Is MWP 1A Real and Could It have Originated in the Northern Hemisphere in Response to Bolling Warming

Meltwater Pulse 1A (MWP 1A) is thought to have encompassed an abrupt rise in sea-level of 19 - 24 m ca. 14,000 calendar years B.P. The postulated rate of sea-level rise during the event was 24 - 50 mrnJyr. In contrast, the average rate of change during the overall glacial termination was 13 mrn/yr. Although MWP 1A is commonly accepted at face value, a compilation of all basic data points casts doubts on its validity. The Laurentide Ice Sheet (LIS) is commonly cited as the source of MWP 1A. Accelerated discharge of freshwater from the ice sheet could have weakened the formation of North Atlantic deep water. However, with the exception of sea-water freshening observed in cores from the Gulf of Mexico and Bermuda Rise, there is little geologic evidence to support a pulse from the LIS during the time of MWP 1A. An alternative suggestion is that MWP 1A originated in Antarctica. One possibility is that a meltwater pulse could have been derived by exploding ice shelves followed by accelerated seaward discharge from ice streams. However, Antarctica contributed at most 18 m, but probably less than 14 m to total LGM sea-level lowering. Furthermore, West Antarctica, which contained two-thirds of the excess Antarctic ice volume at the LGM, did not undergo substantial deglaciation until mid to late-Holocene time, well after MWP 1A. The University of Majne Ice Sheet Model (UMISM) was employed to explore the physical plausibility of MWP 1A coming from the LIS. A GISP2 record of temperature was used as input to drive mass-balance calculations. However, the extremely low temperatures in the climate signal prevented the model from generating a realistic ice sheet. Based on similarities to sea-level reconstructions, the EPICA Dome C ice core record was then used as an alternate proxy for climate. Results indicate that a 10.5 m meltwater pulse is possible in response to climatic warming at the beginning of the Bolling-Allerijd interstadial. However, in order to achieve this result, it was necessary to superimpose on the EPICA forcing a modem climate regime over the ice sheet for 300 model years. Thus both geological and glaciological results cast serious doubts on the existence of MWP 1A. Even if the constraining sea-level data are interpreted as permitting such a pulse, the glaciological model input must be stimulated by artificial warming superimposed on an Antarctic signal to produce both the correct termination and the postulated magnitude and timing of MWP 1A

James L. Fastook, Professor of Computer ScienceIS MWP 1A REAL AND COULD IT HAVE ORIGINATED IN THE NORTHERN HEMISPHERE IN RESPONSE TO

Meltwater Pulse 1A (MWP 1A) is thought to have encompassed an abrupt rise in sea-level of 19- 24 m ca. 14,000 calendar years B.P. The postulated rate of sea-level rise during the event was 24- 50 mm/yr. In contrast, the average rate of change during the overall glacial termination was 13 mm/yr. Although MWP 1A is commonly accepted at face value, a compilation of all basic data points casts doubts on its validity. The Laurentide Ice Sheet (LIS) is commonly cited as the source of MWP 1A. Accelerated discharge of freshwater from the ice sheet could have weakened the formation of North Atlantic deep water. However, with the exception of sea-water freshening observed in cores from the Gulf of Mexico and Bermuda Rise, there is little geologic evidence to support a pulse from the LIS during the time of MWP 1A. An alternative suggestion is that MWP 1A originated in Antarctica. One possibility is that a meltwater pulse could have been derived by exploding ice shelves followed by accelerated seaward discharge from ice streams. However, Antarctica contributed atmost 18 m, but probably less than 14 m to total LGM sea-level lowering. Furthermore, West Antarctica, which contained two-thirds of the excess Antarctic ice volume at th

An Ensemble Mean and Evaluation of Third Generation Global Climate Reanalysis Models

We have produced a global ensemble mean of the four third-generation climate reanalysis models for the years 1981–2010. The reanalysis system models used in this study are National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis Interim (ERA-I), Japan Meteorological Agency (JMA) 55-year Reanalysis (JRA-55), and National Aeronautics and Space Administration (NASA) Modern-Era Retrospective Analysis for Research and Applications (MERRA). Two gridded datasets are used as a baseline, for temperature the Global Historical Climatology Network (GHCN), and for precipitation the Global Precipitation Climatology Centre (GPCC). The reanalysis ensemble mean is used here as a comparison tool of the four reanalysis members. Meteorological fields investigated within the reanalysis models include 2-m air temperature, precipitation, and 500-hPa geopotential heights. Comparing the individual reanalysis models to the ensemble mean, we find that each perform similarly over large domains but exhibit significant differences over particular regions

Extreme weather years drive episodic changes in lake chemistry: implications for recovery from sulfate deposition and long-term trends in dissolved organic carbon

Interannual climate variability is expected to increase over the next century, but the extent to which hydroclimatic variability influences biogeochemical processes is unclear. To determine the effects of extreme weather on surface water chemistry, a 30-year record of surface water geochemistry for 84 lakes in the northeastern U.S. was combined with landscape data and watershed-specific weather data. With these data, responses in sulfate (SO42−) and dissolved organic carbon (DOC) concentrations were characterized during an extreme wet year and an extreme dry year across the region. Redundancy analysis was used to model lake chemical response to extreme weather as a function of watershed features. A response was observed in DOC and SO42− concentration in response to extreme wet and dry years in lakes across the northeastern U.S. Acidification was observed during drought and brownification was observed during wet years. Lake chemical response was related to landscape characteristics in different ways depending on the type of extreme year. A linear relationship between wetland coverage and DOC and SO42− deviations was observed during extreme wet years. The results presented here help to clarify the variability observed in long-term recovery from acidification and regional increases in DOC. Understanding the chemical response to weather variability is becoming increasingly important as temporal variation in precipitation is likely to intensify with continued atmospheric warming

Younger Dryas deglaciation of Scotland driven by warming summers

The Younger Dryas Stadial (YDS; similar to 12,900-11,600 y ago) in the Northern Hemisphere is classically defined by abrupt cooling and renewed glaciation during the last glacial- interglacial transition. Although this event involved a global reorganization of atmospheric and oceanic circulation [Denton GH, Alley RB, Comer GC, Broecker WS (2005) Quat Sci Rev 24: 1159-1182], the magnitude, seasonality, and geographical footprint of YDS cooling remain unresolved and pose a challenge to our understanding of abrupt climate change. Here, we present a deglacial chronology from Scotland, immediately downwind of the North Atlantic Ocean, indicating that the Scottish ice cap disintegrated during the first half of the YDS. We suggest that stratification of the North Atlantic Ocean resulted in amplified seasonality that, paradoxically, stimulated a severe wintertime climate while promoting warming summers through solar heating of the mixed layer. This latter process drove deglaciation of downwind landmasses to completion well before the end of the YDS.This research is supported by grants from the Dan and Betty Churchill Exploration Fund and the Lamont–Doherty Earth Observatory (LDEO) Climate Center. G.R.M.B. was supported by a LDEO postdoctoral fellowship. A.E.P. was supported by the Gary Comer Science and Education Foundation, the National Oceanographic and Atmospheric Administration, and a LDEO postdoctoral fellowship. This is LDEO contribution no. 7772