Jeremy D. Shakun

The last glacial maximum

The Melting Is in the Details Global sea level rises and falls as ice sheets and glaciers melt and grow, providing an integrated picture of the changes in ice volume but little information about how much individual ice fields are contributing to those variations. Knowing the regional structure of ice variability during glaciations and deglaciations will clarify the mechanisms of the glacial cycle. Clark et al. (p. 710) compiled and analyzed more than 5000 radiocarbon and cosmogenic surface exposure ages in order to develop a record of maximum regional ice extent around the time of the Last Glacial Maximum. The responses of the Northern and Southern Hemispheres differed significantly, which reveals how the evolution of specific ice sheets affected sea level and provides insight into how insolation controlled the deglaciation. Regional patterns are presented of the timing of ice-sheet and mountain-glacier maxima near the end of the last ice age. We used 5704 14C, 10Be, and 3He ages that span the interval from 10,000 to 50,000 years ago (10 to 50 ka) to constrain the timing of the Last Glacial Maximum (LGM) in terms of global ice-sheet and mountain-glacier extent. Growth of the ice sheets to their maximum positions occurred between 33.0 and 26.5 ka in response to climate forcing from decreases in northern summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO2. Nearly all ice sheets were at their LGM positions from 26.5 ka to 19 to 20 ka, corresponding to minima in these forcings. The onset of Northern Hemisphere deglaciation 19 to 20 ka was induced by an increase in northern summer insolation, providing the source for an abrupt rise in sea level. The onset of deglaciation of the West Antarctic Ice Sheet occurred between 14 and 15 ka, consistent with evidence that this was the primary source for an abrupt rise in sea level ~14.5 ka.

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation

The covariation of carbon dioxide (CO2) concentration and temperature in Antarctic ice-core records suggests a close link between CO2 and climate during the Pleistocene ice ages. The role and relative importance of CO2 in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO2 during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO2 concentrations is an explanation for much of the temperature change at the end of the most recent ice age.

A Reconstruction of Regional and Global Temperature for the Past 11,300 Years

Exceptional Now The climate has been warming since the industrial revolution, but how warm is climate now compared with the rest of the Holocene? Marcott et al. (p. 1198) constructed a record of global mean surface temperature for more than the last 11,000 years, using a variety of land- and marine-based proxy data from all around the world. The pattern of temperatures shows a rise as the world emerged from the last deglaciation, warm conditions until the middle of the Holocene, and a cooling trend over the next 5000 years that culminated around 200 years ago in the Little Ice Age. Temperatures have risen steadily since then, leaving us now with a global temperature higher than those during 90% of the entire Holocene. Current global average surface air temperature is warmer than that for all but a small fraction of the past 11,300 years. Surface temperature reconstructions of the past 1500 years suggest that recent warming is unprecedented in that time. Here we provide a broader perspective by reconstructing regional and global temperature anomalies for the past 11,300 years from 73 globally distributed records. Early Holocene (10,000 to 5000 years ago) warmth is followed by ~0.7°C cooling through the middle to late Holocene (<5000 years ago), culminating in the coolest temperatures of the Holocene during the Little Ice Age, about 200 years ago. This cooling is largely associated with ~2°C change in the North Atlantic. Current global temperatures of the past decade have not yet exceeded peak interglacial values but are warmer than during ~75% of the Holocene temperature history. Intergovernmental Panel on Climate Change model projections for 2100 exceed the full distribution of Holocene temperature under all plausible greenhouse gas emission scenarios.

Global climate evolution during the last deglaciation

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO2 and CH4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.

Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum

Last Glacial Maximum temperature reconstructions and model simulations can constrain the equilibrium climate sensitivity. Assessing the impact of future anthropogenic carbon emissions is currently impeded by uncertainties in our knowledge of equilibrium climate sensitivity to atmospheric carbon dioxide doubling. Previous studies suggest 3 kelvin (K) as the best estimate, 2 to 4.5 K as the 66% probability range, and nonzero probabilities for much higher values, the latter implying a small chance of high-impact climate changes that would be difficult to avoid. Here, combining extensive sea and land surface temperature reconstructions from the Last Glacial Maximum with climate model simulations, we estimate a lower median (2.3 K) and reduced uncertainty (1.7 to 2.6 K as the 66% probability range, which can be widened using alternate assumptions or data subsets). Assuming that paleoclimatic constraints apply to the future, as predicted by our model, these results imply a lower probability of imminent extreme climatic change than previously thought.

Northern Hemisphere forcing of Southern Hemisphere climate during the last deglaciation

According to the Milankovitch theory, changes in summer insolation in the high-latitude Northern Hemisphere caused glacial cycles through their impact on ice-sheet mass balance. Statistical analyses of long climate records supported this theory, but they also posed a substantial challenge by showing that changes in Southern Hemisphere climate were in phase with or led those in the north. Although an orbitally forced Northern Hemisphere signal may have been transmitted to the Southern Hemisphere, insolation forcing can also directly influence local Southern Hemisphere climate, potentially intensified by sea-ice feedback, suggesting that the hemispheres may have responded independently to different aspects of orbital forcing. Signal processing of climate records cannot distinguish between these conditions, however, because the proposed insolation forcings share essentially identical variability. Here we use transient simulations with a coupled atmosphere–ocean general circulation model to identify the impacts of forcing from changes in orbits, atmospheric CO2 concentration, ice sheets and the Atlantic meridional overturning circulation (AMOC) on hemispheric temperatures during the first half of the last deglaciation (22–14.3 kyr bp). Although based on a single model, our transient simulation with only orbital changes supports the Milankovitch theory in showing that the last deglaciation was initiated by rising insolation during spring and summer in the mid-latitude to high-latitude Northern Hemisphere and by terrestrial snow–albedo feedback. The simulation with all forcings best reproduces the timing and magnitude of surface temperature evolution in the Southern Hemisphere in deglacial proxy records. AMOC changes associated with an orbitally induced retreat of Northern Hemisphere ice sheets is the most plausible explanation for the early Southern Hemisphere deglacial warming and its lead over Northern Hemisphere temperature; the ensuing rise in atmospheric CO2 concentration provided the critical feedback on global deglaciation.

Latest Pleistocene advance of alpine glaciers in the southwestern Uinta Mountains, Utah, USA: Evidence for the influence of local moisture sources

Cosmogenic surface-exposure 10 Be dating of Last Glacial Maximum (LGM) moraines indicates that glaciers in the southwestern Uinta Mountains remained at their maximum positions until ca. 16.8 0.7 ka, 2 k.y. after glaciers in the neighboring Wind River Range and Colorado Rockies began to retreat. The timing of the local LGM in the south- western Uintas overlaps with both the hydrologic maximum of Lake Bonneville and pre- liminary estimates of the local LGM in the western Wasatch Mountains. This broad syn- chroneity indicates that Lake Bonneville and glaciers in northern Utah were responding to similar climate forcing. Furthermore, equilibrium line altitudes (ELAs) for reconstruct- ed LGM alpine glaciers increase with distance from the Lake Bonneville shoreline, rising from 2600 m to 3200 m over the 120 km length of the glaciated Uintas. This pro- nounced ELA gradient suggests that the magnitude of the latest Pleistocene glacial advance in the western Uintas was due, at least in part, to enhanced precipitation derived from Lake Bonneville; thus, the lake acted as a local amplifier of regional climate forcing. This relationship underscores the sensitivity of alpine glaciers to moisture availability during the latest Pleistocene, and further demonstrates the importance of local moisture sources on glacier mass balance.

Regional and global forcing of glacier retreat during the last deglaciation

The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global warming associated with rising greenhouse gas concentrations. By comparison, the importance of greenhouse gases in driving glacier retreat during the most recent deglaciation, the last major interval of global warming, is unclear due to uncertainties in the timing of retreat around the world. Here we use recently improved cosmogenic-nuclide production-rate calibrations to recalculate the ages of 1,116 glacial boulders from 195 moraines that provide broad coverage of retreat in mid-to-low-latitude regions. This revised history, in conjunction with transient climate model simulations, suggests that while several regional-scale forcings, including insolation, ice sheets and ocean circulation, modulated glacier responses regionally, they are unable to account for global-scale retreat, which is most likely related to increasing greenhouse gas concentrations.

A persistent and dynamic East Greenland Ice Sheet over the past 7.5 million years

Climate models show that ice-sheet melt will dominate sea-level rise over the coming centuries, but our understanding of ice-sheet variations before the last interglacial 125,000 years ago remains fragmentary. This is because terrestrial deposits of ancient glacial and interglacial periods are overrun and eroded by more recent glacial advances, and are therefore usually rare, isolated and poorly dated. In contrast, material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time-varying state of major ice sheets. Here we show that the East Greenland Ice Sheet existed over the past 7.5 million years, as indicated by beryllium and aluminium isotopes (10Be and 26Al) in quartz sand removed by deep, ongoing glacial erosion on land and deposited offshore in the marine sedimentary record. During the early Pleistocene epoch, ice cover in East Greenland was dynamic; in contrast, East Greenland was mostly ice-covered during the mid-to-late Pleistocene. The isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions. Although our data are continuous, they are from low-deposition-rate sites and sourced only from East Greenland. Consequently, the signal of extensive deglaciation during short, intense interglacials could be missed or blurred, and we cannot distinguish between a remnant ice sheet in the East Greenland highlands and a diminished continent-wide ice sheet. A clearer constraint on the behaviour of the ice sheet during past and, ultimately, future interglacial warmth could be produced by 10Be and 26Al records from a coring site with a higher deposition rate. Nonetheless, our analysis challenges the possibility of complete and extended deglaciation over the past several million years.

An early Pleistocene Mg/Ca‐δ18O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41‐kyr world

PUBLICATIONS Paleoceanography RESEARCH ARTICLE 10.1002/2016PA002956 Key Points: • Six ice sheet meltwater events identified in Gulf of Mexico seawater δ O record from 2.55-1.70 Ma • Events are typically long, occur late in benthic δ O deglaciations, and line up with summer insolation • This challenges view of early Pleistocene marine δ O as simply recording obliquity-driven Northern Hemisphere ice volume Supporting Information: • Supporting Information S1 • Supporting Information S2 Correspondence to: J. D. Shakun, jeremy.shakun@bc.edu Citation: Shakun, J. D., M. E. Raymo, and D. W. Lea (2016), An early Pleistocene Mg/Ca-δ O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world, Paleoceanography, 31, 1011–1027, doi:10.1002/2016PA002956. Received 25 MAR 2016 Accepted 5 JUL 2016 Accepted article online 11 JUL 2016 Published online 25 JUL 2016 An early Pleistocene Mg/Ca-δ 18 O record from the Gulf of Mexico: Evaluating ice sheet size and pacing in the 41-kyr world Jeremy D. Shakun 1 , Maureen E. Raymo 2 , and David W. Lea 3 Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, USA, 2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA, 3 Department of Earth Science, University of California, Santa Barbara, CA, USA Early Pleistocene glacial cycles in marine δ 18 O exhibit strong obliquity pacing, but there is a perplexing lack of precession variability despite its important influence on summer insolation intensity – the presumed forcing of ice sheet growth and decay according to the Milankovitch hypothesis. This puzzle has been explained in two ways: Northern Hemisphere ice sheets instead respond to insolation integrated over the summer, which is mostly controlled by obliquity, or anti-phased precession-driven variability in ice volume between the hemispheres cancels out in global δ 18 O, leaving the in-phase obliquity signal to dominate. We evaluated these ideas by reconstructing Laurentide Ice Sheet (LIS) meltwater discharge to the Gulf of Mexico from 2.55-1.70 Ma using foraminiferal Mg/Ca and δ 18 O. Our δ 18 O sw record displays six prominent anomalies, which likely reflect meltwater pulses, and they have several remarkable characteristics: (1) their presence suggests that the LIS expanded into the mid-latitudes numerous times; (2) they tend to occur or extend into interglacials in benthic δ 18 O; (3) they generally correlate with summer insolation intensity better than integrated insolation forcing; and (4) they are perhaps smaller in amplitude but longer in duration than their late Pleistocene counterparts, suggesting comparable total meltwater fluxes. Overall, these observations suggest that the LIS was large, sensitive to precession, and decoupled from marine δ 18 O numerous times during the early Pleistocene – observations difficult to reconcile with a straightforward interpretation of the early Pleistocene marine δ 18 O record as a proxy for Northern Hemisphere ice sheet size driven by obliquity forcing at high latitudes. Abstract 1. Introduction The Milankovitch hypothesis holds that ice sheets are sensitive to the intensity of summer insolation, which depends on both the tilt of the earth – which varies with the 41-kyr obliquity cycle – and the seasonal distance to the sun – which varies with the 23-kyr precession cycle. The Milankovitch model has had consid- erable success in explaining late Pleistocene ice volume variations of the past one million years, cycles that are concentrated at eccentricity, precession and obliquity frequencies as well as their multiples [Huybers, 2011; Imbrie and Imbrie, 1980; Raymo, 1997]. Nonetheless, the marine δ 18 O record suggests that the immedi- ately preceding glacial cycles of the late Pliocene-early Pleistocene (3–1 Ma) occurred at the almost purely 41-kyr pacing (Figure 1b) [Huybers, 2007; Pisias and Moore, 1981; Raymo and Nisancioglu, 2003; Ruddiman et al., 1989]. This 41-kyr world is difficult to reconcile with the Milankovitch hypothesis – why is precession variabil- ity absent in the early Pleistocene if summer insolation intensity controls ice sheet mass balance? ©2016. American Geophysical Union. All Rights Reserved. SHAKUN ET AL. Two hypotheses have been suggested to rectify the apparent conflict between the ice volume changes pre- dicted by Milankovitch forcing and those actually observed in the marine δ 18 O record. The Integrated Insolation hypothesis points out that the most intense summers are also the shortest, since the Earth orbits faster when closer to the sun [Huybers, 2006]. Since these competing precession-driven effects, intensity ver- sus duration, nearly cancel out when integrated over the course of the summer, one might not expect to see a strong precession signal in ice volume variability. The Antiphase hypothesis instead argues that ice sheets are driven by both obliquity and precession (as expressed in summer insolation intensity), but while obliquity is in phase between the hemispheres (i.e., increased axial tilt causes stronger summers in both hemispheres), precession forcing is anti-phased (i.e., when one hemisphere’s summer occurs closest to the sun, the other’s summer occurs farthest from the sun six months later) [Raymo et al., 2006]. Therefore, if a record of global ice volume, such as marine δ 18 O or sea level, was recording ice volume changes in both hemispheres, it would EARLY PLEISTOCENE MELTWATER EVENTS