Elizabeth K. Thomas

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.

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.

Leaf wax δ2H and varve-thickness climate proxies from proglacial lake sediments, Baffin Island, Arctic Canada

We present a multiproxy paleoclimate record using leaf wax hydrogen isotopes (δ2Hwax) and varve thickness from Arctic proglacial lake sediments. We also provide one of the first evaluations of the utility of δ2Hwax as a paleoclimate proxy in Arctic proglacial lakes. We compare varve thickness and δ2Hwax at sub-decadal resolution from 1948 to 2004 AD, and at sub-centennial resolution from 1450 to 2004 AD. Varve thickness and δ2Hwax both contain large interannual variability and are anti-correlated during the late twentieth century, suggesting that both proxies respond rapidly, but by different mechanisms, to catchment-scale forcings. At longer time scales, varve thickness exhibits a strong response to Little Ice Age cooling (1661–1827 AD in this record) but does not show evidence for twentieth century warming recorded throughout the Arctic. δ2Hwax does record regional-scale temperature changes, with more 2H-depleted values during the Little Ice Age and an abrupt change to more 2H-enriched values in the twentieth century. This corresponds well with a recent Arctic-wide temperature reconstruction in which the seventeenth century is the coldest interval, and the twentieth century is the warmest interval. Our results suggest that δ2Hwax is a promising proxy that can be applied at high resolution in proglacial Arctic lakes.

Temperature and leaf wax δ2H records demonstrate seasonal and regional controls on Asian monsoon proxies

Orbital-scale precipitation isotope records can elucidate climate forcing mechanisms and provide benchmarks for climate model validation. The ability to differentiate the influence of temperature, seasonality, and vapor transport history on precipitation isotope proxies is critical to both objectives. We present a 300 k.y. leaf wax hydrogen isotope record from the South China Sea with the effects of local condensation temperature removed (δ 2 H wax– T ). δ 2 H wax– T reflects annually integrated precipitation δ 2 H in the Pearl River catchment of southeast China. Depleted δ 2 H wax– T lags minimum precession (P min ) by 1.0 ± 0.7 k.y., reflecting the influence of maximum summer insolation and minimum winter insolation, with a minor influence of global ice volume, which lags P min by 3.3 ± 0.4 k.y. In contrast, annually integrated cave δ 18 O minima in central China, 1000 km north of our site, lag P min by 2.7 ± 0.3 k.y., in phase with ice volume minima. This phase indicates that precipitation δ 18 O in central China is more strongly influenced by ice volume forcing at the precession band, with a lesser influence of Northern Hemisphere insolation. Our new δ 2 H wax– T data demonstrate that precipitation isotopes in Asia have strong regional variability. Interpreting water isotope records within the context of regionally varying temperature, seasonality, and sensitivity to changing glacial boundary conditions is imperative to understanding Asian hydroclimatic change.

A major increase in winter snowfall during the middle Holocene on western Greenland caused by reduced sea ice in Baffin Bay and the Labrador Sea

Precipitation is predicted to increase in the Arctic as temperature increases and sea ice retreats. Yet the mechanisms controlling precipitation in the Arctic are poorly understood and quantified only by the short, sparse instrumental record. We use hydrogen isotope ratios (δ2H) of lipid biomarkers in lake sediments from western Greenland to reconstruct precipitation seasonality and summer temperature during the past 8 kyr. Aquatic biomarker δ2H was 100‰ more negative from 6 to 4 ka than during the early and late Holocene, which we interpret to reflect increased winter snowfall. The middle Holocene also had high summer air temperature, decreased early winter sea ice in Baffin Bay and the Labrador Sea, and a strong, warm West Greenland Current. These results corroborate model predictions of winter snowfall increases caused by sea ice retreat and furthermore suggest that warm currents advecting more heat into the polar seas may enhance Arctic evaporation and snowfall.

Local glaciation in West Greenland linked to North Atlantic Ocean circulation during the Holocene

Recent observations indicate that ice-ocean interaction drives much of the recent increase in mass loss from the Greenland Ice Sheet; however, the role of ocean forcing in driving past glacier change is poorly understood. To extend the observational record and our understanding of the ocean-cryosphere link, we used a multi-proxy approach that combines new data from proglacial lake sediments, 14 C-dated in situ moss that recently emerged from beneath cold-based ice caps, and 10 Be ages to reconstruct centennial-scale records of mountain glacier activity for the past ∼10 k.y. in West Greenland. Proglacial lake sediment records and 14 C dating of moss indicate the onset of Neoglaciation in West Greenland at ca. 5 ka with substantial snowline lowering and glacier expansion at ca. 3.7 ka followed by additional ice expansion phases at ca. 2.9, ca. 1.7, and ca. 1.4 ka and during the Little Ice Age. We find that widespread glacier growth at ca. 3.7 ka in West Greenland coincides with marked cooling and reduced strength of the West Greenland Current in Disko Bugt. The transition to cooler ocean conditions at ca. 3.7 ka identified in Disko Bugt is registered by marine proxy data farther afield in East Greenland and on the northwestern Icelandic shelf, implying large-scale paleoceanographic changes across the North Atlantic during this interval. The similarity between glacier change on West Greenland and multiple marine and terrestrial records across the North Atlantic suggests that glaciers are strongly influenced by changes in ocean circulation and consequently implies that the ocean-cryosphere teleconnection is a persistent feature of the Arctic system.