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Fig. 1. d18O time series during penultimate glacial period for stalagmites A penultimate glacial monsoon record from Hulu Cave and two-phase glacial terminations
Cheng et al. Geology
Vol. 34, Issue 3, pp. 217 - 220, March 2006


Oxygen isotope records of three stalagmites from Hulu Cave, China, extend the previous high-resolution absolute-dated Hulu Asian Monsoon record from the last to the penultimate glacial and deglacial periods. The penultimate glacial monsoon broadly follows orbitally induced insolation variations and is punctuated by at least 16 millennial-scale events. We confirm a Weak Monsoon Interval between 135.5 ± 1.0 and 129.0 ± 1.0 ka, prior to the abrupt increase in monsoon intensity at Asian Monsoon Termination II.
Fig. 1. Idealized North Atlantic surface (red arrows) and deep water (blue arrow) circulation during MIS 3 stadial events Rapid subtropical North Atlantic salinity oscillations across Dansgaard-Oeschger cycles
Schmidt et al. Nature
Vol. 443, No. 7111, pp.561-564, doi:10.1038/nature05121, 5 Oct 2006.


We combine Mg/Ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four Dansgaard-Oeschger cycles (spanning 45.9-59.2 kyr ago) to generate a seawater salinity proxy record from a subtropical gyre deep-sea sediment core. We show that North Atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials, covarying with inferred shifts in the Tropical Atlantic hydrologic cycle and North Atlantic overturning circulation. We hypothesize that increased stadial salinities preconditioned the North Atlantic Ocean for a rapid return to deep overturning circulation and high-latitude warming by contributing to increased North Atlantic surface-water density on interstadial transitions.
Fig. 2A. Actual and reconstructed January-December mean tropical SST Two-hundred-fifty years of reconstructed and modeled tropical temperatures
Wilson et al. Journal of Geophysical Research - Oceans
Vol. 111, No. C10, C10007, doi:10.1029/2005JC003188, 14 October 2006.


Recent large-scale palaeoclimate reconstructions of past temperature have been essentially biased to the extratropics owing to a paucity of proxy data in tropical regions. Herein we describe the first coral-based reconstruction of sea surface temperatures (SSTs) for the whole of the tropics (30°N-30°S). It was developed from 14 disparate coral records located in the Indian and Pacific oceans. Agreement with simulations from two general circulation models indicates that the late 20th century is likely the warmest period in the tropics for the last 250 years, and that this recent warming can only be explained by anthropogenic forcing.
Fig. 5. Comparison of STD and RCS NH reconstructions with mean annual land (20N-90N) temperatures On the long-term context for late twentieth century warming
D'Arrigo et al. Journal of Geophysical Research - Atmospheres
Vol. 111, No. D3, D03103, doi:10.1029/2005JD006352, 07 February 2006.


Previous tree-ring-based Northern Hemisphere temperature reconstructions portray a varying amplitude range between the "Medieval Warm Period"(MWP), "Little Ice Age" (LIA) and present. We describe a new reconstruction, developed using largely different methodologies and additional new data compared to previous efforts. Unlike earlier studies, we quantify differences between more traditional (STD) and Regional Curve Standardization (RCS) methodologies, concluding that RCS is superior for retention of low-frequency trends. Continental North American versus Eurasian RCS series developed prior to merging to the hemispheric scale cohere surprisingly well, suggesting common forcing, although there are notable deviations (e.g., fifteenth to sixteenth century). Results indicate clear MWP (warm), LIA (cool), and recent (warm) episodes.
Fig. 1C. d13CH4 in Pakitsoq ice samples Ice Record of δ13C for Atmospheric CH4 Across the Younger Dryas-Preboreal Transition
Schaefer et al. Science
Vol. 313, Issue 5790, pp. 1109-1112, 10.1126/science.1126562, 25 August 2006.


We report atmospheric methane carbon isotope ratios (δ13CH4) from the Western Greenland ice margin spanning the Younger Dryas-to-Preboreal (YD-PB) transition. Over the recorded ~800 years, δ13CH4 was around -46 per mil; that is, ~1 per mil higher than in the modern atmosphere and ~5.5 per mil higher than would be expected from budgets without 13C-rich anthropogenic emissions. This requires higher natural 13C-rich emissions or stronger sink fractionation than conventionally assumed. Constant δ13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.
Figure 3B. Seasonal variations in insolation at 65N Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing
Huybers Science
Vol. 313, Issue 5786, pp. 508-511, 10.1126/science.1125249, 28 July 2006.


Long-term variations in Northern Hemisphere summer insolation are generally thought to control glaciation. But the intensity of summer insolation is primarily controlled by 20,000-year cycles in the precession of the equinoxes, whereas early Pleistocene glacial cycles occur at 40,000-year intervals, matching the period of changes in Earth's obliquity. The resolution of this 40,000-year problem is that glaciers are sensitive to insolation integrated over the duration of the summer. The integrated summer insolation is primarily controlled by obliquity and not precession because, by Kepler's second law, the duration of the summer is inversely proportional to Earth's distance from the Sun.
Figure 1. Northern Hemisphere mean annual temperature reconstructions Reconstructing hemispheric-scale climates from multiple stalagmite records
Smith et al. International Journal of Climatology
Vol. 26, Issue 10, pp. 1417 - 1424, Aug. 2006


The magnitude of recent warming, and the variability of climate on centennial- millennial time scales are compromised by questions concerning the ability of tree rings to capture low-frequency climate fluctuations. Annually laminated stalagmite records can potentially provide a low-frequency climate archive through variations in annual growth rate. Presented here is an initial attempt to demonstrate the applicability of annually laminated stalagmite series to a large-scale climate reconstruction, by producing a 500-year Northern Hemisphere temperature reconstruction. The reconstruction shows an overall warming trend with a magnitude of 0.65 K and several other low-frequency characteristics consistent with other independent Northern Hemisphere archives. The result is sufficiently encouraging to warrant significant future effort in characterising annual growth rate records from laminated speleothems.
CLIVAR diagram of Atlantic portion of ocean thermohaline circulation Surface and Deep Ocean Interactions During the Cold Climate Event 8200 Years Ago
Ellison et al. Science
Vol. 312, Issue 5782, pp. 1929-1932, 30 June 2006.


Evidence from a North Atlantic deep-sea sediment core reveals that the largest climatic perturbation in our present interglacial, the 8200-year event, is marked by two distinct cooling events in the subpolar North Atlantic at 8490 and 8290 years ago. An associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the Atlantic meridional overturning circulation. The existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying Laurentide Ice Sheet margin.
Figure 3. Winter temperature trends (deg. C per decade) from 1684 to 1738 European Seasonal and Annual Temperature Variability, Trends, and Extremes Since 1500
Luterbacher et al. Science
Vol. 303, Issue 5663, pp. 1499-1503, 5 March 2004.


Multiproxy reconstructions of monthly and seasonal surface temperature fields for Europe back to 1500 show that the late 20th- and early 21st-century European climate is very likely (>95% confidence level) warmer than that of any time during the past 500 years. This agrees with findings for the entire Northern Hemisphere. European winter average temperatures during the period 1500 to 1900 were reduced by ~0.5°C (0.25°C for annual mean temperatures) compared to the 20th century. Summer temperatures did not experience systematic century-scale cooling relative to present conditions. The coldest European winter was 1708/1709; 2003 was by far the hottest summer.
Figure 2. Difference map between the warmest and coldest multi-decadal springs European spring and autumn temperature variability and change of extremes over the last half millennium
Xoplaki et al. Geophysical Research Letters
Vol. 32, L15713, 13 August 2005.


We evaluate variability, trends, uncertainties, and change of extremes of reconstructed and observed European spring and autumn temperature back to 1500. Spring and autumn temperature experienced systematic century-scale cooling compared to present conditions. The coldest springs appeared during the Maunder Minimum (DT = -1 K wrt 1901-2000). The amplitude of spring temperature variations at decadal and multidecadal scales doubles that of autumn and is most expressed in northeastern Europe. The decade 1995-2004 was very likely the warmest of the last half millennium. Anomalously warm springs and autumns have generally become more extreme in recent decades. However, the recent changes are statistically not significant with respect to the pre-industrial period.
Satellite image of Lake Powell, Utah on the Colorado River above Lee's Ferry, Arizona. USGS Landsat Photo. Updated Streamflow Reconstructions for the Upper Colorado River Basin
Woodhouse et al. Water Resources Research
Vol. 42, W05415, 11 May 2006.


Updated proxy reconstructions of water year (October-September) streamflow for four key gauges in the Upper Colorado River Basin were generated using an expanded tree ring network and longer calibration records than in previous efforts. Reconstructed gauges include the Green River at Green River, Utah; Colorado near Cisco, Utah; San Juan near Bluff, Utah; and Colorado at Lees Ferry, Arizona. The reconstructions explain 72-81% of the variance in the gauge records, and results are robust across several reconstruction approaches. Time series plots as well as results of cross-spectral analysis indicate strong spatial coherence in runoff variations across the subbasins. The Lees Ferry reconstruction suggests a higher long-term mean than previous reconstructions but strongly supports earlier findings that Colorado River allocations were based on one of the wettest periods in the past 5 centuries and that droughts more severe than any 20th to 21st century event occurred in the past.
Figure 1. Paleoclimatic proxies from ODP Site 846. Evolution of the Eastern Tropical Pacific Through Plio-Pleistocene Glaciation
Lawrence et al. Science
Vol. 312, pp. 79-83, 7 April 2006.


A tropical Pacific climate state resembling that of a permanent El Nino is hypothesized to have ended as a result of a reorganization of the ocean heat budget ~3 million years ago, a time when large ice sheets appeared in the high latitudes of the Northern Hemisphere. We report a high resolution alkenone reconstruction of conditions in the heart of the eastern equatorial Pacific (EEP) cold tongue that reflects the combined influences of changes in the equatorial thermocline, the properties of the thermocline's source waters, atmospheric greenhouse gas content, and orbital variations on sea surface temperature (SST) and biological productivity over the past 5 million years.
Satellite image of Mongolia. NASA MODIS Photo. Extension of Drought Records for Central Asia Using Tree Rings: West-Central Mongolia
Davi et al. Journal of Climate
Vol. 19, pp. 288-299, 15 January 2006.


Central Asian drought has had drastic impacts on vast regions over recent years. Longer records and insight into temporal drought patterns could aid greatly in anticipating extreme events and agrarian planning. Mongolia is representative of the central Asian region, and tree-ring resources are used herein to extend the climate record and test for solar influence and/or Pacific Ocean teleconnections. Absolutely dated tree-ring-width chronologies from five sampling sites in west-central Mongolia were used in precipitation models and an individual model was made using the longest of the five tree-ring records (1340-2002). The tree-ring sites are in or near the Selenge River basin, the largest river in Mongolia and a major input into Lake Baikal in Siberia. Regression models resulted in a reconstruction of streamflow that extends from 1637 to 1997 and explains 49% of the flow variation.
Figure 1. Measured concentrations from the EPICA Dome C ice core. Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles
Wolff et al. Nature
Vol. 440, pp. 491-496, 23 March 2006.


Sea ice and dust flux increased greatly in the Southern Ocean during the last glacial period. Palaeorecords provide contradictory evidence about marine productivity in this region, but beyond one glacial cycle, data were sparse. Here we present continuous chemical proxy data spanning the last eight glacial cycles (740,000 years) from the Dome C Antarctic ice core. These data constrain winter sea-ice extent in the Indian Ocean, Southern Ocean biogenic productivity and Patagonian climatic conditions. We found that maximum sea-ice extent is closely tied to Antarctic temperature on multi-millennial timescales, but less so on shorter timescales. Biological dimethylsulphide emissions south of the polar front seem to have changed little with climate, suggesting that sulphur compounds were not active in climate regulation. We observe large glacial-interglacial contrasts in iron deposition, which we infer reflects strongly changing Patagonian conditions.
Portion of Figure 2. Fraction of records exceeding thresholds The Spatial Extent of 20th-Century Warmth in the Context of the Past 1200 Years
Osborn and Briffa Science
Vol. 311, Issue 5762, pp. 841 - 844, 10 February 2006.


Periods of widespread warmth or cold are identified by positive or negative deviations that are synchronous across a number of temperature-sensitive proxy records drawn from the Northern Hemisphere. The most significant and longest duration feature during the last 1200 years is the geographical extent of warmth in the middle to late 20th century. Positive anomalies during 890 to 1170 and negative anomalies during 1580 to 1850 are consistent with the concepts of a Medieval Warm Period and a Little Ice Age, but comparison with instrumental temperatures shows the spatial extent of recent warmth to be of greater significance than that during the medieval period.
Glacier National Park, Montana Long-Duration Drought Variability and Impacts on Ecosystem Services: A Case Study from Glacier National Park, Montana
Pederson et al. Earth Interactions
Volume 10, Paper 4, January 2006.


Instrumental climate records suggest that summer precipitation and winter snowpack in Glacier National Park (Glacier NP), Montana, vary significantly over decadal to multidecadal time scales. Because instrumental records for the region are limited to the twentieth century, knowledge of the range of variability associated with these moisture anomalies and their impacts on ecosystems and physical processes are limited. The authors developed a reconstruction of summer (June-August) moisture variability spanning A.D. 1540-2000 from a multispecies network of tree-ring chronologies in Glacier NP.


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