Climate Change During the Last Deglaciation in Antarctica

Greenland ice core records provide clear evidence of rapid changes in climate in a variety of climate indicators. In this work, rapid climate change events in the Northern and Southern hemispheres are compared on the basis of an examination of changes in atmospheric circulation developed from two ice cores. High-resolution glaciochemical series, covering the period 10,000 to 16,000 years ago, from a central Greenland ice core and a new site in east Antarctica display similar variability. These findings suggest that rapid climate change events occur more frequently in Antarctica than previously demonstrated.     

GRIP deuterium excess reveals rapid and orbital-scale changes in Greenland moisture origin

The Northern Hemisphere hydrological cycle is a key factor coupling ice sheets, ocean circulation, and polar amplification of climate change. Here we present a Northern Hemisphere deuterium excess profile covering one climatic cycle, constructed with the use of delta18O and deltaD Greenland Ice Core Project (GRIP) records. Past changes in Greenland source and site temperatures are quantified with precipitation seasonality taken into account. The imprint of obliquity is evidenced in the site-to-source temperature gradient at orbital scale. At the millennial time scale, GRIP source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events, and these rapid shifts are associated with large-scale changes in atmospheric circulation.     

Indian Ocean climate and an absolute chronology over Dansgaard/Oeschger events 9 to 13

Oxygen-isotope ratios of a stalagmite from Socotra Island in the Indian Ocean provide a record of changes in monsoon precipitation and climate for the time period from 42 to 55 thousand years before the present. The pattern of precipitation bears a striking resemblance to the oxygen-isotope record from Greenland ice cores, with increased tropical precipitation associated with warm periods in the high northern latitudes. The largest change, at the onset of interstadial 12, occurred very rapidly, in about 25 years. The chronology of the events found in our record requires a reevaluation of previously published time scales for climate events during this period.     

The atmosphere during the younger dryas

One of the most dramatic climate change events observed in marine and ice core records is the Younger Dryas, a return to near-glacial conditions that punctuated the last deglaciation. High-resolution, continuous glaciochemical records, newly retrieved from central Greenland, record the chemical composition of the arctic atmosphere at this time. This record shows that both the onset and the termination of the Younger Dryas occurred within 10 to 20 years and that massive, frequent, and short-term (decadal or less) changes in atmospheric composition occurred throughout this event. Changes in atmospheric composition are attributable to changes in the size of the polar atmospheric cell and resultant changes in source regions and to the growth and decay of continental biogenic source regions.     

Rapid Variations in Atmospheric Methane Concentration During the Past 110,000 Years

A methane record from the GISP2 ice core reveals that millennial-scale variations in atmospheric methane concentration characterized much of the past 110,00 years. As previously observed in a shorter record from central Greenland, abrupt concentration shifts of about 50 to 300 parts per billion by volume were coeval with most of the interstadial warming events (better known as Dansgaard-Oeschger events) recorded in the GISP2 ice core throughout the last glacial period. The magnitude of the rapid concentration shifts varied on a longer time scale in a manner consistent with variations in Northern Hemisphere summer insolation, which suggests that insolation may have modulated the effects of interstadial climate change on the terrestrial biosphere.     

800,000 years of abrupt climate variability

We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.     

Synchronous tropical South China Sea SST change and Greenland warming during deglaciation

The tropical ocean plays a major role in global climate. It is therefore crucial to establish the precise phase between tropical and high-latitude climate variability during past abrupt climate events in order to gain insight into the mechanisms of global climate change. Here we present alkenone sea surface temperature (SST) records from the tropical South China Sea that show an abrupt temperature increase of at least 1 degrees C at the end of the last glacial period. Within the recognized dating uncertainties, this SST increase is synchronous with the B?lling warming observed at 14.6 thousand years ago in the Greenland Ice Sheet Project 2 ice core.     

Synchronous climate changes in antarctica and the north atlantic

Central Greenland ice cores provide evidence of abrupt changes in climate over the past 100,000 years. Many of these changes have also been identified in sedimentary and geochemical signatures in deep-sea sediment cores from the North Atlantic, confirming the link between millennial-scale climate variability and ocean thermohaline circulation. It is shown here that two of the most prominent North Atlantic events-the rapid warming that marks the end of the last glacial period and the Bolling/Allerod-Younger Dryas oscillation-are also recorded in an ice core from Taylor Dome, in the western Ross Sea sector of Antarctica. This result contrasts with evidence from ice cores in other regions of Antarctica, which show an asynchronous response between the Northern and Southern Hemispheres.     

Temporal relationships of carbon cycling and ocean circulation at glacial boundaries

Evidence from high-sedimentation-rate South Atlantic deep-sea cores indicates that global and Southern Ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice-sheet growth and retreat, respectively. No consistent lead-lag relationships are observed during abrupt millennial warming events during the last ice age, allowing for the possibility that ocean circulation triggered some millenial climate changes. At the major glacial-interglacial transitions, the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets.     

Rapid changes in the hydrologic cycle of the tropical Atlantic during the last glacial

Sedimentary time series of color reflectance and major element chemistry from the anoxic Cariaco Basin off the coast of northern Venezuela record large and abrupt shifts in the hydrologic cycle of the tropical Atlantic during the past 90,000 years. Marine productivity maxima and increased precipitation and riverine discharge from northern South America are closely linked to interstadial (warm) climate events of marine isotope stage 3, as recorded in Greenland ice cores. Increased precipitation at this latitude during interstadials suggests the potential for greater moisture export from the Atlantic to Pacific, which could have affected the salinity balance of the Atlantic and increased thermohaline heat transport to high northern latitudes. This supports the notion that tropical feedbacks played an important role in modulating global climate during the last glacial period.     

Ocean circulation: its effects on seasonal sea-ice simulations

A diagnostic ice-ocean model of the Arctic, Greenland, and Norwegian seas is constructed and used to examine the role of ocean circulation in seasonal sea-ice simulations. The model includes lateral ice motion and three-dimensional ocean circulation. The ocean portion of the model is weakly forced by observed temperature and salinity data. Simulation results show that including modeled ocean circulation in seasonal sea-ice simulations substantially improves the predicted ice drift and ice margin location. Simulations that do not include lateral ocean movment predict a much less realistic ice edge.     

340,000-year centennial-scale marine record of Southern Hemisphere climatic oscillation

In order to investigate rapid climatic changes at mid-southern latitudes, we have developed centennial-scale paleoceanographic records from the southwest Pacific that enable detailed comparison with Antarctic ice core records. These records suggest close coupling of mid-southern latitudes with Antarctic climate during deglacial and interglacial periods. Glacial sections display higher variability than is seen in Antarctic ice cores, which implies climatic decoupling between mid- and high southern latitudes due to enhanced circum-Antarctic circulation. Structural and temporal similarity with the Greenland ice core record is evident in glacial sections and suggests a degree of interhemispheric synchroneity not predicted from bipolar ice core correlations.     

Glacial-interglacial changes in moisture sources for greenland: influences on the ice core record of climate

Large, abrupt shifts in the (l8)O/(16)O ratio found in Greenland ice must reflect real features of the climate system variability. These isotopic shifts can be viewed as a result of air temperature fluctuations, but determination of the cause of the changes-the most crucial issue for future climate concerns-requires a detailed understanding of the controls on isotopes in precipitation. Results from general circulation model experiments suggest that the sources of Greenland precipitation varied with different climate states, allowing dynamic atmospheric mechanisms for influencing the ice core isotope shifts.     

Recent sea-level contributions of the Antarctic and Greenland ice sheets

After a century of polar exploration, the past decade of satellite measurements has painted an altogether new picture of how Earth's ice sheets are changing. As global temperatures have risen, so have rates of snowfall, ice melting, and glacier flow. Although the balance between these opposing processes has varied considerably on a regional scale, data show that Antarctica and Greenland are each losing mass overall. Our best estimate of their combined imbalance is about 125 gigatons per year of ice, enough to raise sea level by 0.35 millimeters per year. This is only a modest contribution to the present rate of sea-level rise of 3.0 millimeters per year. However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers, which has accelerated over the past decade. In both continents, there are suspected triggers for the accelerated ice discharge-surface and ocean warming, respectively-and, over the course of the 21st century, these processes could rapidly counteract the snowfall gains predicted by present coupled climate models.     

The influence of vegetation-atmosphere-ocean interaction on climate during the mid-holocene

Simulations with a synchronously coupled atmosphere-ocean-vegetation model show that changes in vegetation cover during the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly (through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the atmosphere-vegetation feedback is most important. Moreover, a reduction of the thermohaline circulation in the Atlantic Ocean leads to a warming of the Southern Hemisphere.     

Mixed-layer deepening during Heinrich events: a multi-planktonic foraminiferal delta18O approach

Proxies from Greenland ice cores and North Atlantic marine sediment cores document repeated extreme climate swings of a few decades to millennia during the last glacial cycle, including periods of intense ice rafting called Heinrich events (HEs). We have found similar oxygen isotope variations recorded in mixed-layer-and thermocline-dwelling planktonic foraminifera during HEs 0, 1, and 4, suggesting that three foraminiferal taxa calcified their shells at similar temperatures in a homogenized upperwater column. This implies that the surface mixed layer was deeper during HEs. Similar deepening occurred on the northern margin of the ice-rafted-debris belt, implying that these deep mixed layers during HEs were widespread in the region. We suggest that an increase in storminess during HEs intensified the vertical mixing of meltwater from ice rafting in the upper ocean.     

Ice-sheet and sea-level changes

Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level approximately 70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered.     

Mesoscale oceanographic processes beneath the ice of fram strait

A major component of the Fram Strait Marginal Ice Zone Experiment was the investigation of air-sea-ice interactions, processes, and circulation patterns found behind the local ice edge and on scales greater than 10 kilometers (mesoscale and large scale). Neutrally buoyant floats, ice-tethered cyclesondes, and helicopter-based measurements were used to obtain uniquely integrated and consistent views of the mesoscale ocean features beneath the ice cover of Fram Strait. Within the vicinity of the Yermak Plateau, three distinct regions of mesoscale motion were observed that coincided with the shallow topography of the plateau, the northward flowing Atlantic water over the western flank of the plateau, and the strong current-shear zone of the East Greenland Polar Front. A subice meander of the front was also observed, which was probably occluded subsequently.     

Aerial photographs reveal late-20th-century dynamic ice loss in northwestern Greenland

Global warming is predicted to have a profound impact on the Greenland Ice Sheet and its contribution to global sea-level rise. Recent mass loss in the northwest of Greenland has been substantial. Using aerial photographs, we produced digital elevation models and extended the time record of recent observed marginal dynamic thinning back to the mid-1980s. We reveal two independent dynamic ice loss events on the northwestern Greenland Ice Sheet margin: from 1985 to 1993 and 2005 to 2010, which were separated by limited mass changes. Our results suggest that the ice mass changes in this sector were primarily caused by short-lived dynamic ice loss events rather than changes in the surface mass balance. This finding challenges predictions about the future response of the Greenland Ice Sheet to increasing global temperatures.     

Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise

Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.