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What's New on the Paleo Web Pages for 2010


Fig. S1b.  LGM carbonate ion concentration plot Loss of Carbon from the Deep Sea Since the Last Glacial Maximum
Yu et al.
Science

Vol. 330, pp. 1084-1087, 19 November 2010.
DOI: 10.1126/science.1193221

Deep-ocean carbonate ion concentrations ([CO3 2-]) and carbon isotopic ratios (δ13C) place important constraints on past redistributions of carbon in the ocean-land-atmosphere system and hence provide clues to the causes of atmospheric CO2 concentration changes. However, existing deep-sea [CO3 2-] reconstructions conflict with one another, complicating paleoceanographic interpretations. Here, we present deep-sea [CO3 2-] for five cores from the three major oceans quantified using benthic foraminiferal boron/calcium ratios since the last glacial period. Combined benthic δ13C and [CO3 2-] results indicate that deep-sea-released CO2 during the early deglacial period (17.5 to 14.5 thousand years ago) was preferentially stored in the atmosphere, whereas during the late deglacial period (14 to 10 thousand years ago), besides contributing to the contemporary atmospheric CO2 rise, a substantial portion of CO2 released from oceans was absorbed by the terrestrial biosphere.
Fig. 6a. Growing Degree Days above 5C at 6ka Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis
Bartlein et al.
Climate Dynamics

DOI: 10.1007/s00382-010-0904-1

Subfossil pollen and plant macrofossil data derived from 14C-dated sediment profiles can provide quantitative information on glacial and interglacial climates. The data allow climate variables related to growing season warmth, winter cold, and plant-available moisture to be reconstructed. Continental-scale reconstructions have been made for the mid-Holocene (MH, around 6 ka) and Last Glacial Maximum (LGM, around 21 ka), allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change. The synthesis of the available MH and LGM climate reconstructions and their uncertainties, obtained using modern analogue, regression and model-inversion techniques, is presented for four temperature variables and two moisture variables. Reconstructions of the same variables based on surface-pollen assemblages are shown to be accurate and unbiased. Reconstructed LGM and MH climate anomaly patterns are coherent, consistent between variables, and robust with respect to the choice of technique. They support a conceptual model of the controls of Late Quaternary climate change whereby the first-order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance.
Figure 7. Differences in December-March temperature Support for global climate reorganization during the "Medieval Climate Anomaly"
Graham et al.
Climate Dynamics

DOI: 10.1007/s00382-010-0914-z

Widely distributed proxy records indicate that the Medieval Climate Anomaly (MCA; ~900-1350 AD) was characterized by coherent shifts in large-scale Northern Hemisphere atmospheric circulation patterns. Although cooler sea surface temperatures in the central and eastern equatorial Pacific can explain some aspects of medieval circulation changes, they are not sufficient to account for other notable features, including widespread aridity through the Eurasian sub-tropics, stronger winter westerlies across the North Atlantic and Western Europe, and shifts in monsoon rainfall patterns across Africa and South Asia. We present results from a full-physics coupled climate model showing that a slight warming of the tropical Indian and western Pacific Oceans relative to the other tropical ocean basins can induce a broad range of the medieval circulation and climate changes indicated by proxy data, including many of those not explained by a cooler tropical Pacific alone. Important aspects of the results resemble those from previous simulations examining the climatic response to the rapid Indian Ocean warming during the late twentieth century, and to results from climate warming simulations - especially in indicating an expansion of the Northern Hemisphere Hadley circulation. Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST) change responsible for producing the proxy-model similarity in our results agrees well with MCA-LIA SST differences obtained in a recent proxy-based climate field reconstruction. Though much remains unclear, our results indicate that the MCA was characterized by an enhanced zonal Indo-Pacific SST gradient with resulting changes in Northern Hemisphere tropical and extra-tropical circulation patterns and hydroclimate regimes, linkages that may explain the coherent regional climate shifts indicated by proxy records from across the planet. The findings provide new perspectives on the nature and possible causes of the MCA - a remarkable, yet incompletely understood episode of Late Holocene climatic change.
Figure 1b. Sea Surface Salinity in the WPWP Holocene evolution of the Indonesian throughflow and the western Pacific warm pool
Linsley et al.
Nature Geoscience

Vol. 3, pp. 578-583, August 2010.
DOI: 10.1038/NGEO920

High sea surface temperatures in the western Pacific warm pool fuel atmospheric convection and influence tropical climate. This region also hosts the Indonesian throughflow, the network of currents through which surface and thermocline waters are transported from the western equatorial Pacific Ocean into the Indian Ocean. Here we show, using records of the d18O and Mg/Ca of planktonic foraminifera from eight sediment cores, that from about 10,000 to 7,000 years ago, sea surface temperatures in the western sector of the western Pacific warm pool were about 0.5°C higher than during pre-industrial times. We also find that about 9,500 years ago, when the South China and Indonesian seas were connected by rising sea level, surface waters in the Makassar Strait became relatively fresher. We suggest that the permanent reduction of surface salinity initiated the enhanced flow at lower, thermocline depths seen in the modern Indonesian throughflow. However, the uniformly warm sea surface temperatures found upstream and downstream of the Indonesian throughflow indicate that the early Holocene warmth in this region was not directly related to reduced heat transport by the throughflow that may have resulted from surface freshening of the Makassar Strait. Instead, we propose that the elevated temperatures were the result of a westward shift or expansion of the boundaries of the western Pacific warm pool.
Figure 1a. Summer precip reconstruction Multi-centennial summer and winter precipitation variability in southern South America
Neukom et al.
Geophysical Research Letters

Vol. 37, L14708, doi:10.1029/2010GL043680

We present the first spatially and temporally highly resolved gridded reconstruction of multi-centennial precipitation variability for southern South America (SSA). A novel reconstruction approach of deriving 10,000 ensemble members based on varying predictor networks and methodological settings allows the identification of spatiotemporal changes in SSA precipitation and associated uncertainties. The summer and winter reconstructions back to AD 1498 and AD 1590, respectively, provide new evidence for multi-centennial increase in summer precipitation and an opposing decrease in winter precipitation into the 20th century. The drying in winter is significant over large parts of SSA, whereas the patterns for summer, possibly representing convective rainfall, have displayed high spatial variability. The fact that such long-term seasonal and spatial changes have occurred in the past, underlines the complex form that hydroclimatic variability might have in the future. This emphasizes the need for careful adaptation strategies as governments become attuned to the realities of climate change.
Figure 1. SST Map and core locations Tropical Ocean Temperatures Over the Past 3.5 Million Years
Herbert et al.
Science

Vol. 328, no. 5985, pp. 1530-1534, 18 June 2010
DOI: 10.1126/science.1185435

Determining the timing and amplitude of tropical sea surface temperature (SST) change is an important part of solving the puzzle of the Plio-Pleistocene ice ages. Alkenone-based tropical SST records from the major ocean basins show coherent glacial- interglacial temperature changes of 1° to 3°C that align with (but slightly lead) global changes in ice volume and deep ocean temperature over the past 3.5 million years. Tropical temperatures became tightly coupled with benthic d18O and orbital forcing after 2.7 million years. We interpret the similarity of tropical SST changes, in dynamically dissimilar regions, to reflect "top-down" forcing through the atmosphere. The inception of a strong carbon dioxide-greenhouse gas feedback and amplification of orbital forcing at ~2.7 million years ago connected the fate of Northern Hemisphere ice sheets with global ocean temperatures since that time.
mountain lake image The Millennium Project
European Climate of the Last Millennium


The Millennium project is a multidisciplinary consortium of more than 38 partners from 16 European countries bringing together historians, chemists, physicists, biologists, geographers, climate modellers and geologists in a multi-disciplinary effort to reconstruct the climate of Europe over the last 1000 years using historical documents ranging from ships logs, church annals and harvest records, and natural archives such as tree rings, insect and plant remains from lakes and peat bogs, ice cores and sea shells. Reconstructing the climate of the past is important because it will allow us to say whether the changes observed over recent years are unusual when viewed within a longer term regional perspective.
Figure 4. Pan Pacific drought patterns during (A) the Great Drought (1876-1878) and (B) the 1918-1919 ENSO event Asian Monsoon Failure and Megadrought During the Last Millennium
Cook et al.
Science

Vol. 328, No. 5977, pp. 486 - 489, 23 April 2010
DOI: 10.1126/science.1185188

The Asian monsoon system affects more than half of humanity worldwide, yet the dynamical processes that govern its complex spatiotemporal variability are not sufficiently understood to model and predict its behavior, due in part to inadequate long-term climate observations. Here we present the Monsoon Asia Drought Atlas (MADA), a seasonally resolved gridded spatial reconstruction of Asian monsoon drought and pluvials over the past millennium, derived from a network of tree-ring chronologies. MADA provides the spatiotemporal details of known historic monsoon failures and reveals the occurrence, severity, and fingerprint of previously unknown monsoon megadroughts and their close linkages to large-scale patterns of tropical Indo-Pacific sea surface temperatures. MADA thus provides a long-term context for recent monsoon variability that is critically needed for climate modeling, prediction, and attribution.
Figure 2. Long-term temperature variation, amplitude and uncertainty. Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate
Frank et al.
Nature

Vol. 463, pp.527-532, 28 January 2010
doi:10.1038/nature08769

The processes controlling the carbon flux and carbon storage of the atmosphere, ocean and terrestrial biosphere are temperature sensitive and are likely to provide a positive feedback leading to amplified anthropogenic warming. Owing to this feedback, at timescales ranging from interannual to the 20-100-kyr cycles of Earth's orbital variations, warming of the climate system causes a net release of CO2 into the atmosphere; this in turn amplifies warming. But the magnitude of the climate sensitivity of the global carbon cycle (termed γ), and thus of its positive feedback strength, is under debate, giving rise to large uncertainties in global warming projections. Here we quantify the median γ as 7.7 p.p.m.v. CO2 per °C warming, with a likely range of 1.7-21.4 p.p.m.v. CO2 per °C. Sensitivity experiments exclude significant influence of pre-industrial land-use change on these estimates. Our results, based on the coupling of a probabilistic approach with an ensemble of proxy-based temperature reconstructions and pre-industrial CO2 data from three ice cores, provide robust constraints for γ on the policy-relevant multi-decadal to centennial timescales. By using an ensemble of >200,000 members, quantification of γ is not only improved, but also likelihoods can be assigned, thereby providing a benchmark for future model simulations. Although uncertainties do not at present allow exclusion of γ calculated from any of ten coupled carbon-climate models, we find that γ is about twice as likely to fall in the lowermost than in the uppermost quartile of their range. Our results are incompatibly lower (P<0.05) than recent pre-industrial empirical estimates of 40 p.p.m.v. CO2 per °C, and correspondingly suggest 80% less potential amplification of ongoing global warming.
Figure 2. Multiproxy summer and winter surface air temperature field reconstructions for southern South America covering the past centuries
Neukom et al.
Climate Dynamics

Online First March 28, 2010
DOI: 10.1007/s00382-010-0793-3

We statistically reconstruct austral summer (winter) surface air temperature fields back to ad 900 (1706) using 22 (20) annually resolved predictors from natural and human archives from southern South America (SSA). This represents the first regional-scale climate field reconstruction for parts of the Southern Hemisphere at this high temporal resolution. We apply three different reconstruction techniques: multivariate principal component regression, composite plus scaling, and regularized expectation maximization. There is generally good agreement between the results of the three methods on interannual and decadal timescales. The field reconstructions allow us to describe differences and similarities in the temperature evolution of different sub-regions of SSA. The reconstructed SSA mean summer temperatures between 900 and 1350 are mostly above the 1901-1995 climatology. After 1350, we reconstruct a sharp transition to colder conditions, which last until approximately 1700. The summers in the eighteenth century are relatively warm with a subsequent cold relapse peaking around 1850. In the twentieth century, summer temperatures reach conditions similar to earlier warm periods. The winter temperatures in the eighteenth and nineteenth centuries were mostly below the twentieth century average. The uncertainties of our reconstructions are generally largest in the eastern lowlands of SSA, where the coverage with proxy data is poorest. Verifications with independent summer temperature proxies and instrumental measurements suggest that the interannual and multi-decadal variations of SSA temperatures are well captured by our reconstructions. This new dataset can be used for data/model comparison and data assimilation as well as for detection and attribution studies at sub-continental scales.
Figure 2. Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly
Mann et al.
Science

Vol. 326, pp. 1256-1260, 27 November 2009
10.1126/science.1177303

Global temperatures are known to have varied over the past 1500 years, but the spatial patterns have remained poorly defined. We used a global climate proxy network to reconstruct surface temperature patterns over this interval. The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally. This period is marked by a tendency for La Niña-like conditions in the tropical Pacific. The coldest temperatures of the Little Ice Age are observed over the interval 1400 to 1700 C.E., with greatest cooling over the extratropical Northern Hemisphere continents. The patterns of temperature change imply dynamical responses of climate to natural radiative forcing changes involving El Niño and the North Atlantic Oscillation-Arctic Oscillation.
  Atlantic hurricanes and climate over the past 1,500 years
Mann et al.
Nature

Vol. 460, pp. 880-885, 13 August 2009
doi:10.1038/nature08219

Atlantic tropical cyclone activity, as measured by annual storm counts, reached anomalous levels over the past decade. The short nature of the historical record and potential issues with its reliability in earlier decades, however, has prompted an ongoing debate regarding the reality and significance of the recent rise. Here we place recent activity in a longer-term context by comparing two independent estimates of tropical cyclone activity over the past 1,500 years. The first estimate is based on a composite of regional sedimentary evidence of landfalling hurricanes, while the second estimate uses a previously published statistical model of Atlantic tropical cyclone activity driven by proxy reconstructions of past climate changes. Both approaches yield consistent evidence of a peak in Atlantic tropical cyclone activity during medieval times (around AD 1000) followed by a subsequent lull in activity. The statistical model indicates that the medieval peak, which rivals or even exceeds (within uncertainties) recent levels of activity, results fromthe reinforcing effects of La-Niña-like climate conditions and relative tropical Atlantic warmth.
Figure 2C. MARGO LGM SST map Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum
MARGO Project members
Nature GeoScience

Vol. 2, pp. 127-132, February 2009
doi:10.1038/NGEO411

Observation-based reconstructions of sea surface temperature from relatively stable periods in the past, such as the Last Glacial Maximum, represent an important means of constraining climate sensitivity and evaluating model simulations. The first quantitative global reconstruction of sea surface temperatures during the Last Glacial Maximum was developed by the Climate Long-Range Investigation, Mapping and Prediction (CLIMAP) project in the 1970s and 1980s. Since that time, several shortcomings of that earlier effort have become apparent. Here we present an updated synthesis of sea surface temperatures during the Last Glacial Maximum, rigorously defined as the period between 23 and 19 thousand years before present, from the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project. We integrate microfossil and geochemical reconstructions of surface temperatures and include assessments of the reliability of individual records. Our reconstruction reveals the presence of large longitudinal gradients in sea surface temperature in all of the ocean basins, in contrast to the simulations of the Last Glacial Maximum climate available at present.


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