Showing papers by "Nicholas P. McKay published in 2018"

A Wetter Arctic Coincident With Hemispheric Warming 8,000 Years Ago

Abstract: Arctic precipitation is predicted to increase this century, with dramatic consequences for high-latitude systems. Observations remain spatiotemporally limited, hampering determination of the forcings causing wetter Arctic conditions, although two mechanisms have been proposed: enhanced local evaporation and greater poleward atmospheric moisture transport. Here a subcentennial-resolution multiproxy lake sediment record from western Greenland sheds light on these mechanisms. Cool summers throughout the Northern Hemisphere and in western Greenland 9 to 8 ka are associated with aridity in this region, via reductions in local evaporation and in meridional moisture gradients, which suppressed poleward moisture transport. Summers became more humid starting 8.1 ka, mainly due to increased evaporation from warmer Arctic seas but also to increased poleward moisture transport caused by hemispheric warming. This record provides independent support for predictions of both enhanced local evaporation and increased poleward moisture transport causing wetter Arctic summers in step with global ocean and atmosphere warming. Plain Language Summary As the Arctic warms, it is getting wetter. This change can amplify warming worldwide by causing more plants to grow and decompose, releasing heat-trapping gases into the atmosphere. Sparse modern weather records in the Arctic make it difficult to pinpoint the forces causing increased rainfall, but scientists are debating two theories: (1) More water evaporates from warm, ice-free Arctic seas, and then falls locally as precipitation, or (2) as Earth warms, humidity rises more at lower latitudes, creating an imbalance that draws moist air up into the drier Arctic. Our research turns to history for insights. We show that in western Greenland, summers became cooler and drier about 9,000 years ago, coinciding with a drop in moisture imported from lower latitudes. Then, around 8,000 years ago, the region warmed rapidly and summers got wetter. A rise in both local evaporation and incoming moisture from lower latitudes may have fueled this change, according to our interpretation of geologic records. Our study suggests that both processes may contribute to a future, wetter Arctic. In addition, we advance scientific inquiry by using a recently developed technique, analysis of the hydrogen isotopes of ancient precipitation, to examine prehistoric humidity and precipitation trends that previously eluded investigation.

Placing the Common Era in a Holocene context: millennial to centennial patterns and trends in the hydroclimate of North America over the past 2000 years

Abstract: . A synthesis of 93 hydrologic records from across North and Central America, and adjacent tropical and Arctic islands, reveals centennial to millennial trends in the regional hydroclimates of the Common Era (CE; past 2000 years). The hydrological records derive from materials stored in lakes, bogs, caves, and ice from extant glaciers, which have the continuity through time to preserve low-frequency ( > 100 year) climate signals that may extend deeper into the Holocene. The most common pattern, represented in 46 (49 %) of the records, indicates that the centuries before 1000 CE were drier than the centuries since that time. Principal component analysis indicates that millennial-scale trends represent the dominant pattern of variance in the southwestern US, northeastern US, mid-continent, Pacific Northwest, Arctic, and tropics, although not all records within a region show the same direction of change. The Pacific Northwest and the southernmost tier of the tropical sites tended to dry toward present, as many other areas became wetter than before. In 22 records (24 %), the Medieval Climate Anomaly period (800–1300 CE) was drier than the Little Ice Age (1400–1900 CE), but in many cases the difference was part of the longer millennial-scale trend, and, in 25 records (27 %), the Medieval Climate Anomaly period represented a pluvial (wet) phase. Where quantitative records permitted a comparison, we found that centennial-scale fluctuations over the Common Era represented changes of 3–7 % in the modern interannual range of variability in precipitation, but the accumulation of these long-term trends over the entirety of the Holocene caused recent centuries to be significantly wetter, on average, than most of the past 11 000 years.

Fluvial suspended sediment yields over hours to millennia in the High Arctic at proglacial Lake Linnévatnet, Svalbard

Abstract: Sediment yield can be a sensitive indicator of catchment dynamics and environmental change. For a glacierized catchment in the High Arctic, we compiled and analyzed diverse sediment transfer data, spanning a wide range of temporal scales, to quantify catchment yields and explore landscape response to past and ongoing hydroclimatic variability. The dataset integrates rates of lake sedimentation from correlated varve records and repeated annual and seasonal sediment traps, augmented by multi-year lake and fluvial monitoring. Consistent spatial patterns of deposition enabled reconstruction of catchment yields from varve- and trap-based fluxes. We used hydroclimatic data and multivariate modeling to examine annual controls of sediment delivery over almost a century, and to examine shorter-term controls of sediment transfer during peak glacier melt. Particle-size analyses, especially for annual sediment traps, were used to further infer sediment transfer mechanisms and timing. Through the Medieval Warm Period and Little Ice Age, there were no apparent multi-century trends in lake sedimentation rates, which were over three times greater than those during the mid-Holocene when glaciers were diminished. Twentieth-century sedimentation rates were greater than those of previous millennia, with a mid-century step increase in mean yield from 240 to 425 Mg km-2 yr-1. Annual yields through the twentieth century showed significant positive relations with spring/summer temperature, rainfall, and peak discharge conditions. This finding is significant for the future of sediment transfer at Linnevatnet, and perhaps more broadly in the Arctic, where continued increases in temperature and rainfall are projected. For 2004-2010, annual yields ranged from 294 to 1330 Mg km-2 yr-1. Sediment trap volumes and particle-size variations indicate that recent annual yields were largely dominated by spring to early summer transfer of relatively coarse-grained sediment. Fluvial monitoring showed daily to hourly sediment transfer to be related to current and prior discharge, diurnal hysteresis, air temperature, and precipitation.

A high-resolution climate record spanning the past 17 000 years recovered from Lake Ohau, South Island, New Zealand

Abstract: . A new annually resolved sedimentary record of Southern Hemisphere mid-latitude hydroclimate was recovered from Lake Ohau, South Island, New Zealand, in March 2016. The Lake Ohau Climate History (LOCH) project acquired cores from two sites (LOCH-1 and -2) that preserve sequences of laminated mud that accumulated since the lake formed ∼ 17 000 years ago. Cores were recovered using a purpose-built barge and drilling system designed to recover soft sediment from thick sedimentary sequences in lake systems up to 150 m deep. This system can be transported in two to three 40 ft long shipping containers and is suitable for use in a range of geographic locations. A comprehensive suite of data has been collected from the sedimentary sequence using state-of-the-art analytical equipment and techniques. These new observations of past environmental variability augment the historical instrumental record and are currently being integrated with regional climate and hydrological modelling studies to explore causes of variability in extreme/flood events over the past several millennia.