The Greenland spatial fingerprint of Dansgaard-Oeschger events in observations and models

Pleistocene Ice Ages display abrupt Dansgaard-Oeschger (DO) climate oscillations that provide prime examples of Earth System tipping points-abrupt transition that may result in irreversible change. Greenland ice cores provide key records of DO climate variability, but gas-calibrated estimates of the temperature change magnitudes have been limited to central and northwest Greenland. Here, we present ice-core δ15N-N2 records from south (Dye 3) and coastal east Greenland (Renland) to calibrate the local water isotope thermometer and provide a Greenland-wide spatial characterization of DO event magnitude. We combine these data with existing records of δ18O, deuterium excess, and accumulation rates to create a multiproxy "fingerprint" of the DO impact on Greenland. Isotope-enabled climate models have skill in simulating the observational multiproxy DO event impact, and we use a series of idealized simulations with such models to identify regions of the North Atlantic that are critical in explaining DO variability. Our experiments imply that wintertime sea ice variation in the subpolar gyre, rather than the commonly invoked Nordic Seas, is both a sufficient and a necessary condition to explain the observed DO impacts in Greenland, whatever the distal cause. Moisture-tagging experiments support the idea that Greenland DO isotope signals may be explained almost entirely via changes in the vapor source distribution and that site temperature is not a main control on δ18O during DO transitions, contrary to the traditional interpretation. Our results provide a comprehensive, multiproxy, data-model synthesis of abrupt DO climate variability in Greenland.

Antarctic surface temperature and elevation during the Last Glacial Maximum

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

Greenland temperature response to climate forcing during the last deglaciation

Greenland ice core water isotopic composition (δ(18)O) provides detailed evidence for abrupt climate changes but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns. We investigate Greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean-atmosphere climate model. Contrary to the traditional δ(18)O interpretation, the Younger Dryas period was 4.5° ± 2°C warmer than the Oldest Dryas, due to increased carbon dioxide forcing and summer insolation. The magnitude of abrupt temperature changes is larger in central Greenland (9° to 14°C) than in the northwest (5° to 9°C), fingerprinting a North Atlantic origin. Simulated changes in temperature seasonality closely track changes in the Atlantic overturning strength and support the hypothesis that abrupt climate change is mostly a winter phenomenon.

Siple Dome ice reveals two modes of millennial CO2 change during the last ice age

Reconstruction of atmospheric CO₂ during times of past abrupt climate change may help us better understand climate-carbon cycle feedbacks. Previous ice core studies reveal simultaneous increases in atmospheric CO₂ and Antarctic temperature during times when Greenland and the northern hemisphere experienced very long, cold stadial conditions during the last ice age. Whether this relationship extends to all of the numerous stadial events in the Greenland ice core record has not been clear. Here we present a high-resolution record of atmospheric CO₂ from the Siple Dome ice core, Antarctica for part of the last ice age. We find that CO₂ does not significantly change during the short Greenlandic stadial events, implying that the climate system perturbation that produced the short stadials was not strong enough to substantially alter the carbon cycle.

Whether all rapid climate events during the last ice age impacted the global carbon cycle is not clearly understood. Ahn and Brook present a high-resolution record of atmospheric CO₂ from Antarctica and suggest that only Greenland stadials associated with massive iceberg discharge influenced atmospheric CO₂.

Constraining past global tropospheric methane budgets with carbon and hydrogen isotope ratios in ice

Upon closer inspection, the classical view of the synchronous relationship between tropospheric methane mixing ratio and Greenland temperature observed in ice samples reveals clearly discernable variations in the magnitude of this response during the Late Pleistocene (<50kyr BP). During the Holocene this relationship appears to decouple, indicating that other factors have modulated the methane budget in the past 10kyr BP. The delta13CH4 and deltaD-CH4 of tropospheric methane recorded in ice samples provide a useful constraint on the palaeomethane budget estimations. Anticipated changes in palaeoenvironmental conditions are recorded as changes in the isotope signals of the methane precursors, which are then translated into past global delta13CH4 and deltaD-CH4 signatures. We present the first methane budgets for the late glacial period that are constrained by dual stable isotopes. The overall isotope variations indicate that the Younger Dryas (YD) and Preindustrial Holocene have methane that is 13C- and 2H-enriched, relative to Modern. The shift is small for delta13CH4 (approx. 1 per thousand) but greater for deltaD-CH4 (approx. 9 per thousand). The YD delta13CH4-deltaD-CH4 record shows a remarkable relationship between them from 12.15 to 11.52kyr BP. The corresponding C- and H-isotope mass balances possibly indicate fluctuating emissions of thermogenic gas. This delta13CH4-deltaD-CH4 relationship breaks down during the YD-Preboreal transition. In both age cases, catastrophic releases of hydrates with Archaeal isotope signatures can be ruled out. Thermogenic clathrate releases are possible during the YD period, but so are conventional natural gas seepages.

Atmospheric methane and nitrous oxide of the Late Pleistocene from Antarctic ice cores

The European Project for Ice Coring in Antarctica Dome C ice core enables us to extend existing records of atmospheric methane (CH4) and nitrous oxide (N2O) back to 650,000 years before the present. A combined record of CH4 measured along the Dome C and the Vostok ice cores demonstrates, within the resolution of our measurements, that preindustrial concentrations over Antarctica have not exceeded 773 +/- 15 ppbv (parts per billion by volume) during the past 650,000 years. Before 420,000 years ago, when interglacials were cooler, maximum CH4 concentrations were only about 600 ppbv, similar to lower Holocene values. In contrast, the N2O record shows maximum concentrations of 278 +/- 7 ppbv, slightly higher than early Holocene values.

On-line systems for continuous water and gas isotope ratio measurements

New continuous on-line techniques for water and air extracted from ice cores are developed. Water isotope ratio determination on any of the water phases (water vapour, water, ice) is of great relevance in different research fields, such as climate and paleoclimate studies, geological surveys, and hydrological studies. The conventional techniques for water isotopes are available in different layouts but all of them are rather time-consuming. Here we report new fast on-line techniques that process water as well as ice samples. The analysis time is only approximately 5 min per sample which includes equilibration and processing. Measurement precision and accuracy are better than 0.1 per thousand and 1 per thousand for delta18O and deltaD, respectively, comparable to conventional techniques. The new on-line techniques are able to analyze a wide range of aqueous samples. This allows, for the first time, to make continuous isotope measurements on ice cores. Similarly, continuous and fast analysis of aqueous samples can be of great value for hydrological, geological and perhaps medical applications.Furthermore, a new technique for the on-line analysis of air isotopes extracted from ice cores is developed. This technique allows rapid analyses with high resolution of the main air components nitrogen, oxygen, and argon. Measurement precision is comparable to precisions obtained by conventional techniques. It is now possible to measure delta15N and delta18O(atm) over entire ice cores helping to synchronize chronologies, to assess gas age-ice age differences, and to calibrate the paleothermometry for rapid temperature changes. This new on-line air extraction and analyzing technique complements the water methods in an ideal way as it separates the air from the melt-water of an ice sample. The remaining water waste flux can directly be analyzed by the water methods.

High-resolution record of Northern Hemisphere climate extending into the last interglacial period

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 degrees C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

Strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation

The climate of the last glacial period was extremely variable, characterized by abrupt warming events in the Northern Hemisphere, accompanied by slower temperature changes in Antarctica and variations of global sea level. It is generally accepted that this millennial-scale climate variability was caused by abrupt changes in the ocean thermohaline circulation. Here we use a coupled ocean-atmosphere-sea ice model to show that freshwater discharge into the North Atlantic Ocean, in addition to a reduction of the thermohaline circulation, has a direct effect on Southern Ocean temperature. The related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical North Atlantic caused by a fast wave-adjustment process. We present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of Greenland and Antarctic temperature changes, the slow changes in Antarctic temperature and its similarity to sea level, as well as a possible time lag of sea level with respect to Antarctic temperature during Marine Isotope Stage 3.

Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period

A precise relative chronology for Greenland and West Antarctic paleotemperature is extended to 90,000 years ago, based on correlation of atmospheric methane records from the Greenland Ice Sheet Project 2 and Byrd ice cores. Over this period, the onset of seven major millennial-scale warmings in Antarctica preceded the onset of Greenland warmings by 1500 to 3000 years. In general, Antarctic temperatures increased gradually while Greenland temperatures were decreasing or constant, and the termination of Antarctic warming was apparently coincident with the onset of rapid warming in Greenland. This pattern provides further evidence for the operation of a "bipolar see-saw" in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales.

The global methane cycle: isotopes and mixing ratios, sources and sinks

A review of the global cycle of methane is presented with emphasis on its isotopic composition. The history of methane mixing ratios, reconstructed from measurements of air trapped in ice-cores is described. The methane record now extends back to 420 kyr ago in the case of the Vostok ice cores from Antarctica. The trends in mixing ratios and in delta13C values are reported for the two Hemispheres. The increase of the atmospheric methane concentration over the past 200 years, and by 1% per year since 1978, reaching 1.7 ppmv in 1990 is underlined. The various methane sources are presented. Indeed the authors describe the methane emissions by bacterial activity under anaerobic conditions in wet environments (wetlands, bogs, tundra, rice paddies), in ruminant stomachs and termite guts, and that originating from fossil carbon sources, such as biomass burning, coal mining, industrial losses, automobile exhaust, sea floor vent, and volcanic emissions. Furthermore, the main sinks of methane in the troposphere, soils or waters via oxidation are also reported, and the corresponding kinetic isotope effects.

16 degrees C rapid temperature variation in central greenland 70,000 years Ago

Variations in the (29)N(2)/(28)N(2) ratio of air bubbles trapped in polar ice cores and their relation to variations of the (18)O/(16)O of the ice allow past surface temperature variations and ice age-gas age differences to be determined. High-resolution measurements of (29)N(2)/(28)N(2) in Dansgaard-Oeschger event 19 (around 70,000 years before the present) in ice from Central Greenland show that at the beginning of the event, the ice age-gas age difference was 1090 +/- 100 years. With the use of a combined firn densification, temperature, and gas diffusion model, the delta(18)O(ice)-temperature coefficient alpha was determined to be 0. 42 +/- 0.05 per mil per kelvin. This coefficient implies a mean surface temperature change of 16.0 kelvin (between 14.3 and 18.1 kelvin), which differs substantially from values derived from borehole temperatures and modern spatial delta(18)O(ice)-surface temperature correlations.

Abrupt climate change at the end of the last glacial period inferred from trapped air in polar Ice

The last glacial period was terminated by an abrupt warming event in the North Atlantic approximately 15,000 years before the present, and warming events of similar age have been reported from low latitudes. Understanding the mechanism of this termination requires that the precise relative timing of abrupt climate warming in the tropics versus the North Atlantic be known. Nitrogen and argon isotopes in trapped air in Greenland ice show that the Greenland Summit warmed 9 +/- 3 degrees C over a period of several decades, beginning 14,672 years ago. Atmospheric methane concentrations rose abruptly over a approximately 50-year period and began their increase 20 to 30 years after the onset of the abrupt Greenland warming. These data suggest that tropical climate became warmer or wetter (or both) approximately 20 to 80 years after the onset of Greenland warming, supporting a North Atlantic rather than a tropical trigger for the climate event.

Variations in atmospheric N2O concentration during abrupt climatic changes

Nitrous oxide (N2O) is an important greenhouse gas that is presently increasing at a rate of 0.25 percent per year. Records measured along two ice cores from Summit in Central Greenland provide information about variations in atmospheric N2O concentration in the past. The record covering the past millennium reduces the uncertainty regarding the preindustrial concentration. Records covering the last glacial-interglacial transition and a fast climatic change during the last ice age show that the N2O concentration changed in parallel with fast temperature variations in the Northern Hemisphere. This provides important information about the response of the environment to global climatic changes.

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.