315 research outputs found
Brief Communication: Can recent ice discharges following the Larsen-B ice-shelf collapse be used to infer the driving mechanisms of millennial-scale variations of the Laurentide ice sheet?
Discussion paper.The effects of an ice-shelf collapse on inland glacier dynamics have recently been widely studied, especially since the breakup of Antarctic Peninsula's Larsen-B ice shelf in 2002. Several studies have documented acceleration of the ice streams that were flowing into the former ice shelf. The mechanism responsible for such a speed-up lies with the removal of the ice-shelf backforce. Independently, it is also well documented that during the last glacial period, the Northern Hemisphere ice sheets experienced large discharges into the ocean, likely reflecting ice flow acceleration episodes on the millennial time scale. The classic interpretation of the latter is based on the existence of an internal thermo-mechanical feedback with the potential to generate oscillatory behavior in the ice sheets. Here we would like to widen the debate by considering that Larsen-B-like glacial analog episodes could have contributed significantly to the registered millennial-scale variablity.Peer reviewe
La Exposición Universal de París de 1889 : breves noticias acerca de París y de lo más notable que encierra, impresiones sobre la exposicion... estractado de las noticias comunicadas por nuestros corresponsales
Título de la cubierta: La exposición de Paris : reseña de las artes é industrias de mas importanci
Glacial/Interglacial simulations with an Earth System model of intermediate complexity
This is a contribution to the project TIC-MOC (CTM2011-28867) funded by the 2008-2011 Spanish R+D Plan. C. Herrero acknowledges a CSIC JAE-Predoc scholarship co-financed by the European Social Fund (FSE) and wishes to thank all PalMA group members for their help and kindnessPeer Reviewe
A new approach for simulating the paleo-evolution of the Northern Hemisphere ice sheets
Offline forcing methods for ice-sheet models often make use of an index
approach in which temperature anomalies relative to the present are calculated by
combining a simulated glacial–interglacial climatic anomaly field,
interpolated through an index derived from the Greenland ice-core temperature
reconstruction, with present-day climatologies. An important drawback of this
approach is that it clearly misrepresents climate variability at millennial
timescales. The reason for this is that the spatial glacial–interglacial
anomaly field used is associated with orbital climatic variations, while it
is scaled following the characteristic time evolution of the index, which
includes orbital and millennial-scale climate variability. The spatial
patterns of orbital and millennial variability are clearly not the same, as
indicated by a wealth of models and data. As a result, this method can be
expected to lead to a misrepresentation of climate variability and thus of
the past evolution of Northern Hemisphere (NH) ice sheets. Here we illustrate
the problems derived from this approach and propose a new offline climate
forcing method that attempts to better represent the characteristic pattern
of millennial-scale climate variability by including an additional spatial
anomaly field associated with this timescale. To this end, three different
synthetic transient forcing climatologies are developed for the past
120 kyr following a perturbative approach and are applied to an ice-sheet
model. The impact of the climatologies on the paleo-evolution of the NH ice
sheets is evaluated. The first method follows the usual index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial–interglacial climatic anomaly field, interpolated through an index derived from ice-core data, with present-day climatologies.
In the second approach the representation of millennial-scale climate
variability is improved by incorporating a simulated stadial–interstadial
anomaly field. The third is a refinement of the second one in which the
amplitudes of both orbital and millennial-scale variations are tuned to
provide perfect agreement with a recently published absolute temperature
reconstruction over Greenland. The comparison of the three climate forcing
methods highlights the tendency of the usual index approach to overestimate
the temperature variability over North America and Eurasia at millennial
timescales. This leads to a relatively high NH ice-volume variability on
these timescales. Through enhanced ablation, this results in too low an ice
volume throughout the last glacial period (LGP), below or at the lower end of
the uncertainty range of estimations. Improving the representation of
millennial-scale variability alone yields an important increase in ice volume
in all NH ice sheets but especially in the Fennoscandian Ice Sheet (FIS).
Optimizing the amplitude of the temperature anomalies to match the Greenland
reconstruction results in a further increase in the simulated ice-sheet
volume throughout the LGP. Our new method provides a more realistic
representation of orbital and millennial-scale climate variability and
improves the transient forcing of ice sheets during the LGP. Interestingly,
our new approach underestimates ice-volume variations on millennial
timescales as indicated by sea-level records. This suggests that either the
origin of the latter is not the NH or that processes not represented in our
study, notably variations in oceanic conditions, need to be invoked to
explain millennial-scale ice-volume fluctuations. We finally provide here
both our derived climate evolution of the LGP using the three methods as well
as the resulting ice-sheet configurations. These could be of interest for
future studies dealing with the atmospheric or/and oceanic consequences of
transient ice-sheet evolution throughout the LGP and as a source of climate
input to other ice-sheet models
The Antarctic Ice Sheet response to glacial millennial-scale variability
The Antarctic Ice Sheet (AIS) is the largest ice sheet on Earth and hence a
major potential contributor to future global sea-level rise. A wealth of
studies suggest that increasing oceanic temperatures could cause a collapse
of its marine-based western sector, the West Antarctic Ice Sheet, through the
mechanism of marine ice-sheet instability, leading to a sea-level increase of
3–5 m. Thus, it is crucial to constrain the sensitivity of the AIS
to rapid climate changes. The last glacial period is an ideal benchmark
period for this purpose as it was punctuated by abrupt Dansgaard–Oeschger
events at millennial timescales. Because their center of action was in the
North Atlantic, where their climate impacts were largest, modeling studies
have mainly focused on the millennial-scale evolution of Northern Hemisphere
(NH) paleo ice sheets. Sea-level reconstructions attribute the origin of
millennial-scale sea-level variations mainly to NH paleo ice sheets, with a
minor but not negligible role of the AIS. Here we investigate the AIS
response to millennial-scale climate variability for the first time. To this
end we use a three-dimensional, thermomechanical hybrid, ice sheet–shelf
model. Different oceanic sensitivities are tested and the sea-level
equivalent (SLE) contributions computed. We find that whereas atmospheric
variability has no appreciable effect on the AIS, changes in submarine
melting rates can have a strong impact on it. We show that in contrast to the
widespread assumption that the AIS is a slow reactive and static ice sheet
that responds at orbital timescales only, it can lead to ice discharges of
around 6 m SLE, involving substantial grounding line migrations at
millennial timescales.</p
Impact of millennial-scale oceanic variability on the Greenland ice-sheet evolution throughout the last glacial period
Temperature reconstructions from Greenland ice-sheet (GrIS) ice
cores indicate the occurrence of more than 20 abrupt warmings during the
last glacial period (LGP) known as Dansgaard-Oeschger (D-O) events. Although
their ultimate cause is still debated, evidence from both proxy data and
modelling studies robustly links these to reorganisations of the Atlantic
Meridional Overturning Circulation (AMOC). During the LGP, the GrIS expanded
as far as the continental shelf break and was thus more directly exposed to
oceanic changes than in the present. Therefore oceanic temperature
fluctuations on millennial timescales could have had a non-negligible impact
on the GrIS. Here we assess the effect of millennial-scale oceanic
variability on the GrIS evolution from the last interglacial to the present
day. To do so, we use a three-dimensional hybrid ice-sheet–shelf model forced
by subsurface oceanic temperature fluctuations, assumed to increase during
D-O stadials and decrease during D-O interstadials. Since in our model the
atmospheric forcing follows orbital variations only, the increase in total
melting at millennial timescales is a direct result of an increase in basal
melting. We show that the GrIS evolution during the LGP could have been
strongly influenced by oceanic changes on millennial timescales, leading to
oceanically induced ice-volume contributions above 1 m sea level equivalent (SLE). Also, our results
suggest that the increased flux of GrIS icebergs as inferred from North
Atlantic proxy records could have been triggered, or intensified, by peaks in
melting at the base of the ice shelves resulting from increasing subsurface
oceanic temperatures during D-O stadials. Several regions across the GrIS
could thus have been responsible for ice mass discharge during D-O events,
opening the possibility of a non-negligible role of the GrIS in oceanic
reorganisations throughout the LGP.</p
Heinrich event 1: an example of dynamical ice-sheet reaction to oceanic changes
Heinrich events, identified as enhanced ice-rafted
detritus (IRD) in North Atlantic deep sea sediments (Heinrich,
1988; Hemming, 2004) have classically been attributed
to Laurentide ice-sheet (LIS) instabilities (MacAyeal, 1993;
Calov et al., 2002; Hulbe et al., 2004) and assumed to lead to
important disruptions of the Atlantic meridional overturning
circulation (AMOC) and North Atlantic deep water (NADW)
formation. However, recent paleoclimate data have revealed
that most of these events probably occurred after the AMOC
had already slowed down or/and NADW largely collapsed,
within about a thousand years (Hall et al., 2006; Hemming,
2004; Jonkers et al., 2010; Roche et al., 2004), implying that
the initial AMOC reduction could not have been caused by
the Heinrich events themselves.
Here we propose an alternative driving mechanism, specifically
for Heinrich event 1 (H1; 18 to 15 ka BP), by which
North Atlantic ocean circulation changes are found to have
strong impacts on LIS dynamics. By combining simulations
with a coupled climate model and a three-dimensional
ice sheet model, our study illustrates how reduced NADW
and AMOC weakening lead to a subsurface warming in the
Nordic and Labrador Seas resulting in rapid melting of the
Hudson Strait and Labrador ice shelves. Lack of buttressing
by the ice shelves implies a substantial ice-stream acceleration,
enhanced ice-discharge and sea level rise, with peak values 500–1500 yr after the initial AMOC reduction. Our
scenario modifies the previous paradigm of H1 by solving
the paradox of its occurrence during a cold surface period,
and highlights the importance of taking into account the effects
of oceanic circulation on ice-sheets dynamics in order
to elucidate the triggering mechanism of Heinrich events.Peer reviewe
5-HT7 receptors in Alzheimer's disease
Even though the involvement of serotonin (5-hydroxytryptamine; 5-HT) and its receptors in Alzheimer's disease (AD) is widely accepted, data on the expression and the role of 5-HT7 receptors in AD is relatively limited. Therefore, the objective of the present work was to study the expression of serotonergic 5-HT7 receptors in postmortem samples of AD brains and correlate it with neurotransmitter levels, cognition and behavior. The study population consisted of clinically well-characterized and neuropathologically confirmed AD patients (n = 42) and age-matched control subjects (n = 18). Reverse-transcription quantitative polymerase chain reaction (RT-qPCR) and high-performance liquid chromatography were performed on Brodmann area (BA) 7, BA10, BA22, BA24, hippocampus, amygdala, thalamus and cerebellum to measure mRNA levels of 5-HT7 receptors (HTR7), as well as the concentrations of various monoamine neurotransmitters and their metabolites. Decreased levels of HTR7 mRNA were observed in BA10. A significant association was observed between HTR7 levels in BA10 and BEHAVE-AD cluster B (hallucinations) (rs(28) = 0.444, P < 0.05). In addition, a negative correlation was observed between HTR7 levels in BA10 and both MHPG concentrations in this brain region (rs(45) = -0.311; P < 0.05), and DOPAC levels in the amygdala (rs(42) = -0.311; P < 0.05). Quite surprisingly, no association was found between HTR7 levels and cognitive status. Altogether, this study supports the notion of the involvement of 5-HT7 receptors in psychotic symptoms in AD, suggesting the interest of testing antagonist acting at this receptor to specifically treat psychotic symptoms in this illness
Simulating the Laurentide Ice Sheet of the Last Glacial Maximum
In the last decades, great effort has been made to reconstruct the Laurentide Ice Sheet (LIS) during the Last Glacial Maximum (LGM; ca. 21 000 years before present, 21 kyr ago). Uncertainties underlying its modelling have led to notable differences in fundamental features such as its maximum elevation, extent and total volume. As a result, the uncertainty in ice dynamics and thus in ice extent, volume and ice stream stability remains large. We herein use a higher-order three-dimensional ice sheet model to simulate the LIS under LGM boundary conditions for a number of basal friction formulations of varying complexity. Their consequences for the Laurentide ice streams, configuration, extent and volume are explicitly quantified. Total volume and ice extent generally reach a constant equilibrium value that falls close to prior LIS reconstructions. Simulations exhibit high sensitivity to the dependency of the basal shear stress on the sliding velocity. In particular, a regularised Coulomb friction formulation appears to be the best choice in terms of ice volume and ice stream realism. Pronounced differences are found when the basal friction stress is thermomechanically coupled: the base remains colder, and the LIS volume is lower than in the purely mechanical friction scenario counterpart. Thermomechanical coupling is fundamental for producing rapid ice streaming, yet it leads to a similar ice distribution overall.</p
Towards a sustainable and equitable blue economy
The global rush to develop the \u2018blue economy\u2019 risks harming both the marine environment and human wellbeing. Bold policies and actions are urgently needed. We identify five priorities to chart a course towards an environmentally sustainable and socially equitable blue economy
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