Modeling and dating glacier fluctuations and their relation to Pacific Ocean climate

This thesis presents the results of an investigation into the interactions between the present-day South Cascade Glacier and the former Mauna Kea ice cap at short (annual to centennial) and long (millennial and multimillennial) time scales. To quantify the response of South Cascade Glacier to atmospheric conditions, a surface energy balance model has been developed. This model has been applied to annual simulations of the mass balance of South Cascade Glacier and is shown to faithfully simulate ablation on all time scales from daily to seasonal. An investigation into the sensitivity of this model to uncertainties in the physical parameters and input data is conducted and provides a comprehensive indication of the uncertainty associated with surface energy balance model estimates of mass balance. These uncertainties are of the order of 10% of the annual mass flux of the glacier. The model is then used in conjunction with a regional model downscaling of climate data and a high resolution (0.5°) gridded observational data set to compute the long-term mass balance history of South Cascade Glacier. Our simulations show that the greatest rate of volume loss in the history of the glacier was in the late 1930s through the mid 1940s. However, present day mass loss is equivalent despite the more climatologically favorable position of the glacier today. Simulated mass balance is compared with Pacific climate indexes and show that the glacier’s relationship to oceanic conditions peaked in the middle part of the 20th century and currently shows a sharp decline. Finally, we present an investigation of the deglacial chronology of Mauna Kea. Our results establish the age of the local last glacial maximum at an age of 22.1 ± 2.1 kyr BP and complete deglaciation was underway by 14.7 ± 1.4 kyr BP. We present strong evidence that retreat after the LGM was followed by a readvance at 16.1 to 16.8 kyr BP. The timing of this readvance is comparable to that of Heinrich event 1 in the North Atlantic. The connection between the North Atlantic and Hawaii climate is discussed in terms of atmospheric modeling results and proxy evidence

Spring warming in Yukon mountains is not amplified by the snow albedo feedback

Decreasing spring snow cover may amplify Arctic warming through the snow albedo feedback. To examine the impact of snowmelt on increasing temperature we used a 5,000 m elevation gradient in Yukon, Canada, extending from valley-bottom conifer forests, through middle elevation tundra, to high elevation icefields, to compare validated downscaled reanalysis air temperature patterns across elevational bands characterized by different patterns of spring snowmelt. From 2000 to 2014 we observed surface warming of 0.01 °C/a·1,000 m in May (0.14 °C/a at 1,000 m to 0.19 °C/a at 5,000 m), and uniform cooling of 0.09 °C/a in June at all elevations. May temperature trends across elevationally dependent land cover types were highly correlated with each other despite large variations in albedo and snow cover trends. Furthermore, a clear dependency of infrared skin temperature on snow cover mediated albedo decline was observed in tundra, but this was insufficient to influence average diurnal air temperature. We observed negative June temperature trends which we attribute to increasing daytime cloud cover because albedo and snow cover trends were unchanging. We conclude that 8-day and monthly averaged Spring air temperature trends are responding to a synoptic external forcing that is much stronger than the snow albedo feedback in sub-Arctic mountains

Geochronology and paleoclimatic implications of the last deglaciation 2 of the Mauna Kea Ice Cap, Hawaii

This is the author's final peer-reviewed manuscript. It contains no copyediting.We present new 3He surface exposure ages on moraines and bedrock near the summit of Mauna Kea, Hawaii, which refine the age of the Mauna Kea Ice Cap during the Local Last Glacial Maximum (LLGM) and identify a subsequent fluctuation of the ice margin. The 3He ages, when combined with those reported previously, indicate that the local ice-cap margin began to retreat from its LLGM extent at 20.5 ± 2.5 ka, in agreement with the age of deglaciation determined from LLGM moraines elsewhere in the tropics. The ice-cap margin receded to a position at least 3 km upslope for ~5.1 kyr before readvancing nearly to its LLGM extent. The timing of this readvance at ~15.4 ka corresponds to a large reduction of the Atlantic meridional overturning circulation (AMOC) following Heinrich Event 1. Subsequent ice-margin retreat began at 14.6 ± 1.9 ka, corresponding to a rapid resumption of the AMOC and onset of the Bølling warm interval, with the ice cap melting rapidly to complete deglaciation. Additional 3He ages obtained from a flood deposit date the catastrophic outburst of a moraine-dammed lake roughly coeval with the Younger Dryas cold interval, suggesting a more active hydrological cycle on Mauna Kea at this time. A coupled mass balance and ice dynamics model is used to constrain the climate required to generate ice caps of LLGM and readvance sizes. The depression of the LLGM equilibrium line altitude requires atmospheric cooling of 4.5 ± 1 oC, whereas the mass balance modeling indicates an accompanying increase in precipitation of as much as three times that of present. We hypothesize (1) that the LLGM temperature depression was associated with global cooling, (2) that the temperature depression that contributed to the readvance occurred in response to an atmospheric teleconnection to the North Atlantic, and (3) that the precipitation enhancement associated with both events occurred in response to a southward shift in the position of the inter-tropical convergence zone (ITCZ). Such a shift in the ITCZ would have allowed midlatitude cyclones to reach Mauna Kea more frequently which would have increased precipitation at high elevations and caused additional cooling.Keywords: Glacial, Paleoclimate, Geochronolog

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

Rapid early Holocene deglaciation of the Laurentide ice sheet

Author Posting. © Nature Publishing Group, 2008. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 1 (2008): 620-624, doi:10.1038/ngeo285.The early Holocene deglaciation of the Laurentide Ice Sheet (LIS) is the most recent and best constrained disappearance of a large Northern Hemisphere ice sheet. Its demise is a natural experiment for assessing rates of ice sheet decay and attendant contributions to sea level rise. Here we demonstrate with terrestrial and marine records that the final LIS demise occurred in two stages of rapid melting from ~9.0- 8.5 and 7.6-6.8 kyr BP with the LIS contributing ~1.3 and 0.7 cm yr-1 to sea level rise, respectively. Simulations using a fully coupled atmosphere-ocean general circulation model suggest that increased ablation from enhanced early Holocene boreal summer insolation may have been the predominant cause of the LIS contributions to sea level rise. Although the boreal summer surface radiative forcing of early Holocene LIS retreat is twice that of projections for 2100 C.E. greenhouse gas radiative forcing, the associated summer surface air temperature increase is the same. The geologic evidence for rapid LIS retreat under a comparable forcing provides a prehistoric precedent for a possible large negative mass balance response of the Greenland Ice Sheet by the end of the coming century.This research was funded by National Science Foundation grants ATM-05-01351 & ATM-05-01241 to D.W.O. & G.A.S., start-up funds from the University of Wisconsin-Madison and a Woods Hole Oceanographic Institution Postdoctoral Scholarship to A.E.C., and the Woods Hole Oceanographic Institution's Ocean and Climate Change Institute (D.W.O. & R.E.C.)

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

Spring warming in Yukon mountains is not amplified by the snow albedo feedback

Decreasing spring snow cover may amplify Arctic warming through the snow albedo feedback. To examine the impact of snowmelt on increasing temperature we used a 5,000 m elevation gradient in Yukon, Canada, extending from valley-bottom conifer forests, through middle elevation tundra, to high elevation icefields, to compare validated downscaled reanalysis air temperature patterns across elevational bands characterized by different patterns of spring snowmelt. From 2000 to 2014 we observed surface warming of 0.01 °C/a·1,000 m in May (0.14 °C/a at 1,000 m to 0.19 °C/a at 5,000 m), and uniform cooling of 0.09 °C/a in June at all elevations. May temperature trends across elevationally dependent land cover types were highly correlated with each other despite large variations in albedo and snow cover trends. Furthermore, a clear dependency of infrared skin temperature on snow cover mediated albedo decline was observed in tundra, but this was insufficient to influence average diurnal air temperature. We observed negative June temperature trends which we attribute to increasing daytime cloud cover because albedo and snow cover trends were unchanging. We conclude that 8-day and monthly averaged Spring air temperature trends are responding to a synoptic external forcing that is much stronger than the snow albedo feedback in sub-Arctic mountains

UllmanDavidCEOASLaurentideIce-SheetSuppData.zip

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

UllmanDavidCEOASLaurentideIce-SheetSuppInfo.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