An ice–climate oscillatory framework for Dansgaard–Oeschger cycles

Intermediate glacial states were characterized by large temperature changes in Greenland and the North Atlantic, referred to as Dansgaard–Oeschger (D–O) variability, with some transitions occurring over a few decades. D–O variability included changes in the strength of the Atlantic meridional overturning circulation (AMOC), temperature changes of opposite sign and asynchronous timing in each hemisphere, shifts in the mean position of the Intertropical Convergence Zone and variations in atmospheric CO2. Palaeorecords and numerical studies indicate that the AMOC, with a tight coupling to Nordic Seas sea ice, is central to D–O variability, yet, a complete theory remains elusive. In this Review, we synthesize the climatic expression and processes proposed to explain D–O cyclicity. What emerges is an oscillatory framework of the AMOC–sea-ice system, arising through feedbacks involving the atmosphere, cryosphere and the Earth’s biogeochemical system. Palaeoclimate observations indicate that the AMOC might be more sensitive to perturbations than climate models currently suggest. Tighter constraints on AMOC stability are, thus, needed to project AMOC changes over the coming century as a response to anthropogenic carbon emissions. Progress can be achieved by additional observational constraints and numerical simulations performed with coupled climate–ice-sheet models

An Atlantic-Pacific ventilation seesaw across the last deglaciation

It has been proposed that the rapid rise of atmospheric CO2across the last deglaciation was driven by the release of carbon from an extremely radiocarbon-depleted abyssal ocean reservoir that was ‘vented’ to the atmosphere primarily via the deep-and intermediate overturning loops in the Southern Ocean. While some radiocarbon observations from the intermediate ocean appear to confirm this hypothesis, others appear to refute it. Here we use radiocarbon measurements in paired benthic-and planktonic foraminifera to reconstruct the benthic–planktonic14C age offset (i.e. ‘ventilation age’) of intermediate waters in the western equatorial Atlantic. Our results show clear increases in local radiocarbon-based ventilation ages during Heinrich-Stadial 1 (HS1) and the Younger Dryas (YD). These are found to coincide with opposite changes of similar magnitude observed in the Pacific, demonstrating a ‘seesaw’ in the ventilation of the intermediate Atlantic and Pacific Oceans that numerical model simulations of North Atlantic overturning collapse indicate was primarily driven by North Pacific overturning. We propose that this Atlantic–Pacific ventilation seesaw would have combined with a previously identified North Atlantic–Southern Ocean ventilation seesaw to enhance ocean–atmosphere CO2exchange during a ‘collapse’ of the North Atlantic deep overturning limb. Whereas previous work has emphasized a more passive role for intermediate waters in deglacial climate change (merely conveying changes originating in the Southern Ocean) we suggest instead that the intermediate water seesaw played a more active role via relatively subtle but globally coordinated changes in ocean dynamics that may have further influenced ocean–atmosphere carbon exchange.We are grateful to Adam Scrivner for technical assistance in the laboratory, as well as the Royal Society and NERC grant NE/L006421/1 for research support. The UVic ESCM numerical ex-periments were performed on a computational cluster from the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme sup-ported by the Australian Government. A.T. and T.F. acknowledge support from the US NSF grants 1341311, 1400914. L.M. is sup-ported by the Australian Research Council grant DE150100107.This is the final version. It was first published by Elsevier at http://www.sciencedirect.com/science/article/pii/S0012821X15003301

Fast and slow components of interstadial warming in the North Atlantic during the last glacial

AbstractThe abrupt nature of warming events recorded in Greenland ice-cores during the last glacial has generated much debate over their underlying mechanisms. Here, we present joint marine and terrestrial analyses from the Portuguese Margin, showing a succession of cold stadials and warm interstadials over the interval 35–57 ka. Heinrich stadials 4 and 5 contain considerable structure, with a short transitional phase leading to an interval of maximum cooling and aridity, followed by slowly increasing sea-surface temperatures and moisture availability. A climate model experiment reproduces the changes in western Iberia during the final part of Heinrich stadial 4 as a result of the gradual recovery of the Atlantic meridional overturning circulation. What emerges is that Greenland ice-core records do not provide a unique template for warming events, which involved the operation of both fast and slow components of the coupled atmosphere–ocean–sea-ice system, producing adjustments over a range of timescales.</jats:p

Enhanced climate instability in the North Atlantic and southern Europe during the Last Interglacial.

Considerable ambiguity remains over the extent and nature of millennial/centennial-scale climate instability during the Last Interglacial (LIG). Here we analyse marine and terrestrial proxies from a deep-sea sediment sequence on the Portuguese Margin and combine results with an intensively dated Italian speleothem record and climate-model experiments. The strongest expression of climate variability occurred during the transitions into and out of the LIG. Our records also document a series of multi-centennial intra-interglacial arid events in southern Europe, coherent with cold water-mass expansions in the North Atlantic. The spatial and temporal fingerprints of these changes indicate a reorganization of ocean surface circulation, consistent with low-intensity disruptions of the Atlantic meridional overturning circulation (AMOC). The amplitude of this LIG variability is greater than that observed in Holocene records. Episodic Greenland ice melt and runoff as a result of excess warmth may have contributed to AMOC weakening and increased climate instability throughout the LIG

Lower oceanic 13C during the last interglacial period compared to the Holocene

The last time in Earth s history when high latitudes were warmer than during pre-industrial times was the last interglacial period (LIG, 129-116 ka BP). Since the LIG is the most recent and best documented interglacial, it can provide insights into climate processes in a warmer world. However, some key features of the LIG are not well constrained, notably the oceanic circulation and the global carbon cycle. Here, we use a new database of LIG benthic 13C to investigate these two aspects. We find that the oceanic mean 13C was 0.2 lower during the LIG (here defined as 125-120 ka BP) when compared to the Holocene (7-2 ka BP). A lower terrestrial carbon content at the LIG than during the Holocene could have led to both lower oceanic 13C and atmospheric 13CO2 as observed in paleo-records. However, given the multi-millennial timescale, the lower oceanic 13C most likely reflects a long-Term imbalance between weathering and burial of carbon. The 13C distribution in the Atlantic Ocean suggests no significant difference in the latitudinal and depth extent of North Atlantic Deep Water (NADW) between the LIG and the Holocene. Furthermore, the data suggest that the multi-millennial mean NADW transport was similar between these two time periods

Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard-Oeschger events

The glacial climate system transitioned rapidly between cold (stadial) and warm (interstadial) conditions in the Northern Hemisphere. This variability, referred to as Dansgaard–Oeschger variability, is widely believed to arise from perturbations of the Atlantic Meridional Overturning Circulation. Evidence for such changes during the longer Heinrich stadials has been identified, but direct evidence for overturning circulation changes during Dansgaard–Oeschger events has proven elusive. Here we reconstruct bottom water [CO₃²⁻] variability from B/Ca ratios of benthic foraminifera and indicators of sedimentary dissolution, and use these reconstructions to infer the flow of northern-sourced deep water to the deep central sub-Antarctic Atlantic Ocean. We find that nearly every Dansgaard–Oeschger interstadial is accompanied by a rapid incursion of North Atlantic Deep Water into the deep South Atlantic. Based on these results and transient climate model simulations, we conclude that North Atlantic stadial–interstadial climate variability was associated with significant Atlantic overturning circulation changes that were rapidly transmitted across the Atlantic. However, by demonstrating the persistent role of Atlantic overturning circulation changes in past abrupt climate variability, our reconstructions of carbonate chemistry further indicate that the carbon cycle response to abrupt climate change was not a simple function of North Atlantic overturning.J.G. was funded by the Gates Cambridge Trust. L.C.S. would like to acknowledge NERC grant NE/J010545/1 and the Royal Society. S.M. was supported by ERC grant 2010-NEWLOG ADG-267931 HE. L.M. was supported by the Australian Research Council grant DE150100107. A.T. acknowledges support from the US NSF (grants 1400914, 1341311).This is the author accepted manuscript. The final version is available from NPG via http://dx.doi.org/10.1038/ngeo255

Mechanisms of millennial-scale atmospheric CO<inf>2</inf> change in numerical model simulations

Numerical models are important tools for understanding the processes and feedbacks in the Earth system, including those involving changes in atmospheric CO2 (CO2,atm) concentrations. Here, we compile 55 published model studies (consisting of 778 individual simulations) that assess the impact of six forcing mechanisms on millennial-scale CO2,atm variations: changes in freshwater supply to the North Atlantic and Southern Ocean, the strength and position of the southern-hemisphere westerlies, Antarctic sea ice extent, and aeolian dust fluxes. We generally find agreement on the direction of simulated CO2,atm change across simulations, but the amplitude of change is inconsistent, primarily due to the different complexities of the model representation of Earth system processes. When freshwater is added to the North Atlantic, a reduced Atlantic Meridional Overturning Circulation (AMOC) is generally accompanied by an increase in Southern Ocean- and Pacific overturning, reduced Antarctic sea ice extent, spatially varying export production, and changes in carbon storage in the Atlantic (rising), in other ocean basins (generally decreasing) and on land (more varied). Positive or negative CO2,atm changes are simulated during AMOC minima due to a spatially and temporally varying dominance of individual terrestrial and oceanic drivers (and compensating effects between them) across the different models. In contrast, AMOC recoveries are often accompanied by rising CO2,atm levels, which are mostly driven by ocean carbon release (albeit from different regions). The magnitude of simulated CO2,atm rise broadly scales with the duration of the AMOC perturbation (i.e., the stadial length). When freshwater is added to the Southern Ocean, reduced deep-ocean ventilation drives a CO2,atm drop via reduced carbon release from the Southern Ocean. Although the impacts of shifted southern-hemisphere westerlies are inconsistent across model simulations, their intensification raises CO2,atm via enhanced Southern Ocean Ekman pumping. Increased supply of aeolian dust to the ocean, and thus iron fertilisation of marine productivity, consistently lowers modelled CO2,atm concentrations via more efficient nutrient utilisation. The magnitude of CO2,atm change in response to dust flux variations, however, largely depends on the complexity of models' marine ecosystem and iron cycle. This especially applies to simulations forced by Antarctic sea ice changes, in which the direction of simulated CO2,atm change varies greatly across model hierarchies. Our compilation highlights that no single (forcing) mechanism can explain observed past millennial-scale CO2,atm variability, and identifies important future needs in coupled carbon cycle-climate modelling to better understand the mechanisms governing CO2,atm changes in the past.Includes NERC funding

Estimated strength of the Atlantic overturning circulation during the last deglaciation

The Atlantic meridional overturning circulation affects the latitudinal distribution of heat, and is a key component of the climate system. Proxy reconstructions, based on sedimentary Pa-231/Th-230 ratios and the difference between surface-and deep-water radiocarbon ages, indicate that during the last glacial period, the overturning circulation was reduced during millennial-scale periods of cooling(1-5). However, much debate exists over the robustness of these proxies(6-8). Here e combine proxy reconstructions of sea surface and air temperatures and a global climate model to quantitatively estimate changes in the strength of the Atlantic meridional overturning circulation during the last glacial period. We find that, relative to the Last Glacial Maximum, the overturning circulation was reduced by approximately 14 Sv during the cold Heinrich event 1. During the Younger Dryas cold event, the overturning circulation was reduced by approximately 12 Sv, relative to the preceding warm interval. These changes are consistent with qualitative estimates of the overturning circulation from sedimentary Pa-231/Th-230 ratios. In addition, we find that the strength of the overturning circulation during the Last Glacial Maximum and the Holocene epoch are indistinguishable within the uncertainty of the reconstruction

Insolation-induced mid-Brunhes transition in Southern Ocean ventilation and deep-ocean temperature

Glacial–interglacial cycles characterized by long cold periods interrupted by short periods of warmth are the dominant feature of Pleistocene climate, with the relative intensity and duration of past and future interglacials being of particular interest for civilization. The interglacials after 430,000 years ago were characterized by warmer climates and higher atmospheric concentrations of carbon dioxide than the interglacials before, but the cause of this climatic transition (the so-called mid-Brunhes event (MBE)) is unknown. Here I show, on the basis of model simulations, that in response to insolation changes only, feedbacks between sea ice, temperature, evaporation and salinity caused vigorous pre-MBE Antarctic bottom water formation and Southern Ocean ventilation. My results also show that strong westerlies increased the pre-MBE overturning in the Southern Ocean via an increased latitudinal insolation gradient created by changes in eccentricity during austral winter and by changes in obliquity during austral summer. The stronger bottom water formation led to a cooler deep ocean during the older interglacials. These insolation-induced differences in the deep-sea temperature and in the Southern Ocean ventilation between the more recent interglacials and the older ones were not expected, because there is no straightforward systematic difference in the astronomical parameters between the interglacials before and after 430,000 years ago. Rather than being a real ‘event’, the apparent MBE seems to have resulted from a series of individual interglacial responses—including notable exceptions to the general pattern—to various combinations of insolation conditions. Consequently, assuming no anthropogenic interference, future interglacials may have pre- or post-MBE characteristics without there being a systematic change in forcings. These findings are a first step towards understanding the magnitude change of the interglacial carbon dioxide concentration around 430,000 years ago