Mark B. Abbott

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Holocene paleohydrology and glacial history of the central Andes using multiproxy lake sediment studies

Here we document at century to millennial scale the regional changes of precipitation^evaporation from the late Pleistocene to present with multiproxy methods on a north^south transect of lake sites across the eastern cordillera of the central Andes. The transect of study sites covers the area from V14‡S to 20‡S and includes core studies from seven lakes and modern calibration water samples from twenty-three watersheds analyzed to constrain the down-core interpretations of stable isotopes and diatoms. We selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance. These include: (1) lakes directly receiving glacial meltwater, (2) overflowing lakes in glaciated watersheds, (3) overflowing lakes in watersheds without active glaciers, and (4) lakes that become closed basins during the dry season. The results of our current work on multiple lakes in the Bolivian Andes show that while the overall pattern of Holocene environmental change is consistent within the region, conditions were not always stable over centennial to over millennial timescales and considerable decadal- to centuryscale climate variability is evident [Abbott et al., Quat. Res. 47 (1997) 70^80, Quat. Res. 47 (1997) 169^180, Quat. Sci. Rev. 19 (2000) 1801^1820; Polissar, Master’s thesis, University of Massachusetts (1999)]. Comparison of the paleoclimate record from one well-studied site, Lago Taypi Chaka Kkota (LTCK), with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle Holocene. Previous work noted a difference between the timing of water-level rise in Lake Titicaca V5.0^3.5 ka B.P. [Abbott et al., Quat. Res. 47 (1997) 169^180; Cross et al., Holocene 10 (2000) 21^32; Rowe et al., Clim. Change 52 (2002) 175^199] and the onset of wetter conditions at 2.3 ka B.P. in LTCK, a lake that drains into the southern end of Lake Titicaca [Abbott et al., Quat. Res. 47 (1997) 70^80]. Sedimentary and oxygen isotope evidence from Paco Cocha (13‡54PS) located in the northern reaches of the expansive 57 000 km 2 Titicaca watershed, which spans V14‡S to 17‡S, indicates that

A 2,300-year-long annually resolved record of the South American summer monsoon from the Peruvian Andes

Decadal and centennial mean state changes in South American summer monsoon (SASM) precipitation during the last 2,300 years are detailed using an annually resolved authigenic calcite record of precipitation δ18O from a varved lake in the Central Peruvian Andes. This unique sediment record shows that δ18O peaked during the Medieval Climate Anomaly (MCA) from A.D. 900 to 1100, providing evidence that the SASM weakened considerably during this period. Minimum δ18O values occurred during the Little Ice Age (LIA) between A.D. 1400 and 1820, reflecting a prolonged intensification of the SASM that was regionally synchronous. After the LIA, δ18O increased rapidly, particularly during the current warm period (CWP; A.D. 1900 to present), indicating a return to reduced SASM precipitation that was more abrupt and sustained than the onset of the MCA. Diminished SASM precipitation during the MCA and CWP tracks reconstructed Northern Hemisphere and North Atlantic warming and a northward displacement of the Intertropical Convergence Zone (ITCZ) over the Atlantic, and likely the Pacific. Intensified SASM precipitation during the LIA follows reconstructed Northern Hemisphere and North Atlantic cooling, El Niño-like warming in the Pacific, and a southward displacement of the ITCZ over both oceans. These results suggest that SASM mean state changes are sensitive to ITCZ variability as mediated by Western Hemisphere tropical sea surface temperatures, particularly in the Atlantic. Continued Northern Hemisphere and North Atlantic warming may therefore help perpetuate the recent reductions in SASM precipitation that characterize the last 100 years, which would negatively impact Andean water resources.

Late-glacial and Holocene vegetational history from two sites in the western Cordillera of southwestern Ecuador

The deciphering of the late-glacial and Holocene vegetation, glacial-geological, and climatic history of Cajas National Park in southwestern Ecuador is undertaken focusing on close-interval sampling of sediment cores from two high elevations lakes, Lagunas Chorreras (3700 m) and Pallcacocha (4060 m). This study involves extensive dating (both accelerator mass spectrometry and tephra), palynological, and sediment analyses of lakes and bogs. Basal dates for the two cores analyzed for pollen range from about 17 000 to about 15 500 cal. yr BP. Vegetation surveys and surface sample pollen and spore analyses were accomplished for two transects in the western Cordillera. One began east of the main divide near Cuenca, Ecuador and the other began at the crest of the western Cordillera, descending towards the Pacific Ocean. These vegetation surveys coupled with pollen analyses of surface samples were used to establish pollen analogues to help in the interpretation of the fossil pollen records. The results of the pollen analyses from the two lake cores indicate two major climatic periods. (1) The late-glacial pollen record, beginning about 17 000 yr BP and ending near 11 000, is characterized by an herb paramo dominated by pteridophytes (primarily Huperzia spp.) with Asteraceae (assumed to be woody species) and Puya spp. The climate inferred from these pollen records was cooler and moister than today. The sediments for this time period are characterized by low loss-on-ignition percentages, and high magnetic susceptibility values (with secondary peaks between 12 000 and 13 000 yr BP). Low concentrations of charcoal fragments indicate a low incidence of fire, and several pronounced fluctuations in pollen assemblages could be interpreted as changes in the prevailing wind direction and/or climate. (2) The beginning of the Holocene, as represented in the pollen record, is characterized by the disappearance of Puya pollen, vastly diminished Huperzia representation, high charcoal concentrations, and the expansion of moist montane forest pollen. Asteraceae (possibly Gynoxys) are replaced as dominant timberline taxa by Polylepis-dominated timberline forest. The charcoal record suggests that fires were much more prevalent during the early to middle Holocene than during the late-glacial. Changes in Isoe «tes concentrations and trends in upland vegetation may be related to fluctuations in lake levels and precipitation.

Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordillera Real, Bolivia

Multiproxy analyses of sediment cores from Lago Taypi Chaka Kkota (LTCK) Cordillera Real, Bolivia, provide a record of drier conditions following late Pleistocene deglaciation culminating in pronounced aridity between 6.2 and 2.3 ka B.P. Today LTCK is a glacial-fed lake that is relatively insensitive to changes in P}E because it is largely bu!ered from dry season draw-down through the year-round supply of glacial meltwater. This was not the case during the middle to late Holocene when glaciers were absent from the watershed. Lake-water d18O values inferred from d18O analysis of sediment cellulose range from !12.9 to !5.3& and average !8.7& between 6.2 and 2.3 ka B.P. Modern lake-water d18O from LTCK averages !14.8& which is compatible with the d18O -8 value of !14.3& for the surface sediment cellulose. Analyses of d18O from modern surface waters in 23 lakes that span the range from glacial-fed to closed basin vary from !16.6 to !2.5&. This approximates the magnitude of the down-core shift in d18O -8 values in LTCK during the middle to late Holocene from !12.9 to !5.3&. Strong paleohydrologic change during the middle Holocene is also evident in diatom assemblages that consist of shallow-water, non-glacial periphytic taxa and bulk organic d13C and d15N that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions. Neoglaciation began after 2.3 ka B.P. as indicated by changes in the composition of the sediments, lower d18O values, and a return to diatom assemblages characteristic of the glacial sediments that formed during the Late Pleistocene. Collectively, these data indicate that the past 2.3 ka B.P. have been the wettest interval during the Holocene. Millennial-scale shifts in the paleohydrologic record of LTCK during the early to middle Holocene conform to other regional paleoclimatic time-series, including Lake Titicaca and Nevado Sajama, and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply. In contrast, an apparent 1200-year lag in the onset of wetter conditions at LTCK (2.3 ka B.P.) compared to Lake Titicaca (3.5 ka B.P.) provides evidence for variable sub-regional hydrologic response to climate change during the middle to late Holocene. ( 2000 Elsevier Science Ltd. All rights reserved.

Reconstruction of paleohydrology and paleohumidity from oxygen isotope records in the Bolivian Andes

Cellulose-inferred lake water N 18 O( N 18 Olw) records from Lago Potosi (LP), a seasonally closed lake in a watershed that is not currently glaciated, and Lago Taypi Chaka Kkota (LTCK) [previously reported in Abbott et al., 2000. Quat. Sci. Rev. 19, 1801^1820], an overflowing lake in a glaciated watershed, provide the basis for late Pleistocene and Holocene paleoclimatic reconstruction in the Bolivian Andes. Deconvolution of the histories of changing evaporative isotopic enrichment from source water N 18 O in the lake sediment records is constrained by comparison to the Sajama ice core oxygen isotope profile, whereas local hydrological influence is distinguished from the regional moisture balance history by the response of the different catchments to climate change. Overall, variations in the LP N 18 Olw record appear to be dominantly controlled by evaporative 18 O-enrichment, reflecting shifts in local effective moisture. This record is used to generate a preliminary quantitative reconstruction of summer relative humidity spanning the past 11 500 cal yr on the basis of an isotope-mass balance model. Results indicate that the late Pleistocene was moist with summer relative humidity values estimated at 10^20% greater than present. Increasing aridity developed in the early Holocene with maximum prolonged dryness spanning 7500^6000 cal yr BP at LP, an interval characterized by summer relative humidity values that may have been 20% lower than present. Highly variable but dominantly arid conditions persist in the mid- to late Holocene, with average summer relative humidity values estimated at 15% below present, which then increase to about 10^20% greater than present by 2000 cal yr BP. Slightly more arid conditions characterize the last millennium with summer relative humidity values ranging from 5^10% lower than present. Similar long-term variations are evident in the LTCK N 18 Olw profile, except during the early Holocene when lake water evaporative 18 O-enrichment in response to low relative humidity appears to have been offset by enhanced inflow from 18 O-depleted snowmelt or groundwater from the large catchment. Although some temporal offset is evident, significant correspondence occurs between the isotope-inferred paleohumidity reconstruction and other paleohydrological proxies from the region. These

Abrupt Climate Change and Pre-Columbian Cultural Collapse

Publisher Summary Holocene climate changes in the circum-Caribbean, and Andean Altiplano are inferred by using paleolimnological methods. Paleoenvironmental data provides a climatic context in which the Maya, and Tiwanaku cultures arose, persisted, and collapsed prior to European contact. In the circum-Caribbean, the arid late Pleistocene period was followed by a relatively moist early to middle Holocene period probably related to large differences between summer, and winter insolation. The earliest Maya settlement was associated with reduced seasonality, and regional drying. In the northern part of the Yucatan Peninsula, the climate became even drier during the Classic period. The Andean Altiplano experienced low seasonality, and dry conditions in the early, and middle Holocene. The southern basin of Lake Titicaca displayed a low stage. Chiripa culture developed, was associated with greater seasonality, increased moisture availability, and rising lake level. Tiwanaku culture depended on raised-field agricultural technology. A prolonged dry period began in the Altiplano, prompting abandonment of raised fields, and cultural decline. Climate changes in the Northern Hemisphere Maya lowlands, and the Southern Hemisphere Andean Altiplano were out of phase on millennial timescales, when climate was apparently forced by shifts in seasonal insolation driven by the precession of the Earths orbit. Shorter frequency climate changes in the Maya, and Tiwanaku regions were driven by factors other than Milankovitch forcing. In both areas, population growth, and cultural development occurred under favorable conditions for agriculture. Paleoenvironmental data indicates cultural development is limited by climatic thresholds, and the abrupt, unpredictable climate changes disrupt agricultural production, and human populations.

1,500 year quantitative reconstruction of winter precipitation in the Pacific Northwest

Multiple paleoclimate proxies are required for robust assessment of past hydroclimatic conditions. Currently, estimates of drought variability over the past several thousand years are based largely on tree-ring records. We produced a 1,500-y record of winter precipitation in the Pacific Northwest using a physical model-based analysis of lake sediment oxygen isotope data. Our results indicate that during the Medieval Climate Anomaly (MCA) (900–1300 AD) the Pacific Northwest experienced exceptional wetness in winter and that during the Little Ice Age (LIA) (1450–1850 AD) conditions were drier, contrasting with hydroclimatic anomalies in the desert Southwest and consistent with climate dynamics related to the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These findings are somewhat discordant with drought records from tree rings, suggesting that differences in seasonal sensitivity between the two proxies allow a more compete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.

Late-Holocene atmospheric lead deposition in the Peruvian and Bolivian Andes

The analysis of lead (Pb) in lake-sediment cores is a useful method to reconstruct local histories of Pb pollution. Here, we use Pb concentration profiles from lake sediments to reconstruct local trajectories of pre-Colonial smelting from three metallurgical centres in the South American Andes: the Morococha mining district, Peru; the Bolivian Altiplano around Lake Titicaca; and the silver-mining centre of Potosí, Bolivia. The earliest evidence for Pb enrichment from smelting is on the Altiplano beginning ~AD 400, coincident with the rise of the pre-Incan Tiwanaku and Wari Empires. Coeval with the collapse of these Empires ~AD 1000, there is a dramatic decrease in Pb pollution on the Altiplano, suggesting metallurgical activity was closely tied to the Tiwanaku state. In contrast, metallurgy at Morococha, Peru and Potosí, Bolivia began ~AD 1000—1200, likely as the result of the diaspora generated by the collapses of Tiwanaku and Wari. The independent chronologies of these records suggest asynchronous metallurgical activity between mining centres, and local-scale control of mineral resources. Following Inca conquest of the Andes ~AD 1450, strong increases in Pb are noted at all three study sites, suggesting an increase in silver production to meet Inca imperial demand. Following Hispanic conquest (AD 1532), large increases in Pb pollution are noted at Morococha and Potosí, only to be superseded by industrial development. The records presented here have implications for the reconstruction of Andean prehistory, and demonstrate the sensitivity of lake sediment geochemistry to pre-Colonial smelting activity. The technique has much potential for exploring the timing and magnitude of pre-industrial metallurgy in the New World.

Drought variability in the Pacific Northwest from a 6,000-yr lake sediment record

We present a 6,000-yr record of changing water balance in the Pacific Northwest inferred from measurements of carbonate δ18O and grayscale on a sediment core collected from Castor Lake, Washington. This subdecadally resolved drought record tracks the 1,500-yr tree-ring-based Palmer Drought Severity Index reconstructions of Cook et al. [Cook ER, Woodhouse CA, Eakin CM, Meko DM, Stahle DW (2004) Science 306:1015–1018] in the Pacific Northwest and extends our knowledge back to 6,000 yr B.P. The results demonstrate that low-frequency drought/pluvial cycles, with occasional long-duration, multidecadal events, are a persistent feature of regional climate. Furthermore, the average duration of multidecadal wet/dry cycles has increased since the middle Holocene, which has acted to increase the amplitude and impact of these events. This is especially apparent during the last 1,000 yr. We suggest these transitions were driven by changes in the tropical and extratropical Pacific and are related to apparent intensification of the El Niño Southern Oscillation over this interval and its related effects on the Pacific Decadal Oscillation. The Castor Lake record also corroborates the notion that the 20th century, prior to recent aridity, was a relatively wet period compared to the last 6,000 yr. Our findings suggest that the hydroclimate response in the Pacific Northwest to future warming will be intimately tied to the impact of warming on the El Niño Southern Oscillation.