Yarrow Axford

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Response of Jakobshavn Isbrae, Greenland, to Holocene climate change

Rapid fluctuations in the velocity of Greenland Ice Sheet (GIS) outlet glaciers over the past decade have made it difficult to extrapolate ice-sheet change into the future. This significant short-term variability highlights the need for geologic records of preinstrumental GIS margin fluctuations in order to better predict future GIS response to climate change. Using 10 Be surface exposure ages and radiocarbon-dated lake sediments, we constructed a detailed chronology of ice-margin fluctuations over the past 10 k.y. for Jakobshavn Isbrae, Greenland9s largest outlet glacier. In addition, we present new estimates of corresponding local temperature changes using a continuous record of insect (Chironomidae) remains preserved in lake sediments. We find that following an early Holocene advance just prior to 8 ka, Jakobshavn Isbrae retreated rapidly at a rate of ∼100 m yr −1 , likely in response to increasing regional and local temperatures. Ice remained behind its present margin for ∼7 k.y. during a warm period in the middle Holocene with sustained temperatures ∼2 °C warmer than today, then the land-based margin advanced at least 2–4 km between A.D. 1500–1640 and A.D. 1850. The ice margin near Jakobshavn thus underwent large and rapid adjustments in response to relatively modest centennial-scale Holocene temperature changes, which may foreshadow GIS response to future warming.

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 todays 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.

Response of a marine‐terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago

Greenland’s largest outlet glacier, using 10 Be surface exposure ages and 14 C‐dated lake sediments. Our chronology of ice‐ margin change demonstrates that Jakobshavn Isbrae advanced to deposit moraines in response to abrupt cooling recorded in central Greenland ice cores ca. 8,200 and 9,300 years ago. While the rapid, dynamically aided retreat of many Greenland outlet glaciers in response to warming is well documented, these results indicate that marine‐terminating outlet glaciers are also able to respond quickly to cooling. We suggest that short lag times of high ice flux margins enable a greater magnitude response of marine‐terminating outlets to abrupt

Chironomids can be reliable proxies for Holocene temperatures. A comment on Velle et al. (2010)

Velle et al. (2010) discussed discrepancies between Scandinavian Holocene chironomid-inferred temperature estimates, which they attribute to the response of chironomids to environmental variables other than temperature and to taxonomic shortcomings. They suggest ways in which the reliability of chironomid-based paleotemperature reconstructions could be improved by taking into account ecological complexity. While we agree with many of their recommendations, based on the results of other work, we think their paper is unnecessarily pessimistic regarding the ability of existing chironomid-based temperature inference models to provide reliable estimates of past temperature. We offer a critique of the main points discussed by Velle et al. (2010) and provide evidence that chironomid-based temperature inference models can reliably reconstruct mean July air temperature in the Lateglacial and Holocene over millennial and centennial timescales.

Late Quaternary eolian and alluvial response to paleoclimate, Canyonlands, southeastern Utah

began at ca. 6 ka and ended by ca. 3‐2 ka, followed by a shift to drier modern conditions; localized mobilization of dune sand has persisted to the present. These interpretations are similar to those of studies at the Chaco dune fi eld, New Mexico, and the Tusayan dune fi eld, Arizona, and are consistent with paleoclimate interpretations of pollen and packrat middens in the region. A period of rapid deposition and infi ltration of eolian dust derived from distant igneous source terranes occurred between ca. 12 and 8 ka. Before ca. 17 ka, and apparently back to at least 45 ka, paleosols contain little or no such infi ltrated dust. After ca. 8 ka, either the supply of dust was reduced or the more arid climate inhibited translocation of dust into the soils.

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.

Chironomids record terrestrial temperature changes throughout Arctic interglacials of the past 200,000 yr

Quaternary interglacial periods provide glimpses of a warmer Arctic and useful perspectives on possible future conditions, but records of Arctic terrestrial conditions over multiple interglacial periods are rare. Here, we take advantage of a site in the Canadian Arctic where lacustrine sediments representing the past three interglacial periods are preserved in an extant lake. We use subfossil insects (chironomids) preserved in this exceptional sedimentary archive to derive temperature reconstructions through the Holocene up to A.D. 2005, through the Last Interglacial sensu stricto (marine isotope stage or MIS 5e), and a portion of the penultimate interglacial (MIS 7). Chironomid-inferred temperatures are warmest for the early Holocene and MIS 5e, two periods with enhanced Northern Hemisphere insolation forcing relative to today. Twentieth-century warming at this site apparently caused the recent extirpation of cold stenothermous chironomid taxa. Assemblages from MIS 5e have close analogs in modern training set data as determined by squared-chord distance, and MIS 5e species assemblages are very similar to Holocene assemblages at this site. MIS 7 sediments record summer temperatures similar to those of the mid- to late Holocene, followed by a descent into glacial conditions. Even MIS 7 chironomid assemblages, dating back ∼200,000 yr, have close modern analogs. These lake sediments also provide direct evidence for a period of regional deglaciation between MIS 5e and the Holocene (most likely MIS 5a). To our knowledge, the data presented here represent the longest paleotemperature record thus far generated using chironomids. The existence of close modern analogs for ancient chironomid assemblages at Lake CF8 suggests that this method can provide useful paleotemperature estimates extending back hundreds of millennia.

Multiple generations of interglacial lake sediment preserved beneath the Laurentide Ice Sheet

It is generally assumed that regions glaciated by continental ice sheets offer little promise for long paleoenvironmental records due to erosional processes associated with glaciation. We show that beneath portions of the northeastern Laurentide Ice Sheet, characterized by cold- based glaciation, sediment sequences representing multiple interglaciations have been pre- served within extant lake basins. Radiocarbon and optically stimulated luminescence dating confi rm the antiquity of the sediments, thereby extending the terrestrial paleoenvironmental record of the Canadian Arctic by hundreds of thousands of years. The lake sediment record presented here corroborates numerous recent cosmogenic exposure dating studies indicating complex patterns of erosion beneath polar ice sheets. It also demonstrates that the presence of intact interglacial sediments does not demand unglaciated refugia. Similarly ancient sedi- ments may be preserved in many regions formerly covered by Pleistocene ice sheets.

Late Glacial and Holocene Glacier and Vegetation Fluctuations at Little Swift Lake, Southwestern Alaska, U.S.A

Abstract Multiproxy data from Little Swift Lake, an alpine lake in southwestern Alaska, provide evidence for pronounced late glacial and Holocene environmental change. An alpine glacier upvalley of Little Swift Lake retreated following the Younger Dryas chronozone, as evidenced by sedimentological changes in the lake record. Glacier retreat was accompanied by local and regional vegetation changes, including the expansion of Betula and contraction of Cyperaceae, in response to climatic amelioration. Warm, moist conditions between ∼9800 and 8000 cal yr B.P. supported abundant Betula shrubs and high lake and watershed productivity. Alnus rapidly expanded near Little Swift Lake while the region cooled between 8000 and 7500 cal yr B.P. Environmental changes at Little Swift Lake appear to have been roughly synchronous with similar changes elsewhere in southwestern Alaska, but late glacial and Holocene changes in other parts of Alaska were different in nature and timing. The complex spatial and temporal patterns of late glacial and Holocene environmental change throughout Alaska point to the importance of local- and regional-scale factors, especially controls on moisture availability, as modulators of site-specific responses to hemispheric- and global-scale climate forcing.