Keith A. Henderson

Late Glacial Stage and Holocene Tropical Ice Core Records from Huascarán, Peru

Two ice cores from the col of Huascar�n in the north-central Andes of Peru contain a paleoclimatic history extending well into the Wisconsinan (W�rm) Glacial Stage and include evidence of the Younger Dryas cool phase. Glacial stage conditions at high elevations in the tropics appear to have been as much as 8� to 12�C cooler than today, the atmosphere contained about 200 times as much dust, and the Amazon Basin forest cover may have been much less extensive. Differences in both the oxygen isotope ratio ζ18O (8 per mil) and the deuterium excess (4.5 per mil) from the Late Glacial Stage to the Holocene are comparable with polar ice core records. These data imply that the tropical Atlantic was possibly 5� to 6�C cooler during the Late Glacial Stage, that the climate was warmest from 8400 to 5200 years before present, and that it cooled gradually, culminating with the Little Ice Age (200 to 500 years before present). A strong warming has dominated the last two centuries.

Ice-core palaeoclimate records in tropical South America since the Last Glacial Maximum

Ice-core records spanning the last 25 000 yr from the tropical Andes of South America are reviewed. These records from Quelccaya, Huascaran and Sajama present a high temporal resolution picture of both the Late Glacial Stage (LGS) and the Holocene climatic and environmental conditions in the South American Andes. Late Glacial Stage conditions at high elevations appear to have been cooler than today, although the magnitude of the inferred cooling differs with the particular proxy used (e.g. snowline depression, pollen, ice cores). Insoluble dust and anion concentrations in the ice cores reveal that LGS hydrological conditions in the tropics (9°S) were much drier than today, whereas in the subtropics (18°S) LGS conditions were much wetter. This probably reflects the migration of the tropical Hadley Cell in response to a different meridional temperature gradient. Low nitrate concentrations in the LGS ice from both Huascaran and Sajama suggest that the Amazon Basin forest cover may have been much less extensive. Discussed is the conundrum surrounding the use of d 18 O as a palaeothermometer in the tropics, where temperatures exhibit little seasonal variation yet the ice-core records suggest that d 18 O records temperature variations on decadal to millennial time-scales. Finally evidence is presented for a strong twentieth century warming. Copyright

Tropical Glacier and Ice Core Evidence of Climate Change on Annual to Millennial Time Scales

This paper examines the potential of the stable isotopic ratios, 18O/16O (δ 18Oice)and 2H/1H (δ Dice), preserved in mid to low latitude glaciers as a toolfor paleoclimate reconstruction. Ice cores are particularly valuable as they contain additional data, such as dust concentrations, aerosol chemistry, and accumulation rates, that can be combined with the isotopic information to assist with inferences about the regional climate conditions prevailing at the time of deposition. We use a collection of multi-proxy ice core histories to explore the δ 18O-climate relationship over the last 25,000 years that includes both Late Glacial Stage (LGS) and Holocene climate conditions. These results suggest that on centennial to millennial time scales atmospheric temperature is the principal control on the δ 18Oice of the snowfall that sustains these high mountainice fields.Decadally averaged δ 18Oice records from threeAndean and three Tibetan ice cores are composited to produce a low latitude δ 18Oice history for the last millennium. Comparison ofthis ice core composite with the Northern Hemisphere proxy record (1000–2000A.D.) reconstructed by Mann et al. (1999) and measured temperatures(1856–2000) reported by Jones et al. (1999) suggests the ice cores have captured the decadal scale variability in the global temperature trends. These ice cores show a 20th century isotopic enrichment that suggests a large scale warming is underway at low latitudes. The rate of this isotopically inferred warming is amplified at higher elevations over the Tibetan Plateau while amplification in the Andes is latitude dependent with enrichment (warming) increasing equatorward. In concert with this apparent warming, in situobservations reveal that tropical glaciers are currently disappearing. A brief overview of the loss of these tropical data archives over the last 30 years is presented along with evaluation of recent changes in mean δ18Oice composition. The isotopic composition of precipitation should be viewed not only as a powerful proxy indicator of climate change, but also as an additional parameter to aid our understanding of the linkages between changes in the hydrologic cycle and global climate.

Recording of El Niño in ice core δ18O records from Nevado Huascarán, Peru

The 68-year monthly resolved time series of δ 18 O from ice cores retrieved from the glaciated col of Nevado Huascaran, Peru (9°S, 77°W, 6050 m) reflects climate variability over Amazonia and the western tropical Atlantic. Over the 25-year period (1968-1993) of midtroposphere observations, the interannual variations in Huascaran δ 18 O relate closely with the zonal wind variations over tropical South America at the 500 hPa level. Additionally, there is some evidence that the spatial distribution of temperature anomalies in the western tropical Atlantic has influence on the 500 hPa circulation and hence the isotopic fractionation of moisture advected across Amazonia. During El Nino warming, the moisture convergence axis over the Atlantic Ocean is commonly diverted northward, leading to unusual warm and dry conditions over northeast Brazil, and 18 O-enriched snowfall at Huascaran. This enrichment phase is enhanced when the peak Pacific warming occurs during the first half of the calendar year, coincident with the wet season over Amazonia. Approximately 12 months later, the El Nino demise is affiliated with a reprisal of strong trade wind circulation, and the resultant cool, pluvial environment over Amazonia triggers a reversal to strongly depleted isotope anomalies.