What's New on the Paleo Web Pages for 2007
Southern Hemisphere and Deep-Sea Warming Led Deglacial Atmospheric
CO2 Rise and Tropical Warming
Stott et al. Science
Vol. 318. No. 5849, pp. 435 - 438, 19 October 2007, doi:10.1126/science.1143791.
Establishing what caused Earth's largest climatic changes in the past requires a precise knowledge of both the forcing and the regional responses. We determined the chronology of high- and low-latitude climate change at the last glacial termination by radiocarbon dating benthic and planktonic foraminiferal stable isotope and magnesium/calcium records from a marine core collected in the western tropical Pacific. Deep-sea temperatures warmed by ~2°C between 19 and 17 thousand years before the present (ky B.P.), leading the rise in atmospheric CO2 and tropical-surface-ocean warming by ~1000 years. The cause of this deglacial deep-water warming does not lie within the tropics, nor can its early onset between 19 and 17 ky B.P. be attributed to CO2 forcing. Increasing austral-spring insolation combined with sea-ice albedo feedbacks appear to be the key factors responsible for this warming.
Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years.
Jouzel et al. Science
Vol. 317, No. 5839, pp.793-797, 10 August 2007.
A high-resolution deuterium profile is now available along the entire European Project for Ice Coring in Antarctica Dome C ice core, extending this climate record back to marine isotope stage 20.2, ~800,000 years ago. Experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation. We assessed the general correspondence between Dansgaard-Oeschger events and their smoothed Antarctic counterparts for this Dome C record, which reveals the presence of such features with similar amplitudes during previous glacial periods. We suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records.
Millennial-scale trends in west Pacific warm pool
hydrology since the Last Glacial Maximum
Partin et al. Nature
Vol. 449, No. 7161, pp. 452-455, 27 September 2007, doi:10.1038/nature06164.
Here we present three absolutely-dated oxygen isotope records from stalagmites in northern Borneo that reflect changes in west Pacific warm pool hydrology over the past 27,000 years. Convective activity, as inferred from oxygen isotopes, reached a minimum during Heinrich event 1, when the Atlantic meridional overturning circulation was weak, pointing to feedbacks between the strength of the overturning circulation and tropical Pacific hydrology. During the Holocene epoch, convective activity appears to track changes in spring and autumn insolation, highlighting the sensitivity of tropical Pacific convection to external radiative forcing. Together, these findings demonstrate that the tropical Pacific hydrological cycle is sensitive to high-latitude climate processes in both hemispheres, as well as to external radiative forcing, and that it may have a central role in abrupt climate change events.
A matter of divergence: Tracking recent warming at hemispheric scales
using tree ring data
Wilson et al. Journal of Geophysical Research - Atmospheres
Vol. 112, D17103, 11 September 2007. doi:10.1029/2006JD008318.
No current tree ring (TR) based reconstruction of extratropical Northern Hemisphere (ENH) temperatures that extends into the 1990s captures the full range of late 20th century warming observed in the instrumental record. Over recent decades, a divergence between cooler reconstructed and warmer instrumental large-scale temperatures is observed. In this study, we compiled TR data and published local/regional reconstructions that show no divergence against local temperatures. Utilizing this data set, we developed a new, completely independent reconstruction of ENH annual temperatures (1750-2000). This record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that TR data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale.
Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years
Kawamura et al. Nature
Vol. 448, No. 7156, pp. 912-917, 23 August 2007. doi:10.1038/nature06015.
Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. The accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation. Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations.
Intense hurricane activity over the past 5,000 years controlled by
El Niño and the West African monsoon
Donnelly and Woodruff Nature
Vol. 447, Number 7143, pp. 465-468, 24 May 2007. doi:10.1038/nature05834.
The processes that control the formation, intensity and track of hurricanes are poorly understood. It has been proposed that an increase in sea surface temperatures caused by anthropogenic climate change has led to an increase in the frequency of intense tropical cyclones. Here we present a record of intense hurricane activity in the western North Atlantic Ocean over the past 5,000 years based on sediment cores from a Caribbean lagoon that contain coarse-grained deposits associated with intense hurricane landfalls. Comparison of the sediment record with palaeo-climate records indicates that this variability was probably modulated by atmospheric dynamics associated with variations in the El Niño/Southern Oscillation and the strength of the West African monsoon, and suggests that sea surface temperatures as high as at present are not necessary to support intervals of frequent intense hurricanes.
155,000 Years of West African Monsoon and Ocean Thermal Evolution
Weldeab et al. Science
Vol. 316, pp. 1303-1307, 1 June 2007, doi:10.1126/science.1140461.
A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial- scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.
Low Atlantic hurricane activity in the 1970s and
1980s compared to the past 270 years
Nyberg et al. Nature
Vol. 447, Number 7145, pp. 698-702, 7 June 2007
Hurricane activity in the North Atlantic Ocean has increased significantly since 1995. This trend has been attributed to both anthropogenically induced climate change and natural variability, but the primary cause remains uncertain. Here we construct a record of the frequency of major Atlantic hurricanes over the past 270 years using proxy records of vertical wind shear and sea surface temperature (the main controls on the formation of major hurricanes in this region) from corals and a marine sediment core. The record indicates that the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s.
Five hundred years of gridded high-resolution precipitation
reconstructions over Europe and the connection to large-scale
Pauling et al. Climate Dynamics
Volume 26, No. 4, pp. 387-405, March 2006
We present seasonal precipitation reconstructions for European land areas (30°W to 40°E/30-71°N; given on a 0.5° x 0.5° resolved grid) covering the period 1500-1900 together with gridded reanalysis from 1901 to 2000 (Mitchell and Jones 2005). A large variety of long instrumental precipitation series, precipitation indices based on documentary evidence and natural proxies (tree-ring chronologies, ice cores, corals and a speleothem) that are sensitive to precipitation signals were used as predictors. Precipitation variability over the last half millennium reveals both large interannual and decadal fluctuations. Using scaled composite analysis, we show that precipitation extremes over central Europe and southern Spain are linked to distinct pressure patterns.
Medieval drought in the upper Colorado River Basin
Meko et al. Geophysical Research Letters
Vol. 34, L10705, 24 May 2007.
New tree-ring records of ring-width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low-frequency feature of the new reconstruction, covering A.D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades.
Climate and hydrographic variability in the Indo-Pacific Warm Pool
during the last millennium
Newton et al. Geophysical Research Letters
Vol. 33, L19710, 12 October 2006, doi:10.1029/2006GL027234.
Planktonic foraminiferal Mg/Ca and δ18O derived sea surface temperature and salinity records from the Makassar Strait, Indonesia, show a long- term cooling and freshening trend, as well as considerable centennial- scale variability during the last millennium. The warmest temperatures and highest salinities occurred during the Medieval Warm Period (MWP), while the coolest temperatures and lowest salinities occurred during the Little Ice Age (LIA). These changes in the western Pacific, along with observations from other high resolution records indicate a regionally coherent southern displacement of the Inter-tropical Convergence Zone during the LIA, with more arid conditions in the northern tropics and wetter conditions in the southern tropics.
Tree-ring carbon isotope data and drought maps
for the U.S. Southwest
Leavitt et al. EOS
Volume 88, Number 4, pp. 39-40, 23 January 2007.
Analysis of short-term fluctuations in δ13C in tree rings indicates they are related to climate, notably the moisture status of the plant. The mechanism for this relationship is related to low moisture promoting stomatal closure (reduced stomatal conductance), reduced discrimination against δ13C in leaf photosynthesis, and therefore higher δ13C values of the resulting photosynthate (and tree rings). This study utilizes 14 pinyon pine sites collected across 6 states in the American Southwest to develop Drought Indices using δ13C analysis.
Influence of the intertropical convergence zone on the East Asian monsoon.
Yancheva et al. Nature
Vol 445, pp. 74-77, 4 January 2007, doi:10.1038/nature05431.
The Asian-Australian monsoon is an important component of the Earth's climate system that influences the societal and economic activity of roughly half the world's population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high- resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds.
Atlas of Pollen-Vegetation-Climate Relationships
for the United States and Canada
Williams et al.
American Association of Stratigraphic Palynologists
Contribution Series No. 43 , December 2006
This atlas displays the distribution of modern pollen abundances in North America, both geographically and relative to various climatic, bioclimatic, and vegetational variables. It is intended to aid analyses of modern pollen-environment relationships as well as the climatic and ecological interpretations of fossil pollen diagrams. The atlas is based upon a recent compilation of pollen samples from surface sediments and polsters that includes 4549 samples, 134 pollen types, and independent climatic and vegetational attributes for all locations. Atlas pages are presented for 106 pollen types, representing all of the major and many minor pollen types found in North America.
Millennial-scale temperature variations in North America during the Holocene
Viau et al. Journal of Geophysical Research
Vol. 111, D09102, doi:10.1029/2005JD006031, May 2006.
A mean continental July temperature reconstruction based on pollen records from across North America quantifies temperature variations of several timescales for the past 14,000 cal yr BP. In North America, temperatures increased nearly 4°C during the late glacial, reaching maximum values between 6000 and 3000 cal yr BP, after which mean July temperatures decreased. These results provide important insight to the global warming debate, as the observed twentieth century temperature increase appears unprecedented compared to our mean North American temperature reconstruction of the past 14,000 years.
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