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What's New on the Paleo Web Pages for 2011


Villars Cave, France Global speleothem oxygen isotope measurements since the Last Glacial Maximum
Shah et al.
Oxygen isotope measurements from cave deposits provide some of the highest-resolution and best-dated information about past fluctuations in temperature and precipitation. Over the past decade, a relatively dense network of sites has been measured spanning the time period from the Last Glacial Maximum to present. These sites yield data that address key scientific questions surrounding climate sensitivity to greenhouse gas concentrations, non-linear responses and thresholds in the climate system, and the skill of state-of-the-art climate models in reproducing states different from the present one. This compilation of speleothem oxygen isotope records includes quality-controlled values from sixty cores spanning part or all of the last deglaciation and Holocene, provided on a common age scale (calendar years before present, where present = 1950 A.D.) and with common measurement units (per mil PDB).
Tree Ring and Snow Course Sites Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum
Schmittner et al.
Science

Vol. 334, No. 6061, pp. 1385-1388, 9 December 2011.

Assessing impacts of future anthropogenic carbon emissions is currently impeded by uncertainties in our knowledge of equilibrium climate sensitivity to atmospheric carbon dioxide doubling. Previous studies suggest 3 K as best estimate, 2-4.5 K as the 66% probability range, and non-zero probabilities for much higher values, the latter implying a small but significant chance of high-impact climate changes that would be difficult to avoid. Here, combining extensive sea and land surface temperature reconstructions from the Last Glacial Maximum with climate model simulations, we estimate a lower median (2.3 K) and reduced uncertainty (1.7-2.6 K 66% probability). Assuming paleoclimatic constraints apply to the future as predicted by our model, these results imply lower probability of imminent extreme climatic change than previously thought.
Tree Ring and Snow Course Sites The unusual nature of recent snowpack declines in the North American Cordillera
Pederson et al.
Science

Vol. 333, no. 6040, pp. 332-335, 15 July 2011

In western North America snowpack has declined in recent decades, and further losses are projected through the 21st century. Here we evaluate the uniqueness of recent declines using snowpack reconstructions from 66 tree-ring chronologies in key runoff generating areas of the Colorado, Columbia and Missouri River drainages. Over the past millennium, late-20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains, and in their north-south synchrony across the cordillera. Both the snowpack declines and their synchrony result from unparalleled springtime warming due to positive reinforcement of the anthropogenic warming by decadal variability. The increasing role of warming on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western USA.
Fig. 1C. Spatial correlation of NADA PC1 with global JFM SSTs during 1870-2002. Interdecadal modulation of El Niño amplitude during the past millennium
Li et al.
Nature Climate Change

Vol. 1, Issue 2, pp. 114-118, May 2011
doi:10.1038/nclimate1086

The El Niño/Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on Earth, alternating between anomalously warm (El Niño) and cold (La Niña) conditions in the tropical Pacific at intervals of 2-8 years. The amplitude of ENSO variability affects the occurrence and predictability of climate extremes around the world, but our ability to detect and predict changes in ENSO amplitude is limited by the fact that the instrumental record is too short to characterize its natural variability. Here we use the North American Drought Atlas - a database of drought reconstructions based on tree-ring records - to produce a continuous, annually resolved record of ENSO variability over the past 1,100 years. Our record is in broad agreement with independent, ENSO-sensitive proxy records in the Pacific and surrounding regions. Together, these records indicate that ENSO amplitude exhibits a quasi-regular cycle of 50-90 years that is closely coupled to the tropical Pacific mean state. Anomalously warm conditions in the eastern Pacific are associated with enhanced ENSO variability, consistent with model simulations. The quasi-periodic ENSO amplitude modulation reported here offers a key observational constraint for improving models and their prediction of ENSO behaviour linked to global warming.
PR Challenge pseudo-proxy Site Map Paleoclimate Reconstruction Challenge
Graham et al.
The last two millennia Paleoclimate Reconstruction (PR) Challenge is designed to engage the scientific PR community in examining its methods in a common framework for the purpose of evaluating their relative strengths and weaknesses. NOAA Paleoclimatology will distribute pseudoproxy data sets that researchers can use to create reconstructions, and NOAA will also distribute the contributed reconstructions so that they can be cross-compared. A key portion of the design of the Challenge is to allow true "apples to apples" comparison of methods across identical experimental platforms. The ultimate goal is to improve last two millennia PR methods so that paleoclimate science can offer the best possible information to help understand both natural and anthropogenic climate change.
Valles Caldera. US NPS image Extended megadroughts in the southwestern United States during Pleistocene interglacials
Fawcett et al.
Nature

Vol. 470, pp. 518-521, 24 February 2011
DOI: 10.1038/nature09839

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, 'dust-bowl-like' megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeo- temperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2°C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4-6°C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.
Fig. 1. Tree Ring Sites and Forest Cover 2500 Years of European Climate Variability and Human Susceptibility
Büntgen et al.
Science

Vol. 331, pp. 578-583, 4 February 2011
DOI: 10.1126/science.1197175

Climate variations influenced the agricultural productivity, health risk, and conflict level of preindustrial societies. Discrimination between environmental and anthropogenic impacts on past civilizations, however, remains difficult because of the paucity of high-resolution paleoclimatic evidence. We present tree ring-based reconstructions of central European summer precipitation and temperature variability over the past 2500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration. Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~250 to 600 C.E. coincided with the demise of the western Roman Empire and the turmoil of the Migration Period. Such historical data may provide a basis for counteracting the recent political and fiscal reluctance to mitigate projected climate change.


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