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Once in a While Many regions at the edge of the Antarctic Ice Sheet have rapidly increased the rates at which they are sliding into the sea and thinning, raising concerns that global warming might cause the sudden collapse of some sections. Johnson et al. (p. 999 , published online 20 February) present data from Pine Island Glacier, which has been thinning and retreating rapidly over the past two decades. The glacier experienced another rapid thinning around 8000 years ago, which occurred about as quickly as is happening now, and which lasted for 25 to 100 years.

The progress in the method of cosmogenic nuclide surface exposure dating has drastically accelerated over the last years, in part fueled by the National Science Foundation (NSF)-funded Cosmic-Ray Produced Nuclide Systematics on Earth (CRONUS-Earth) initiative. This progress has affected the sensitivity of the method, its overall precision and accuracy, and its robustness, and opened up new scientific fields in the Earth sciences and beyond. This article attempts to highlight some of the most recent and important areas of progress to give an overview of the state of the method and to outline the next line of progress. While the technique is clearly surging ahead, cosmogenic nuclide surface exposure dating is far from reaching its limits.

Glacierized change in the Himalayas affects river-discharge, hydro-energy and agricultural production, and Glacial Lake Outburst Flood potential, but its quantification and extent of impacts remains highly uncertain. Here we present conservative, comprehensive and quantitative predictions for glacier area and meltwater flux changes in Bhutan, monsoonal Himalayas. In particular, we quantify the uncertainties associated with the glacier area and meltwater flux changes due to uncertainty in climate data, a critical problem for much of High Asia. Based on a suite of gridded climate data and a robust glacier melt model, our results show that glacier area and meltwater change projections can vary by an order of magnitude for different climate datasets. However, the most conservative results indicate that, even if climate were to remain at the present-day mean values, almost 10% of Bhutan s glacierized area would vanish and the meltwater flux would drop by as much as 30%. Under the conservative scenario of an additional 1 C regional warming, glacier retreat is going to continue until about 25% of Bhutan s glacierized area will have disappeared and the annual meltwater flux, after an initial spike, would drop by as much as 65%. Citation

Mountain glaciers worldwide have undergone net recession over the past century in response to atmospheric warming(1), but the extent to which this warming reflects natural versus anthropogenic climate change remains uncertain(2,3). Between about 11,500 years ago and the nineteenth century, progressive atmospheric cooling over the European Alps induced glacier expansion(2,4-6), culminating with several large-scale advances during the seventeen to nineteenth centuries(3). However, it is unclear whether this glacier behaviour reflects global or a more regional forcing. Here we reconstruct glacier fluctuations in the Southern Alps of New Zealand for the past 11,000 years using Be-10 exposure ages. We use those fluctuations to estimate the associated temperature variations. On orbital to submillennial timescales, changes in glacier snowlines in New Zealand were linked to regional climate and oceanographic variability and were asynchronous with snowline variations in European glaciers. We attribute this asynchrony to the migration of the intertropical convergence zone. In light of this persistent asynchrony, we suggest that the net glacier recession and atmospheric warming in both regions over the past century is anomalous in the context of earlier Holocene variability and corresponds with anthropogenic emissions of greenhouse gases.

International audience; The natural variability of Holocene climate defines the baseline to assess ongoing climate change. Greenland ice-core records indicate warming superimposed by abrupt climate oscillations in the early Holocene, followed by a general cooling trend throughout the middle and late Holocene that culminated during the Little Ice Age (LIA). Tropical precipitation changes correlate with these patterns throughout the Holocene. Here we use mountain glaciers in the European Alps to reconstruct the regional Holocene climate evolution and to test for a link between mid-latitude, North Atlantic, and tropical climate. Our precise 10Be chronology from Tsidjiore Nouve Glacier, western Swiss Alps, indicates a glacier culmination during the earliest Holocene ∼11.4 k.y. ago, likely related to the Preboreal Oscillation. Based on our data, no Holocene glacier advance of similar amplitude occurred until ∼3.8 k.y. ago, when the glacier reached LIA limits. The 10Be ages between 500 and 170 yr correspond to the LIA, while the youngest 10Be ages overlap with the historically recorded post-LIA glacier positions. Integrating our data with existing records, we propose a hemispheric climate link between the Alps, North Atlantic temperature, and tropical precipitation patterns for the Holocene, supporting the concept of a pervasive climate driver. These findings from northern mid-latitudes are consistent with the hypothesis formulated for the tropics that the Earth’s thermal equator, responding to North Atlantic temperature changes, might have migrated southward throughout the Holocene, reaching the southern turning point toward the end of the LIA.

of the region. A novel application of 10 Be surface exposure dating is presented to determine the timing of phreatomagmatic eruptions at Ubehebe Crater and to test the idea that volcanism may relate to a wetter than present hydro-climate. Twelve of the fifteen ages obtained lie between 0.8 and 2.1 ka, while three samples give older, mid-Holocene ages. The cluster between 0.8 and 2.1 ka is interpreted as encompassing the intervalof volcanicactivityduringwhich Ubehebe Crater was formed. The remaining older ages are inferred to date eruptions at the older neighboring craters. The main and most recent period of activity encompasses the Medieval Warm Period, an interval of prolonged drought in the American southwest, as well as slightly wetter conditions prior to the Medieval Warm Period. Phreatomagmatic activity under varied hydrologic conditions casts doubt on the idea that eruptive timing relates to a wetter hydro-climate. Instead, the presence of a relatively shallow modern water table suggests that sufficient groundwater was generally available for phreatomagmatic eruptions at the Ubehebe site, in spite of prevailing arid conditions. This and the youth of the most recent activity suggest that the Ubehebe volcanic field may constitute a more significant hazard than generally appreciated. Citation: Sasnett, P., B. M. Goehring, N. Christie-Blick, and J. M. Schaefer (2012), Do phreatomagmatic eruptions at Ubehebe Crater (Death Valley, California) relate to a wetter than present hydro-climate?, Geophys. Res. Lett., 39, L02401

In situ cosmogenic 14 C( in situ 14 C) analysis from quartz-bearing rocks is a novel isotopic tool useful for quantifying recent surface exposure histories (up to ! 25ka). It is particularly powerful when combined with longer- lived cosmogenic isotopes such as 10 Be. Recent advances in the extraction of in situ 14 C from quartz now permit the routine application of this method. However, only a few experiments to calibrate the production rate of in situ 14 C in quartz have been published to date. Here, we present a new in situ 14 C production rate estimate derived from a well- dated debris flow deposit in the Southern Alps, New Zealand, previously used to calibrate 10 Be production rates. For example, based on a geomagnetic implementation of the Lal/Stone scaling scheme we derive a spallogenic production rate of 11.4" 0.9 atoms 14 C (g quartz) # 1 a # 1 and a 14 C/ 10 Be spallogenic production rate ratio of 3.0" 0.2. The results are comparable with production rates from previous calibrations in the northern hemisphere. Copyright # 2012 John Wiley & Sons, Ltd.

Warming Up For the past half-million years, our planet has passed through a cycle of glaciation and deglaciation every 100,000 years or so. Each of these cycles consists of a long and irregular period of cooling and ice sheet growth, followed by a termination—a period of rapid warming and ice sheet decay—that precedes a relatively short warm interval. But what causes glacial terminations? Denton et al. (p. 1652 ) review the field and propose a chain of events that may explain the hows and whys of Earth's emergence from the last glacial period. Pulling together many threads from both hemispheres suggests a unified causal chain involving ice sheet volume, solar radiation energy, atmospheric carbon dioxide concentrations, sea ice, and prevailing wind patterns.

The now acknowledged thinning of the Greenland Ice Sheet raises concerns about its potential contribution to future sea level rise. In order to appreciate the full extent of its contribution to sea level rise, reconstruction of the ice sheet's most recent last deglaciation could provide key information on the timing and the height of the ice sheet at a time of rapid climate readjustment. We measured 10Be concentrations in 12 samples collected along longitudinal and altitudinal transects from Sisimiut to within 10 km of the Isunguata Sermia Glacier ice margin on the western coast of Greenland. Along the longitudinal transect, we collected three perched boulders and two bedrocks. In addition, we sampled seven perched boulders along a vertical transect in a valley within 10 km of the Isunguata Sermia Glacier ice margin. Our pilot dataset constrains the height of the ice sheet during the Last Glacial Maximum (LGM) between 500 m and 840 m (including the 120 m relative sea level depression at the time of the LGM, 21 ka BP). From the transect we estimate the thinning of the ice sheet at the end of the deglaciation between 12.3 ± 1.5 10Be ka (n = 2) and 8.3 ± 1.2 10Be ka (n = 3) to be ∼6 cm a−1 over this time period. Direct dating of the retreat of the western margin of the Greenland Ice Sheet has the potential to better constrain the retreat rate of the ice margin, the thickness of the former ice sheet as well as its response to climate change. Copyright © 2008 John Wiley & Sons, Ltd.

Isotopic records from polar ice cores imply globally asynchronous warming at the end of the last glaciation. However, 10 Be exposure dates show that large-scale retreat of mid-latitude Last Glacial Maximum glaciers commenced at about the same time in both hemispheres. The timing of retreat is consistent with the onset of temperature and atmospheric CO 2 increases in Antarctic ice cores. We suggest that a global trend of rising summer temperatures at the end of the Last Glacial Maximum was obscured in North Atlantic regions by hypercold winters associated with unusually extensive winter sea ice.

(1) Calculation of the sea level high latitude production rate based on the samples from the Herdla-Halsnoy Younger Dryas end moraine required accounting for the isostatic rebound of the site following deglaciation (Lohne et al., 2007). In the process of calculating exposure ages of samples from nearby sites, it was realized that the original calibration of the production rates from Halsnoy were calculated using a correction based on the relative sea level history (70m maximum) plus the eustatic sea level history (50m at maximum), i.e. that the amount of isostatic depression had been over corrected. Sea level high latitude Be production rates based on the correct relative sea level history are presented below in a corrected version of Table 2. (2) In the original publication, the ‘Combined’ production rates were determined using a chi-squared minimization based on the data from both calibration sites. However, because the two studied sites have significantly different ages (Halsnoy 11650 100 and Oldedalen 6070 111), and therefore integrate production over different lengths of time, it is inappropriate to determine a sea level high latitude production rate for the entire data set, using the chi-squared minimization. Instead, the arithmetic mean of the production rates from the two sites is the more appropriate average production rate. We present in the corrected Table 2 below both the updated production rate values for the samples from Halsnoy and average production rates using the arithmetic mean and standard deviation of the production rates from the two sites. We apologize for any inconvenience caused. References

The retreat of the Laurentide Ice Sheet at the end of the last ice age represents a dramatic sequence of environmental change that is evident in geological records across North America. The details of when, where, how, and why the Laurentide Ice Sheet collapsed are critical to understanding ocean-atmosphere-ice dynamics during this period of significant global climate change. The retreat sequence along the eastern margin of the Laurentide Ice Sheet is especially relevant for understanding climate variability. Enormous amounts of freshwater were released into the North Atlantic following deglaciation, and an influx of freshwater into the North Atlantic Deep Water formation zone can potentially trigger abrupt climate changes. Determining the mechanisms and feedbacks involved in climate change at the end of the last ice age therefore requires an understanding of the relationship between the southern margin ice retreat and connected meltwater events to atmospheric and sea surface temperatures, ice-rafting Heinrich events, sea level rise, and atmospheric greenhouse gas concentrations.

Meltwater drainage from glacial Lake Agassiz has been implicated for nearly 15 years as a trigger for thermohaline circulation changes producing the abrupt cold period known as the Younger Dryas. On the basis of initial field reconnaissance to the lake's proposed outlets, regional geomorphic mapping, and preliminary chronological data, an alternative hypothesis may be warranted. Should ongoing data collection continue to support preliminary results, it could be concluded that Lake Agassiz did not flood catastrophically into the Lake Superior basin preceding the Younger Dryas (Figure 1). All preliminary findings imply a retreating ice sheet margin approximately 1000 years younger than previously thought, which would have blocked key meltwater corridors at the start of the Younger Dryas.