Current Warm Period Research Articles

On the Relationship between Holocene Environmental Variability and the Diatom Composition in the Peña Lagoon, Se Uruguay

Uruguayan Southern lagoons exhibit high Holocene resolution paleoenvironmental-paleoclimatic records for inferring long-term regional changes. The multiproxy analysis of three sediment cores enabled the recognition of Holocene climatic variability from the paleolimnological record of the Pena lagoon over the last 2,458 cal. years BP. Four main stages were identified throughout the record. The first stage (2,458–1,500 cal. years BP) was characterized as a shallow meso-eutrophic system with high abundances of aerophilic benthic species, epiphytic taxa and planktonic taxa. The second stage (1,415–390 cal. years BP) showed a noticeable change in the diatom assemblage dominated by the fresh-brackish benthic species Staurosira construens, but also presented fluctuations in the abundance of Aulacoseira ambigua and Aulacoseira granulata, which indicate the occurrence of temperate to cold and semiarid climatic conditions including intervals of high rainfall. The core chronology allowed us to ascribe this stage to the Little Ice Age (LIA). The third stage, post 390 cal. years BP, showed the highest proportion of freshwater planktonic species throughout the entire core, thus indicating the development of an eutrophic system under relatively warm and wet conditions, which were assigned to the Current Warm Period. After ca. 1,962 AD, a sharp increase in the abundance of epiphytic species highlights the onset of the fourth stage, which was characterized by littoral expansion and, consequently, the proliferation of associated macrophytes due to anthropogenic impacts.

Variability in landfalling trends of cyclonic disturbances over North Indian Ocean region during current and pre-warming climate

Cyclonic disturbances (CDs) are one of the deadliest systems formed over ocean basins in the world. Impact of warming climate over these basins including that of North Indian Ocean (NIO) is quite prominent in terms of enhanced intensity and re-curvature of the cyclonic systems. However, the impact of warming climate on landfall activity is not yet studied for the NIO basin. Therefore, the current study is performed by dividing the climate into current and pre-warming periods based on sea surface temperature (SST) anomaly variation. The study reveals that Bangladesh, Andhra Pradesh (AP), and Tamil Nadu (TN) are more vulnerable to severe cyclones formed over Bay of Bengal (BOB) during the current warming climate. Gujrat is prone to severe cyclones and Arabian Peninsula countries are vulnerable to cyclonic storms formed over the Arabian Sea during the current warming climate as well. During CWP, Bangladesh and Arakan are more vulnerable to CD landfall in pre-monsoon season, whereas in post-monsoon months, AP, TN and Bangladesh are more prone coastal areas of BOB. Gujrat and IAA are more vulnerable coastal areas of AS irrespective of seasons considered. The enhanced genesis over southern and middle sector of BOB is mainly responsible for more landfall over AP, TN and Bangladesh. In addition, change in wind direction from NW to N-NW and increased meridional SST over BOB are found to be encouraging the landfall activity near AP and TN coasts. The W-SW and zonally distributed SST supports landfall over Gujrat. There is less impact of change in genesis location over AS landfalling CDs. Over AS near to 12° N, a well-organised wind circulation is observed enhancing the dissipation of the cyclonic systems over the basin during current warming period.

Comparative Reaction of Larch (Larix sibirica Ledeb.) Radial Increment on Climate Change in the Forest Steppe and Highlands of Southern Siberia

The influence of climate changes on larch (Larix sibirica Ledeb.) radial increment under conditions of a limited (forest steppe) and sufficient (high-altitude Kuznetsk Alatau, floodplain stands) humidification is considered. The relationship between growth index of larch trees (N = 257) and ecological and climatic variables is analyzed. In the forest steppe, with the onset of warming, a decrease in the aridity of the climate, an increase in the duration of the growing season (1980s), and an increase in the larch growth index followed by its depression in the 1990s have been observed. Radial-increment depression is caused by an increase in vapor-pressure deficit and arid climate due to a rising air temperature. In the 2000s, radial-increment fluctuations with average values not exceeding those before the beginning of current climate warming period occurred. In the highlands, since the 1970s, there has been a general increase in the larch radial increment closely associated with the main limiting factor of growth—air temperature. At the same time, in arid years, the radial-increment depression of larch trees in highland and floodplain larch forests is also noted. When implementing “hard” climate scenarios (RCP 6.0 and RCP 8.5), it is likely that the larch growth index in a forest steppe will decrease further and its increase in areas of sufficient moisture will be observed.

The Little Ice Age was 1.0–1.5 °C cooler than current warm period according to LOD and NAO

We study the yearly values of the length of day (LOD, 1623-2016) and its link to the zonal index (ZI, 1873-2003), the Northern Atlantic oscillation index (NAO, 1659-2000) and the global sea surface temperature (SST, 1850-2016). LOD is herein assumed to be mostly the result of the overall circulations occurring within the ocean-atmospheric system. We find that LOD is negatively correlated with the global SST and with both the integral function of ZI and NAO, which are labeled as IZI and INAO. A first result is that LOD must be driven by a climatic change induced by an external (e.g. solar/astronomical) forcing since internal variability alone would have likely induced a positive correlation among the same variables because of the conservation of the Earth's angular momentum. A second result is that the high correlation among the variables implies that the LOD and INAO records can be adopted as global proxies to reconstruct past climate change. Tentative global SST reconstructions since the 17th century suggest that around 1700, that is during the coolest period of the Little Ice Age (LIA), SST could have been about 1.0-1.5 {\deg}C cooler than the 1950-1980 period. This estimated LIA cooling is greater than what some multiproxy global climate reconstructions suggested, but it is in good agreement with other more recent climate reconstructions including those based on borehole temperature data.

Warm Arctic−cold Siberia: comparing the recent and the early 20th-century Arctic warmings

The Warm Arctic–cold Siberia surface temperature pattern during recent boreal winter is suggested to be triggered by the ongoing decrease of Arctic autumn sea ice concentration and has been observed together with an increase in mid-latitude extreme events and a meridionalization of tropospheric circulation. However, the exact mechanism behind this dipole temperature pattern is still under debate, since model experiments with reduced sea ice show conflicting results. We use the early twentieth-century Arctic warming (ETCAW) as a case study to investigate the link between September sea ice in the Barents–Kara Sea (BKS) and the Siberian temperature evolution. Analyzing a variety of long-term climate reanalyses, we find that the overall winter temperature and heat flux trend occurs with the reduction of September BKS sea ice. Tropospheric conditions show a strengthened atmospheric blocking over the BKS, strengthening the advection of cold air from the Arctic to central Siberia on its eastern flank, together with a reduction of warm air advection by the westerlies. This setup is valid for both the ETCAW and the current Arctic warming period.

Science and Poetry in Deep Time

The ice core sample that marks the beginning of the geological epoch known as the Holocene is archived in a freezer at the University of Copenhagen.1 Drilled in 2003 from the central Greenland ice sheet, the sample provided enough evidence for the International Union of Geological Sciences to endorse the proposal that the Pleistocene–Holocene boundary occurred 11,700 calendar years ago.2 In scientific terms, the 2-kilometre-long ice core exhibits ‘an abrupt shift in deuterium excess values, accompanied by more gradual changes in δ18O, dust concentration, a range of chemical species, and annual layer thickness’, marking the distinction between two geological ages. To the layperson, the climactic event is better recognised as the end of the Ice Age, and the beginning of the current warm period in which we live.3 Or, in which we believed we lived. On 29 August 2016, the International Geological Congress’s Working Group on the Anthropocene (WGA) announced the culmination of seven years’ research into the subdivision of recent geological time: humanity’s impact on the Earth – encompassing its various and interconnected ecological, biological, and geological systems – is now recognised as so momentous that the relatively young Holocene has been superseded.4 The Anthropocene has arrived.

Speleothem records decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium

This study presents the first well-dated high resolution stable isotope (δ18O and δ13C) and trace element (Mg and Sr) speleothem records from southwestern Morocco covering the last 1000 yrs. Our records reveal substantial decadal to multidecadal swings between dry and humid periods, consistent with regional paleorecords with prevailing dry conditions during the Medieval Climate Anomaly (MCA), wetter conditions during the second part of the Little Ice Age (LIA), and a trend towards dry conditions during the current warm period. These coherent regional climate signals suggest common climate controls. Statistical analyses indicate that the climate of southwestern Morocco remained under the combined influence of both the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) over the last millennium. Interestingly, the generally warmer MCA and colder LIA at longer multidecadal timescales probably influenced the regional climate in North Africa through the influence on Sahara Low which weakened and strengthened the mean moisture inflow from the Atlantic Ocean during the MCA and LIA respectively.

Temperature variations over the past 600 years documented by a <italic>δ</italic><sup> 13</sup>C record from terrestrial plant remains from Kanas Lake, Altai Mountains, Northwestern China

Arid central Asia (ACA) is characterized by relatively dry climates and fragile ecosystems, which restrict the survival and development of human beings. A full understanding of the climatic and environmental evolution in the region is not only significant for the thorough understanding of global climate change, but is of importance for the sustainable development of the area as well, especially in the context of ongoing global warming. The Altai Mountains are located in the central part of the ACA and numerous lakes are distributed in the Mountains. Kanas Lake, one of the largest lake in the Altai Mountains of Northwestern China, is surrounded by dense forests. The climate of the area is characterized by relatively abundant precipitation and cold temperatures and the vegetations are very sensitive to climate changes. Recent temperature reconstructions based on the width and short-term series of carbon isotopes of tree-rings from the area did not exhibit an obvious warming trend during the past century. This result is unexpected in the context of ongoing global warming, but it remains uncertain whether the regional climate did not response to global warming due to complex unknown processes, or whether the tree-ring data did not record the signal of long-term warming. A 81-cm-long sediment core (KNS15E) was recovered from a water depth of 29 m near the lakeside of Kanas Lake using a modified piston corer. Here we present the results of stable carbon isotope analysis of a sequence of C3 terrestrial plant remains from core KNS15E. A chronology is constructed from five AMS 14C dates from wood fragments (twigs) and it reveals that the whole core spans the past 570 years. From the context of the sampling site, the soil of the adjacent forest would have been well-supplied with water from rainfall and snowmelt from the mountains which would have been supplied to site as ground water. In addition, based on the previous studies of carbon isotopes of tree-rings and of α -cellulose of sedge plants, the δ 13C variations are likely to be mainly related to temperature changes. The raw δ 13C values are corrected to pre-industrial values to account for changes in the δ 13C value of atmospheric CO2 and for changes in CO2 concentration. The stable carbon isotope record provides a record of temperature changes over the last 570 years which can be clearly divided into the Little Ice Age and the Current Warm Period. The temperature increased continuously since the early 19th century and the 20th century was the warmest period in the sequence. The temperature variations revealed in our study are consistent with temperature reconstructions from ice cores, tree-rings and from Teletskoe Lake in the Russian Altai, as well as with the extratropical NH mean temperature record. In addition, a trend of decreasing temperature is indicated by the decreasing stable carbon isotope ratios of both bulk plant remains and conifer needles in the upper part of the core, which corresponds with the widely-reported phenomenon of a hiatus in global warming in central Asia and with meteorological data from the study region.

Changes in biological productivity and ocean-climatic fluctuations during the last ~ 1.5 kyr in the Humboldt ecosystem off northern Chile (27°S): A multiproxy approach

A sedimentary box core (BIAC072014) collected in a coastal environment in the Humboldt Current Ecosystem (HCE, Inglesa Bay, ~27°S, northern Chile) was used to reconstruct the changes in biological productivity and ocean-climate fluctuations during the last ~1.5kyr via geochemical, mineralogical and micropaleontological analyses. From ~510CE to ~930CE, “El Niño-like” conditions prevailed, while during the Medieval Climate Anomaly (MCA, ~950CE to ~1450CE), “La Niña-like” conditions predominated. This pattern might have resulted from weakening/strengthening of the Walker cell, contraction/expansion of the South Pacific Sub-tropical High (SPSH) and a predominance of warm/cold phases of the Pacific Decadal Oscillation (PDO). Between ~1820CE and the present, Inglesa Bay experienced a reduction in the Oxygen Minimum Zone (OMZ) intensity and biological productivity, an increase in the detritic input and a decrease in sea surface temperature (SST). Despite the regional intensification and secular cooling (~14°S and ~27°S) of coastal upwelling, the similarity between the reduction in biological productivity and OMZ intensity in this region and that in the central zone of Chile (~36°S) suggests that Inglesa Bay acts a transition zone in relation to the oceanographic behaviour of the HCE. The reduction in the OMZ intensity and biological productivity during the last ~200years could be explained by greater subsurface ventilation in intermediate depths due to more northward displacement of the subduction region of the Eastern South Pacific Intermediate Water (ESPIW) near ~30°S in response to the strengthening of the circulation of the Subtropical Gyre. The coastal cooling observed in the northern section of HCE during the current Warm Period suggests that it is not possible to demonstrate that El Niño-like conditions promoted the reduction in biological productivity and OMZ intensity in Inglesa Bay.

Recently submerged tidal notches in the wider Makarska area (Central Adriatic, Croatia)

The active tidal notches in the Makarska area, which were observed and described in historical records from the beginning of the 20th century (Andrijašević, 1910), have been reanalysed 107 years later. Detailed mapping of three locations in the wider Makarska area revealed their level of submergence, which varied between 30 and 37 ± 5 cm below the recent mean sea-level (MSL) according to location. This investigation points to the recent acceleration of relative sea-level change during the Current Warm Period (CWP) and also to the past temporary still-stand. According to data from Central Adriatic islands of Vis, Ravnik, and Biševo, this still-stand probably lasted around 500 years – during the Little Ice Age (LIA) period. However, the notch in the Dubrovnik area, which was, as stated in historical data (Suess, 1885–1908), active at the end of the 19th century, is still active today, indicating tectonic uplift of the Dubrovnik area.

Growth response of Scots pines in polar-alpine tree-line to a warming climate

Coniferous trees at the alpine and polar tree lines of the Northern Hemisphere represent the outermost limit of their ecological range. Under such conditions, even small temperature variations may cause growth responses, which therefore can be used as indicators for changing environmental conditions. In this study we analysed the radial growth of Scots pine (Pinus sylvestris L.) along a latitudinal and altitudinal gradient at six locations in the polar and alpine tree-line ecotone in Finnish Lapland. We evaluated the effect of climatic trends on pine growth in relation to tree age and region, specifying a northern and a southern region in the study area. We found a response of Scots pine to climatic variations until the 1980s, but not to the current warming period. Increasing growth trends could be detected since 2000 in the radial growth of southern located trees, predominantly of juvenile ages, while the northern trees did not respond significantly to the current warming. In the north recent warmer and wetter conditions during winter time, inducing snow loads, wind damages, diseases and frost damages possibly masked the benefits of warmer conditions. The missing link between warming and radial growth would affect the use of tree-rings as proxy for past climate and for predictions for forest extension in polar-alpine tree-line sites.

Decoupling of coral skeletal δ13C and solar irradiance over the past millennium caused by the oceanic Suess effect

AbstractMany factors influence the seasonal changes in δ13C levels in coral skeletons; consequently, the climatic and environmental significance of such changes is complicated and controversial. However, it is widely accepted that the secular declining trend of coral δ13C over the past 200 years reflects the changes in the additional flux of anthropogenic CO2 from the atmosphere into the surface oceans. Even so, the centennial‐scale variations, and their significance, of coral δ13C before the Industrial Revolution remain unclear. Based on an annually resolved coral δ13C record from the northern South China Sea, the centennial‐scale variations of coral δ13C over the past millennium were studied. The coral δ13C and total solar irradiance (TSI) have a significant positive Pearson correlation and coupled variation during the Medieval Warm Period and Little Ice Age, when natural forcing controlled the climate and environment. This covariation suggests that TSI controls coral δ13C by affecting the photosynthetic activity of the endosymbiotic zooxanthellae over centennial timescales. However, there was a decoupling of the coral skeletal δ13C and TSI during the Current Warm Period, the period in which the climate and environment became linked to anthropogenic factors. Instead, coral δ13C levels have a significant Pearson correlation with both the atmospheric CO2 concentration and δ13C levels in atmospheric CO2. The correlation between coral δ13C and atmospheric CO2 suggests that the oceanic 13C Suess effect, caused by the addition of increasing amounts of anthropogenic 12CO2 to the surface ocean, has led to the decoupling of coral δ13C and TSI at the centennial scale.

A comparison of the climates of the Medieval Climate Anomaly, Little Ice Age, and Current Warm Period reconstructed using coral records from the northern South China Sea

AbstractFor the global oceans, the characteristics of high‐resolution climate changes during the last millennium remain uncertain because of the limited availability of proxy data. This study reconstructs climate conditions using annually resolved coral records from the South China Sea (SCS) to provide new insights into climate change over the last millennium. The results indicate that the climate of the Medieval Climate Anomaly (MCA, AD 900–1300) was similar to that of the Current Warm Period (CWP, AD 1850‐present), which contradicts previous studies. The similar warmth levels for the MCA and CWP have also been recorded in the Makassar Strait of Indonesia, which suggests that the MCA was not warmer than the CWP in the western Pacific and that this may not have been a globally uniform change. Hydrological conditions were drier/saltier during the MCA and similar to those of the CWP. The drier/saltier MCA and CWP in the western Pacific may be associated with the reduced precipitation caused by variations in the Pacific Walker Circulation. As for the Little Ice Age (LIA, AD 1550–1850), the results from this study, together with previous data from the Makassar Strait, indicate a cold and wet period compared with the CWP and the MCA in the western Pacific. The cold LIA period agrees with the timing of the Maunder sunspot minimum and is therefore associated with low solar activity. The fresher/wetter LIA in the western Pacific may have been caused by the synchronized retreat of both the East Asian Summer Monsoon and the Australian Monsoon.

Wet and cold climate conditions recorded by coral geochemical proxies during the beginning of the first millennium CE in the northern South China Sea

The past two millennia include some distinct climate intervals, such as the Medieval Warm Period (MWP) and the Little Ice Age (LIA), which were caused by natural forcing factors, as well as the Current Warm Period (CWP) that has been linked to anthropogenic factors. Therefore, this period has been of great interest to climate change researchers. However, most studies are based on terrestrial proxy records, historical documentary data, and simulation results, and the ocean and the tropical record are very limited. The Eastern Han, Three Kingdoms, and Western Jin periods (25–316 CE) cover the beginning first millennium CE in China, and were characterized by a cold climate and frequent wars and regime changes. This study used paired Sr/Ca and δ18O series recovered from a fossil coral to reconstruct the sea surface water conditions during the late Eastern Han to Western Jin periods (167–309 CE) at Wenchang, eastern Hainan Island in the northern South China Sea (SCS), to investigate climate change at this time. The long-term sea surface temperature (SST) during the study interval was 25.1°C, which is about 1.5°C lower than that of the CWP (26.6°C). Compared with the average value of 0.40‰ during the CWP, the long-term average seawater δ18O (−0.06‰) was more negative. These results indicate that the climate conditions during the study period were cold and wet and comparable with those of the LIA. This colder climate may have been associated with the weaker summer solar irradiance. The wet conditions were caused by the reduced northward shift of the intertropical convergence zone/monsoon rainbelt associated with the retreat of the East Asian summer monsoon. Interannual and interdecadal climate variability may also have contributed to the variations in SST and seawater δ18O recorded over the study period.

The inter-annual variations and the long-term trends of monthly air temperatures in Iraq over the period 1941–2013

Mean annual and monthly averages of mean (Tmean), maximum (Tmax) and minimum (Tmin) air temperature from seven stations in Iraq were analysed to detect the inter-annual variation, long-term temporal and spatial trends distribution over the period 1941–2013. Due to non-homogeneous problems, this period has been divided into two short separated periods (1941–1980 and 1995–2013), in order to compute temperature trends. In this context two statistical tests were used: linear regression and Mann-Kendall. The time series of mean annual temperature indicate the current warming period over Iraq identical with the global warming, which has started since the middle of seventh decade of the last century. The 2010 was the warmest year in all stations. Three distinct inter-annual temperature variation patterns were observed. These were probably the effects of micro-scale and meso-scale factors. The first one represents central and northern Iraq. The second represents the south of Iraq and Kirkuk station and the third one is a characteristic for eastern Iraq. Temperature trend analysis revealed that there are general upward trends with the strongest warming trends identified in the summer months which are around 89 % of the total significant monthly trends. Spatially, in both periods the southern region of Iraq is most affected by the warming trend in Tmean and Tmax. When considering Tmin, the southern and northern regions both are affected by warming with more pronounced trend intensity in the northern stations. No significant trend occurs Hai station in both periods and Baghdad has the still less trend value. In the first period the highest rise of Tmean and Tmax values are observed in July and June in Nasiriya station at 0.61 °C/decade and 0.63 °C/decade, respectively and in Mosul station for Tmin in August is 1.41 °C/decade. Moreover, in the period from 1995 to 2013, the highest warming trend of Tmean and Tmax were in Hai station for March at 1.48 °C/decade and at 1.85 °C/decade, respectively. Baghdad station experienced the highest significant trend for Tmin in August at 1.89 °C/decade.

Hydroclimatic changes over the past 900 years documented by the sediments of Tiewaike Lake, Altai Mountains, Northwestern China

The reconstruction of variations in effective moisture over the past millennium are important for assessing potential future climatic trends in environmentally-sensitive arid Central Asia (ACA), especially in the context of ongoing global warming. However, high-resolution time series reflecting past moisture variability in the region are scarce, thus preventing a full understanding of the hydroclimatic evolution. Here we present the results of multi-proxy analysis of a sediment core spanning the past 900 years from Tiewaike Lake in the Altai Mountains, northern Xinjiang. The climatic proxies measured include grain size, organic matter and carbonate content, C/N ratio, and the stable carbon isotope composition of bulk organic matter. The results show that over the past 900 years the climatic evolution of the study area can be divided into distinct hydroclimatic phases: the arid Medieval Climate Anomaly (MCA, ∼1090–1430 AD), the humid Little Ice Age (LIA, 1430–1940 AD), and the arid Current Warm Period (CWP, 1940–2012 AD). During the LIA, the water level at Tiewaike Lake was overall higher than during the other climatic intervals; in addition, the LIA climate was unstable and characterized by six intervals with the occurrence of heavy precipitation events on a multi-decadal timescale. A humid, unstable climate during the LIA is also suggested by climatic records from adjacent regions. Our results support the view that over the past millennium, on a multi-centennial timescale, the moisture variations in ACA were generally out-of-phase with those in the region affected by the Asian summer monsoon. The humid, unstable LIA climate in ACA may have been associated with changes in the North Atlantic Oscillation (NAO) index and/or with variations in solar irradiance.

Climate change evidences from the end of the Little Ice Age to the Current Warm Period registered by Melincué Lake (Northern Pampas, Argentina)

Abstract Geochemical, sedimentological, historical and instrumental information from Melincue Lake (33°43′S/61°28′W) was analyzed in order to reconstruct main hydro-climatic changes along the Pampean region, considering the end of the Little Ice Age (LIA) and the Current Warm Period (CWP) climatic phases. Elemental X-ray fluorescence determinations were performed on a dated short core extracted from the deepest part of the lake. Chemical (Ca, Sr, Al, K, Si, Ti and Fe) variation was analyzed along with physical (magnetic susceptibility, dry bulk density and medium particle size) and mineralogical proxies, as well as carbon and nitrogen contents (organic and inorganic carbon and organic nitrogen). Multicomponent statistical analyses allowed the identification of the main environmental processes ruling sedimentation, which included carbonate precipitation, detrital input and primary productivity. Homogeneous, carbonate-rich, detrital-rich and organic matter-poor deposits reflect very shallow and ephemeral lake conditions during the last years of the LIA period. Very low sedimentation rates and the presence of discontinuities in the sedimentary record pointed to episodes of extensive lake shrinkages and probable lake-floor subaerial exposures. Bedded and laminated organic-rich muds accumulated in perennial lake conditions after ca. AD 1880 reveal an important environmental change concurrent with the onset of the Current Warm Period. The contrast of sediment composition and lithological characteristics with the instrumental record of the CWP allowed us to consider the development of a shallow saline-lake system, enhanced detrital input and carbonate precipitation during dry periods. In turn, higher lake levels, and thus more diluted lake-waters during wetter periods, resulted in enhanced organic lake productivity and lower carbonate precipitation. The comparison of the paleolimnological reconstruction from Melincue Lake to different hydrological and paleoclimatic records (i.e. limnimetric measurements and ocean temperature changes) suggests that the moisture variation registered between the LIA and the CWP was consistent with (and likely forced by) large-scale climatic oscillations. The present study provides new evidences to evaluate the climate and environmental changes in central Argentina related to the dynamics of the South American Monsoon system during the last five centuries.

Rapid climate fluctuations over the past millennium: evidence from a lacustrine record of Basomtso Lake, southeastern Tibetan Plateau.

Abrupt climate changes and fluctuations over short time scales are superimposed on long-term climate changes. Understanding rapid climate fluctuations at the decadal time scale over the past millennium will enhance our understanding of patterns of climate variability and aid in forecasting climate changes in the future. In this study, climate changes on the southeastern Tibetan Plateau over the past millennium were determined from a 4.82-m-long sediment core from Basomtso Lake. At the centennial time scale, the Medieval Climate Anomaly (MCA), Little Ice Age (LIA) and Current Warm Period (CWP) are distinct in the Basomtso region. Rapid climate fluctuations inferred from five episodes with higher sediment input and likely warmer conditions, as well as seven episodes with lower sediment input and likely colder conditions, were well preserved in our record. These episodes with higher and lower sediment input are characterized by abrupt climate changes and short time durations. Spectral analysis indicates that the climate variations at the centennial scale on the southeastern Tibetan Plateau are influenced by solar activity during the past millennium.

Terrigenous material supply to the Peruvian central continental shelf (Pisco, 14° S) during the last 1000 years: paleoclimatic implications

Abstract. In the eastern Pacific, lithogenic input to the ocean responds to variations in the atmospheric and oceanic system and their teleconnections over different timescales. Atmospheric (e.g., wind fields), hydrological (e.g., fresh water plumes) and oceanic (e.g., currents) conditions determine the transport mode and the amount of lithogenic material transported from the continent to the continental shelf. Here, we present the grain size distribution of a composite record of two laminated sediment cores retrieved from the Peruvian continental shelf that record the last ∼ 1000 years at a sub-decadal to centennial time-series resolution. We propose novel grain size indicators of wind intensity and fluvial input that allow reconstructing the oceanic–atmospheric variability modulated by sub-decadal to centennial changes in climatic conditions. Four grain size modes were identified. Two are linked to aeolian inputs (M3: ∼ 54; M4: ∼ 91 µm on average), the third is interpreted as a marker of sediment discharge (M2: ∼ 10 µm on average), and the last is without an associated origin (M1: ∼ 3 µm). The coarsest components (M3 and M4) dominated during the Medieval Climate Anomaly (MCA) and the Current Warm Period (CWP) periods, suggesting that aeolian transport increased as a consequence of surface wind stress intensification. In contrast, M2 displays an opposite behavior, exhibiting an increase in fluvial terrigenous input during the Little Ice Age (LIA) in response to more humid conditions associated with El Niño-like conditions. Comparison with other South American paleoclimate records indicates that the observed changes are driven by interactions between meridional displacement of the Intertropical Convergence Zone (ITCZ), the South Pacific Subtropical High (SPSH) and Walker circulation at decadal and centennial timescales.

Pervasive multidecadal variations in productivity within the Peruvian Upwelling System over the last millennium

There is no agreement on the pluri-decadal expression of El Niño–Southern Oscillation (ENSO) in the Pacific over the last millennium. Marine records from the Peruvian margin indicate humid conditions (El Niño-like mean conditions) over the Little Ice Age, while precipitation records from the eastern equatorial Pacific infer arid conditions (La Niña-like mean conditions) for the same period. We here studied diatom assemblages, nitrogen isotopes, and major and minor elements at the lamination level in three laminated trigger cores located between 11°S and 15°S on the Peruvian shelf within the oxygen minimum zone (OMZ) to reconstruct precipitation and ocean productivity at the multiannual to multidecadal timescales over the last millennium. We respected the sediment structure, thus providing the first records of the mean climatic conditions at the origin of the lamination deposition, which ones represent several years. Light laminations were deposited under productive and dry conditions, indicative of La Niña-like mean conditions in the system, while dark laminations were deposited under non-productive and humid conditions, representative of El Niño-like mean conditions. La Niña-like mean conditions were predominant during the Medieval Warm Period (MWP; 1000–600 years BP) and Current Warm Period (CWP; 150 years BP to present), while El Niño-like mean conditions prevailed over the Little Ice Age (LIA; 600–150 years BP). We provide evidence for persistent multidecadal variations in productivity over the last millennium, which were disconnected from the mean climate state. Multidecadal variability has been stronger over the last 450 years concomitantly to increased variability in the NAO index. Two intervals of strong multidecadal variability were also observed over the MWP, congruent to decreased solar irradiance and increased volcanic activity.