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Author(s): Margot J. Cramwinckel [sup.1] , Matthew Huber [sup.2] , Ilja J. Kocken [sup.1] , Claudia Agnini [sup.3] , Peter K. Bijl [sup.1] , Steven M. Bohaty [sup.4] , Joost [...], Palaeoclimate reconstructions of periods with warm climates and high atmospheric CO.sub.2 concentrations are crucial for developing better projections of future climate change. Deep-ocean.sup.1,2 and high-latitude.sup.3 palaeotemperature proxies demonstrate that the Eocene epoch (56 to 34 million years ago) encompasses the warmest interval of the past 66 million years, followed by cooling towards the eventual establishment of ice caps on Antarctica. Eocene polar warmth is well established, so the main obstacle in quantifying the evolution of key climate parameters, such as global average temperature change and its polar amplification, is the lack of continuous high-quality tropical temperature reconstructions. Here we present a continuous Eocene equatorial sea surface temperature record, based on biomarker palaeothermometry applied on Atlantic Ocean sediments. We combine this record with the sparse existing data.sup.4-6 to construct a 26-million-year multi-proxy, multi-site stack of Eocene tropical climate evolution. We find that tropical and deep-ocean temperatures changed in parallel, under the influence of both long-term climate trends and short-lived events. This is consistent with the hypothesis that greenhouse gas forcing.sup.7,8, rather than changes in ocean circulation.sup.9,10, was the main driver of Eocene climate. Moreover, we observe a strong linear relationship between tropical and deep-ocean temperatures, which implies a constant polar amplification factor throughout the generally ice-free Eocene. Quantitative comparison with fully coupled climate model simulations indicates that global average temperatures were about 29, 26, 23 and 19 degrees Celsius in the early, early middle, late middle and late Eocene, respectively, compared to the preindustrial temperature of 14.4 degrees Celsius. Finally, combining proxy- and model-based temperature estimates with available CO.sub.2 reconstructions.sup.8 yields estimates of an Eocene Earth system sensitivity of 0.9 to 2.3 kelvin per watt per square metre at 68 per cent probability, consistent with the high end of previous estimates.sup.11.A 26-million-year record of equatorial sea surface temperatures reveals synchronous changes of tropical and polar temperatures during the Eocene epoch forced by variations in concentrations of atmospheric carbon dioxide, with a constant degree of polar amplification. more...

Author(s): Taylor F. Schildgen [sup.1] [sup.2] , Pieter A. van der Beek [sup.3] , Hugh D. Sinclair [sup.4] , Rasmus C. Thiede [sup.2] [sup.5] Author Affiliations: (1) GFZ German Research [...], The potential link between erosion rates at the Earth's surface and changes in global climate has intrigued geoscientists for decades.sup.1,2 because such a coupling has implications for the influence of silicate weathering.sup.3,4 and organic-carbon burial.sup.5 on climate and for the role of Quaternary glaciations in landscape evolution.sup.1,6. A global increase in late-Cenozoic erosion rates in response to a cooling, more variable climate has been proposed on the basis of worldwide sedimentation rates.sup.7. Other studies have indicated, however, that global erosion rates may have remained steady, suggesting that the reported increases in sediment-accumulation rates are due to preservation biases, depositional hiatuses and varying measurement intervals.sup.8-10. More recently, a global compilation of thermochronology data has been used to infer a nearly twofold increase in the erosion rate in mountainous landscapes over late-Cenozoic times.sup.6. It has been contended that this result is free of the biases that affect sedimentary records.sup.11, although others have argued that it contains biases related to how thermochronological data are averaged.sup.12 and to erosion hiatuses in glaciated landscapes.sup.13. Here we investigate the 30 locations with reported accelerated erosion during the late Cenozoic.sup.6. Our analysis shows that in 23 of these locations, the reported increases are a result of a spatial correlation bias--that is, combining data with disparate exhumation histories, thereby converting spatial erosion-rate variations into temporal increases. In four locations, the increases can be explained by changes in tectonic boundary conditions. In three cases, climatically induced accelerations are recorded, driven by localized glacial valley incision. Our findings suggest that thermochronology data currently have insufficient resolution to assess whether late-Cenozoic climate change affected erosion rates on a global scale. We suggest that a synthesis of local findings that include location-specific information may help to further investigate drivers of global erosion rates. Reported acceleration of erosion in mountainous landscapes during the late Cenozoic is the result of combining thermochronology data with disparate exhumation histories, thereby converting spatial variations in erosion rates into temporal increases. more...

About 34 million years ago, Earth's climate shifted from a relatively ice-free world to one with glacial conditions on Antarctica characterized by substantial ice sheets. How Earth's temperature changed during this climate transition remains poorly understood, and evidence for Northern Hemisphere polar ice is controversial. Here, we report proxy records of sea surface temperatures from multiple ocean localities and show that the high-latitude temperature decrease was substantial and heterogeneous. High-latitude (45 degrees to 70 degrees in both hemispheres) temperatures before the climate transition were ~20[degrees]C and cooled an average of ~5[degrees]C. Our results, combined with ocean and ice-sheet model simulations and benthic oxygen isotope records, indicate that Northern Hemisphere glaciation was not required to accommodate the magnitude of continental ice growth during this time. more...

Byline: Henry F. Diaz (1), David W. Stahle (2) Abstract: There is abundant historical evidence that climatic extremes in the past have led to significant and sometimes severe societal impacts. The severity of these impacts depends on the intensity and duration of the climatic event, social organization, and the prevailing socioeconomic conditions at the time of the climatic extreme. In this issue of Climatic Change we present the results from 12 studies, which document climatic extremes on different time scales and provide interesting evidence for direct and indirect social impacts of climatic changes in the Americas during the pre-Hispanic, colonial, and modern eras. Author Affiliation: (1) NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO, 80305, USA (2) Tree-Ring Laboratory, Department of Geosciences, University of Arkansas, Fayetteville, AR, 72701, USA Article History: Registration Date: 04/01/2007 Received Date: 14/07/2006 Accepted Date: 18/12/2006 Online Date: 10/03/2007 more...

Byline: Matthew D. Therrell (1,4), David W. Stahle (1), Jose Villanueva Diaz (2), Eladio H. Cornejo Oviedo (3), Malcolm K. Cleaveland (1) Abstract: Maize was domesticated more than 6,000 years ago in central Mexico, and remains a vital staple food and cultural symbol in Mesoamerica. Maize yield in the central highlands is strongly dependant on adequate rainfall early in the growing season (April--June) because late maturation of the crop may result in damage from autumn frost. Climate-induced crop failures with profound socioeconomic impacts have punctuated Mexican history. However, reliable records of maize harvest have not been available until very recently, and historical records of crop yield and price are discontinuous and can be difficult to interpret. We have developed a continuous, exactly dated, tree-ring reconstruction of maize yield variability in central Mexico from 1474 to 2001 that provides new insight into the history of climate and food availability in the heartland of the Mesoamerican cultural province. The reconstruction indicates that seven of the most severe agricultural crises in Mexican history occurred during decadal-scale episodes of reconstructed maize shortfalls. Author Affiliation: (1) Department of Geosciences, University of Arkansas, Fayetteville, Arkansas, 72701, U.S.A. (2) Instituto Nacional de Investigaciones Forestales y Agropecurias (INIFAP), KM 6.5, Margen Derecha Canal Sacramento, Gomez Palacio, Durango, 35140, Mexico (3) Departamento Forestal, Universidad Autonoma Agraria Antonio Narro, Buenavista, Saltillo, Coahuila, 25315, Mexico (4) Department of Geography, The University of Iowa, 316 Jessup Hall, Iowa City, Iowa, 52240, U.S.A. Article History: Registration Date: 01/01/2006 Received Date: 19/05/2003 Online Date: 08/03/2006 more...