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Wang P.X.,Tongji University | Wang B.,University of Hawaii at Manoa | Cheng H.,Xi'an Jiaotong University | Cheng H.,University of Minnesota | And 5 more authors.
Climate of the Past | Year: 2014

Monsoon has earned increasing attention from the climate community since the last century, yet only recently have regional monsoons been recognized as a global system. It remains a debated issue, however, as to what extent and at which timescales the global monsoon can be viewed as a major mode of climate variability. For this purpose, a PAGES (Past Global Changes) working group (WG) was set up to investigate the concept of the global monsoon and its future research directions. The WG's synthesis is presented here. On the basis of observation and proxy data, the WG found that the regional monsoons can vary coherently, although not perfectly, at various timescales, varying between interannual, interdecadal, centennial, millennial, orbital and tectonic timescales, conforming to the global monsoon concept across timescales. Within the global monsoon system, each subsystem has its own features, depending on its geographic and topographic conditions. Discrimination between global and regional components in the monsoon system is a key to revealing the driving factors in monsoon variations; hence, the global monsoon concept helps to enhance our understanding and to improve future projections of the regional monsoons. This paper starts with a historical review of the global monsoon concept in both modern and paleo-climatology, and an assessment of monsoon proxies used in regional and global scales. The main body of the paper is devoted to a summary of observation data at various timescales, providing evidence of the coherent global monsoon system. The paper concludes with a projection of future monsoon shifts in a warming world. The synthesis will be followed by a companion paper addressing driving mechanisms and outstanding issues in global monsoon studies.


Galbraith E.D.,McGill University | Kienast M.,Dalhousie University | Albuquerque A.L.,Federal University of Fluminense | Altabet M.A.,University of Massachusetts Dartmouth | And 36 more authors.
Nature Geoscience | Year: 2013

Over much of the ocean's surface, productivity and growth are limited by a scarcity of bioavailable nitrogen. Sedimentary δ 15 N records spanning the last deglaciation suggest marked shifts in the nitrogen cycle during this time, but the quantification of these changes has been hindered by the complexity of nitrogen isotope cycling. Here we present a database of δ 15 N in sediments throughout the world's oceans, including 2,329 modern seafloor samples, and 76 timeseries spanning the past 30,000 years. We show that the δ 15 N values of modern seafloor sediments are consistent with values predicted by our knowledge of nitrogen cycling in the water column. Despite many local deglacial changes, the globally averaged δ 15 N values of sinking organic matter were similar during the Last Glacial Maximum and Early Holocene. Considering the global isotopic mass balance, we explain these observations with the following deglacial history of nitrogen inventory processes. During the Last Glacial Maximum, the nitrogen cycle was near steady state. During the deglaciation, denitrification in the pelagic water column accelerated. The flooding of continental shelves subsequently increased denitrification at the seafloor, and denitrification reached near steady-state conditions again in the Early Holocene. We use a recent parameterization of seafloor denitrification to estimate a 30-120% increase in benthic denitrification between 15,000 and 8,000 years ago. Based on the similarity of globally averaged δ 15 N values during the Last Glacial Maximum and Early Holocene, we infer that pelagic denitrification must have increased by a similar amount between the two steady states. © 2013 Macmillan Publishers Limited. All rights reserved.


Elbert J.,University of Bern | Grosjean M.,University of Bern | von Gunten L.,PAGES International Project Office | Urrutia R.,University of Concepción | And 5 more authors.
Holocene | Year: 2012

High-resolution climate reconstructions from a range of natural archives across the world are fundamental to place current climate change into perspective. Paleoclimate records for the Southern Hemisphere are scarce and only a few quantitative high-resolution reconstructions exist for the past millennium. We present a record of annually laminated sediments of Lago Plomo (46°59′S, 72°52′W,203 m a.s.l.) located east of the Northern Patagonian Ice Field (NPI). Radiometric dating (210Pb, 137Cs, 14C AMS) is consistent with counts of millimetre-scale laminae, confirming the annual nature of the laminae couplets with a light summer and a dark winter layer. The varves were analyzed for thickness, mass accumulation rate (MAR), scanning x-ray fluorescence (XRF) and scanning reflectance spectroscopy in the visible range (VIS-RS). MAR data were calibrated against austral winter (JJA) precipitation data (CRU TS 3.0) for the period ad 1930-2002 (r = 0.67, p(aut) < 0.05). Using a linear inverse regression model we reconstructed winter precipitation for Lago Plomo back to ad 1530. The root mean squared error of prediction (RMSEP) is small (13.3 mm/month; 12% of the average precipitation) compared with the pronounced decadal and multidecadal variability, suggesting that most of the reconstructed variability is significant. Wetter phases (reference ad 1930-2002) were observed around ad 1600, ad 1630-1690 and ad 1780-1850, and a prolonged drier period ad 1690-1780 with a multidecadal minimum centered on ad 1770. The spatial correlation for South America suggests that the JJA precipitation record from Lago Plomo is representative for large areas in the southwest between c. 41°S and 51°S. © The Author(s) 2011.


Martel-Cea A.,Austral University of Chile | Maldonado A.,University of La Serena | Maldonado A.,Católica del Norte University | Grosjean M.,University of Bern | And 4 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2016

We present a reconstruction of environmental changes from sediments of high-altitude Laguna Chepical in the subtropical Andes of Central Chile (32°16′S; 70°30′W, 3050 m a.s.l.) for the past 3100 years. Based on subfossil pollen, microscopic charcoal and diatoms, we inferred changes in moisture (related to precipitation) and ice-cover/ice-free season (related to summer temperature) at decadal to millennial scales. Sustained wetter and colder summer temperatures than today prevailed between 1100 BC and ca. AD 1. Afterward, decreasing pollen accumulation rates and increased fire activity suggest drier conditions and possibly enhanced seasonality and/or inter-annual climate variability. Frequent changes between cold and warm summers were observed, particularly for the last 1000 years. About AD 1250 (during the Medieval Climate Anomaly), wet years and early break up of ice-cover occurred in central Chile, which is today typical for El Niño-like mean conditions. Conversely, and with the exception of a few wet pulses, a generally dry period with extended ice-cover (cool summers) was observed between AD 1400 and AD 1850 (Little Ice Age). This can be interpreted as a trend toward more La Niña-like mean conditions. Recent climate change and human disturbances during the last 100 years have prompted changes in diatom and plant communities that are unprecedented in the late Holocene. First, planktonic diatoms increased as a result of hydraulic interventions in the lake during the late 19th century, and secondly, the Andean vegetation shifted upward as result of recent warming, and the frequency of arboreal taxa was significantly reduced. At the same time peaks of fire activity were observed. © 2016 Elsevier B.V.


Waelbroeck C.,CEA Saclay Nuclear Research Center | Kiefer T.,PAGES International Project Office | Dokken T.,University of Bergen | Chen M.-T.,National Taiwan Ocean University | And 5 more authors.
Quaternary Science Reviews | Year: 2014

Estimates of the change in surface seawater δ18O (δ18Osw) between the Last Glacial Maximum (LGM) and Late Holocene (LH) are derived from homogenous data sets with rigorous age control, namely MARGO sea surface temperature (SST) estimates and oxygen isotopic ratios (δ18O) of planktonic foraminifers. Propagation of uncertainties associated with each proxy allows the identification of robust patterns of change in δ18Osw. Examination of these patterns on a regional scale highlights which changes in surface currents and hydrological cycle are consistent with both planktonic isotopic data and reconstructed SST. Positive local annual mean LGM-LH δ18Osw anomalies characterize the glacial tropical Indian Ocean, portions of the western and eastern margins of the North Pacific, the Iberian margin and the tropical North Atlantic, as well as the South African margin. Although reduced precipitation during the LGM with respect to the LH may have contributed to some extent to these local enrichments in surface seawater 18O, the largest positive anomalies appear to be related to changes in ocean circulation. Large local negative annual mean LGM-LH δ18Osw anomalies are found in the South Pacific and North Atlantic, reflecting the equatorward migration of surface temperature fronts during the LGM with respect to the LH. In the northern North Atlantic, a region characterized by large discrepancies between SST estimates based on different proxies, only SST estimates based on planktonic foraminifer counts yield annual mean LGM-LH δ18Osw anomalies consistent with a southward shift of the polar front at the LGM relative to the LH. © 2014 Elsevier Ltd.


De Jong R.,University of Bern | Von Gunten L.,PAGES International Project Office | Maldonado A.,University of La Serena | Grosjean M.,University of Bern
Climate of the Past | Year: 2013

High-resolution reconstructions of climate variability that cover the past millennia are necessary to improve the understanding of natural and anthropogenic climate change across the globe. Although numerous records are available for the mid-and high-latitudes of the Northern Hemisphere, global assessments are still compromised by the scarcity of data from the Southern Hemisphere. This is particularly the case for the tropical and subtropical areas. In addition, high elevation sites in the South American Andes may provide insight into the vertical structure of climate change in the mid-troposphere. This study presents a 3000 yr-long austral summer (November to February) temperature reconstruction derived from the 210Pb-and 14C-dated organic sediments of Laguna Chepical (32 16' S, 70 30' W, 3050 m a.s.l.), a high-elevation glacial lake in the subtropical Andes of central Chile. Scanning reflectance spectroscopy in the visible light range provided the spectral index iR/i570/iR/i630, which reflects the clay mineral content in lake sediments. For the calibration period (AD 1901-2006), the iR/i570/iR/i630 data were regressed against monthly meteorological reanalysis data, showing that this proxy was strongly and significantly correlated with mean summer (NDJF) temperatures (iR/i3 yr Combining double low line -0.63, ip/iadj Combining double low line 0.01). This calibration model was used to make a quantitative temperature reconstruction back to 1000 BC. brbr The reconstruction (with a model error RMSEPboot of 0.33 C) shows that the warmest decades of the past 3000 yr occurred during the calibration period. The 19th century (end of the Little Ice Age (LIA)) was cool. The prominent warmth reconstructed for the 18th century, which was also observed in other records from this area, seems systematic for subtropical and southern South America but remains difficult to explain. Except for this warm period, the LIA was generally characterized by cool summers. Back to AD 1400, the results from this study compare remarkably well to low altitude records from the Chilean Central Valley and southern South America. However, the reconstruction from Laguna Chepical does not show a warm Medieval Climate Anomaly during the 12-13th century, which is consistent with records from tropical South America. The Chepical record also indicates substantial cooling prior to 800 BC. This coincides with well-known regional as well as global glacier advances which have been attributed to a grand solar minimum. This study thus provides insight into the climatic drivers and temperature patterns in a region for which currently very few data are available. It also shows that since ca. AD 1400, long-term temperature patterns were generally similar at low and high altitudes in central Chile. © Author(s) 2013.


Verleyen E.,Ghent University | Hodgson D.A.,Natural Environment Research Council | Sabbe K.,Ghent University | Cremer H.,TNO | And 19 more authors.
Earth-Science Reviews | Year: 2011

We review the post-glacial climate variability along the East Antarctic coastline using terrestrial and shallow marine geological records and compare these reconstructions with data from elsewhere. Nearly all East Antarctic records show a near-synchronous Early Holocene climate optimum (11.5-9 ka BP), coinciding with the deglaciation of currently ice-free regions and the optimum recorded in Antarctic ice and marine sediment cores. Shallow marine and coastal terrestrial climate anomalies appear to be out of phase after the Early Holocene warm period, and show complex regional patterns, but an overall trend of cooling in the terrestrial records. A Mid to Late Holocene warm period is present in many East Antarctic lake and shallow coastal marine records. Although there are some differences in the regional timing of this warm period, it typically occurs somewhere between 4.7 and 1 ka BP, which overlaps with a similar optimum found in Antarctic Peninsula terrestrial records. The differences in the timing of these sometimes abrupt warm events in different records and regions points to a number of mechanisms that we have yet to identify. Nearly all records show a neoglacial cooling from 2 ka BP onwards. There is no evidence along the East Antarctic coastline for an equivalent to the Northern Hemisphere Medieval Warm Period and there is only weak circumstantial evidence in a few places for a cool event crudely equivalent in time to the Northern Hemisphere's Little Ice Age. There is a need for well-dated, high resolution climate records in coastal East Antarctica and particularly in Terre Adélie, Dronning Maud Land and Enderby Land to fully understand the regional climate anomalies, the disparity between marine and terrestrial records, and to determine the significance of the heterogeneous temperature trends being measured in the Antarctic today. © 2010 Elsevier B.V.


Abram N.J.,Australian National University | McGregor H.V.,University of Wollongong | Tierney J.E.,University of Arizona | Tierney J.E.,Woods Hole Oceanographic Institution | And 27 more authors.
Nature | Year: 2016

The evolution of industrial-era warming across the continents and oceans provides a context for future climate change and is important for determining climate sensitivity and the processes that control regional warming. Here we use post-ad 1500 palaeoclimate records to show that sustained industrial-era warming of the tropical oceans first developed during the mid-nineteenth century and was nearly synchronous with Northern Hemisphere continental warming. The early onset of sustained, significant warming in palaeoclimate records and model simulations suggests that greenhouse forcing of industrial-era warming commenced as early as the mid-nineteenth century and included an enhanced equatorial ocean response mechanism. The development of Southern Hemisphere warming is delayed in reconstructions, but this apparent delay is not reproduced in climate simulations. Our findings imply that instrumental records are too short to comprehensively assess anthropogenic climate change and that, in some regions, about 180 years of industrial-era warming has already caused surface temperatures to emerge above pre-industrial values, even when taking natural variability into account. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.


Johnstone H.J.H.,University of Bremen | Kiefer T.,PAGES International Project Office | Elderfield H.,University of Cambridge | Schulz M.,University of Bremen
Geochemistry, Geophysics, Geosystems | Year: 2014

A record of deep-sea calcite saturation (Δ[CO 32-]), derived from X-ray computed tomography-based foraminifer dissolution index, XDX, was constructed for the past 150 ka for a core from the deep (4157 m) tropical western Indian Ocean. G. sacculifer and N. dutertrei recorded a similar dissolution history, consistent with the process of calcite compensation. Peaks in calcite saturation (∼15 μmol/kg higher than the present-day value) occurred during deglaciations and early in MIS 3. Dissolution maxima coincided with transitions to colder stages. The mass record of G. sacculifer better indicated preservation than did that of N. dutertrei or G. ruber. Dissolution-corrected Mg/Ca-derived SST records, like other SST records from marginal Indian Ocean sites, showed coolest temperatures of the last 150 ka in early MIS 3, when mixed layer temperatures were ∼4°C lower than present SST. Temperatures recorded by N. dutertrei showed the thermocline to be ∼4°C colder in MIS 3 compared to the Holocene (8 ka B.P.). Key Points Foraminifer dissolution index XDX gives deep water calcite saturation XDX used to correct dissolution bias in Mg/Ca and shell mass records "Corrected" records for the core (WIND28K) are compared to published data © 2014. American Geophysical Union. All Rights Reserved.


Newman L.,PAGES International Project Office | Kiefer T.,PAGES International Project Office | Otto-Bliesner B.,U.S. National Center for Atmospheric Research | Wanner H.,University of Bern
Current Opinion in Environmental Sustainability | Year: 2010

The Past Global Changes (PAGES) project was founded in 1991 with the mission to address past changes in the Earth System in a quantitative and process-oriented way in order to improve projections of future climate and environment, and inform strategies for sustainability. Toward this goal, PAGES has identified four sets of questions aimed at developing a better understanding of climate-environment sensitivity, regional variability, global system behavior and human interaction with climate and environment. These questions are addressed by scientific Working Groups that hold workshops and other activities, toward the production of syntheses and products. Furthermore, PAGES supports the international paleoscience community by fostering collaboration and communication, and ensuring access to and dissemination of results, data, and other relevant information. © 2010 Elsevier B.V.

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