Holocene abrupt climate shifts recorded in Gun Nuur lake core, northern Mongolia

A continuous 7.44 m lake core was successfully drilled at Gun Nuur Lake, northern Mongolia, and analyses on environment magnetic parameters, organic matter content and organic δ^13C were conducted in an attempt to retrieve the Holocene chronosequence of climatic changes based on 6 AMS ^14C dates. We found that the Holocene climate in northern Mongolia has been alternating between cold (or cool)/wet conditions and warm/arid conditions, and also punctuated with a series of abrupt climate shifts. The abrupt climate shifts occurred around 1750, 2800, 4000, 5200, 7200, and 9200 aBP (^14C age), being chronologically correlative to those abrupt climatic events recorded in the high-latitude North Atlantic Ocean. The correlation indicates that the climatic changes in northern Mongolia were linked with those in the North Atlantic Ocean probably via the North Atlantic Oscillation-affected westerly winds. The strength and position of westerly winds might have modulated the Siberian-Mongolian high pressure system (winter monsoon), directly influencing the climate in China.     

Climatic instability recorded by the mollusk assemblages from the late glacial loess deposits in China

The loess-paleosol sequence in China records abundant and valuable information on the global and regional climate changes. Biological record from the loess sequence is the most direct evidence on variation in pattern of paleo-atmospheric circulation and changes in winter and summer monsoon. A new record of climatic instability, which occurred in the Loess Plateau during the late glacial period, is presented. Through the study of terrestrial mollusks from three loess sequences, the authors intend to characterize the biological response process to rapid climate change and to learn the mechanisms driving the instable climate changes and the possible linkage in different regions. The result shows the striking consistent variability in the ratio records of three mollusk sequences of the late glacial, indicating apparent rapid climate fluctuations. Correlation of our three mollusk records with the oxygen isotopic records from Greenland ice cores and foraminifera1 records from the Northeast Pacific Ocean reveals similar instability climate during the late glacial period, which provides us a new thought probing the climate instability observed in the North Atlantic, the North Pacific and the Loess Plateau. The low-level atmospheric circulation in the Northern Hemisphere may be one possible way to link the unstable climate patterns observed in the above three regions.     

Interconnections between the Asian monsoon, ENSO, and high northern latitude climate during the Holocene

The International Geosphere Biosphere Program (IGBP), which promotes better understanding of the living environment, was initiated in the early 1990s. IGBP and other programs have uncovered much evi-dence that the Earth system is complex and nonlinear, ex…     

Increased seasonality in Middle East temperatures during the last interglacial period

The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate. Owing to changes in the Earth's orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales, which would have led to corresponding changes in the seasonal temperature cycle. Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation, a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere-ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records, contributed to the larger amplitude of the seasonal cycle in the Middle East.     

Progress in studies of the climate of humid period and the impacts of changing precession in early-mid Holocene

Studies on the climate of humid period and the impacts of changing precession in the early-mid Holocene are reviewed in this paper. High-resolution proxy data indicated that the African Humid Period, strong summer monsoon from the Arabian Sea to South Asia, northward migration of ITCZ （Intertropical Convergence Zone） over the northern South America, and the humid period of China appeared in 10.5-5.5 kaBP, 10.0--6.0 kaBP, 10.5-5.4 kaBP, and 11.0-8.0 kaBP, respectively. Modeling studies proved that summer insolation over the Northern Hemisphere increased following the changes of precession in the early Holocene, which increased the land-sea temperature contrasts, intensified the summer monsoon circulation over the area under the influence of summer monsoon. However, in the Northern Hemisphere, and finally induced a humid climate modeling results underestimated the increase of precipitation and the degree of northward extension of monsoon rain belt compared with palaeo-environmental data. These discrepancies between the modeling results and the palaeo-environmental data may be associated with the changes of North Atlantic circulation, sea ice and vegetation covers. Moreover, climate of the humid period was not stable, in which several droughts were inlaid on centennial scale. In this review, perspectives for further studies of the climate change of the humid period in the early-mid Holocene are also proposed. demy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved.     

Centennial-scale climate cooling with a sudden cold event around 8,200 years ago

The extent of climate variability during the current interglacial period, the Holocene, is still debated. Temperature records derived from central Greenland ice cores show one significant temperature anomaly between 8,200 and 8,100 years ago, which is often attributed to a meltwater outflow into the North Atlantic Ocean and a slowdown of North Atlantic Deep Water formation--this anomaly provides an opportunity to study such processes with relevance to present-day freshening of the North Atlantic. Anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in Greenland. But the anomalies in many of these records span 400 to 600 years, start from about 8,600 years ago and form part of a repeating pattern within the Holocene. More sudden climate changes around 8,200 years ago appear superimposed on this longer-term cooling. The compounded nature of the signals implies that far-field climate anomalies around 8,200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of North Atlantic Deep Water formation, and the geographical extent of the rapid cooling event 8,200 years ago remains to be determined.     

Long-term trend and abrupt events of the Holocene Asian monsoon inferred from a stalagmite δ18O record from Shennongjia in Central China

The middle-Holocene was a period of profound cul- ture transitions: the Neolithic culture around Central China[1], Mesopotamia[2] and India[3] all mysteriously collapsed around 4 ka . It is plausible that this civiliza-tion collapse can be contributed to …     

An abrupt climate event in a coupled ocean-atmosphere simulation without external forcing

Temperature reconstructions from the North Atlantic region indicate frequent abrupt and severe climate fluctuations during the last glacial and Holocene periods. The driving forces for these events are unclear and coupled atmosphere-ocean models of global circulation have only simulated such events by inserting large amounts of fresh water into the northern North Atlantic Ocean. Here we report a drastic cooling event in a 15,000-yr simulation of global circulation with present-day climate conditions without the use of such external forcing. In our simulation, the annual average surface temperature near southern Greenland spontaneously fell 6-10 standard deviations below its mean value for a period of 30-40 yr. The event was triggered by a persistent northwesterly wind that transported large amounts of buoyant cold and fresh water into the northern North Atlantic Ocean. Oceanic convection shut down in response to this flow, concentrating the entire cooling of the northern North Atlantic by the colder atmosphere in the uppermost ocean layer. Given the similarity between our simulation and observed records of rapid cooling events, our results indicate that internal atmospheric variability alone could have generated the extreme climate disruptions in this region.     

Strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation

The climate of the last glacial period was extremely variable, characterized by abrupt warming events in the Northern Hemisphere, accompanied by slower temperature changes in Antarctica and variations of global sea level. It is generally accepted that this millennial-scale climate variability was caused by abrupt changes in the ocean thermohaline circulation. Here we use a coupled ocean-atmosphere-sea ice model to show that freshwater discharge into the North Atlantic Ocean, in addition to a reduction of the thermohaline circulation, has a direct effect on Southern Ocean temperature. The related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical North Atlantic caused by a fast wave-adjustment process. We present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of Greenland and Antarctic temperature changes, the slow changes in Antarctic temperature and its similarity to sea level, as well as a possible time lag of sea level with respect to Antarctic temperature during Marine Isotope Stage 3.     

Rapid oscillations of Chinese monsoon climate since the last deglaciation and its comparison with GISP2 record

The geographic extent of the climate oscillations during the last deglaciation is an open question in the world. Here a high resolution climate record from a 16 000 C-14 a loess sequence in the northwestern margin of the Chinese Loess Plateau is reported. Comparison with GISP2 record shows that all major climate oscillation (e. g. Oldest Dryas/ Boiling/ Older Dryas/ Allerod/ Younger Dryas) in the North Atlantic region also registered in the record. In addition, the sequence also contains some other distinctive strengthened winter monsoon periods. It is suggested that a coupling mechanism operate between the two areas, and climate oscillations in the North Atlantic region may not be a local phenomenon.     

Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African monsoon

The processes that control the formation, intensity and track of hurricanes are poorly understood. It has been proposed that an increase in sea surface temperatures caused by anthropogenic climate change has led to an increase in the frequency of intense tropical cyclones, but this proposal has been challenged on the basis that the instrumental record is too short and unreliable to reveal trends in intense tropical cyclone activity. Storm-induced deposits preserved in the sediments of coastal lagoons offer the opportunity to study the links between climatic conditions and hurricane activity on longer timescales, because they provide centennial- to millennial-scale records of past hurricane landfalls. Here we present a record of intense hurricane activity in the western North Atlantic Ocean over the past 5,000 years based on sediment cores from a Caribbean lagoon that contain coarse-grained deposits associated with intense hurricane landfalls. The record indicates that the frequency of intense hurricane landfalls has varied on centennial to millennial scales over this interval. Comparison of the sediment record with palaeo-climate records indicates that this variability was probably modulated by atmospheric dynamics associated with variations in the El Ni?o/Southern Oscillation and the strength of the West African monsoon, and suggests that sea surface temperatures as high as at present are not necessary to support intervals of frequent intense hurricanes. To accurately predict changes in intense hurricane activity, it is therefore important to understand how the El Ni?o/Southern Oscillation and the West African monsoon will respond to future climate change.     

Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods

The magnitude of heat and salt transfer between the Indian and Atlantic oceans through 'Agulhas leakage' is considered important for balancing the global thermohaline circulation. Increases or reductions of this leakage lead to strengthening or weakening of the Atlantic meridional overturning and associated variation of North Atlantic Deep Water formation. Here we show that modern Agulhas waters, which migrate into the south Atlantic Ocean in the form of an Agulhas ring, contain a characteristic assemblage of planktic foraminifera. We use this assemblage as a modern analogue to investigate the Agulhas leakage history over the past 550,000 years from a sediment record in the Cape basin. Our reconstruction indicates that Indian-Atlantic water exchange was highly variable: enhanced during present and past interglacials and largely reduced during glacial intervals. Coherent variability of Agulhas leakage with northern summer insolation suggests a teleconnection to the monsoon system. The onset of increased Agulhas leakage during late glacial conditions took place when glacial ice volume was maximal, suggesting a crucial role for Agulhas leakage in glacial terminations, timing of interhemispheric climate change and the resulting resumption of the Atlantic meridional overturning circulation.     

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean-atmosphere feedbacks or by changes in the Atlantic thermohaline circulation. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America, as well as the Indian and East Asian monsoon systems, but not with tropical Pacific sea surface temperatures. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of approximately 1 degrees C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of approximately 0.5 degrees C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans' thermohaline circulation, although possibly amplified by tropical ocean-atmosphere interaction as suggested before.     

Climate instability during the last glaciation recorded in the Yuanbu loess section

Abrupt events within the Linxia Yuanbu loess section in the western Loess Plateau in China were investigated. The climatic proxy index of the content of mean grain-size and >4 μm grain-size fraction of the section was used as a climatic indicator, and a moving average method was applied in the statistical analysis to extract the abrupt events recorded in the section. It was found that the loess of the last glaciation recorded not only the abrupt events teleconnected with the high-latitude North Atlantic and Greenland regions but also as many as 15 abrupt events in addition to the Heinrich (H) and Younger Dryas (YD) events. This indicates that abrupt climate changes were typical of the climate changes occurring in the Loess Plateau during the last glaciation. The widely recognized mechanisms of the abrupt climate changes, including the heat transfer theory of the North Atlantic Ocean, the atmospheric circulation theory, and the theory of the interaction between low-latitude ocean and the atmosphere remains to be discussed. Perhaps this problem could be resolved by revealing the relation between the evolution of the features of the East Asian monsoon climate and the climate in the surrounding areas. Through a detailed study of the features of the Heinrich and YD events, criteria on which the abrupt events recorded in the loess could be distinguished were tentatively defined. The events were associated with a large grain size, high carbonate content, high lightness, low susceptibility, low redness and low yellowness. In addition, the variation amplitude of the adjacent peak and valley on the curve of the content of the mean grain-size and >40 μm grain-size fraction was larger than 3.4 μm and the content of >40 μm grain-size fraction was larger than 2.0 μm. Lastly, the length of the variation time was less than 500 years. If all these criteria were met, the event was viewed as an abrupt event.     

Holocene palynological records and their responses to the controversies of climate system in the Shiyang River drainage basin

The Shiyang River drainage basin is located in the northwest margin of the Asian monsoon region. Previous studies reached different conclusions about Holocene climatic changes in the basin. Some studies suggested the Holocene climatic changes were mainly controlled by the Asian monsoon and that the climate was relatively humid during the early Holocene (11.6-7.1 cal ka BP). Other studies found the mid-Holocene Climatic Optimum (7.0-5.0 cal ka BP), and this climate condition was similar to the Holocene westerly wind pattern in arid Central Asia. The modern climate is affected by the Asian monsoon and westerly wind in the drainage basin, and the Holocene climatic records showed two different Holocene climatic patterns-a westerly wind pattern and monsoonal pattern. However, it remains unclear what caused the two different Holocene climatic patterns to co-exist in the region. The palynological records are the main evidence for the Holocene climatic changes in the basin. This paper focuses on palynological records for different parts of the drainage basin. Among them, QTH02, QTL-03 and Sanjiaocheng records are located in the terminal lake, and the Hongshuihe record is located in the middle reaches of the basin. In the terminal lake, the palynological records of QTH02 and QTL-03 are similar, but the Sanjiaocheng record differs. The difference is mainly affected by the variable pollen assemblages in the different locations of the lake basin. From comparison and synthesis of the four palynological records, we concluded that the millennial-scale Holocene climatic changes were affected by the combined effects of the Asian monsoon and westerly wind in the drainage basin, which show the complicated Holocene climatic pattern in the northwest margin of the Asian monsoon.     

Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes

The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.     

Moisture transport across Central America as a positive feedback on abrupt climatic changes

Moisture transport from the Atlantic to the Pacific ocean across Central America leads to relatively high salinities in the North Atlantic Ocean and contributes to the formation of North Atlantic Deep Water. This deep water formation varied strongly between Dansgaard/Oeschger interstadials and Heinrich events-millennial-scale abrupt warm and cold events, respectively, during the last glacial period. Increases in the moisture transport across Central America have been proposed to coincide with northerly shifts of the Intertropical Convergence Zone and with Dansgaard/Oeschger interstadials, with opposite changes for Heinrich events. Here we reconstruct sea surface salinities in the eastern equatorial Pacific Ocean over the past 90,000 years by comparing palaeotemperature estimates from alkenones and Mg/Ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity. We detect millennial-scale fluctuations of sea surface salinities in the eastern equatorial Pacific Ocean of up to two to four practical salinity units. High salinities are associated with the southward migration of the tropical Atlantic Intertropical Convergence Zone, coinciding with Heinrich events and with Greenland stadials. The amplitudes of these salinity variations are significantly larger on the Pacific side of the Panama isthmus, as inferred from a comparison of our data with a palaeoclimate record from the Caribbean basin. We conclude that millennial-scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes, modulating the North Atlantic freshwater budget and hence North Atlantic Deep Water formation.     

Abrupt Holocene changes of the Asian monsoon at millennial- and centennial-scales: Evidence from lake sediment document in Minqin Basin, NW China

Considerable research is now focusing on abrupt Holocene changes at millennial- and centennial-scales all over the world. This study shows that the changes in the Holocene summer monsoon can be divided into two main periods, based on climatic proxy records of lake sediments from the Sanjiaocheng section located at the NW boundary of the summer monsoon, Gansu Province. The early and middle Holocene was humid with stronger summer monsoon, while the late Holocene is dry with weak summer monsoon. Detailed results also show a series of millennial- and centennial-scale changes in the intensity of the summer monsoon, with a periodicity of {dy1600} years during the whole Holocene, and with a periodicity of 800 years during the early Holocene. Ten dry events during the Holocene are associated with the weakening of the summer monsoon. These rapid climatic changes may be representative of a global climatic change pattern during the Holocene.     

Peat record reflecting Holocene climatic change in the Zoige Plateau and AMS radiocarbon dating

Through the use of reliable AMS dating of high resolution (15-30 years) peat and the establishment of monsoon climate proxies sequence, we have been able to recognize several cold, dry events in the Tibetan Plateau during the Holocene. The more obvious ones occurred around 12800, 11300, 10200, 9580, 8900, 6400, 4400, 3700, 2800 and 1500 cal. aBP. These events correlate well with both ice rafting events recorded in high latitude North Atlantic Ocean sediment cores and cooling events in the low latitude SST. Spectral analysis indicates high frequency climate variation on centennial-millennial time scale during the Holocene. This further reflects Holocene climate instability and the existence of centennial-millenium scale rhythm in mid latitude areas as well.     

Millennial and orbital variations of El Niño/Southern Oscillation and high-latitude climate in the last glacial period

The El Ni?o/Southern Oscillation (ENSO) phenomenon is believed to have operated continuously over the last glacial-interglacial cycle. ENSO variability has been suggested to be linked to millennial-scale oscillations in North Atlantic climate during that time, but the proposals disagree on whether increased frequency of El Ni?o events, the warm phase of ENSO, was linked to North Atlantic warm or cold periods. Here we present a high-resolution record of surface moisture, based on the degree of peat humification and the ratio of sedges to grass, from northern Queensland, Australia, covering the past 45,000 yr. We observe millennial-scale dry periods, indicating periods of frequent El Ni?o events (summer precipitation declines in El Ni?o years in northeastern Australia). We find that these dry periods are correlated to the Dansgaard-Oeschger events--millennial-scale warm events in the North Atlantic climate record--although no direct atmospheric connection from the North Atlantic to our site can be invoked. Additionally, we find climatic cycles at a semiprecessional timescale (approximately 11,900 yr). We suggest that climate variations in the tropical Pacific Ocean on millennial as well as orbital timescales, which determined precipitation in northeastern Australia, also exerted an influence on North Atlantic climate through atmospheric and oceanic teleconnections.