• Energy Research

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth's climate dynamics. For the last glacial period, ice core studies have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard-Oeschger events in Greenland through the Atlantic meridional overturning circulation. It has been unclear, however, whether the shorter Dansgaard-Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard-Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

SignificancePolar ice is a unique archive of past atmosphere. Here, we present methane stable isotope records (used as source fingerprint) for the current and two past interglacials and their preceding glacial maxima. Our data are used to constrain global emissions of methane. Tropical wetlands and floodplains seem to be the dominant sources of atmospheric methane changes, steered by past variations in sea level, monsoon intensity, temperature, and the water table. In contrast, geologic emissions of methane are stable over a wide range of climatic conditions. The long-term shift seen in both isotopes for the last 25,000 y compared with older intervals is likely connected to changes in the terrestrial biosphere and fire regimes as a consequence of megafauna extinction.

SignificanceEarth’s radiative imbalance determines whether energy is flowing into or out of the ocean–atmosphere system. The present, anthropogenic, positive imbalance drives global warming. This study reconstructs the radiative imbalance for the last deglaciation, ∼20,000 to 10,000 y ago. During the deglaciation, a positive imbalance was maintained for several thousand years, which brought the climate system from the last ice age into the Holocene warm period. We show that the imbalance varied significantly during this time, possibly due to changes in ocean circulation that affect the radiative energy fluxes, highlighting the importance of internal variability in Earth’s energy budget.

Understanding the causes of past atmospheric methane (CH4) variability is important for characterizing the relationship between CH4, global climate and terrestrial biogeochemical cycling. Ice core records of atmospheric CH4 contain rapid variations linked to abrupt climate changes of the last glacial period known as Dansgaard-Oeschger (DO) events and Heinrich events (HE)1,2. The drivers of these CH4 variations remain unknown but can be constrained with ice core measurements of the stable isotopic composition of atmospheric CH4, which is sensitive to the strength of different isotopically distinguishable emission categories (microbial, pyrogenic and geologic)3-5. Here we present multi-decadal-scale measurements of δ13C-CH4 and δD-CH4 from the WAIS Divide and Talos Dome ice cores and identify abrupt 1‰ enrichments in δ13C-CH4 synchronous with HE CH4 pulses and 0.5‰ δ13C-CH4 enrichments synchronous with DO CH4 increases. δD-CH4 varied little across the abrupt CH4 changes. Using box models to interpret these isotopic shifts6 and assuming a constant δ13C-CH4 of microbial emissions, we propose that abrupt shifts in tropical rainfall associated with HEs and DO events enhanced 13C-enriched pyrogenic CH4 emissions, and by extension global wildfire extent, by 90-150%. Carbon cycle box modelling experiments7 suggest that the resulting released terrestrial carbon could have caused from one-third to all of the abrupt CO2 increases associated with HEs. These findings suggest that fire regimes and the terrestrial carbon cycle varied contemporaneously and substantially with past abrupt climate changes of the last glacial period.

Atmospheric methane (CH4) records reconstructed from polar ice cores represent a integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here we present dual stable isotopic methane records (d13CH4 and dD(CH4)) from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation and the water table, as modulated by insolation, (local) sea level and monsoon intensity. Based on our new dD(CH4) constraint, it appears that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes, and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 years compared to older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly due to biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial. All methane isotope data are given as measured (d13C wrt VPDB, dD wrt VSMOW). All data are given corrected for gravitational settling in the firn. d13CH4 data are additionally given corrected for diffusional fractionation in the firn. All relevant information concerning data and corrections are given in the main article, the Materials and Methods section and in the Supporting Information (SI).All data are free of krypton interference during the mass spectrometric analysis. For details see Schmitt et al. (2014, doi:10.5194/amt-7-2645-2014).All data are given on a depth scale and the gas age according to AICC 2012: Veres et al. (2013, doi:10.5194/cp-9-1733-2013 and Bazin et al. (2013, doi:10.5194/cp-9-1715-2013). Supplement to: Bock, Michael; Schmitt, Jochen; Beck, Jonas; Seth, Barbara; Chappellaz, Jérôme A; Fischer, Hubertus (2017): Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records. Proceedings of the National Academy of Sciences, 201613883

AbstractThe European Project for Ice Coring in Antarctica Dome ice core from Dome C (EDC) has allowed for the reconstruction of atmospheric CO2 concentrations for the last 800,000 years. Here we revisit the oldest part of the EDC CO2 record using different air extraction methods and sections of the core. For our established cracker system, we found an analytical artifact, which increases over the deepest 200 m and reaches 10.1 ± 2.4 ppm in the oldest/deepest part. The governing mechanism is not yet fully understood, but it is related to insufficient gas extraction in combination with ice relaxation during storage and ice structure. The corrected record presented here resolves partly ‐ but not completely ‐ the issue with a different correlation between CO2 and Antarctic temperatures found in this oldest part of the records. In addition, we provide here an update of 800,000 years atmospheric CO2 history including recent studies covering the last glacial cycle.