Logan E. Mitchell

Onset of deglacial warming in West Antarctica driven by local orbital forcing

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

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.

Carbon and hydrogen isotopic composition of methane over the last 1000 years

the common era (CE)). The d 13 Co f CH4 data corroborate the record from Law Dome, Antarctica, with high fidelity. The new d Do f CH4 data set covaries with the d 13 Co f CH4 record. Both d 13 Co f CH4 and d Do f CH4 were relatively stable and close to the present-day values from � 1000 to � 1500 CE. Both isotopic ratios decreased to minima around 1700 CE, remained low until the late 18th century, and then rose exponentially to present-day values. Our new d Do f CH4 data provide an additional independent constraint for evaluating possible CH4 source histories. We searched a broad range of source scenarios using a simple box model to identify histories consistent with the constraints of the CH4 concentration and isotope data from 990–1730 CE. Results typically show a decrease over time in the biomass-burning source (found in 85% of acceptable scenarios) and an increase in the agricultural source (found in 77% of acceptable scenarios), indicating preindustrial human influence on atmospheric methane as proposed in previous studies.

Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget

Bipolar Signature Atmospheric methane has increased approximately 2.5-fold since the start of the industrial revolution, a consequence of human activity. However, a smaller and more gradual rise began around 6000 years ago, near the time when human agriculture began to develop and expand. Mitchell et al. (p. 964) present two, high-resolution ice core methane records of the past 2500 years, one from each pole. Methane emissions were primarily from the tropics, with secondary contributions from the higher latitudes where most humans lived. Thus, both natural and human sources are needed to explain the late-Holocene atmospheric methane record. Records derived from polar ice cores provide constraints on methane emissions during the late preindustrial Holocene. The origin of the late preindustrial Holocene (LPIH) increase in atmospheric methane concentrations has been much debated. Hypotheses invoking changes in solely anthropogenic sources or solely natural sources have been proposed to explain the increase in concentrations. Here two high-resolution, high-precision ice core methane concentration records from Greenland and Antarctica are presented and are used to construct a high-resolution record of the methane inter-polar difference (IPD). The IPD record constrains the latitudinal distribution of emissions and shows that LPIH emissions increased primarily in the tropics, with secondary increases in the subtropical Northern Hemisphere. Anthropogenic and natural sources have different latitudinal characteristics, which are exploited to demonstrate that both anthropogenic and natural sources are needed to explain LPIH changes in methane concentration.

Observing and modeling the influence of layering on bubble trapping in polar firn

Interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn. Much work has been done to understand the processes affecting air transport in the open pores of the firn, but a paucity of data from air trapped in bubbles in the firn-ice transition region has limited the ability to constrain the effect of bubble closure processes. Here we present high-resolution measurements of firn density, methane concentrations, nitrogen isotopes, and total air content that show layering in the firn-ice transition region at the West Antarctic Ice Sheet (WAIS) Divide ice core site. Using the notion that bubble trapping is a stochastic process, we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach. We include the process of bubble trapping into an open-porosity firn air transport model and obtain a good fit to the firn core data. We find that layering broadens the depth range over which bubbles are trapped, widens the modeled gas age distribution of air in closed bubbles, reduces the mean gas age of air in closed bubbles, and introduces stratigraphic irregularities in the gas age scale that have a peak-to-peak variability of ~10 years at WAIS Divide. For a more complete understanding of gas occlusion and its impact on ice core records, we suggest that this experiment be repeated at sites climatically different from WAIS Divide, for example, on the East Antarctic plateau.

Minimal geological methane emissions during the Younger Dryas–Preboreal abrupt warming event

Methane (CH4) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions) and top-down approaches (measuring atmospheric mole fractions and isotopes) for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane (14CH4) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today, our results indicate that current estimates of today’s natural geological methane emissions (about 52 teragrams per year) are too high and, by extension, that current estimates of anthropogenic fossil methane emissions are too low. Our results also improve on and confirm earlier findings that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas–Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates, permafrost and methane trapped under ice) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas–Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur.

Ice stratigraphy at the Pakitsoq ice margin, West Greenland, derived from gas records

Horizontal ice-core sites, where ancient ice is exposed at the glacier surface, offer unique opportunities for paleo-studies of trace components requiring large sample volumes. Following previous work at the Pakitsoq ice margin in West Greenland, we use a combination of geochemical parameters measured in the ice matrix (d 18 Oice) and air occlusions (d 18 Oatm, d 15 No f N 2 and methane concentration) to date ice layers from specific climatic intervals. The data presented here expand our understanding of the stratigraphy and three-dimensional structure of ice layers outcropping at Pakitsoq. Sections containing ice from every distinct climatic interval during Termination I, including Last Glacial Maximum, Bolling/Allerod, Younger Dryas and the early Holocene, are identified. In the early Holocene, we find evidence for climatic fluctuations similar to signals found in deep ice cores from Greenland. A second glacial-interglacial transition exposed at the extreme margin of the ice is identified as another outcrop of Termination I (rather than the onset of the Eemian interglacial as postulated in earlier work). Consequently, the main structural feature at Pakitsoq is a large-scale anticline with accordion-type folding in both exposed sequences of the glacial-Holocene transition, leading to multiple layer duplications and age reversals.