These initial results for long-period radiative variations (a) provide a first comparative range of values in future UV radiative change study related to ozone depletion; (b) generate baseline values for researchers interested in the potential environmental and biological effects of past UV radiative variations; and (c) aid in analyses of the influence of solar radiative changes based upon orbital mechanisms on climatic change.
 
Article
Our field surveys show that a wetland/swamp layer is widely distributed in the western part of the Chinese Loess Plateau. The grayish-blue and aquatic mollusk-enriched layer at Sujianwan section (typical in the major valleys) is dated between ∼10,000 and 4000 cal. years B.P. A wetland/swamp sub-layer and a pedogenically altered wetland/swamp sub-layer bracket a middle complex of wetland/swamp and fluvial alternating couplets at Dadiwan section (typical in the branch valleys). The middle complex formed between ∼8000 and 6000 cal. years B.P. and contains abundant aquatic mollusks, as well as the highest percentage of the tree and shrub pollens. We propose that four mechanisms might have shared the responsibility for generating and maintaining the Megehumid climate in the western part of the Chinese Loess Plateau. They are (1) the insolation peak (7% more than today's) between 12,000 and 8000 years B.P., (2) the increased late summer insolation about 6000 cal. years B.P. in the Northern Hemisphere, (3) the shift of the long-term El Nino-like system towards the Asian side of the Pacific, and (4) positive vegetation feedbacks under a wet and warm climate.
 
Article
The mainly endemic phytoplankton record of Lake Baikal has been used in this study to help interpret climate variability during the last 1000 years in central Asia. The diatom record was derived from a short core taken from the south basin and has been shown to be free from any sedimentary heterogeneities. We employ here a diatom-based inference model of snow accumulation on the frozen lake for the first time (r2boot=0.709; RMSEP=0.120 log cm). However, palaeoenvironmental reconstructions have been improved by the use of correction factors, specifically developed for the dominant phytoplankton (Aulacoseira baicalensis, Aulacoseira skvortzowii, Cyclotella minuta, Stephanodiscus meyerii and Synedra acus) in the south basin of Lake Baikal. Cluster analysis identifies three significant zones in the core, zone 1 (c. 880 AD–c. 1180 AD), zone 2 (c. 1180–1840 AD) and zone 3 (c. 1840–1994 AD), coincident with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the period of recent warming, respectively. Our results indicate that S. acus dominated the diatom phytoplankton within zone 1 coincident with the MWP. S. acus is an opportunistic species that is able to increase its net growth when A. baicalensis does not. During this period, conditions are likely to have been unfavourable for the net increases in A. baicalensis growth due to the persistence of warm water in the lake, together with an increased length of summer stratification and delay in timing of the autumnal overturn. In zone 2, spring diatom crops blooming under the ice declined in abundances due in part to increased winter severity and snow cover on the lake. Accumulating snow on the lake is likely to have arisen from increased anticyclonic activity, resulting in prolonged winters expressed during the LIA. Thick, accumulating snow cover inhibits light penetration through the ice, thereby having negative effects on cell division rate and extent of turbulence underneath the ice. Consequently, only taxa whose net growth occurs during autumn overturn (C. minuta) predominate in the lake at this time. Diatom census data and reconstructions of snow accumulation suggest that warming in the Lake Baikal region started as early as c. 1750 AD, with a shift from taxa that bloom during autumn overturn to assemblages that begin to grow underneath the frozen lake in spring. Very recent increases and subsequent decline of S. acus in the surface sediments of the lake mirror monitoring records of this species over the last 50 years. Our study confirms that, over the last 1000 years, physical processes are important in determining planktonic diatom populations in the lake and highlights the value of integrated plankton, trap, and sediment studies for improving quantitative palaeoenvironmental reconstructions from fossil material.
 
Article
Short-term (shorter than a century) temperature and rainfall variations correlate roughly along the Afrasian arid belt over the last 500 years. Among various external forcings, solar activity might be one of the most important. The spectral analysis of climatic change in China and auroral number series of the last 1700 years shows some potential relations at the time-scale of decades to century.
 
Article
During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation.
 
Article
A wide range of climatic, geologic and archeological records can be characterized by measuring their 14C and 10Be concentrations, using the accelerator mass spectrometry (AMS). These records are found not only in the traditional sampling sites such as lake sediments and ice cores, but also in diverse natural accumulates and biogeochemical products such as: loess/paleosol deposits, corals, speleothems, forest-fire horizons and weathered meteorites. The in-situ production of cosmogenic radionuclides in terrestrial materials provides several possibilities of determining their chronology. The purpose of this review is to highlight selected applications of AMS, which have bearing to our understanding of both chronology of archival materials, and learning about climatic changes in the past.
 
Article
Past analogues for our present interglacial or even warmer periods have been sought in order to better understand our present and future climate. Marine Isotope Stage (MIS) 5, more precisely substage 5e, has long been considered to be a good candidate. However, there were some elements against this analogy in the data themselves [Kukla et al. Quat. Sci. Rev. 16 (6) (1997) 605], as well as in the mechanisms [Berger, 1989 Response of the climate system to CO2 and astronomical forcings. In: Paleo-Analogs, IPCC Working Group I, Bath, 20–21 November 1989] and forcing related to both periods. Here we suggest that the period from 405 to 340 ka before present (BP), including a large part of Marine Isotope Stage 11, could be a good analogue for future climate. The insolation over this interval shows a strong linear correlation with the insolation signal over the recent past and the future. In addition, simulations using the climate model developed in Louvain-la-Neuve (LLN 2-D NH) show that both MIS 11 and the future are characterized by small amount (if any) of continental ice, with almost no variation during the whole interval. In contrast, MIS 5 is exhibiting larger variability in simulated ice volume. This confirms that the interval [405–340 ka BP] may lead to a better understanding of our present and future warm climate.
 
Article
This study addresses changes in the absolute magnitude and spatial geometry of particle flux and export production in a meridional transect across the central equatorial Pacific Ocean's upwelling system during oxygen isotope Stage 11 and Stage 12 and compares these time periods to the current Holocene interglacial system. Temporal and spatial variability in several chemical proxies of export production, and in particular the distributions of Ba, scavenged Al, and P, are studied in a suite of sediment cores gathered along a cross-equator transect at 5°S, 2°S, 0°, 2°N, and 4°N. Because this latitudinal range preserves strong gradients in biogenic particle flux in the modern equatorial Pacific Ocean, we are able to assess variations in the relative magnitude of export production as well as the meridional width of the equatorial system through the late Quaternary glacial/interglacial cycles. During interglacial oxygen isotope Stage 11 the chemical proxies each indicate lower particle flux and export production than during Stage 12. These records are consistent throughout the transect during this time period, but geographic narrowing (during the interglacial) and widening (during the glacial) of the meridional gradient also occurs. Although carbonate concentration varies dramatically through glacial/interglacial cycles at all latitudes studied, the productivity proxies record only minimal glacial/interglacial change at 5°S and 4°N, indicating that the carbonate minima at these latitudes is controlled dominantly by dissolution rather than production. The chemical data indicate that although the spatial geometry of the system during Stages 11 and 12 indicates maximum productivity at the equator during both glacial and interglacial conditions, the absolute magnitude of export production integrated from 5°S to 4°N during Stage 11 was 25–50% less than during Stage 12, and also was 25–50% less than it is now.
 
Article
A sediment core from the high latitude of the Northern Atlantic (Nordic seas) was intensively studied by means of biogeochemical, sedimentological, and micropaleontological methods. The proxy records of interglacial marine oxygen isotope stage (MIS) 11 are directly compared with records from the Holocene (MIS 1), revealing that many features of MIS 11 are rather atypical for an interglaciation at these latitudes.Full-interglacial conditions without deposition of ice-rafted debris existed in MIS 11 for about 10 kyr (∼398–408 ka). This time is marked by the lightest d18O values in benthic foraminifera, indicating a small global ice volume, and by the appearance of subpolar planktic foraminifera, indicating a northward advection of Atlantic surface water. A comparison with MIS 1, using the same proxies, implies that surface temperatures were lower and global ice volume was larger during MIS 11. A comparative study of the ratio between planktic and benthic foraminifera also reveals strong differences among the two intervals. These data imply that the coupling between surface and bottom bioproductivity, i.e., the vertical transportation of the amount of fresh organic matter, was different in MIS 11. This is corroborated by a benthic fauna in MIS 11, which contains no epifaunally-living species. Despite comparable values in carbonate content (%), reflectance analyses of the total sediment (greylevel) show much higher values for MIS 11 than for MIS 1. These high values are attributed to increased corrosion of foraminiferal tests, directly affecting the sediment greylevel. The reason for this enhanced carbonate corrosion in MIS 11 remains speculative, but may be linked to the global carbon cycle.
 
Article
During the last deglaciation two distinct warming phases occurred in the N Atlantic region at ∼14.7 and ∼11.5 ka cal BP. These two shifts are the transitions from (1) GS-2a (Greenland Stadial 2a) to GI-1e (Greenland Interstadial 1e) and (2) GS-1 to the Preboreal. In this study we characterise these two important climate transitions by comparing maps of January and July temperatures for Europe acquired with two independent methods: (1) simulations with the ECHAM4 atmospheric general circulation model in T42 resolution and (2) temperature reconstructions based on geological and palaeoecological data. We also compare estimated lake level changes with simulated P-E (effective precipitation) values. These comparisons enable quantification of the climate change during the two phases. January temperatures increased by as much as 20°C in NW Europe from values between −25°C and −15°C in both GS-2a and GS-1 to temperatures between −5°C and 5°C in both GI-1e and the Preboreal. During July the changes were smaller, as the July temperatures increased in NW Europe by 3–5°C from about 10°C to 15°C in both GS-2a and GS-1 to values of 13°C to 17°C in both GI-1e and the Preboreal. In S Europe the increase in July temperature was less intense. Our analysis suggests that the effective precipitation remained at the same level during the 14.7 ka cal BP transition, whereas a small increase is inferred for some regions for the 11.5 ka cal BP shift. This small effect in effective precipitation is explained by comparable increases in precipitation and evaporation during both transitions. We infer that the strong increase in January temperatures was forced by changes in the N Atlantic Ocean, as the variations in sea surface temperatures and the position of the sea ice margin determined the temperature change over land. The increase in July temperatures was mainly driven by two factors: the increase in insolation and the deglaciation in Scotland and Scandinavia. The insolation changes were gradual (2 to 3 W/m2) compared to the changes in the N Atlantic Ocean, explaining the relatively small temperature increase during July compared to January. In regions that were deglaciated during the two climate transitions, July temperatures appeared to have increased by up to 10°C. Our results suggest that the registration of the magnitude of the two climate shifts in terrestrial proxy records was geographically different due to the changing environmental conditions; variations in the N Atlantic sea ice limit appear to be the most important. This implies that reconstructed temperature curves from different places in Europe should show different magnitudes. Moreover, it is to be expected that the timing of the major warming phases is spatially different, as this timing is mainly determined by the position of the sea ice and land ice margins relative to the place of interest.
 
Article
Ocean Drilling Program Leg 188, Prydz Bay, East Antarctica is part of a larger initiative to explore the Cenozoic history of the Antarctic Ice Sheet through direct drilling and sampling of the continental margins. In this paper, we present stable isotopic results from Ocean Drilling Program (ODP) Site 1167 located on the Prydz Channel Trough Mouth Fan (TMF), the first Antarctic TMF to be drilled. The foraminifer-based δ18O record is interpreted along with sedimentary and downhole logging evidence to reconstruct the Quaternary glacial history of Prydz Bay and the adjacent Lambert Glacier Amery Ice Shelf System (LGAISS). We report an electron spin resonance age date of 36.9±3.3 ka at 0.45 m below sea floor and correlate suspected glacial–interglacial cycles with the global isotopic stratigraphy to improve the chronology for Site 1167. The δ18O record based on planktonic (Neogloboquadrina pachyderma (s.)) and limited benthic results (Globocassidulina crassa), indicates a trend of ice sheet expansion that was interrupted by a period of reduced ice volume and possibly warmer conditions during the early–mid-Pleistocene (0.9–1.38 Ma). An increase in δ18O values after ∼900 ka appears to coincide with the mid-Pleistocene climate transition and the expansion of the northern hemisphere ice sheet. The δ18O record in the upper 50 m of the stratigraphic section indicates as few as three glacial–interglacial cycles, tentatively assigned as marine isotopic stages (MIS) 16–21, are preserved since the Brunhes/Matuyama paleomagnetic reversal (780 ka). This suggests that there is a large unconformity near the top of the section and/or that there may have been few extreme advances of the ice sheet since the mid-Pleistocene climate transition resulting in lowered sedimentation rates on the Prydz Channel TMF. The stable isotopic record from Site 1167 is one of the few available from the area south of the Antarctic Polar Front that has been linked with the global isotopic stratigraphy. Our results suggest the potential for the recovery of useful stable isotopic records in other TMFs.
 
Article
We present a new inventory of global sulfur dioxide emissions from anthropogenic activities for the years 1980–2000. Emissions were estimated in 11 world regions using country-level emissions inventories and regional fossil fuel sulfur content information. Estimated global emissions in 1990 are 72 TgS with an estimated uncertainty of ±8% due to random errors with additional systematic errors that suggest that true emissions may be higher than this central value. We estimate that 56% of 1990 world emissions are from coal, 24% from oil, 15% from industrial processes and 3% from biomass burning. When our results are compared with other studies, they are similar at the global-mean level, but show marked differences at the regional level. Globally, emissions have been roughly constant from 1980 to the present. However, a significant shift has occurred in the spatial distribution of emissions. While 60% of global emissions in 1980 were from around the North Atlantic basin, this region contributed less than 40% of the global total by 1995 and will contribute even less in the future. Currently, based on our estimates, the centrally planned Asia (CPA) region, dominated by China, is the largest contributor to global sulfur dioxide emissions. A gridded data set for 1990 emissions is also produced, including a consistent seasonal cycle and a stratification of emissions into low and elevated releases. Implications for climate modeling and detection studies are discussed.
 
Article
New pollen and radiocarbon data from an 8.6-m coastal section, Cape Shpindler (69°43′N; 62°48′E), Yugorski Peninsula, document the latest Pleistocene and Holocene environmental history of this low Arctic region. Twelve AMS 14C dates indicate that the deposits accumulated since about 13,000 until 2000 radiocarbon years BP. A thermokarst lake formed ca. 13,000–12,800 years BP, when scarce arctic tundra vegetation dominated the area. By 12,500 years BP, a shallow lake existed at the site, and Arctic tundra with Poaceae, Cyperaceae, Salix, Saxifraga, and Artemisia dominated nearby vegetation. Climate was colder than today. Betula nana became dominant during the Early Preboreal period about 9500 years BP, responding to a warm event, which was one of the warmest during the Holocene. Decline in B. nana and Salix after 9500 years BP reflects a brief event of Preboreal cooling. A subsequent increase in Betula and Alnus fruticosa pollen percentages reflects amelioration of environmental conditions at the end of Preboreal period (ca. 9300 years BP). A decline in arboreal taxa later, with a dramatic increase in herb taxa, reflects a short cold event at about 9200 years BP. The pollen data reflect a northward movement of tree birch, peaking at the middle Boreal period, around 8500 years BP. Open Betula forest existed on the Kara Sea coast of the Yugorski Peninsula during the Atlantic period (8000–4500 years BP), indicating that climate was significantly warmer than today. Deteriorating climate around the Atlantic–Subboreal boundary (ca. 4500 years BP) is recorded by a decline in Betula percentages. Sedimentation slowed at the site, and processes of denudation and/or soil formation started at the beginning of the Subatlantic period, when vegetation cover on Yugorski Peninsula shifted to near-modern assemblages.
 
Article
A 58-m-long sediment core IMAGES MD01-2412 was recovered in the southwestern part of the Okhotsk Sea for high resolution paleocenography. An age model of the core was obtained by accelerator mass spectrometry (AMS) 14C dating of planktonic foraminifer shells, oxygen–isotope stratigraphy of benthic foraminifer calcite, and tephrochronology, resulting in a core-bottom age of 115 kyr. Sea-ice expansion in the Okhotsk Sea was reconstructed by ice-rafted debris (IRD) based on measurement of dropstone, coarse fraction, sand fractions of terrigenous particles, and the magnetic properties. The SW Okhotsk Sea has not had perennial but seasonal sea-ice conditions during the 115 kyr. Seasonal sea ice fluctuated with large amplitudes on millennial scale during the glacials (Marine isotope stage: MIS 2, 3, and 4) and varied relatively little during the Holocene (MIS 1) and the last interglacial (MIS 5). Enhanced polar atmospheric circulation during the glacial resulted in strong wind fields over the Okhotsk Sea and accelerated the large sea-ice expansion during the glacials (MIS 2, 3, and 4). During the interglacials (MIS 1 and 5), sea ice also expanded by small amplitudes. During these periods, decrease of the Amur River discharge would be one of the possible factors for sea-ice expansion. The two main factors of polar atmospheric circulation and Amur River discharge would be responsible for sea-ice expansion during 120 kyr.
 
Article
Sea-ice expansion in the Okhotsk Sea in winter is sensitively affected by global warming and cooling. Regionally, the southwestern Okhotsk Sea is closely linked to climate change in East Asia, including Japan, because the cold sea surface temperature (SST) in the southwestern Okhotsk Sea influences directly the development of the Okhotsk atmospheric high-pressure system, and the activated Okhotsk high causes cold climatic conditions in northern Japan. Therefore, environmental change in the Okhotsk Sea indicates two-way interactions as a sensitive mirror reflecting global climate change and as a driving force of regional climate change. To better understand how surface environmental changes in the Okhotsk Sea can influence climate change in East Asia, SSTs were estimated in the southwestern Okhotsk Sea for the past 120 ky with millennial to centennial time resolution using the long-chain unsaturated alkyl ketone (alkenone) thermometer. The alkenone temperature, which corresponds to the SST to 20 m depth in autumn, showed repeated abrupt changes at a centennial timescale, especially during the last glacial period, 20–60 ky before present (BP). The alkenone temperature changed concurrently with changes from interstadials (warm events) to stadials (cold events) in the δ18O record of the ice cores from Greenland, although some interstadials could not be identified in the alkenone temperature record. A wavelet power spectrum analysis showed that a periodicity of about 8 ky was prominent during 10–90 ky BP, and a 4- to 5-ky cycle was characteristic during 30–40 ky BP in the alkenone temperature records. These periodicities were both similar and dissimilar to those in the Polar Circulation Index, which is based on the atmospheric circulation intensity at high latitudes, as recorded by major-ion concentrations in GISP2. Both the similarity and dissimilarity imply that the SST in the southwestern Okhotsk Sea is controlled mainly by the atmosphere–ocean circulation system in the Northern Hemisphere; however, the relationship between the SST in the Okhotsk Sea and the climate in the Greenland is not linear. Anomalously high alkenone temperatures occurred repeatedly in the glacial period. These warm alkenone temperature episodes would have had multiple causes. In particular, high alkenone temperatures during the last glacial maximum (LGM) have been reported previously for locations near this study site. More investigations are necessary to understand what happened in the Okhotsk Sea and in adjacent seas at the time of the LGM.
 
Article
A 198-m-long core was obtained from Lake Poukawa, Hawkes Bay, New Zealand for paleoclimatic analysis. A chronology extending back to ca. 120 ka has been developed using a combination of tephrostratigraphy, radiocarbon, optical, and U–Th disequilbrium dating. The core contains a new record of tephra beds, including temporal intervals poorly recorded elsewhere, and revises the dispersal for some known events. Thirty macroscopic tephra beds were identified, comprising 20 rhyolites with compositions consistent with previously studied tephra from Taupo and Okataina calderas, and 10 andesites–dacites compositionally similar to Tongariro and Egmont centre eruptions. Electron microprobe data provides evidence for a total of 24 rhyolite eruptions amongst the 20 macroscopic beds. Four widespread rhyolitic marker beds: Whakatane (4.6 ka), Kawakawa (22.6 ka), Tahuna (ca. 43 ka), and Rotoehu (ca. 50 ka) provide temporal constraints for the upper 40 m of the core. The occurrence of Opepe (9 ka) and Okaia (23 ka) tephra beds in this core extends their known dispersal to southern North Island. A previously unrecognised and chemically distinct rhyolite tephra (ca. 35 ka) was also found in the sequence. Twelve rhyolitic tephra occur in the interval 50–120 ka, a period in which the timing and nature of volcanic events is poorly understood at proximal sites of the Taupo Volcanic Zone.
 
Article
We present a mass balance model for Eurasia which is based on the calculation of accumulation from a moisture balance concept. The model is forced with 500 hPa temperatures from GCM time slices at LGM and present day. The model simulates key characteristics, such as control on the size of ice sheets through the advection of moisture, asymmetric ice sheets due to advection of moisture and orography, and the drying of ice sheets when they grow. A simulation of the Eurasian Ice Sheet through a full glacial cycle shows that the model reproduces realistic ice sheets that compare well with geomorphological data. During the Middle Weichselian and the Late Weichselian, the model picks up the trend that the Scandinavian part of the ice grows towards the south and east whilst the ice sheet covering the Barents and Kara Seas remains relatively stable. However, the model seriously underestimates the observed ice extent in the Baltic area. Uncertainties in the temperature and the wind field limit the reliability of regional modelling results.
 
Article
The concentration of methane in the atmosphere has varied considerably during the last 125,000 years. Boreal wetlands represent one of the main sources of methane emissions into the atmosphere, the rate of which is largely controlled by climate. Changes in climate (mainly in the duration of the frost-free period) and in the extent of wetlands presumably caused variations in the methane production from boreal ecosystems. We chose Northern Eurasia to estimate both climatic changes and the area of methane-producing ecosystems, as it plays a leading role in methane emission. Palaeobotanic and palaeocryological data were used for the reconstruction. The two most recent warm stages: the Holocene Optimum (5500–6000 years BP) and the Last Interglacial Optimum (ca. 125,000 years BP) were studied. During these warm periods, both an area of tundra and the proportion of the wetlands within the boreal forest zone were considerably reduced. On the other hand, a longer frost-free period and higher precipitation would have caused higher methane production. The precipitation rise was apparently in part compensated by an increase in potential evaporation due to higher summer temperatures. Compared to methane emissions of about 9×106 t per year from modern forests of Northern Eurasia, emissions amounted to 86 and 44% of modern values for the region during the Holocene Optimum and Last Interglacial Optimum respectively. Under the greenhouse warming expected early in the 21st century, the climatic conditions may lead to a considerable increase of methane emission.
 
Article
Within the period 13-10 ka, the Earth experienced high-amplitude fluctuations in climate and to a lesser degree, also in sea level. These high-amplitude changes occurred within the period of superposition of two exponential curves in the eustatic rise in sea level. This intermediate period seems to represent the Earth's geodynamic response to the general deceleration due to the sea level rise. The deceleration caused water-masses to move polewards. At a critical point, the symmetry axes of the Earth's core and mantle were displaced with respect to each other along a meridional path recorded in a trans-polar shift of the axis of the geomagnetic dipole field. At about the same time, the Earth came into a new mode with large-scale interchanges of angular momentum between the “solid” Earth and the hydrosphere. These speeding-ups and slowing-downs of the hydrosphere caused increases and decreases in the ocean current system. The Gulf Stream affecting climate and sea level in Europe, the Labrador Current controlling climate and ice marginal changes in the Hudson Strait region. The Humboldt Current controlling climate and precipitation in South America, the coastal upwelling and the marine productivity and in that way also affecting the atmospheric CO2 content. These ocean current changes are the main controlling factors of the high-amplitude changes within the intermediate period from about 13.5 to 9.5 ka.
 
Article
Recent geologically-based reconstructions of the Eurasian ice sheet show that during the peak Saalian (≈ 140 kya) the ice sheet was larger over Eurasia than during the Last Glacial Maximum (LGM) at ≈ 21 kya. To address this problem we use the LMDZ4 atmospheric general circulation model to evaluate the impact on the Saalian ice sheet's surface mass balance (SMB) from proglacial lakes, dust deposition on snow, vegetation and sea surface temperatures (SST) since geological records suggest that these environmental parameters were different during the two glacial periods. Seven model simulations have been carried out. Dust deposition decreases the mean SMB by intensifying surface melt during summer while proglacial lakes cool the summer climate and reduce surface melt on the ice sheet. A simulation including both proglacial lakes and dust shows that the presence of the former parameter reduces the impact of the latter, in particular, during summer. A switch from needle-leaf to tundra vegetation affects the regional climate but not enough to significantly influence the SMB of the nearby ice margin. However, a steady-state vegetation in equilibrium with the climate should be computed to improve the boundary conditions for further evaluations of the vegetation impact on the ice sheet's SMB. Finally, changes of the SST broadly affect the regional climate with significant consequences for the SMB.
 
Sedimentary zones, selected mineralogical composition and concentrations of chemical compounds in the sediments of Laguna Miscanti. 
Article
Lake sediments and pollen, spores and algae from the high-elevation endorheic Laguna Miscanti (22°45′S, 67°45′W, 4140 m a.s.l., 13.5 km2 water surface, 10 m deep) in the Atacama Desert of northern Chile provide information about abrupt and high amplitude changes in effective moisture. Although the lack of terrestrial organic macrofossils and the presence of a significant 14C reservoir effect make radiocarbon dating of lake sediments very difficult, we propose the following palaeoenvironmental history. An initial shallow freshwater lake (ca. 22,000 14C years BP) disappeared during the extremely dry conditions of the Last Glacial Maximum (LGM; 18,000 14C years BP). That section is devoid of pollen. The late-glacial lake transgression started around 12,000 14C years BP, peaked in two phases between ca. 11,000 and <9000 14C years BP, and terminated around 8000 14C years BP. Effective moisture increased more than three times compared to modern conditions (∼200 mm precipitation), and a relatively dense terrestrial vegetation was established. Very shallow hypersaline lacustrine conditions prevailed during the mid-Holocene until ca. 3600 14C years BP. However, numerous drying and wetting cycles suggest frequent changes in moisture, maybe even individual storms during the mid-Holocene. After several humid spells, modern conditions were reached at ca. 3000 14C years BP. Comparison between limnogeological data and pollen of terrestrial plants suggest century-scale response lags. Relatively constant concentrations of long-distance transported pollen from lowlands east of the Andes suggest similar atmospheric circulation patterns (mainly tropical summer rainfall) throughout the entire period of time. These findings compare favorably with other regional paleoenvironmental data.
 
Article
Four sediment cores located at the Eurasian continental margin underlying the Atlantic layer have been studied for their dinoflagellate cyst content. Concentrations of distinct dinoflagellate cyst taxa display fluctuations in the late Quaternary, which are linked to changes in the inflow of relatively warm Atlantic surface and near-surface waters, resulting in increased local production of cysts in certain time intervals. Based on the assumption that marked changes in strength of inflow occurred synchronously at the Eurasian continental margin, concentration maxima can be used to correlate sediment cores. A dinoflagellate cyst record from the northern Barents Sea continental margin has been related to the stable oxygen isotope and paleomagnetic records to provide direct chronological information. The combination of these methods permits definition of stratigraphic sections equivalent to oxygen isotope stages in carbonate-poor sequences from the Eurasian continental margin. Previous age models of sediment cores are revised, based on dinoflagellate cyst abundance peaks and species distribution, but a firm chronostratigraphy of sedimentary sequences at the eastern Laptev Sea continental margin cannot be established because of the weak signal at the sites furthest from Fram Strait.In the past 150,000 years, the influence of Atlantic (sub-) surface waters generally decreased from west to east along the Eurasian continental margin, in particular during the glacials. Pronounced concentration maxima of cosmopolitan and temperate–subpolar dinoflagellate cysts indicate the inflow of Atlantic waters and seasonally increased production of cysts in the Holocene and Eemian. The Holocene is well-marked at the entire Eurasian continental margin but it is more difficult to assess the extent of (sub-) surface water inflow during the Eemian, which may have only reached the western Laptev Sea continental margin.
 
Article
Rogation (ceremonies to ask God for rain: pro-pluvia, or to stop raining: pro-serenitate) analysis is an effective method to derive information about climate extremes from documentary data. Weighted annual sum by levels has been a widespread technique to analyze such data but this analysis is liable to be biased to spring values as these ceremonies are strongly related to farming activities. The analysis of the length of pro-pluvia periods (the time span during which rogations are carried out in relation to a drought event) and the combination of annual and seasonal information offers a more objective criterion for the analysis of the drought periods and an increase in the resolution of the study.Analysis by the pro-pluvia periods method of the rogation series from the Toledo (central Spain) Cathedral Chapter allows a good characterization of the droughts during the 1506–1900 period. Two drought maxima appear during the 1600–1675 and 1711–1775 periods, characterized by rogations during almost all the year, with a middle stage (1676–1710) when droughts were less frequent and their length shortened.Sea level pressure patterns for the instrumental and documentary periods show that droughts were mostly related to a north-eastern position of the Azores High that displaced the Atlantic low pressure systems towards a northern position. There is a weak relation with the North Atlantic Oscillation but this fact is related to the local character of the series that increases the weight of the local factors.Comparison of rainfall/drought records around Spain and the Western Mediterranean reveals the heterogeneity of their distribution in time and space as well as stresses the need of more and longer reconstructions. Better knowledge of drought variability would help to improve regional models of climate extremes and the understanding of the atmospheric patterns related to their development.
 
Article
In this work, a reconstruction of seasonal and annual rainfall characteristics in the Iberian Peninsula from the 16th century to the present is described. This reconstruction is based on the analysis of a wide variety of documentary data. Weather information was taken from original documentary sources in different Spanish localities, representative of the main pluviometric regions in the Iberian Peninsula (western area, Mediterranean coast, northern coast). Documentary data were translated into numerical indices to elaborate time series of rainfall for each season of the year. Two calibration procedures were performed: indexing the instrumental series when an overlapping period was not available, and regression with instrumental data corresponding to an overlapping period in the 19th century. Results of both methods were compared. Preliminary results show time variations in the seasonal and annual rainfall regime as anomalies from the reference period 1961–1990. Results are compared with other analyses reported in the literature. Scientific challenges for future research are outlined.
 
Article
A glacier–climate model was used to calculate climatic conditions in a test site on the east Andean slope around Cochabamba (17°S, Bolivia) for the time of the maximum Late Pleistocene glaciation. Results suggest a massive temperature reduction of about − 6.4 °C (+ 1.4/− 1.3 °C), combined with annual precipitation rates of about 1100 mm (+ 570 mm/− 280 mm). This implies no major change in annual precipitation compared with today. Summer precipitation was the source for the humidity in the past, as is the case today. This climate scenario argues for a maximum advance of the paleo-glaciers in the eastern cordillera during the global Last Glacial Maximum (LGM, 20 ka BP), which is confirmed by exposure age dates. In a synthesized view over the central Andes, the results point to an increased summer precipitation-driven Late Glacial (15–10 ka BP) maximum advance in the western part of the Altiplano (18°S–23°S), a temperature-driven maximum advance during full glacial times (LGM) in the eastern cordillera, and a pre- and post-LGM (32 ka BP/14 ka BP) maximum advance around 30°S related to increased precipitation and reduced temperature on the western slope of the Andes. The results indicate the importance of understanding the seasonality and details of the mass balance–climate interaction in order to disentangle drivers for the observed regionally asynchronous past glaciations in the central Andes.
 
Article
We modeled the two most extreme highstands of Lake Naivasha during the last 175 k.y. to estimate potential precipitation/evaporation changes in this basin. In a first step, the bathymetry of the paleolakes at ∼135 and 9 k.y. BP was reconstructed from sediment cores and surface outcrops. Second, we modeled the paleohydrologic budget during the highstands using a simplified coupled energy mass-balance model. Our results show that the hydrologic and hence the climate conditions at ∼135 and 9 k.y. BP were similar, but significantly different from today. The main difference is a ∼15% higher value in precipitation compared to the present. An adaptation and migration of vegetation in the cause of climate changes would result in a ∼30% increase in precipitation. The most likely cause for such a wetter climate at ∼135 and 9 k.y. BP is a more intense intertropical convergence and increased precipitation in East Africa.
 
Map showing the location of the ODP Sites 1095 and 1096 on the continental rise Drift 7 located on the Pacific margin of the Antarctic Peninsula.  
Map showing the location of the ODP Site 1165 located on the continental rise Wild Drift off Prydz Bay.  
Sedimentological, magnetic susceptibility (MST), siliceous microfossil abundances, biogenic opal, and mineralogical and geochemical parameters plotted vs. depth and age for Site 1165s.
A) Benthic foraminifera δ 18 O record for the past 5 Ma (Lisiecki and Raymo, 2005). The shadowed interval indicates the studied interval. B) Isotopic model after Lisiecki and Raymo (2005) for the studied age interval. This curve shows the four warm circum-Antarctic events correlated to interglacial isotopic stages based in our age model and the paleoclimate indicators used for this study. Refer to text for details about paleoenvironmental conditions (i.e., sea ice extent, inferred Surface Water temperatures, and ice sheet behavior) during these circum-Antarctic warm intervals. The MG11 stage coincides with the sedimentary section in our cores dated by the magnetic polarity reversal Gilbert -Gauss at 3.58 Ma.  
Article
Our study characterizes glacial and interglacial deposition on two Antarctic margins in order to discriminate between regional and continent-wide early to middle Pliocene warm intervals that caused sea-ice reduction and continental ice sheet retreat. We use a multi-proxy (i.e., sediment facies and grain size, siliceous microfossils, biogenic opal, geochemical composition and clay mineralogy) approach to examine sediments recovered in drill holes from the West Antarctic Peninsula and the East Antarctic Prydz Bay margins, focusing on the climatic record between 4 and 3.5 Ma.
 
Article
The North Atlantic Oscillation (NAO) is the leading mode of atmospheric variability in the North Atlantic region, influencing storm tracks and creating a dipole pattern of precipitation from north to south across Western Europe. This distinct spatial distribution of precipitation provides a framework that can be potentially used to identify and reconstruct patterns of past NAO-forced climate variability. In this study we use tree-ring width series from Western Europe, in conjunction with principal components analysis and advanced spectral methods, to prospect for quasi-periodic climate signals that are forced by the NAO. We identify a robust 25-yr anti-phased synchronization in climate variability between Scandinavia and the Mediterranean during the 17th–20th centuries. The amplitude of the 25-yr beat displays a long-term modulation in northern and southern Europe, with minimum amplitude during the late Maunder Minimum. This amplitude minimum coincides with a maximum in Δ14C, suggesting a potential solar or oceanic influence on the intensity of the 25-yr band of quasi-periodic variability.
 
Article
The inland lake level changes in arid or semi-arid mainland Asia at 6 ka and 18 ka have been reconstructed by means of survey of literature through compilations of sedimentological, biogeological and hydrological data. The results show that most lakes identical in low water stands at present. But the lakes from East and South Asia are characterised by a lower water levels at full glaciation while the lakes in Northwest Asia are registered higher stands. Conversely, comparative higher lake levels are registered in East and South Asia and lower lake levels appeared in Northwest Asia at 6 ka. Between above two regions, there occurred a climatic transitional zone centred in the area ranged from Central Asia north-eastward to west Mongolian Plateau and south-westward to the Middle East, where the lake behaviors are close to the lakes from Southeast Asia at 6 ka but change to be similar to the lakes from Northwest Asia at 18 ka. This lake level changes in temporal and spatial pattern indicated the global-scale alternation of atmospheric circulation associated with the shifts of westerlies and variation of summer monsoon intensity. The interpretation of changes in lake level and the reconstructions of atmospheric circulation are comparable with GCMs simulated moisture change induced by the change of internal and external forcing during last glacial maximum and mid-Holocene.
 
Article
One of the fundamental problems to quantifying past impact of anthropogenic activities is that long series of observational data for pollutant deposition and changes in the nutrient cycling of ecosystems (eutrophication) are often not available. Lake sediments may provide suitable archives to decipher the history of local and regional pollution and eutrophication.Here we provide quantitative high-resolution data for the history of airborne pollutants and eutrophication from sediments of five lakes in Central Chile between ca. AD 1800–2005. We use spheroidal carbonaceous particles (SCPs) from fossil fuel combustion and excess atmospheric Cu deposition from mining activities as a proxy for atmospheric deposition. Organic carbon and nitrogen flux rates to the sediments and C/N ratios are used as a proxy for aquatic primary production and eutrophication.We show that the lake sediment SCP and Cu records are highly consistent and depict in great detail the local and regional history of urban, industrial and transportation history as reported in independent documentary sources and statistics. The pre-industrial and pre-1950 background concentrations (and flux rates) of the substances can be quantified. We can also show that technical measures taken in the early 1980s to trim down Cu emissions from the copper mines reduced the excess atmospheric Cu fallout to the lakes by about 50%.Eutrophication of the lakes did not start before ca. 1980. Prior to that time, warm season temperatures explain most of the variance in TOC and N flux to the sediments. The three dimictic lakes show only moderate eutrophication responses to enhanced N supply (as atmospheric fallout; enrichment factors for TOC and N 1.1–2.6), suggesting that mainly phosphorus controls aquatic primary production. The meromictic lake, where phosphorus recycling is likely, shows the largest response (enrichment factors for TOC and N between 9–20).While all five lakes show overall consistent and similar trends for the pollution history during the 19th and 20th century, there are significant differences in the details of the individual profiles. This suggests that local sources are highly important and the common regional (background) signal is relatively marginal. This is very different from Europe.
 
Article
The relationship between the stable carbon isotope composition of tree rings from Mt. Helan, China, and precipitation was investigated for the period AD 1804–1997. The results reveal that in the Mt. Helan region, δ13C of tree rings negatively correlates with the total precipitation from February to July. We calculated discrimination (Δ) to remove the effects of δ13C of atmospheric CO2 on the tree-ring δ13C series, and reconstructed February to July precipitation back to 1804 using linear regression (R2=0.415, R2adj=0.394, F=19.88, P<0.0001), with best chronology replication for AD 1845–1991. The reconstruction shows annual to decadal wetness/dryness fluctuations. The historically documented drought of 1926–1930, and the wet period in the 1940s were found in our reconstruction.
 
Article
Using1948–2006 surface 2 m daily temperature, daily maximum temperature and daily minimum temperature of National Centers for Environmental Prediction (NCEP) reanalysis dataset, summer warm extreme indices, warm days (TG90P), warm-spell days (WSFI), warm day-times (TX90P) and warm nights (TN90P) are calculated for Gaussian grids, a complete Northern Hemisphere (NH) picture of changes of summer warm extremes is presented, and their links with El Niño/La Niña & Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and North Atlantic Oscillation (NAO) are also examined in this paper. The results on the NH summer seasonal basis are as follows. Warm days, warm-spell days, warm day-times and warm nights increase at rates of 2.18, 1.23, 2.31 and 2.00 days/decade respectively during 1948–2006. A broader area is exposed to frequent occurrence of warm extremes in the recent 30 years than in the last 30 years. The 59-year long-term upward trend is characterized by a positive trend reversion in the late 1970s, with a slight downward trend in the last 30 years and a rapid upward trend in the recent 30 years, representing the main form of interdecadal variance of NH warm extremes. Warm days, warm-spell days, warm day-times and warm nights increase at rates of 4.53, 3.36, 4.44 and 4.21 days/decade respectively during 1977–2006. During 1948–2006, the largest increasing rate is at central tropical Atlantic and the largest decreasing rate in Mongolia and north China. Significant (level of 0.05) upward trends cover about half of the NH during 1948–2006 and about a third of the NH during 1977–2006 with the very significant upward trends more focused, while very sparse regions have significant downward trends during these two periods. In the recent 30 years, although NH-land summer warms at a faster rate than NH-water, warm extremes on NH-water increase much faster than those on NH-land, the average warm extreme indices and their increasing trends on NH are most modulated by those on NH-water. TX90P increases more rapidly than TN90P and significant upward (downward) trends in TX90P have more (less) areal coverage than significant upward (downward) trends in TN90P on NH-land during 1948–2006. ENSO mainly modulates interannual variations, especially during PDO warm phase after the 1980s, and it has opposite impacts in Indian Ocean, South China Sea, south and southeast Asia coasts and in the interior Asia. PDO mainly controls the long-term significant upward trend in the central tropical Atlantic and downward trend in Mongolia and north China. NAO indices of different seasons have different impacted locations, the most obvious positive trend reversion in the late 1970s in south Greenland offshore is related to the interdecadal variation of summer NAO. An abrupt shift of NH summer diurnal temperature range in NCEP reanalysis data in 1967 suggests that the derived results related to the very high minimum temperature on NH-land before 1967 be accepted with care.
 
Article
The sediment load delivered from the Huanghe (Yellow River) to the sea has decreased sharply to 0.15 × 109 metric tons per year (0.15 Gt/yr) between 2000 and 2005, and now represents only 14% of the widely cited estimate of 1.08 Gt/yr. The river seems to be reverting to the pristine levels characteristic of the middle Holocene, prior to human intervention. Datasets from 1950 to 2005 from four key gauging stations in the main stream reveal distinct stepwise decreases in sediment load, which are attributed to both natural and anthropogenic impacts over the past 56 yr. Completions of two reservoirs, Liujiaxia (1968) and Longyangxia (1985), in the upper reaches of the river and their joint operations have resulted in stepwise decreases in sediment load coming from the upper reaches. Effective soil conservation practices in the middle reaches since the late 1970s, combined with the operation of the Sanmenxia and Xiaolangdi reservoirs, have also caused stepwise decreases in sediment load at Huayuankou in the middle reaches, but the decrease differs from that observed in the upper reaches. Decrease in precipitation is responsible for 30% of the decrease in sediment load at Huayuankou, while the remaining 70% is ascribed to human activities in the river basin, of which soil conservation practices contribute 40% to the total decrease. Sediment retention within reservoirs accounts for 20% of the total sediment load decrease, although there was notable sediment retention within the Xiaolangdi reservoir from 2000 to 2005. The remaining 10% of the decrease in sediment load is a result of the operation of reservoirs in the upper reaches. In the lower reaches, 20% of the sediment passing Huayuankou has been lost as a result of channel deposition and water abstraction. Soil conservation practices and the operation of reservoirs have lowered the content of coarser sediment (D > 0.05 mm) at Huayuankou, and reduced channel deposition in the lower reaches. In contrast, sediment loss owing to water abstraction in the lower reaches has increased considerably as water consumption for agricultural needs has increased. Therefore, the combined effects of climate change and human activities in the upper, middle, and lower reaches have resulted in stepwise decreases in the sediment load delivered from the Huanghe to the sea. The Huanghe provides an excellent example of the altered river systems impacted by climate change and extensive human activities over the past 56 yr. Further dramatic decreases in sediment load and water discharge in the Huanghe will trigger profound geological, morphological, ecological, and biogeochemical responses in the estuary, delta, and coastal sea.
 
Article
The objective of this statistical–climatological study is to identify characteristic weather situations in which tornadoes in Germany preferably occur. Tornado reports have been taken from the TorDACH database for the period 1950–2003. Weather situations are defined by the objective weather types classification of the German Meteorological Service (DWD), described by the direction of air mass advection, the cyclonality and the humidity (precipitable water) of the troposphere. In addition the number of thunderstorm days and aerological parameters, derived from temperature and dewpoint in 850 and 500 hPa, are taken into account. Higher frequencies of tornadoes in Germany are found in recent years 1998–2003, particularly in the years 2003 (40 tornadoes) and 2000 (warmest year in Germany since the beginning of the 20th century). There is no shift of the seasonal cycle detectable for the period 1980–2003 compared to 1950–2003 and also no shift of the intensity distribution. The majority of the tornadoes can be attributed to 3 specific weather types, all with a south-westerly advection and high humidity. This can be found for weak as well as for strong tornadoes. Anti/cyclonality in 950 hPa reduces/enhances the tornado frequency roughly by a factor of 2, while in 500 hPa cyclonality shows no significant influence. The fraction of the number of registered tornado days to the number of thunderstorm days has a maximum of one tornado in 50 thunderstorm days in the north-western part of Germany. Specific ranges of temperature and dewpoint in 850/500 hPa, which are not the highest values, are favoured for the occurrence of tornadoes.
 
Article
During the last half of the 20th century, cumulative annual discharge from 137 representative rivers (watershed areas ranging from 0.3 to 6300 × 103 km2) to the global ocean remained constant, although annual discharge from about one-third of these rivers changed by more than 30%. Discharge trends for many rivers reflected mostly changes in precipitation, primarily in response to short- and longer-term atmospheric–oceanic signals; with the notable exception of the Parana, Mississippi, Niger and Cunene rivers, few of these “normal" rivers experienced significant changes in either discharge or precipitation. Cumulative discharge from many mid-latitude rivers, in contrast, decreased by 60%, reflecting in large part impacts due to damming, irrigation and interbasin water transfers. A number of high-latitude and high-altitude rivers experienced increased discharge despite generally declining precipitation. Poorly constrained meteorological and hydrological data do not seem to explain fully these “excess” rivers; changed seasonality in discharge, decreased storage and/or decreased evapotranspiration also may play important roles.
 
Article
Daily temperature and precipitation data since 1960 are selected from 735 weather stations that are scattered over China. After comparatively analyzing relative interpolation methods, gradient-plus-inverse distance squared (GIDS) is selected to create temperature surfaces and Kriging interpolation method is selected to create precipitation surfaces. Digital elevation model of China is combined into Holdridge Life Zone (HLZ) model on the basis of simulating relationships between temperature and elevation in different regions of China. HLZ model is operated on the created temperature and precipitation surfaces in ARC/INFO environment. Spatial pattern of major terrestrial ecosystems in China and its change in the four decades of 1960s, 1970s, 1980s and 1990s are analyzed in terms of results from operating HLZ model. The results show that HLZ spatial pattern in China has had a great change since 1960. For instance, nival area and subtropical thorn woodland had a rapid decrease on an average and they might disappear in 159 years and 96 years, respectively, if their areas would decrease at present rate. Alpine dry tundra and cool temperate scrub continuously increased in the four decades and the decadal increase rates are, respectively, 13.1% and 3.4%. HLZ patch connectivity has a continuous increase trend and HLZ diversity has a continuous decrease trend on the average. Warm temperate thorn steppe, subtropical wet forest and cool temperate wet forest shifted 1781.45 km, 1208.14 km and 977.43 km in the four decades, respectively. These HLZ types are more sensitive to climate change than other ones. These changes reflect the great effects of climate change on terrestrial ecosystems in China.
 
Article
Two Mediterranean sea level distributions spanning the last decades are examined. The first one is a reconstruction of sea level obtained by a reduced-space optimal interpolation applied to tide gauge and altimetry data. The second distribution is obtained from a 3D (baroclinic) regional circulation model. None of the two representations includes the mechanical atmospheric forcing. Results are presented for two different periods: 1993–2000 (for which altimetry data are available) and 1961–2000 (the longest period common to both distributions).The first period is examined as a test period for the model, since the reconstruction is very similar to altimetry observations. The modelled sea level is in fair agreement with the reconstruction in the Western Mediterranean and in the Aegean Sea (except in the early nineties), but in the Ionian Sea the model departs from observations. For the whole period 1961–2000 the main feature is a marked positive trend in the Ionian Sea (up to 1.8 mm yr− 1), observed both in the reconstruction and in the model. Also the distribution of positive trends in the Western Mediterranean (mean value of 1.1 mm yr− 1) and the smaller trends in the Aegean Sea (0.5 mm yr− 1) are similar in the reconstruction and in the model, despite the first implicitly accounts for sea level variations due to remote sources such as ice melting and the second does not. The interannual sea level variability associated with key regional events such as the Eastern Mediterranean Transient is apparently captured by the reconstruction but not by the model (at least in its present configuration). Hence, the reconstruction can be envisaged as a useful tool to validate further long-term numerical simulations in the region.
 
Click here to download high resolution image
Summary of change in swale width (m) 1967-2007 (width relative to 2007)
Summary of historic volume change (m3/m) 1967-2007
Article
Raine Island is a vegetated coral cay located on the far northern outer Great Barrier Reef (GBR), recognised as a globally significant turtle rookery. Cay geomorphology, specifically the morphology of the beach and swale, dictate the availability of nesting sites and influence nesting success. Understanding short and long-term shoreline change is critical for managers charged with protecting the nesting habitat, particularly as climate change progresses. Historical topographic surveys, a simple numerical model and geographic information system (GIS) techniques were used to reconstruct a 40-year (1967–2007) shoreline history of Raine Island. Results show that significant shoreline change has occurred on 78% of the island's shoreline between 1967 and 2007; 34% experienced net retreat and 44% net progradation during the study interval. Shoreline retreat is mainly concentrated on the east–southeast section of the shoreline (average annual rate of − 0.3 ± 0.3 m/yr), while the shore on the western side of the island prograded at a similar rate (0.4 ± 0.2 m/yr). A seasonal signal was detected relating to oscillations in wind direction and intensity, with the southeast and west–southwest shorelines migrating an average of ∼ 17 m from season to season. The volume of sediment deposited on Raine Island between 1967 and 2007 increased by ∼ 68,000 m3 net, but accretion rates varied significantly seasonally and from year to year. The largest volumetric changes have typically occurred over the last 23 years (1984–2007). Despite the recent concern that Raine Island is rapidly eroding, our data demonstrate net island growth (6% area, 4% volume) between 1967 and 2007. Perceptions of erosion probably reflect large morphological changes arising from seasonal, inter-annual and inter-decadal patterns of sediment redistribution rather than net loss from the island's sediment budget.
 
Article
The spectacular drying of Lake Chad around 1970 has been widely reported in the media. The lake is now divided into different individual seasonal or perennial lake pools. Paradoxically, our understanding of the hydrology of the lake in its present state remains poor. In particular there is a lack of fundamental hydrological data to quantify the drought and limited information about water management options. The hydrologic changes are the driving forces for the natural resources associated with the lake i.e. fisheries, recession cultivation on the lake floor and green vegetation for livestock. Whereas the yearly cycle of the natural resources has become fairly predictable in the southern pool of the lake, variability and vulnerability have increased greatly in the northern pool.Here we use a hydrological model of Lake Chad developed recently to i) reconstruct the past levels and inundated areas in order to compensate for the lack of hydrological data and, ii) simulate the effects on the lake of modifications in the entering fluxes, either by abstraction for irrigation purposes, addition from an inter basin transfer or impact of climate change. The study period runs from 1973 to 2011 with a total inundated area of the lake ranging between 1800 and 15,000 km2.Highlights► Lake Chad has split into different pools during the drought period 1973–2011. ► A hydrologic model is used to reconstruct water levels and areas for this period. ► The impact on the lake of different interbasin transfer scenarios is simulated. ► A limited transfer (5 km3/yr) would provide the best benefits to the populations.
 
Article
Pio XI glacier, with an area of 1263 km2 and a length of 65.7 km, is the largest of the Southern Patagonia Icefield (SPI). During the period 1945–1995 it experienced a net advance of ca. 10 km., a unique behavior considering that virtually all neighboring glaciers are retreating. Two maps at 1:50,000 scale, produced from 1975 and 1995 aerial photography, are used to compute volume changes for this glacier. This is done by digitizing the maps and creating digital terrain models at 200-m resolution. Volume changes for a reduced sample area, in combination with the hypsometric curve of the glacier, are analyzed in terms of the recent advance experienced by the glacier. The sample area (4.5% of the glacier area) for the volume change analysis shows an average thickening on the glacier for the period 1975–1995 of 44.1 m. which represents 2.2 m a−1. The corresponding volume change was 2.52 km 3, with a larger thickening in the lower part of the ablation area. The hypsometric curve of the glacier shows a low sensitivity of the glacier to ELA variations. Nevertheless, the increase in temperature has shifted the ELA upwards, reaching close to a threshold, from where the glacier would begin to retreat in the future if the ELA elevation trend continues.
 
Article
The TIROS-N microwave sounding units (MSU's) have demonstrated an ability to measure globally averaged tropospheric brightness temperature anomalies to a precision of 0.01°C per month. This has been revealed by five separate overlap periods of concurrently operating morning and afternoon satellites. For MSU channel 2, comparisons to ten years of radiosonde data show that these anomalies are closely related to anomalies in 850-300 mbar thickness, with gridpoint correlations to radiosondes usually ranging between 0.85 and 0.95. The comparisons also suggest that there is no drift or bias in the MSU's (or US-controlled radiosonde system) between 1979–1988 to about 0.01°C when results are averaged over the Pacific or Caribbean. The non-trivial amount of lower stratospheric influence on MSU channel 2, combined with evidence for cooling in that layer during 1979–1990, suggests the need for removal of this effect from the measurements before interpretation in terms of a “tropospheric” temperature. Because the NOAA-6 MSU channel 3 was not usable for precision climate monitoring due to a sizeable drift, we have performed a retrieval on MSU channel 2 at different view angles to achieve stratospheric cancellation. We present global brightness temperature anomalies of both raw channel and the troposheric retrieval for the period 1979–1990. It appears that the MSU's (and presumably the advanced microwave sounding units as well) will provide a valuable resource for not only monitoring intraseasonal to interannual climate change, but also for long-term monitoring of potential global warming.
 
Article
Since 1981, hourly values of albedo have been measured routinely at Norwegian Polar Institute's research station in Ny-Ålesund, Svalbard. We have undertaken statistical analysis of the time series 1981–1997 to investigate potential long-term variability and trends in the albedo data set. The following questions have been raised and answered by regression analysis: (i) Has the time of beginning of snow melt changed? (ii) Have melt rates changed? (iii) Has the time of snow arrival in fall changed? (iv) Has the period without snow cover changed? The period without snow on the ground is studied because of its importance for tundra characteristics as a habitat for biota, e.g. length of the growth season. Our data show that albedo varies seasonally, with large variations in spring and autumn and much smaller variations in winter and summer. The variability is reasonably constant within each season. Density estimates of the albedo data suggest that the dates with highest likelihood for (i) start of snow melt and (ii) start of snow formation are 5th of June and 17th of September, respectively. Highest probability for the length of snow-free season is 94 days. None of the tests indicated any significant trends (or indications of climate change) in the 17-year record of albedo, that means that the four questions above were all answered by “no.” Correlation with the North Atlantic Oscillation (NAO) index is also investigated. No correlation between the NAO index and albedo nor temperature or precipitation was found. Even so, because of the short duration that our data set spans, we cannot rule out that such a correlation exists on decadal time scales.
 
Article
A simulation model based on satellite observations of monthly vegetation cover was used to estimate monthly carbon fluxes in terrestrial ecosystems from 1982 to 1998. The NASA–CASA model was driven by vegetation properties derived from the Advanced Very High Resolution Radiometer (AVHRR) and radiative transfer algorithms that were developed for Moderate Resolution Imaging Spectroradiometer (MODIS). For the terrestrial biosphere, predicted net ecosystem production (NEP) flux for atmospheric CO2 has varied widely between an annual source of −0.9 Pg C per year and a sink of +2.1 Pg C per year. The southern hemisphere tropical zones (SHT, between 0° and 30°S) have a major influence over the predicted global trends in interannual variability of NEP. In contrast, the terrestrial NEP sink for atmospheric CO2 on the North American (NA) continent has been fairly consistent between +0.2 and +0.3 Pg C per year, except during relatively cool annual periods when continental NEP fluxes are predicted to total to nearly zero. The predicted NEP sink for atmospheric CO2 over Eurasia (EA) increased notably in the late 1980s and has been fairly consistent between +0.3 and +0.55 Pg C per year since 1988. High correlations can be detected between the El Niño Southern Oscillation (ENSO) and predicted NEP fluxes on the EA continent and for the SHT latitude zones, whereas NEP fluxes for the North American continent as a whole do not correlate strongly with ENSO events over the same time series since 1982. These observations support the hypothesis that regional climate warming has had notable but relatively small-scale impacts on high latitude ecosystem (tundra and boreal) sinks for atmospheric CO2.
 
Top-cited authors
P. Lionello
  • Università del Salento
Jaia Syvitski
  • University of Colorado Boulder
Michel Meybeck
  • Sorbonne Université
Dennis Lettenmaier
  • University of California, Los Angeles
Anny Cazenave
  • Paul Sabatier University - Toulouse III