scholarly journals Climate bifurcation during the last deglaciation?

2012 ◽  
Vol 8 (4) ◽  
pp. 1127-1139 ◽  
Author(s):  
T. M. Lenton ◽  
V. N. Livina ◽  
V. Dakos ◽  
M. Scheffer
Keyword(s):  
Climate Variability ◽  
Early Warning ◽  
Bifurcation Point ◽  
Climate Changes ◽  
Younger Dryas ◽  
Cold Climate ◽  
Last Deglaciation ◽  
Overturning Circulation ◽  
Slowing Down ◽  
The Last Deglaciation

Abstract. There were two abrupt warming events during the last deglaciation, at the start of the Bølling-Allerød and at the end of the Younger Dryas, but their underlying dynamics are unclear. Some abrupt climate changes may involve gradual forcing past a bifurcation point, in which a prevailing climate state loses its stability and the climate tips into an alternative state, providing an early warning signal in the form of slowing responses to perturbations, which may be accompanied by increasing variability. Alternatively, short-term stochastic variability in the climate system can trigger abrupt climate changes, without early warning. Previous work has found signals consistent with slowing down during the last deglaciation as a whole, and during the Younger Dryas, but with conflicting results in the run-up to the Bølling-Allerød. Based on this, we hypothesise that a bifurcation point was approached at the end of the Younger Dryas, in which the cold climate state, with weak Atlantic overturning circulation, lost its stability, and the climate tipped irreversibly into a warm interglacial state. To test the bifurcation hypothesis, we analysed two different climate proxies in three Greenland ice cores, from the Last Glacial Maximum to the end of the Younger Dryas. Prior to the Bølling warming, there was a robust increase in climate variability but no consistent slowing down signal, suggesting this abrupt change was probably triggered by a stochastic fluctuation. The transition to the warm Bølling-Allerød state was accompanied by a slowing down in climate dynamics and an increase in climate variability. We suggest that the Bølling warming excited an internal mode of variability in Atlantic meridional overturning circulation strength, causing multi-centennial climate fluctuations. However, the return to the Younger Dryas cold state increased climate stability. We find no consistent evidence for slowing down during the Younger Dryas, or in a longer spliced record of the cold climate state before and after the Bølling-Allerød. Therefore, the end of the Younger Dryas may also have been triggered by a stochastic perturbation.

scholarly journals Climate bifurcation during the last deglaciation

2012 ◽  
Vol 8 (1) ◽  
pp. 321-348 ◽  
Author(s):  
T. M. Lenton ◽  
V. N. Livina ◽  
V. Dakos ◽  
M. Scheffer
Keyword(s):  
Climate Variability ◽  
Early Warning ◽  
Bifurcation Point ◽  
Younger Dryas ◽  
Cold Climate ◽  
Meridional Overturning Circulation ◽  
Last Deglaciation ◽  
Overturning Circulation ◽  
Slowing Down ◽  
The Last Deglaciation

Abstract. The last deglaciation was characterised by two abrupt warming events, at the start of the Bølling-Allerød and at the end of the Younger Dryas, but their underlying causes are unclear. Some abrupt climate changes may involve gradual forcing past a bifurcation point, in which a prevailing climate state loses its stability and the climate tips into an alternative state, providing an early warning signal in the form of slowing responses to perturbations. However, the abrupt Dansgaard-Oeschger (DO) events during the last ice age were probably triggered by stochastic fluctuations without bifurcation or early warning, and whether the onset of the Bølling-Allerød (DO event 1) was preceded by slowing down or not is debated. Here we show that the interval from the Last Glacial Maximum to the end of the Younger Dryas, as recorded in three Greenland ice cores with two different climate proxies, was accompanied by a robust slowing down in climate dynamics and an increase in climate variability, consistent with approaching bifurcation. Prior to the Bølling warming there was a robust increase in climate variability but no consistent slowing down signal, suggesting this abrupt change was probably triggered by a stochastic fluctuation. The Bølling warming marked a distinct destabilisation of the climate system, which excited an internal mode of variability in Atlantic meridional overturning circulation strength, causing multi-centennial climate fluctuations. There is some evidence for slowing down in the transition to and during the Younger Dryas. We infer that a bifurcation point was finally approached at the end of the Younger Dryas, in which the cold climate state, with weak Atlantic overturning circulation, lost its stability, and the climate tipped irreversibly into a warm interglacial state. The lack of a large triggering perturbation at the end of the Younger Dryas, and the fact that subsequent meltwater perturbations did not cause sustained cooling, support the bifurcation hypothesis.

The chronology of the last Deglaciation: Implications to the cause of the Younger Dryas Event

1988 ◽  
Vol 3 (1) ◽  
pp. 1-19 ◽  
Author(s):  
W. S. Broecker ◽  
M. Andree ◽  
W. Wolfli ◽  
H. Oeschger ◽  
G. Bonani ◽  
...  
Keyword(s):  
Younger Dryas ◽  
Last Deglaciation ◽  
The Last Deglaciation ◽  
Younger Dryas Event

Timing of the last Deglaciation in the Sierra Nevada of the Mérida Andes, Venezuela

2013 ◽  
Vol 80 (3) ◽  
pp. 482-494 ◽  
Author(s):  
Julien Carcaillet ◽  
Isandra Angel ◽  
Eduardo Carrillo ◽  
Franck A. Audemard ◽  
Christian Beck
Keyword(s):  
Sierra Nevada ◽  
Late Pleistocene ◽  
Younger Dryas ◽  
Ice Cover ◽  
Published Data ◽  
Last Deglaciation ◽  
The Last Deglaciation ◽  
Absolute Age ◽  
Glacial Landforms

In the tropical Mérida Andes (northwestern Venezuela), glacial landforms were found at altitudes between 2600 and 5000 m, corresponding to 600 km2 of ice cover during the maximum glacial extension. However, the lack of sufficient absolute age data prevents detailed reconstruction of the timing of the last deglaciation. On the northwestern flank of the Mucuñuque Massif, successive moraines and striated eroded basement surfaces were sampled for cosmogenic 10Be investigation. Their compilation with published data allows the establishment of a detailed chronology of the post-LGM glacier history. The oldest moraines (18.1 and 16.8 ka) correspond to the Oldest Dryas. Successive moraine ridges indicate stops in the overall retreat between the LGM and the Younger Dryas. The cold and short Older Dryas stadial has been identified. Results indicate that most of the ice withdrew during the Pleistocene. The dataset supports an intensification of the vertical retreat rate from ~ 25 m/ka during the late Pleistocene to ~ 310 m/ka during the Pleistocene/Holocene. Afterwards, the glacier was confined and located in the higher altitude zones. The altitude difference of the Younger Dryas moraines in the Mucubají, La Victoria and Los Zerpa valleys indicates a strong effect of valley orientation on the altitude of moraine development.

scholarly journals A New 14C Calibration Data Set for the Last Deglaciation Based on Marine Varves

Radiocarbon ◽  
1997 ◽  
Vol 40 (1) ◽  
pp. 483-494 ◽  
Author(s):  
Konrad A. Hughen ◽  
Jonathan T. Overpeck ◽  
Scott J. Lehman ◽  
Michaele Kashgarian ◽  
John R. Southon ◽  
...  
Keyword(s):  
Global Climate ◽  
Ice Core ◽  
Younger Dryas ◽  
Core Layer ◽  
Cariaco Basin ◽  
Calibration Data ◽  
Last Deglaciation ◽  
Data Set ◽  
The Last Deglaciation ◽  
Calibration Data Set

Varved sediments of the tropical Cariaco Basin provide a new 14C calibration data set for the period of deglaciation (10,000 to 14,500 years before present: 10–14.5 cal ka bp). Independent evaluations of the Cariaco Basin calendar and 14C chronologies were based on the agreement of varve ages with the GISP2 ice core layer chronology for similar high-resolution paleoclimate records, in addition to 14C age agreement with terrestrial 14C dates, even during large climatic changes. These assessments indicate that the Cariaco Basin 14C reservoir age remained stable throughout the Younger Dryas and late Allerød climatic events and that the varve and 14C chronologies provide an accurate alternative to existing calibrations based on coral U/Th dates. The Cariaco Basin calibration generally agrees with coral-derived calibrations but is more continuous and resolves century-scale details of 14C change not seen in the coral records. 14C plateaus can be identified at 9.6, 11.4, and 11.7 14C ka bp, in addition to a large, sloping “plateau” during the Younger Dryas (∼10 to 11 14C ka bp). Accounting for features such as these is crucial to determining the relative timing and rates of change during abrupt global climate changes of the last deglaciation.

scholarly journals Estimated strength of the Atlantic overturning circulation during the last deglaciation

2013 ◽  
Vol 6 (3) ◽  
pp. 208-212 ◽  
Author(s):  
Stefan P. Ritz ◽  
Thomas F. Stocker ◽  
Joan O. Grimalt ◽  
Laurie Menviel ◽  
Axel Timmermann
Keyword(s):  
Last Deglaciation ◽  
Overturning Circulation ◽  
The Last Deglaciation

Rapid changes in the oceanic fronts in the Norwegian Sea during the last deglaciation: implications for the Younger Dryas cooling event

1998 ◽  
Vol 152 (1-3) ◽  
pp. 177-188 ◽  
Author(s):  
D Klitgaard-Kristensen ◽  
T.L Rasmussen ◽  
H.P Sejrup ◽  
H Haflidason ◽  
Tj.C.E van Weering
Keyword(s):  
Younger Dryas ◽  
Last Deglaciation ◽  
Norwegian Sea ◽  
Oceanic Fronts ◽  
Rapid Changes ◽  
The Last Deglaciation

scholarly journals Drivers of the evolution and amplitude of African Humid Periods

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Laurie Menviel ◽  
Aline Govin ◽  
Arthur Avenas ◽  
Katrin J. Meissner ◽  
Katharine M. Grant ◽  
...  
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Interglacial Period ◽  
Last Deglaciation ◽  
Last Interglacial ◽  
Overturning Circulation ◽  
Last Interglacial Period ◽  
Meridional Overturning ◽  
Northern Hemispheric ◽  
The Last Deglaciation

AbstractDuring orbital precession minima, the Sahara was humid and more vegetated, providing potential corridors for Hominins migration. Uncertainties remain over the climatic processes controlling the initiation, demise and amplitude of these African Humid Periods. Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period, and compare the results with a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of African Humid Periods as the Atlantic Meridional Overturning Circulation modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the African Humid Period. Through its impact on Northern Hemispheric ice-sheets disintegration and thus Atlantic Meridional Overturning Circulation, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the African Humid Period during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions.

scholarly journals Towards understanding potential atmospheric contributions to abrupt climate changes: characterizing changes to the North Atlantic eddy-driven jet over the last deglaciation

2019 ◽  
Vol 15 (4) ◽  
pp. 1621-1646
Author(s):  
Heather J. Andres ◽  
Lev Tarasov
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Changes ◽  
Ice Sheet ◽  
Wind Forcing ◽  
Last Deglaciation ◽  
Sea Ice Extent ◽  
The North ◽  
The Last Deglaciation ◽  
Background Climate

Abstract. Abrupt climate shifts of large amplitudes were common features of the Earth's climate as it transitioned into and out of the last full glacial state approximately 20 000 years ago, but their causes are not yet established. Midlatitude atmospheric dynamics may have played an important role in these climate variations through their effects on heat and precipitation distributions, sea ice extent, and wind-driven ocean circulation patterns. This study characterizes deglacial winter wind changes over the North Atlantic (NAtl) in a suite of transient deglacial simulations using the PlaSim Earth system model (run at T42 resolution) and the TraCE-21ka (T31) simulation. Though driven with yearly updates in surface elevation, we detect multiple instances of NAtl jet transitions in the PlaSim simulations that occur within 10 simulation years and a sensitivity of the jet to background climate conditions. Thus, we suggest that changes to the NAtl jet may play an important role in abrupt glacial climate changes. We identify two types of simulated wind changes over the last deglaciation. Firstly, the latitude of the NAtl eddy-driven jet shifts northward over the deglaciation in a sequence of distinct steps. Secondly, the variability in the NAtl jet gradually shifts from a Last Glacial Maximum (LGM) state with a strongly preferred jet latitude and a restricted latitudinal range to one with no single preferred latitude and a range that is at least 11∘ broader. These changes can significantly affect ocean circulation. Changes to the position of the NAtl jet alter the location of the wind forcing driving oceanic surface gyres and the limits of sea ice extent, whereas a shift to a more variable jet reduces the effectiveness of the wind forcing at driving surface ocean transports. The processes controlling these two types of changes differ on the upstream and downstream ends of the NAtl eddy-driven jet. On the upstream side over eastern North America, the elevated ice sheet margin acts as a barrier to the winds in both the PlaSim simulations and the TraCE-21ka experiment. This constrains both the position and the latitudinal variability in the jet at LGM, so the jet shifts in sync with ice sheet margin changes. In contrast, the downstream side over the eastern NAtl is more sensitive to the thermal state of the background climate. Our results suggest that the presence of an elevated ice sheet margin in the south-eastern sector of the North American ice complex strongly constrains the deglacial position of the jet over eastern North America and the western North Atlantic as well as its variability.

scholarly journals The last deglaciation: timing the bipolar seesaw

2011 ◽  
Vol 7 (2) ◽  
pp. 671-683 ◽  
Author(s):  
J. B. Pedro ◽  
T. D. van Ommen ◽  
S. O. Rasmussen ◽  
V. I. Morgan ◽  
J. Chappellaz ◽  
...  
Keyword(s):  
Climate Changes ◽  
Ice Core ◽  
Time Lag ◽  
Last Deglaciation ◽  
Atmospheric Teleconnections ◽  
The Last Deglaciation ◽  
The Antarctic ◽  
First Time ◽  
Coupling Mechanisms ◽  
Ice Core Records

Abstract. Precise information on the relative timing of north-south climate variations is a key to resolving questions concerning the mechanisms that force and couple climate changes between the hemispheres. We present a new composite record made from five well-resolved Antarctic ice core records that robustly represents the timing of regional Antarctic climate change during the last deglaciation. Using fast variations in global methane gas concentrations as time markers, the Antarctic composite is directly compared to Greenland ice core records, allowing a detailed mapping of the inter-hemispheric sequence of climate changes. Consistent with prior studies the synchronized records show that warming (and cooling) trends in Antarctica closely match cold (and warm) periods in Greenland on millennial timescales. For the first time, we also identify a sub-millennial component to the inter-hemispheric coupling. Within the Antarctic Cold Reversal the strongest Antarctic cooling occurs during the pronounced northern warmth of the Bølling. Warming then resumes in Antarctica, potentially as early as the Intra-Allerød Cold Period, but with dating uncertainty that could place it as late as the onset of the Younger Dryas stadial. There is little-to-no time lag between climate transitions in Greenland and opposing changes in Antarctica. Our results lend support to fast acting inter-hemispheric coupling mechanisms, including recently proposed bipolar atmospheric teleconnections and/or rapid bipolar ocean teleconnections.

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