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Papers by Raymond Bradley

We analyze global patterns of reconstructed surface temperature for insights into the behavior of... more We analyze global patterns of reconstructed surface temperature for insights into the behavior of the El Niño/Southern Oscillation (ENSO) and related climatic variability during the past three centuries. The global temperature reconstructions are based on calibrations of a large set of globally distributed proxy records, or "multiproxy" data, against the dominant patterns of surface temperature during the past century. These

Science, 2001

The archeological and historical record is replete with evidence for prehistoric, ancient and pre... more The archeological and historical record is replete with evidence for prehistoric, ancient and pre-modern societal collapse. These collapses occurred quite suddenly and frequently involved regional abandonment, replacement of one subsistence base by another (such as agriculture by pastoralism) or conversion to a lower energy socio- political organization (such as local state from interregional empire). Each of these collapse episodes has

Arctic glaciers have globally experienced a negative mass balance because of the changing climate... more Arctic glaciers have globally experienced a negative mass balance because of the changing climate over the past decades. The Lofoten archipelago at 68°N has one of the largest air temperature anomalies in the world relative to latitude; this is because of the warm waters of the Norwegian Atlantic Current (NAC). Hence, the climate in Lofoten is very sensitive to the ocean-atmosphere interaction between variations in the magnitude and strength of the NAC and the prevailing westerly wind direction. Here we present the first high-resolution late-Holocene glacier record from the Lofoten archipelago in northern Norway. The study is based on analyses of lacustrine sediments in the proglacial lake Kveitvikvatnet (31 m a.s.l.), and glacial-geomorphological mapping of the ~4.2 km2 large surrounding catchment. At present, Kveitvikvatnet receives glacial meltwater from three small cirque glaciers. In total, five sediment cores have been retrieved from the lake. The cores (~2-5 m long) have been...

We present high-resolution geochemical data at 200 micrometer using ITRAX core scanning X-ray flu... more We present high-resolution geochemical data at 200 micrometer using ITRAX core scanning X-ray fluorescence, and physical properties obtained by scanning electron microscope images of the sediment thin sections in order to reveal the depositional history during the Holocene. The core MD02-2490 has been taken from Saanich Inlet, BC, a silled fjord on southeastern Vancouver Island, as part of the IMAGES VIII (International Marine Past Global Changes Study) program. Annual laminations in Saanich Inlet are mainly couplets made of a biogenic layer and a terrigenous layer. Lamina thickness measured from 2,000 to 7,000 yr BP reveals an abrupt increase in biogenic input around 6,500 yr BP, while the annual accumulation rate is likely to be constant. During this period, the relative abundances of major and minor elements also change significantly. The spectral analysis performed on the thickness of both biogenic and terrigenous layers shows that there are strong 2-3 year and 16 year cycles ar...

A variable Holocene climate has been shown for most areas on globe. In the Atlantic region ocean ... more A variable Holocene climate has been shown for most areas on globe. In the Atlantic region ocean circulation is one key factor in these climate fluctuations but high-resolution records are lacking due to low sedimentation rates in the marine environments. Here we present a biomarker study of peat and lake sediments from Nightingale Island, situated in the Tristan da Cunha Islands in the central South Atlantic. The sequence has previously been analyzed for its pollen, elemental carbon and nitrogen content, and chronologically constrained by a large number of radiocarbon dates. These results have revealed a variable hydrology, with periods of increased erosion rates and precipitation, throughout the Holocene. In the present study we focus on changes in lipid biomarker distributions, and use them as proxies for past climate and environmental changes. The variations of chain lengths of n-alkanes reflect the change of vegetation in the watershed or growing in the lake while soil bacteria...

Geochmica et Cosmochimica Acta

Approximately 8.47 ± 0.3 ky ago, ~163,000 km3 of freshwater was released from glacial Lakes Agass... more Approximately 8.47 ± 0.3 ky ago, ~163,000 km3 of freshwater was released from glacial Lakes Agassiz and Ojibway into the North Atlantic, triggering sudden and widespread cooling in the North Atlantic region. Temperatures decreased by 1.5 to 3°C in Europe and North America and, further afield, the hydrological cycle in the Northern Hemisphere tropics weakened considerably. Marine sediments and climate model simulations suggest that this climatic anomaly termed the "8.2 ky event" was triggered by a slowdown of the meridional overturning circulation (MOC) by ~50% in response to a meltwater-induced freshening of the North Atlantic. The meltwater pulse (MWP) responsible for the 8.2 ky event is the final one in a series of at least 14 similar events documented for the early Holocene, but the possible climatic impacts of these smaller outbursts are not well documented. Based on data from our own speleothem records from southern Arabia and an ensemble of recently published and rev...

Journal of Paleolimnology

We analyzed Holocene sedimentary records from two lakes in the Lofoten Islands, northern Norway t... more We analyzed Holocene sedimentary records from two lakes in the Lofoten Islands, northern Norway to evaluate environmental changes during the Holocene related to northern North Atlantic climate dynamics. The lakes are located in different geomorphological settings, and thus provide a contrast in their response to regional climate change. Environmental changes at both lakes were interpreted based on magnetic susceptibility, organic-matter flux, C/N, δ13Corg, Ti concentrations, and mass accumulation rates. Chronologies were established using 16 AMS radiocarbon dates, and average deposition rates in both environments are higher than 0.2 mm/year throughout the Holocene. At Vikjordvatnet, sedimentary geochemical properties define three distinct phases of sedimentation related to changes in aquatic productivity and gradual landscape development. Following deglaciation, during the early Holocene (11.6–7.2 ka), aquatic productivity increased and the landscape stabilized as regional temperatu...

Journal of Climate

The northeastern United States is one of the most variable climates in the world, and how climate... more The northeastern United States is one of the most variable climates in the world, and how climate extremes are changing is critical to populations, industries, and the environment in this region. A long-term (1870-2005) temperature and precipitation dataset was compiled for the northeastern United States to assess how the climate has changed. Adjustments were made to daily temperatures to account for changes in mean, variance, and skewness resulting from inhomogeneities, but precipitation data were not adjusted. Trends in 17 temperature and 10 precipitation indices at 40 stations were evaluated over three time periods-1893-2005, 1893-1950, and 1951-2005 and over 1870-2005 for a subset of longer-term stations. Temperature indices indicate strong warming with increases in the frequency of warm events (e.g., warm nights and warm summer days) and decreases in the frequency of cold events (e.g., ice days, frost days, and the cold spell duration indicator). The strongest warming is exhibi...

Quaternary Research

Seasonality of precipitation is an important yet elusive climate parameter in paleoclimatological... more Seasonality of precipitation is an important yet elusive climate parameter in paleoclimatological reconstructions. This parameter can be inferred qualitatively from pollen and other paleoecological methods, but is difficult to assess quantitatively. Here, we have assessed seasonality of precipitation and summer surface wetness using compound specific hydrogen and carbon isotope ratios of vascular plant leaf waxes and Sphagnum biomarkers extracted from the sediments of an ombrotrophic peatland, Bøstad Bog, Nordland, Norway. Our reconstructed precipitation seasonality and surface wetness are consistent with regional vegetation reconstructions. During the early Holocene, 11.5–7.5 ka, Fennoscandia experienced a cool, moist climate. The middle Holocene, 7.5–5.5 ka, was warm and dry, transitioning towards cooler and wetter conditions from the mid-Holocene to the present. Changes in seasonality of precipitation during the Holocene show significant coherence with changes in sea surface temp...

Boreas

Pilcher, J., Bradley, R. S., Francus, P. & Anderson, L. 2005 (May): A Holocene tephra record from... more Pilcher, J., Bradley, R. S., Francus, P. & Anderson, L. 2005 (May): A Holocene tephra record from the Lofoten Islands, Arctic Norway. Boreas, Vol. 34, pp. 136–156. Oslo. ISSN 0300–9483.A tephrochronology has been established for a peat bog in the Lofoten Islands that provides a dating framework for future lake and bog studies of climate variation in this climatically sensitive area. Twenty-three tephra layers were identified, all apparently of Icelandic origin. These included the historically dated tephras of AD 1875 (Askja), AD 1362 (Öraefajökull), AD 1158 (Hekla), AD 1104 (Hekla) and the Landnam tephra identified at AD 875 in the GRIP ice core. Other layers, previously radiocarbon dated in Ireland and elsewhere, include the Hekla eruptions of c. 2310 BC and c. 5990 BC. The basal clays below the peat contain tephra of both the Askja eruption of c. 9500 BC (10 000 radiocarbon years BP) and the well-known Vedde Ash of c. 12 000 BP (10 030 80 BC in GRIP ice core).

1] We derive an optimal Northern Hemisphere mean surface temperature reconstruction from terrestr... more 1] We derive an optimal Northern Hemisphere mean surface temperature reconstruction from terrestrial borehole temperature profiles spanning the past five centuries. The pattern of borehole ground surface temperature (GST) reconstructions displays prominent discrepancies with instrumental surface air temperature (SAT) estimates during the 20th century, suggesting the presence of a considerable amount of noise and/or bias in any underlying spatial SAT signal. The vast majority of variance in the borehole dataset is efficiently retained by its two leading eigenvectors. A sizable share of the variance in the first eigenvector appears to be associated with non-SAT related bias in the borehole data. A weak but detectable SAT signal appears to be described by a combination of the first two eigenvectors. Exploiting this eigendecomposition, application of optimal signal estimation methods yields a hemispheric borehole SAT reconstruction that is largely consistent with instrumental data available in past centuries, and is indistinguishable in it smajor features from several published long-term temperature estimates based on both climate proxy data and model simulations. Optimal surface temperature reconstructions using terrestrial borehole data,

Journal of Climatology, 1981

Developments in Paleoenvironmental Research, 2005

Results are presented from a set experiments designed to control for the various factors that may... more Results are presented from a set experiments designed to control for the various factors that may influence reconstructions of large-scale temperature patterns in past centuries, including (a) the choice actual proxy data used, (b) the reconstruction methodology, (c) the spatial domain of the reconstruction and (d) the seasonal window targeted. These experiments compare results based both on the global multiproxy

Advances in Global Change Research, 2003

Linear trend analysis of observational data combined with model diagnostics from an atmospheric g... more Linear trend analysis of observational data combined with model diagnostics from an atmospheric general circulation model are employed to search for potential mechanisms related to the observed glacier retreat in the tropical Andes between 1950 and 1998. Observational evidence indicates that changes in precipitation amount or cloud cover over the last decades are minor in most regions and are therefore rather unlikely to have caused the observed retreat. The only exception is in southern Peru and western Bolivia where there is a general tendency toward slightly drier conditions. Near-surface temperature on the other hand has increased significantly throughout most of the tropical Andes. The temperature increase varies markedly between the eastern and western Andean slopes with a much larger temperature increase to the west. Simulations with the ECHAM-4 model, forced with observed global sea surface temperatures (SST) realistically reproduce the observed warming trend as well as the spatial trend pattern. Model results further suggest that a significant fraction of the observed warming can be traced to a concurrent rise in SST in the equatorial Pacific and that the markedly different trends in cloud cover to the east and west of the Andes contributed to the weaker warming east of the Andes in the model. The observed increase in relative humidity, derived from CRU 05 data, is also apparent in the model simulations, but on a regional scale the results between model and observations vary significantly. It is argued that changes in temperature and humidity are the primary cause for the observed glacier retreat during the 2nd half of the 20th century in the tropical Andes.

Economic Geology, 1992

The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold min... more The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold mineralization is hosted by a thin (0.5-2.0 km) but regionally extensive sequence of basalt and andesite, interbedded graywacke and argillite, and andesitic-lamprophyric intru-sions. ...