55 research outputs found
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Assessing the impact of Laurentide Ice Sheet topography on glacial climate
Simulations of past climates require altered
boundary conditions to account for known shifts in the Earth
system. For the Last Glacial Maximum (LGM) and subsequent
deglaciation, the existence of large Northern Hemisphere
ice sheets caused profound changes in surface topography
and albedo. While ice-sheet extent is fairly well
known, numerous conflicting reconstructions of ice-sheet topography
suggest that precision in this boundary condition is
lacking. Here we use a high-resolution and oxygen-isotope-enabled
fully coupled global circulation model (GCM)
(GISS ModelE2-R), along with two different reconstructions
of the Laurentide Ice Sheet (LIS) that provide maximum and
minimum estimates of LIS elevation, to assess the range of
climate variability in response to uncertainty in this boundary
condition. We present this comparison at two equilibrium
time slices: the LGM, when differences in ice-sheet topography
are maximized, and 14 ka, when differences in maximum
ice-sheet height are smaller but still exist. Overall, we
find significant differences in the climate response to LIS
topography, with the larger LIS resulting in enhanced Atlantic
Meridional Overturning Circulation and warmer surface
air temperatures, particularly over northeastern Asia and
the North Pacific. These up- and downstream effects are associated
with differences in the development of planetary
waves in the upper atmosphere, with the larger LIS resulting
in a weaker trough over northeastern Asia that leads to the
warmer temperatures and decreased albedo from snow and
sea-ice cover. Differences between the 14 ka simulations are
similar in spatial extent but smaller in magnitude, suggesting that climate is responding primarily to the larger difference
in maximum LIS elevation in the LGM simulations. These
results suggest that such uncertainty in ice-sheet boundary
conditions alone may significantly impact the results of paleoclimate
simulations and their ability to successfully simulate
past climates, with implications for estimating climate
sensitivity to greenhouse gas forcing utilizing past climate
states
Recommended from our members
Laurentide ice-sheet instability during the last deglaciation
Changes in the amount of summer incoming solar radiation (insolation) reaching the Northern Hemisphere are the underlying pacemaker of glacial cycles. However, not all rises in boreal summer insolation over the past 800,000 years resulted in deglaciation to present-day ice volumes, suggesting that there may be a climatic threshold for the disappearance of land-based ice. Here we assess the surface mass balance stability of the Laurentide ice sheet—the largest glacial ice mass in the Northern Hemisphere—during the last deglaciation (24,000 to 9,000 years ago). We run a surface energy balance model with climate data from simulations with a fully coupled atmosphere–ocean general circulation model for key time slices during the last deglaciation. We find that the surface mass balance of the Laurentide ice sheet was positive throughout much of the deglaciation, and suggest that dynamic discharge was mainly responsible for mass loss during this time. Total surface mass balance became negative only in the early Holocene, indicating the transition to a new state where ice loss occurred primarily by surface ablation. We conclude that the Laurentide ice sheet remained a viable ice sheet before the Holocene and began to fully deglaciate only once summer temperatures and radiative forcing over the ice sheet increased by 6–7 °C and 16–20 W m⁻², respectively, relative to full glacial conditions
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Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent
Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use ¹⁰Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing ¹⁰Be ages. At least in south Greenland, these ¹⁰Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.Keywords: Early Holocene climate, Greenland ice sheet, Cosmogenic datin
Epithelial effects on the neural regulation of airway smooth muscle : studied in the ferret trachea
Comparison of clinical and physical measures of image quality in chest PA and pelvis AP views at varying tube voltages
Image quality in digital chest PA and pelvis AP was assessed using two different methods; one based on observations of images of an anthropomorphic phantom, one based on computer modelling using an anthropomorphic voxel phantom. The tube voltage was varied within a broad range (50-150 kV), including those values typically used with screen-film radiography. The tube charge was altered so that approximately the same effective dose was achieved in the modelled patient (anthropomorphic phantom). Two x-ray units were employed using a digital image detector (computed radiography, CR, system) with standard tube filtration and anti-scatter device. Clinical image quality was assessed by a group of radiologists using a visual grading analysis (VGA) technique based on the revised CEC image criteria. Physical image quality was derived from the computer model in terms of the signal-to-noise ratio, SNR for fixed effective dose in the voxel phantom. The computer model uses Monte Carlo simulations of the patient and complete imaging system. Both the VGAS (visual grading analysis score) and SNR increase with decreasing tube voltage in both chest PA and pelvis AP examinations, indicating superior performance if lower tube voltages than used today are employed in digital radiology. A positive correlation between clinical and physical measures of image quality was found. The pros and cons of using lower tube voltages with CR digital radiography than typically used in analogue screen-film radiography are discussed as well as the relevance of using VGAS and quantum noise SNR as measures of image quality
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UllmanDavidCEOASAssessingImpactLaurentide_SupplementaryMaterial.zip
Simulations of past climates require altered
boundary conditions to account for known shifts in the Earth
system. For the Last Glacial Maximum (LGM) and subsequent
deglaciation, the existence of large Northern Hemisphere
ice sheets caused profound changes in surface topography
and albedo. While ice-sheet extent is fairly well
known, numerous conflicting reconstructions of ice-sheet topography
suggest that precision in this boundary condition is
lacking. Here we use a high-resolution and oxygen-isotope-enabled
fully coupled global circulation model (GCM)
(GISS ModelE2-R), along with two different reconstructions
of the Laurentide Ice Sheet (LIS) that provide maximum and
minimum estimates of LIS elevation, to assess the range of
climate variability in response to uncertainty in this boundary
condition. We present this comparison at two equilibrium
time slices: the LGM, when differences in ice-sheet topography
are maximized, and 14 ka, when differences in maximum
ice-sheet height are smaller but still exist. Overall, we
find significant differences in the climate response to LIS
topography, with the larger LIS resulting in enhanced Atlantic
Meridional Overturning Circulation and warmer surface
air temperatures, particularly over northeastern Asia and
the North Pacific. These up- and downstream effects are associated
with differences in the development of planetary
waves in the upper atmosphere, with the larger LIS resulting
in a weaker trough over northeastern Asia that leads to the
warmer temperatures and decreased albedo from snow and
sea-ice cover. Differences between the 14 ka simulations are
similar in spatial extent but smaller in magnitude, suggesting that climate is responding primarily to the larger difference
in maximum LIS elevation in the LGM simulations. These
results suggest that such uncertainty in ice-sheet boundary
conditions alone may significantly impact the results of paleoclimate
simulations and their ability to successfully simulate
past climates, with implications for estimating climate
sensitivity to greenhouse gas forcing utilizing past climate
states
Recommended from our members
UllmanDavidCEOASLaurentideIce-Sheet.pdf
Changes in the amount of summer incoming solar radiation (insolation) reaching the Northern Hemisphere are the underlying pacemaker of glacial cycles. However, not all rises in boreal summer insolation over the past 800,000 years resulted in deglaciation to present-day ice volumes, suggesting that there may be a climatic threshold for the disappearance of land-based ice. Here we assess the surface mass balance stability of the Laurentide ice sheet—the largest glacial ice mass in the Northern Hemisphere—during the last deglaciation (24,000 to 9,000 years ago). We run a surface energy balance model with climate data from simulations with a fully coupled atmosphere–ocean general circulation model for key time slices during the last deglaciation. We find that the surface mass balance of the Laurentide ice sheet was positive throughout much of the deglaciation, and suggest that dynamic discharge was mainly responsible for mass loss during this time. Total surface mass balance became negative only in the early Holocene, indicating the transition to a new state where ice loss occurred primarily by surface ablation. We conclude that the Laurentide ice sheet remained a viable ice sheet before the Holocene and began to fully deglaciate only once summer temperatures and radiative forcing over the ice sheet increased by 6–7 °C and 16–20 W m⁻², respectively, relative to full glacial conditions
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