Shaun A. Marcott

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.

Ice-shelf collapse from subsurface warming as a trigger for Heinrich events

Episodic iceberg-discharge events from the Hudson Strait Ice Stream (HSIS) of the Laurentide Ice Sheet, referred to as Heinrich events, are commonly attributed to internal ice-sheet instabilities, but their systematic occurrence at the culmination of a large reduction in the Atlantic meridional overturning circulation (AMOC) indicates a climate control. We report Mg/Ca data on benthic foraminifera from an intermediate-depth site in the northwest Atlantic and results from a climate-model simulation that reveal basin-wide subsurface warming at the same time as large reductions in the AMOC, with temperature increasing by approximately 2 °C over a 1–2 kyr interval prior to a Heinrich event. In simulations with an ocean model coupled to a thermodynamically active ice shelf, the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the HSIS by a factor of approximately 6. By analogy with recent observations in Antarctica, the resulting ice-shelf loss and attendant HSIS acceleration would produce a Heinrich event.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

Significance Antarctic ice cores provide a precise, well-dated history of increasing atmospheric CO2 during the last glacial to interglacial transition. However, the mechanisms that drive the increase remain unclear. Here we reconstruct a key indicator of the sources of atmospheric CO2 by measuring the stable isotopic composition of CO2 in samples spanning the period from 22,000 to 11,000 years ago from Taylor Glacier, Antarctica. Improvements in precision and resolution allow us to fingerprint CO2 sources on the centennial scale. The data reveal two intervals of rapid CO2 rise that are plausibly driven by sources from land carbon (at 16.3 and 12.9 ka) and two others that appear fundamentally different and likely reflect a combination of sources (at 14.6 and 11.5 ka). An understanding of the mechanisms that control CO2 change during glacial–interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO2 (δ13C-CO2) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data demarcate a decrease in δ13C-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO2 rise of 40 ppm is associated with small changes in δ13C-CO2, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ13C-CO2 that suggest rapid oxidation of organic land carbon or enhanced air–sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO2 coincident with increases in atmospheric CH4 and Northern Hemisphere temperature at the onset of the Bølling (14.6–14.3 ka) and Holocene (11.6–11.4 ka) intervals are associated with small changes in δ13C-CO2, suggesting a combination of sources that included rising surface ocean temperature.

Northern Hemisphere forcing of the last deglaciation in southern Patagonia

Although the general patterns of deglacial climate change are relatively well constrained, how, and to what magnitude, large parts of the Southern Hemisphere responded to deglacial forcings remains unknown, particularly for the early part of the last deglaciation. We investigate the timing and magnitude of early deglacial climate change using cosmogenic 10 Be surface exposure ages of moraines deposited by glaciers in the Rio Guanaco Valley, adjacent to the Southern Patagonian Ice Field at 50°S. We demonstrate that the beginning of ice retreat from the local last glacial maximum occurred at 19.7 ± 1.1 ka, with significant retreat commencing at 18.9 ± 0.4 ka, concurrent with glacier retreat elsewhere in southern Patagonia and New Zealand and with warming of Southern Hemisphere middle to high latitudes. A third moraine shows that half of the deglacial retreat upvalley had occurred by 17.0 ± 0.3 ka. Equilibrium line altitudes and climate simulations show ∼1.5 °C of warming in southern Patagonia between 18.9 ± 0.4 ka and 17.0 ± 0.3 ka, one-third of the total estimated deglacial warming relative to present. The climate model links this warming to retreat of Northern Hemisphere ice sheets ca. 19 ka through changes in ocean circulation that caused a bipolar seesaw response resulting in Southern Hemisphere warming and driving initial deglaciation across southern Patagonia.

Effects of seafloor diagenesis on planktic foraminiferal radiocarbon ages

Radiocarbon (14C) ages obtained from planktic foraminiferal calcite are a mainstay for reconstructing ocean-climate change and carbon cycle dynamics of the past 30 k.y., yet the effects of diagenesis on this vital chronometer are poorly constrained. Here, we address this shortcoming by comparing 14C ages and trace element ratios (Mg/Ca, Mn/Ca) of planktic foraminifera with white, opaque shells deemed well preserved by traditional standards to those with exquisitely preserved translucent shells. Results support a diagenetic mechanism as opaque shells yield 14C ages invariably older and trace element ratios consistently higher than those of translucent shells. Radiocarbon age offsets are particularly pronounced in mono-specific samples taken from stratigraphic horizons proximal to the δ18O maximum marking the Last Glacial Maximum (LGM) and the subsequent deglacial. Radiocarbon-based calendar ages of translucent shells from the two intervals are congruent with the established age ranges for these climate events, whereas those of co-occurring opaque shells overestimate the LGM and deglacial by 8–15 k.y. and 14–22 k.y., respectively. These results demonstrate that the use of translucent foraminifera enhances reproducibility and accuracy of 14C ages by minimizing the deleterious effects of diagenesis. This study serves as a cautionary tale since white, opaque foraminifera are common in pelagic sediments, and 14C ages derived from their ostensibly well-preserved shells can lead to discrepancies in the timing of Quaternary climate events and ocean circulation reconstructions.

Persistent millennial-scale glacier fluctuations in Ireland between 24 ka and 10 ka

We report 80 10Be ages on 14 moraines from Irish cirques that show a previously unrecognized signal of at least eight millennial-scale fluctuations between 24.5±0.7 ka and 11.0±0.3 ka. Several moraine ages may be correlative with abrupt warming at the onset of the Bolling (14.7 ka) and the end of the Younger Dryas (11.7 ka), suggesting a forced response. Our ages also identify glacier fluctuations that occurred when regional temperatures were relatively stable. This finding is consistent with modeling results showing several hundred-meter-scale glacier fluctuations in response to interannual variability. At the same time, our composite record of cirque-glacier average equilibrium line altitudes (ELAs) shows a response to warming due to increasing greenhouse gases and summer insolation modulated by abrupt climate changes. Our new 10Be chronology thus records both forced and unforced millennial-scale glacier fluctuations superimposed on a lower frequency ELA signal of forced response to climate change.

Asynchronous warming and δ18O evolution of deep Atlantic water masses during the last deglaciation

Significance The reorganizations of deep Atlantic water masses are widely thought to regulate glacial–interglacial climate changes. However, the pattern of reorganizations and their impact on ocean tracer transport remain poorly constrained by marine proxies. Our modeling study, which simulates the coevolution of water masses and oxygen isotopes during the last deglaciation, suggests that deglacial meltwater input causes both northern- and southern-sourced deep water transports to decrease. This reorganization pattern leads to asynchronous warming between the deep North and South Atlantic, which might have caused the observed deglacial phasing difference in deep water oxygen isotope records between these ocean basins. We further propose a mechanism to explain the early warming in the northern North Atlantic. The large-scale reorganization of deep ocean circulation in the Atlantic involving changes in North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) played a critical role in regulating hemispheric and global climate during the last deglaciation. However, changes in the relative contributions of NADW and AABW and their properties are poorly constrained by marine records, including δ18O of benthic foraminiferal calcite (δ18Oc). Here, we use an isotope-enabled ocean general circulation model with realistic geometry and forcing conditions to simulate the deglacial water mass and δ18O evolution. Model results suggest that, in response to North Atlantic freshwater forcing during the early phase of the last deglaciation, NADW nearly collapses, while AABW mildly weakens. Rather than reflecting changes in NADW or AABW properties caused by freshwater input as suggested previously, the observed phasing difference of deep δ18Oc likely reflects early warming of the deep northern North Atlantic by ∼1.4 °C, while deep Southern Ocean temperature remains largely unchanged. We propose a thermodynamic mechanism to explain the early warming in the North Atlantic, featuring a strong middepth warming and enhanced downward heat flux via vertical mixing. Our results emphasize that the way that ocean circulation affects heat, a dynamic tracer, is considerably different from how it affects passive tracers, like δ18O, and call for caution when inferring water mass changes from δ18Oc records while assuming uniform changes in deep temperatures.

Early to Late Holocene Surface Exposure Ages From Two Marine‐Terminating Outlet Glaciers in Northwest Greenland

Terrestrial chronologies from southern Greenland provide a detailed deglacial history of the Greenland Ice Sheet (GIS). The northern GIS margin history, however, is less established. Here we present surface exposure ages from moraines associated with two large outlet glaciers, Petermann and Humboldt, in the northwestern sector of the GIS. These moraine chronologies indicate a Little Ice Age advance of the ice sheet margin before ~0.3 ka and a possible equivalent advance of similar magnitude prior to ~2.8 ka. An early Holocene moraine at Humboldt Glacier was abandoned by 8.3 ± 1.7 ka and is contemporaneous with other moraines deposited along the entire western GIS margin. This widespread ice margin stability between ~9 and 8 ka indicates that while this margin was influenced by warming atmospheric temperatures during the early Holocene, the warming was likely overprinted with the effect of the abrupt climate cooling at 9.3 and 8.2 ka. Plain Language Summary The global climate is warming, and the Greenland Ice Sheet is responding. A more complete understanding of this process is needed to better predict its future response to climate change. We determine how the ice sheet changed following the last ice age in northwest Greenland. The northwest sector of the ice sheet retreated to the coast by ~10,000 years ago during a period of warming atmospheric temperatures. About 8,300 years ago the ice stopped retreating despite relatively high atmospheric temperatures. A similar standstill occurred in areas along western Greenland between ~9,000 and 8,000 years ago. This suggests that despite the long-term warming, well-known abrupt cooling events that occurred in the region at this time influenced the ice sheet margin and temporarily stopped the long-term pattern of ice retreat. The ice sheet retreated after 8,300 years ago and then advanced during the latest cold period, the Little Ice Age (1350–1850 CE), in a fashion similar to elsewhere in Greenland. Our study finds that the Greenland Ice Sheet margins are sensitive to both long-term (>1,000 years) and short-term (<100 years) atmospheric temperature changes. This sensitivity of the ice margin has important implications when assessing ongoing and future ice loss today.