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 Inter-comparison 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.