M. E. Davis

Holocene—Late Pleistocene Climatic Ice Core Records from Qinghai-Tibetan Plateau

Three ice cores to bedrock from the Dunde ice cap on the north-central Qinghai-Tibetan Plateau of China provide a detailed record of Holocene and Wisconsin-W�rm late glacial stage (LGS) climate changes in the subtropics. The records reveal that LGS conditions were apparently colder, wetter, and dustier than Holocene conditions. The LGS part of the cores is characterized by more negative δ18O ratios, increased dust content, decreased soluble aerosol concentrations, and reduced ice crystal sizes than the Holocene part. These changes occurred rapidly ∼10,000 years ago. In addition, the last 60 years were apparently one of the warmest periods in the entire record, equalling levels of the Holocene maximum between 6000 and 8000 years ago.

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.

Recent warming' ice core evidence from tropical ice cores with emphasis on Central Asia

Thompson, L G., Mosley-Thompson, E, Davis, M, Lm, P N, Yao, T, Dyurgerov, M and Dal, J, 1993 Recent warming ice core evidence from tropical ice cores w~th emphasis on Central Asia Global Planet_ Change, 7 145-156 Ice cores from the tropics and subtropics, Jn conjunction with those from the polar regions, provide a multtfaceted record (dust, chemistry, stable isotopes, accumulation) of environmental changes which can be viewed both spatially and temporally This paper emphasizes the oxygen isotopic record (8l~O) preserved in cores from the poles to the tropics and assesses the evtdence for global warming In the last 50-100 years From north to south these records include Camp Century, Greenland, Dunde and Guhya Ice Caps, China, Gregonev Ice Cap, Kirghxzia (formerly part of USSR), Quelccaya Ice Cap, Peru and Slple Station and South Pole, Antarctica The central Asian records along with that from Quelccaya provide strong evidence of recent and rapid warming In the troptcs and subtropics For the Dunde Ice Cap, where a long paleochmanc record is available, the warming m this century appears to be unprecedented in the Holocene These tropical and subtropical records contrast sharply with those from polar cores which show little evtdence of a recent warming These data suggest that either the recent warming Is a middle and lower latitude phenomenon or that these high altitude troptcal and subtropical glaciers may be more sensitive to climate changes than the massive polar ice sheets Regardless, the current rapid disintegration of many tropical and subtropical glaciers may result in the permanent loss of numerous umque archwes

Annually Resolved Ice Core Records of Tropical Climate Variability over the Past ~1800 Years

Quelccaya Ice Cap Ice cores drilled in the ice sheets of Greenland and Antarctica are some of the most important sources of information about the paleoclimate of high latitudes. Comparable records from the tropics are rare, however, because there are so few locations at which long-lived, undisturbed ice can be found. Thompson et al. (p. 945, published online 4 April) report results obtained from one of the few such sites, the Quelccaya ice cap in the Peruvian Andes. The annually resolved data, extending back 1800 years, provide a detailed chronicle of changes in the isotopic composition of the oxygen in the ice, which are related to the sea surface temperature of the waters source. Analyses of a collection of major ions such as ammonium and nitrate reveal how atmospheric circulation in the region varied over that period. Finally, the radiocarbon content of ancient plants—recently exposed by the retreat of the ice sheet—reveals that Quelccaya has not been smaller for at least six thousand years. A record from the Quelccaya ice cap in Peru shows the variability of climate in the tropical Andes. Ice cores from low latitudes can provide a wealth of unique information about past climate in the tropics, but they are difficult to recover and few exist. Here, we report annually resolved ice core records from the Quelccaya ice cap (5670 meters above sea level) in Peru that extend back ~1800 years and provide a high-resolution record of climate variability there. Oxygen isotopic ratios (δ18O) are linked to sea surface temperatures in the tropical eastern Pacific, whereas concentrations of ammonium and nitrate document the dominant role played by the migration of the Intertropical Convergence Zone in the region of the tropical Andes. Quelccaya continues to retreat and thin. Radiocarbon dates on wetland plants exposed along its retreating margins indicate that it has not been smaller for at least six millennia.

A 1000 year climate ice-core record from the Guliya ice cap, China: its relationship to global climate variability

In 1992, an American-Chinese expedition successfully recovered three ice cores (308.6, 93.2 and 34.5m) from the Guliya ice cap (summit 6710 m a.s.l) in the far western Kunlun on the Qinghai–Tibetan Plateau, China. Guliya resembles a “polar” icecap with 10 m, 200 m and basal temperatures of –15.6°, –5.9° and –2.1°C, respectively. The 308.6 m core to bedrock is the longest ice core retrieved from an elevation greater than 4000 m a.s.l. and provides the first ice-core history from the western side of the Qinghai Plateau. The Plateau experiences a pronounced annual precipitation cycle during which 70–80% of annual total precipitation falls in the summer monsoon season. This leads to a marked visible stratigraphy in the glaciers which allows accurate dating of the ice cores and reconstruction of the net mass accumulation. This paper presents (1) the results of the geophysical program to determine ice thickness, ice flow and surface topography, (2) an assessment of net accumulation from stake measurements, snow pits and shallow core samples, and (3) the analyses of the upper 100 m of the 308.6 m core which provide a 1000 year history, including the ‘“Little Ice Age”, which is compared with Chinese historical records. Extended periods of positive accumulation on Guliya are closely contemporaneous with dry periods in eastern China. A trans-Pacific teleconnection is suggested by the strong temporal coherence between extended wet and dry phases on Guliya and on the Quelccaya ice cap, Peru.

GLACIAL STAGE ICE-CORE RECORDS FROM THE SUBTROPICAL DUNDE ICE CAP. CHINA

The first ice-core record of both the Holocene and Wisconsin/Wiirm Late Glacial Stage (LGS) from the subtropics has been extracted from three ice cores to bedrock from the Dunde ice cap on the north-central Qinghai- Tibetan Plateau. Ice thicknesses at the ice-cap summit average 138 m, the bedrock surface is relatively flat, surface and basal temperatures are -7.3 and -4.7 °C, respectively and the ice cap exhibits radial flow away from the summit dome. These records reveal a major change in the climate of the plateau -10 000 years ago and suggest that LGS conditions were colder, wetter and dustier than Holocene conditions. This is inferred from the more negative 6180 ratios, increased dust content, decreased soluble aerosol concentrations, and reduced ice-crystal sizes, which characterize the LGS part of the cores. Total B radioactivity from shallow ice cores indicates that over the last 24 years the average accumulation rate has been -400 mm a-1 at the summit. The ice cores have been dated using a combination of annual layers in the insoluble dust and 6180 in the upper sections of core, visible dust layers which are annual, and ice-flow modeling. The oxygenxad isotope record which serves as a temperature proxy indicates that the last 60 years have been the warmest in the entire record.

Climate of the last 500 years: High resolution ice core records

Precipitation accumulating on the Earths ice sheets and ice caps records a variety of physical and chemical information about the atmosphere and, in some cases, provides unique insight to both the history and the mechanics of the Earths environmental system. High resolution (well-dated) dust records from both polar ice sheets suggest a linkage between increased atmospheric dust and cooler temperatures over Antarctica, but a similar relationship is not observed in ice cores from either Greenland or China. Net accumulation histories for the last 490 years reveal no discemible global pattern although interesting regional differences and similarities exist. Excess (non-sea salt) sulfate profiles provide

Influence of variations in NAO and SO on air temperature over the northern Tibetan Plateau as recorded by δ18O in the Malan ice core

[1]xa0An isotopic (δ18O) enrichment trend from 1887 to 1998 recorded in the Malan ice core from the northern Tibetan Plateau indicates that temperatures there have warmed in the past century. Statistical analyses suggest that the annual δ18O in this ice core largely reflects air temperature variations in the warm season. Statistically significant modest to weak correlations were found between the annual δ18O and the North Atlantic Oscillation (NAO) index from May through October and the Southern Oscillation (SO) index from August through November, respectively. A multivariate linear regression shows that when combined, the NAO and SO account for 55% of the total variance in δ18O over the past century. Moreover, the correlations between δ18O and the NAO and the SO vary over the study period, as might be expected due to changes in the strength and amplitude of the NAO and SO over the past century.

A 1000 year history of atmospheric sulfate concentrations in southern Asia as recorded by a Himalayan ice core

[1]xa0A sulfate record covering the period A.D. 1000–1997 from the Dasuopu glacier in the Himalayas reveals that this site is sensitive to anthropogenic activity originating in southern Asia. Prior to 1870 atmospheric sulfate concentrations were relatively low and constant, but thereafter concentrations have increased and since 1930 the rate of increase has accelerated rapidly. This accelerating trend in sulfate deposition is paralleled by growing SO2 emissions over southern Asia resulting from the increased energy demand. The concentration of sulfate deposited in the last 50 years exceeds that for any prior 50-year period in the last millennium. Unlike the Greenland ice core-derived sulfate concentrations that have declined since the 1970s, sulfate concentrations deposited on the Himalayan ice fields continue to increase, having nearly doubled since 1970. This reflects regional differences between Europe and Asia in source strength and transport pathways for atmospheric sulfate, as well as differing degrees of environmental regulation.

Recent ice-core climate records from the Cordillera Blanca, Peru

Ice cores recently drilled to bedrock on the col of Huascaran (9°06′ S, 77°36′ W, 6047 m a.s.l.) offer the potential for a long, annually resolved climate record from tropical South America. This paper presents the record from 1950 to 1993 preserved in microparticle and nitrate concentrations and oxygen-isotopic ratios. Average monthly temperatures from a satellite-linked automatic weather station installed on nearby Hualcan in 1991 are presented. Annual temperatures from local high-altitude meteorological stations, along with the annual Huascaran isotopic record, show a warming trend over the last two decades. The marked preservation of the climate record in oxygen-isotopic ratios on Huascaran is absent at lower-elevation sites, which have been affected by the recent warming. This paper demonstrates the establishment of a time-scale for the Huascaran core, the preservation of the climatic signal with depth and the linkage of the ice-core “proxy-climate” parameters with measured climatic variations.