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Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake-levels and geochemistry |
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| Authors: | I Farrera S P Harrison I C Prentice G Ramstein J Guiot P J Bartlein R Bonnefille M Bush W Cramer U von Grafenstein K Holmgren H Hooghiemstra G Hope D Jolly S-E Lauritzen Y Ono S Pinot M Stute G Yu |
| Institution: | Laboratoire Paléoenvironnements et Palynologie, USTL, Place Eugene Bataillon, F-34095 Montpellier cédex 5, France, FR Max Planck Institute for Biogeochemistry, PO Box 10?01?64, D-07701 Jena, Germany, DE Laboratoire des Sciences du Climat et de l’Environnement, CEA Saclay, Batiment 709, Orme des Merisiers, F-91191 Gif-sur-Yvette cédex, France, FR IMEP CNRS, Case 451, Faculté de St Jér?me, F-13397 Marseille cédex 20, France, FR Department of Geography, University of Oregon, Eugene, OR 97403, USA, US Palynology Laboratory, French Institute of Pondichery, 11 St. Louis Street., P.B. 33, Pondicherry 605 001, India, IN Department of Biological Sciences, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA, US Potsdam Institute for Climate Impact Research (PIK), Telegrafenberg, P.O. Box 60?12?03, D-14412 Potsdam, Germany, DE Department of Physical Geography, Stockholm University, S-106 91 Stockholm, Sweden, SE Department of Palynology and Paleo/Actuo-ecology, University of Amsterdam, Kruislaan 318, NL-1098 SM Amsterdam, The Netherlands, NL Research School of Pacific Studies, Australian National University, Canberra 0200, ACT, Australia, AU Department of Geology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway, NO Laboratory of Geoecology, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan, JP Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964, USA, US Nanjing Institute of Geography and Limnology, Chinese Academy of Science, Nanjing 210093, China, CN |
| Abstract: | Palaeodata in synthesis form are needed as benchmarks for the Palaeoclimate Modelling Intercomparison Project (PMIP). Advances since the last synthesis of terrestrial palaeodata from the last glacial maximum (LGM) call for a new evaluation, especially of data from the tropics. Here pollen, plant-macrofossil, lake-level, noble gas (from groundwater) and δ18O (from speleothems) data are compiled for 18±2 ka (14C), 32 °N–33 °S. The reliability of the data was evaluated using explicit criteria and some types of data were re-analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month (MTCO), mean annual temperature (MAT), plant available moisture (PAM) and runoff (P-E). Cold-month temperature (MAT) anomalies from plant data range from −1 to −2 K near sea level in Indonesia and the S Pacific, through −6 to −8 K at many high-elevation sites to −8 to −15 K in S China and the SE USA. MAT anomalies from groundwater or speleothems seem more uniform (−4 to −6 K), but the data are as yet sparse; a clear divergence between MAT and cold-month estimates from the same region is seen only in the SE USA, where cold-air advection is expected to have enhanced cooling in winter. Regression of all cold-month anomalies against site elevation yielded an estimated average cooling of −2.5 to −3 K at modern sea level, increasing to ≈−6 K by 3000 m. However, Neotropical sites showed larger than the average sea-level cooling (−5 to −6 K) and a non-significant elevation effect, whereas W and S Pacific sites showed much less sea-level cooling (−1 K) and a stronger elevation effect. These findings support the inference that tropical sea-surface temperatures (SSTs) were lower than the CLIMAP estimates, but they limit the plausible average tropical sea-surface cooling, and they support the existence of CLIMAP-like geographic patterns in SST anomalies. Trends of PAM and lake levels indicate wet LGM conditions in the W USA, and at the highest elevations, with generally dry conditions elsewhere. These results suggest a colder-than-present ocean surface producing a weaker hydrological cycle, more arid continents, and arguably steeper-than-present terrestrial lapse rates. Such linkages are supported by recent observations on freezing-level height and tropical SSTs; moreover, simulations of “greenhouse” and LGM climates point to several possible feedback processes by which low-level temperature anomalies might be amplified aloft. Received: 7 September 1998 / Accepted: 18 March 1999 |
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