Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake levels and geochemistry

Palaeodata in synthesis form are needed as benchmarks for the Palaeoclimate Modelling Intercomparison Project (PMIP), Advances since the last synthesi s of terrestrial palaeodata from the last glacial maximum (LGM) call for a new evaluation, especially of data from the tropics. Here pollen, plant-mac rofossil, lake-level, noble gas (from groundwater) and delta(18)O (from spe leothems) data are compiled for 18 +/- 2 ka (C-14), 32 degrees N-33 degrees S. The reliability of the data was evaluated using explicit criteria and s ome types of data were re-analysed using consistent methods in order to der ive a set of mutually consistent palaeoclimate estimates of mean temperatur e of the coldest month (MTCO), mean annual temperature (MAT), plant availab le 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 -15K 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 divergen ce between MAT and cold-month estimates from the same region is seen only i n the SE USA, where cold-air advection is expected to have enhanced cooling in winter. Regression of all cold-month anomalies against site elevation y ielded an estimated average cooling of - 2.5 to - 3 K at modern sea level, increasing to approximate to - 6 K by 3000m. How ever, Neotropical sites sh owed larger than the average sea-level cooling (- 5 to - 6 K) and a non-sig nificant elevation effect, whereas W and S Pacific sites showed much less s ea-level cooling (- 1 K) and a stronger elevation effect. These findings su pport the inference that tropical sea-surface temperatures (SSTs) were lowe r than the CLIMAP estimates, but they limit the plausible average tropical sea-surface cooling, and they support the existence of CLIMAP-like geograph ic patterns in SST anomalies. Trends of PAM and lake levels indicate wet LG M conditions in the W USA, and at the highest elevations, with generally dr y conditions elsewhere. These results suggest a colder-than-present ocean s urface producing a weaker hydrological cycle, more arid continents, and arg uably steeper-than-present terrestrial lapse rates. Such linkages are suppo rted by recent observations on freezing-level height and tropical SSTs, mor eover, simulations of "greenhouse" and LGM climates point to several possib le feedback processes by which low-level temperature anomalies might be amp lified aloft.