Ancient plant DNA reveals High Arctic greening during the Last Interglacial.

Summer warming is driving a greening trend across the Arctic, with the potential for large-scale amplification of climate change due to vegetation-related feedbacks [Pearson et al., Nat. Clim. Chang. (3), 673-677 (2013)]. Because observational records are sparse and temporally limited, past episodes of Arctic warming can help elucidate the magnitude of vegetation response to temperature change. The Last Interglacial ([LIG], 129,000 to 116,000 y ago) was the most recent episode of Arctic warming on par with predicted 21st century temperature change [Otto-Bliesner et al., Philos. Trans. A Math. Phys. Eng. Sci. (371), 20130097 (2013) and Post et al., SciAdv (5), eaaw9883 (2019)]. However, high-latitude terrestrial records from this period are rare, so LIG vegetation distributions are incompletely known. Pollen-based vegetation reconstructions can be biased by long-distance pollen transport, further obscuring the paleoenvironmental record. Here, we present a LIG vegetation record based on ancient DNA in lake sediment and compare it with fossil pollen. Comprehensive plant community reconstructions through the last and current interglacial (the Holocene) on Baffin Island, Arctic Canada, reveal coherent climate-driven community shifts across both interglacials. Peak LIG warmth featured a ∼400-km northward range shift of dwarf birch, a key woody shrub that is again expanding northward. Greening of the High Arctic-documented here by multiple proxies-likely represented a strong positive feedback on high-latitude LIG warming. Authenticated ancient DNA from this lake sediment also extends the useful preservation window for the technique and highlights the utility of combining traditional and molecular approaches for gleaning paleoenvironmental insights to better anticipate a warmer future.

Rate of mass loss from the Greenland Ice Sheet will exceed Holocene values this century.

The Greenland Ice Sheet (GIS) is losing mass at a high rate1. Given the short-term nature of the observational record, it is difficult to assess the historical importance of this mass-loss trend. Unlike records of greenhouse gas concentrations and global temperature, in which observations have been merged with palaeoclimate datasets, there are no comparably long records for rates of GIS mass change. Here we reveal unprecedented mass loss from the GIS this century, by placing contemporary and future rates of GIS mass loss within the context of the natural variability over the past 12,000 years. We force a high-resolution ice-sheet model with an ensemble of climate histories constrained by ice-core data2. Our simulation domain covers southwestern Greenland, the mass change of which is dominated by surface mass balance. The results agree favourably with an independent chronology of the history of the GIS margin3,4. The largest pre-industrial rates of mass loss (up to 6,000 billion tonnes per century) occurred in the early Holocene, and were similar to the contemporary (AD 2000-2018) rate of around 6,100 billion tonnes per century5. Simulations of future mass loss from southwestern GIS, based on Representative Concentration Pathway (RCP) scenarios corresponding to low (RCP2.6) and high (RCP8.5) greenhouse gas concentration trajectories6, predict mass loss of between 8,800 and 35,900 billion tonnes over the twenty-first century. These rates of GIS mass loss exceed the maximum rates over the past 12,000 years. Because rates of mass loss from the southwestern GIS scale linearly5 with the GIS as a whole, our results indicate, with high confidence, that the rate of mass loss from the GIS will exceed Holocene rates this century.

Deglaciation of the Pacific coastal corridor directly preceded the human colonization of the Americas.

The route and timing of early human migration to the Americas have been a contentious topic for decades. Recent paleogenetic analyses suggest that the initial colonization from Beringia took place as early as 16 thousand years (ka) ago via a deglaciated corridor along the North Pacific coast. However, the feasibility of such a migration depends on the extent of the western Cordilleran Ice Sheet (CIS) and the available resources along the hypothesized coastal route during this timeframe. We date the culmination of maximum CIS conditions in southeastern Alaska, a potential bottleneck region for human migration, to ~20 to 17 ka ago with cosmogenic 10Be exposure dating and 14C dating of bones from an ice-overrun cave. We also show that productive marine and terrestrial ecosystems were established almost immediately following deglaciation. We conclude that CIS retreat ensured that an open and ecologically viable pathway through southeastern Alaska was available after 17 ka ago, which may have been traversed by early humans as they colonized the Americas.

Greenland was nearly ice-free for extended periods during the Pleistocene.

The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres of global sea-level rise. Its stability in our warming climate is therefore a pressing concern. However, the sparse proxy evidence of the palaeo-stability of the GIS means that its history is controversial (compare refs 2 and 3 to ref. 4). Here we show that Greenland was deglaciated for extended periods during the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago), based on new measurements of cosmic-ray-produced beryllium and aluminium isotopes (10Be and 26Al) in a bedrock core from beneath an ice core near the GIS summit. Models indicate that when this bedrock site is ice-free, any remaining ice is concentrated in the eastern Greenland highlands and the GIS is reduced to less than ten per cent of its current volume. Our results narrow the spectrum of possible GIS histories: the longest period of stability of the present ice sheet that is consistent with the measurements is 1.1 million years, assuming that this was preceded by more than 280,000 years of ice-free conditions. Other scenarios, in which Greenland was ice-free during any or all Pleistocene interglacials, may be more realistic. Our observations are incompatible with most existing model simulations that present a continuously existing Pleistocene GIS. Future simulations of the GIS should take into account that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing.

Glacier maxima in Baffin Bay during the Medieval Warm Period coeval with Norse settlement.

The climatic mechanisms driving the shift from the Medieval Warm Period (MWP) to the Little Ice Age (LIA) in the North Atlantic region are debated. We use cosmogenic beryllium-10 dating to develop a moraine chronology with century-scale resolution over the last millennium and show that alpine glaciers in Baffin Island and western Greenland were at or near their maximum LIA configurations during the proposed general timing of the MWP. Complimentary paleoclimate proxy data suggest that the western North Atlantic region remained cool, whereas the eastern North Atlantic region was comparatively warmer during the MWP-a dipole pattern compatible with a persistent positive phase of the North Atlantic Oscillation. These results demonstrate that over the last millennium, glaciers approached their eventual LIA maxima before what is considered the classic LIA in the Northern Hemisphere. Furthermore, a relatively cool western North Atlantic region during the MWP has implications for understanding Norse migration patterns during the MWP. Our results, paired with other regional climate records, point to nonclimatic factors as contributing to the Norse exodus from the western North Atlantic region.

Glacier extent during the Younger Dryas and 8.2-ka event on Baffin Island, Arctic Canada.

Greenland ice cores reveal that mean annual temperatures during the Younger Dryas (YD) cold interval--about 12.9 to 11.7 thousand years ago (ka)--and the ~150-year-long cold reversal that occurred 8.2 thousand years ago were ~15° and 3° to 4°C colder than today, respectively. Reconstructing ice-sheet response to these climate perturbations can help evaluate ice-sheet sensitivity to climate change. Here, we report the widespread advance of Laurentide Ice Sheet outlet glaciers and independent mountain glaciers on Baffin Island, Arctic Canada, in response to the 8.2-ka event and show that mountain glaciers during the 8.2-ka event were larger than their YD predecessors. In contrast to the wintertime bias of YD cooling, we suggest that cooling during the 8.2-ka event was more evenly distributed across the seasons.

A holocene perspective on algal mercury scavenging to sediments of an Arctic lake.

Anthropogenic activities have increased the amount of mercury (Hg) transported atmospherically to the Arctic. At the same time, recent climate warming is altering the limnology of arctic lakes and ponds, including increases in aquatic primary production. It has been hypothesized that climate-driven increases in aquatic production have enhanced Hg scavenging from the water column, and that this mechanism may account for much of the recent rise in lake sediment Hg. Here, we test the relationship between climate, algal production, and sediment Hg using a well-dated and multiproxy lake sediment record spanning the Holocene from Lake CF3 (Baffin Island, Nunavut, Canada). During the early Holocene, peak (summer) insolation drove July air temperatures higher than present, and resulted in increased autochthonous primary production as recorded by total organic matter, spectrally inferred Chl-a, diatom abundance, and carbon stable isotopic signatures. However, there are no relationships between any of these proxies and sediment Hg concentrations during this interval. Given that the behavior of preindustrial Hg was relatively stable during past intervals of naturally mediated high production, we surmise that postindustrial increases in Hg accumulation within CF3 reflect a multiplicative effect of atmospheric deposition of anthropogenic Hg and increased sedimentation rates.

Recent changes in a remote Arctic lake are unique within the past 200,000 years.

The Arctic is currently undergoing dramatic environmental transformations, but it remains largely unknown how these changes compare with long-term natural variability. Here we present a lake sediment sequence from the Canadian Arctic that records warm periods of the past 200,000 years, including the 20th century. This record provides a perspective on recent changes in the Arctic and predates by approximately 80,000 years the oldest stratigraphically intact ice core recovered from the Greenland Ice Sheet. The early Holocene and the warmest part of the Last Interglacial (Marine Isotope Stage or MIS 5e) were the only periods of the past 200,000 years with summer temperatures comparable to or exceeding today's at this site. Paleoecological and geochemical data indicate that the past three interglacial periods were characterized by similar trajectories in temperature, lake biology, and lakewater pH, all of which tracked orbitally-driven solar insolation. In recent decades, however, the study site has deviated from this recurring natural pattern and has entered an environmental regime that is unique within the past 200 millennia.

Temporal trends of pollution Pb and other metals in east-central Baffin Island inferred from lake sediment geochemistry.

Concentrations and stable isotope ratios of lead (Pb) from lake sediments were used to quantify temporal patterns of anthropogenic Pb pollution in the Clyde River region of Baffin Island, Arctic Canada. Surface sediments from eight lakes on eastern Baffin Island and one from northern-most Greenland, spanning a gradient of 20 degrees latitude, showed great variability with respect to Pb concentration and stable isotopic Pb ratios, with little apparent latitudinal trend. To constrain the temporal evolution of regional Pb pollution, a well-dated core from one of the sites, Lake CF8 on east-central Baffin Island, was analyzed geochemically at high stratigraphic resolution. A pronounced decrease in the (206)Pb/(207)Pb ratio occurs in sediments deposited between 1923 and the mid-1970s, likely reflecting alkyl-Pb additives derived from the combustion of fossil fuels at a global scale. A two-component mixing model indicates that 17-26% of the Pb in the labile fraction of sediments deposited in Lake CF8 between 2001 and 2005 is from anthropogenic input. A Pb-Pb co-isotopic plot ((206)Pb/(207)Pb vs.(208)Pb/(206)Pb ratios) of the Lake CF8 time series data indicates multiple possible sources of industrial Pb pollution. Despite widespread reductions in industrial Pb emissions since the 1970s, there is no evidence for attendant reductions of pollution Pb at Lake CF8. Enhanced scavenging from increased primary production as well as changing precipitation rates as climate warms may represent important factors that modulate Pb deposition to Lake CF8, and Arctic lakes elsewhere.