Millennial scale feedbacks determine the shape and rapidity of glacial termination

Within the Late Pleistocene, terminations describe the major transitions marking the end of glacial cycles. While it is established that abrupt shifts in the ocean/atmosphere system are a ubiquitous component of deglaciation, significant uncertainties remain concerning their specific role and the likelihood that terminations may be interrupted by large-amplitude abrupt oscillations. In this perspective we address these uncertainties in the light of recent developments in the understanding of glacial terminations as the ultimate interaction between millennial and orbital timescale variability. Innovations in numerical climate simulation and new geologic records allow us to highlight new avenues of research and identify key remaining uncertainties such as sea-level variability

A paleo-perspective on the AMOC as a tipping element

Ocean circulation within the Atlantic is capable of changing rapidly, with important consequences for global climate. Evidence from various climate archives suggests that abrupt transitions in the past were preceded by systematic behavior that could have provided early warning indicators

Combined impact of shifts in Southern Ocean westerlies and Antarctic sea ice during LGM on atmospheric CO2

A significant influence of changes in the westerly winds over the Southern Ocean was proposed as a mechanism to explain a large portion of the glacial atmospheric pCO2 drawdown (Toggweiler et al., 2006). However, additional modelling studies with Earth System Models of Intermediate Complexity do not confirm the size and sometimes even the sign of the impact of southern hemispheric winds on the glacial pCO2 as suggested by Toggweiler (Men- viel et al., 2008; Tschumi et al., 2008, d’Orgeville et al., 2010). We here add to this discussion and explore the potential contribution of changes in the latitudinal position of the winds on Southern Ocean physics and the carbon cycle by using a state-of-the-art ocean general circulation model (MITgcm) in a spatial resolution increasing in the Southern Ocean (2◦ longitude; northern hemisphere: 2◦ latitude; southern hemisphere: 2◦cos(α)). We discuss how the change in carbon cycling is related to the upwelling strength and pattern in the Southern Ocean and how they depend on the changing wind fields and/or the sea ice coverage. While the previous studies explored the impact of the westlies starting from present day or pre-industrial back- ground conditions, we here perform simulations from LGM background climate. Ocean surface conditions are for reasons of consistency taken from output of the COSMOS Earth System model for a pre-industrial control and two LGM runs (Zhang et al., in preparation). Additionally, a northwards shift (by 10◦) of the westerly wind belt as proposed by Toggweiler is investigated

Rapid changes in ice core gas records Part 2: Understanding the rapid rise in atmospheric CO2 at the onset of the Bølling/Allerød

During the last glacial/interglacial transition the Earth's climate underwent rapid changes around 14.6 kyr ago. Temperature proxies from ice cores revealed the onset of the Bølling/Allerød (B/A) warm period in the north and the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A is accompanied by a rapid sea level rise of about 20 m during meltwater pulse (MWP) 1A, whose exact timing is matter of current debate. In situ measured CO₂ in the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10±1 ppmv in 230 yr at the same time. Allowing for the age distribution of CO₂ in firn we here show, that atmospheric CO₂ rose by 20–35 ppmv in less than 200 yr, which is a factor of 2–3.5 larger than the CO₂ signal recorded in situ in EDC. Based on the estimated airborne fraction of 0.17 of CO₂ we infer that 125 Pg of carbon need to be released to the atmosphere to produce such a peak. Most of the carbon might have been activated as consequence of continental shelf flooding during MWP-1A. This impact of rapid sea level rise on atmospheric CO₂ distinguishes the B/A from other Dansgaard/Oeschger events of the last 60 kyr, potentially defining the point of no return during the last deglaciation

Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød

One of the most abrupt and yet unexplained past rises in atmospheric CO2 (10 ppmv in two centuries in the EPICA Dome C [EDC] ice core) occurred in quasi-synchrony with abrupt northern hemispheric warming into the Bølling/Allerød, about 14.6 ka ago. In Köhler et al. (2014) we used a U/Th-dated record of atmospheric Δ14C from Tahiti corals to provide an independent and precise age control for this CO2 rise. We also used model simulations to show that the release of old (nearly 14C-free) carbon can explain these changes in CO2 and Δ14C. The Δ14C record provides an independent constraint on the amount of carbon released (125 PgC). We suggest, in line with observations of atmospheric CH4 and terrigenous biomarkers, that thawing permafrost in high northern latitudes could have been the source of carbon, possibly with contribution from flooding of the Siberian continental shelf during meltwater pulse 1A. Our findings highlight the potential of the permafrost carbon reservoir to modulate abrupt climate changes via greenhouse-gas feedbacks. These calculations and conclusions were challenged by the new CO2 data (Marcott et al. 2014) from the West Antarctic Ice Sheet Divide Ice Core (WDC), which have a higher temporal resolution. We therefore revised our carbon release experiments in order to meet these new WDC CO2 data. We furthermore used a new age distribution during gas enclosure in ice which includes the most recent understanding of firn densification. We then can align EDC and WDC CO2 data and propose a peak amplitude in atmospheric CO2 of about 15 ppmv around 14.6 ka BP corresponding to a C pulse of 85 PgC released in 200 years (0.425 PgC per year). This is 68% of the initial suggested strength of the C pulse of 125 PgC, that then led to a peak amplitude in true atmospheric CO2 of 22 ppmv. CO2 data from other ice cores suggest that the amplitude in atmospheric CO2 was in-between both these scenarios. The revised scenario proposes a carbon release that is still large enough to explain the atmospheric Δ14C anomaly of – (50 – 60) ‰ in 200 –250 years derived from Tahiti corals. However, in the revised scenario the released carbon needs to be essentially free of 14C, while in the previously suggested scenario there was still the possibility that the released carbon still contained some 14C and had a difference in the Δ14C signature to the atmosphere Δ(Δ14C) of –700 ‰. The previous scenario, therefore, contained a larger possibility that the released carbon might eventually been released from the deep ocean. The revised interpretation proposed here strengthens the idea that the carbon was released from permafrost thawing, since this had more likely a nearly 14C-free signature than any other known source. We therefore conclude, that the new WDC CO2 data are not in conflict with our permafrost thawing hypothesis, but indicate only that the magnitude of the released carbon might have been smaller than initially suggested. References: Köhler, P., Knorr, G., and Bard, E. 2014. Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød. Nature Communications 5, 5520. DOI: 10.1038/ncomms6520. Marcott, S. A., Bauska, T. K., Buizert, C., Steig, E. J., Rosen, J. L., Cuffey, K. M., Fudge, T. J., Severing­ haus, J. P., Ahn, J., Kalk, M. L., McConnell, J. R., Sowers, T., Taylor, K. C., White, J. W. C., and Brook, E. J. 2014. Centennial scale changes in the global carbon cycle during the last deglaciation. Nature 514: 616–619. DOI: 10.1038/nature13799

Abrupt carbon release at the onset of the Bølling/Allerød: Permafrost thawing with inter-hemispheric impact

Atmospheric carbon dioxide (CO2) during the last deglaciation (∼18–10 kyr BP) switched around 14.6 kyr BP from a rather gradual rise to an abrupt jump, which is recorded in ice cores as an increase of 10 ppmv in less than two centuries. So far the source of that CO2 excursion could not be identified and the climatic implications are largely unknown. Here we use highly resolved U/Th dated atmospheric ∆14C from Tahiti corals as independent age control for CO2 changes. This provides a temporal framework to show that the northern high latitude warming into the Bølling/Allerød occurred quasi-synchronous to this CO2 rise within a few decades. Furthermore we show that an abrupt release (within two centuries) of long-term immobile nearly 14C-free carbon (∼125 PgC) from thawing permafrost might explain the observed anomalies in atmospheric CO2 and ∆14C, in line with CH4 and biomarker records from ice and sediment cores. In transient climate simulations we show that the abrupt carbon release in the northern high latitudes and associated CO2 changes bear the potential to modulate Antarctic temperature. These findings are in agreement with the observed onset of the Antarctic Cold Reversal about two centuries after the beginning of the Bølling/Allerød, as detected in independent annual layer-counted ice cores from both hemispheres. Based on the timing, magnitude, origin and the inter-hemispheric impact we speculate that this abrupt deglacial release of long-term stored carbon via thawing permafrost might have provided the final push out of the last ice age

The impact of the temperature-CO2 decoupling on the state-dependency of paleo climate sensitivity during the late Pleistocene

Climate change projections for the future are uncertain, also due to inter-model differences. The application of these models to paleo times, which can be constrained by reconstructions, is therefore essential, not only to gain a better understanding of past climate changes, but also for model validation purposes. In this respect both data- and model-based approaches have been used to generate time series of global temperature changes, ∆Tg. The ratio of ∆Tg over radiative forcing, ∆R, defines the specific equilibrium climate sensitivity S, and has been suggested to be state-dependent, potentially increasing towards warming climates, and therefore suggesting climate sensitivity for the future to be at the upper end of the range of published results (Köhler et al., 2015, 2017). Here we reanalyse existing time series of ∆Tg and ∆R for the last 800,000 years and show that this proposed state-dependency of S is only found if ∆Tg is based on data (reconstructions), and not if ∆Tg is based on models (simulations). We furthermore identify that in data-based reconstructions ∆Tg is decoupled from atmospheric CO2 predominantely during times of decreasing obliquity (identical to periods of land-ice sheet growth and sea level fall), while in model simulations ∆Tg and CO2 vary in phase throughout. This multi-millennial decoupling of CO2 and temperature has been suggested to be partially caused by a sea level-induced surge in magma and CO2 fluxes from oceanic hotspot volcanoes and mid ocean ridges (Hasenclever et al., 2017). The neglection of these feedbacks between the solid Earth and the climate system in recent Earth system models is partly responsible for the data/model misfit, and illustrates our current limitation in the model-based interpretation of the paleo records. Paleo-based estimates of S might be restricted to data without this ∆Tg-CO2-decoupling leading to a 20% smaller quantification of S for interglacial conditions of the late Pleistocene

Abrupt North Atlantic circulation changes in response to gradual CO2 forcing in a glacial climate state

Glacial climate is marked by abrupt, millennial-scale climate changes known as Dansgaard–Oeschger cycles. The most pronounced stadial coolings, Heinrich events, are associated with massive iceberg discharges to the North Atlantic. These events have been linked to variations in the strength of the Atlantic meridional overturning circulation. However, the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear. Here we show that, in a fully coupled atmosphere–ocean model, gradual changes in atmospheric CO2 concentrations can trigger abrupt climate changes, associated with a regime of bi-stability of the Atlantic meridional overturning circulation under intermediate glacial conditions. We find that changes in atmospheric CO2 concentrations alter the transport of atmospheric moisture across Central America, which modulates the freshwater budget of the North Atlantic and hence deep-water formation. In our simulations, a change in atmospheric CO2 levels of about 15 ppmv—comparable to variations during Dansgaard–Oeschger cycles containing Heinrich events—is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode. Because changes in the Atlantic meridional overturning circulation are thought to alter atmospheric CO2 levels, we infer that atmospheric CO2 may serve as a negative feedback to transitions between strong and weak circulation modes

A modern prototype three-layer stratification in the Arctic Ocean since the Miocene

The tectonic opening of the Fram Strait (FS) was critical to the water exchange between the Atlantic Ocean and the Arctic Ocean, and caused the transition from a restricted to a ventilated Arctic Ocean during early Miocene. If and how the water exchange between the Arctic Ocean and the North Atlantic influenced the global current system is still disputed. We apply a fully coupled atmosphere-ocean-sea-ice model to investigate stratification and ocean circulation in the Arctic Ocean in response to the opening of the FS during early to middle Miocene. Progressive widening of the FS gateway in our simulation causes a moderate warming, while salinity conditions in the Nordic Seas remain similar. On the contrary, with increasing FS width Arctic temperatures remain unchanged and salinity changes appear to steadily become stronger. For a sill depth of ~1500 m, we achieve ventilation of the Arctic Ocean due to enhanced import of saline Atlantic water through a FS width of ~105 km. Moreover, at this width and depth, we detect a modern-like three-layer stratification in the Arctic Ocean. The exchange flow through FS is characterized by vertical separation of a low salinity cold outflow from the Arctic Ocean confined to a thin upper layer, an intermediate saline inflow from the Atlantic Ocean below and a cold bottom Arctic outflow. Using a significantly shallower and narrower FS during the early Miocene, our study suggests that the ventilation mechanisms and stratification in the Arctic Ocean are comparable to the present-day characteristics

Role of sediments in controlling the dynamics of paleo-ice sheets

The motion of glacial ice is predominantly controlled by basal conditions, which include a variety of parameters such as ice rheology, temperature, water content, the presence of sediments, and topography. Soft sediment deformation has long been hypothesized to be a dominant control on the size and dynamics of temperate ice sheets such as the Laurentide Ice Sheet. The transition from hard-bedded regions (areas that lack significant sediment cover) to soft sediment areas put a limit on the maximum volume of these ice sheets. When the ice sheet margin reached soft sediment cover, it may have caused the ice sheet to surge, with global-scale climatic impacts. Current generation ice sheet models only have limited control on how sediments modify the behavior of an ice sheet. We present a model of sediment deformation that can take into account the thickness, lithology and hydrology at the base of the ice sheet using the Parallel Ice Sheet Model (PISM). We assess how changes in sediment properties affect the advance and retreat of the ice sheet, including standstills in the margin when the ice sheet becomes restricted to the hard-bedded interior areas. We apply this model to the Wisconsin Glaciation (~85-11 kyrs ago) of the Laurentide ice sheet. We show how the distribution of sediments affect its growth and retreat. We specifically focus on how the soft bedded Hudson Bay impeded the growth of the ice sheet, up to the lead up to the Last Glacial Maximum. We also investigate the relationship between Dansgaard–Oeschger and Heinrich events and the basal dynamics of the ice sheets