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Nathaniel B. Miller
,
Matthew D. Shupe
,
Christopher J. Cox
,
Von P. Walden
,
David D. Turner
, and
Konrad Steffen

Abstract

The surface energy budget plays a critical role in determining the mass balance of the Greenland Ice Sheet, which in turn has significant implications for global sea levels. Nearly three years of data (January 2011–October 2013) are used to characterize the annual cycle of surface radiative fluxes and cloud radiative forcing (CRF) from the central Greenland Ice Sheet at Summit Station. The annual average CRF is 33 W m−2, representing a substantial net cloud warming of the central Greenland surface. Unlike at other Arctic sites, clouds warm the surface during the summer. The surface albedo is high at Summit throughout the year, limiting the cooling effect of the shortwave CRF and thus the total CRF is dominated by cloud longwave warming effects in all months. All monthly mean CRF values are positive (warming), as are 98.5% of 3-hourly cases. The annual cycle of CRF is largely driven by the occurrence of liquid-bearing clouds, with a minimum in spring and maximum in late summer. Optically thick liquid-bearing clouds [liquid water path (LWP) > 30 g m−2] produce an average longwave CRF of 85 W m−2. Shortwave CRF is sensitive to solar zenith angle and LWP. When the sun is well above the horizon (solar zenith angle < 65°), a maximum cloud surface warming occurs in the presence of optically thin liquid-bearing clouds. Ice clouds occur frequently above Summit and have mean longwave CRF values ranging from 10 to 60 W m−2, dependent on cloud thickness.

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Sebastian H. Mernild
,
David M. Holland
,
Denise Holland
,
Aqqalu Rosing-Asvid
,
Jacob C. Yde
,
Glen E. Liston
, and
Konrad Steffen

Abstract

The distribution of terrestrial surface runoff to Ilulissat Icefjord, west Greenland, is simulated for the period 2009–13 to better emphasize the spatiotemporal variability in freshwater flux and the link between runoff spikes and observed hydrographic conditions at the Greenland Ice Sheet tidewater glacier margins. Runoff model simulations were forced with automatic weather station data and verified against snow water equivalent depth, equilibrium line altitude, and quasi-continuous salinity and temperature observations obtained by ringed seals. Instrumented seals provide a novel platform to examine the otherwise inaccessible waters beneath the dense ice mélange within the first 0–10 km of the calving front. The estimated mean freshwater flux from land was 70.6 ± 4.2 km3 yr−1, with an 85% contribution of ice discharge from Jakobshavn Isbrae (also known as Sermeq Kujalleq), 14% from runoff, and the remaining 1% from precipitation on the fjord surface area, subglacial geothermal melting, and frictional melting due to basal ice motion. Runoff was simulated to be present from May to November and to vary spatially according to glacier cover and individual catchment size. Salinity and temperature observations correlate (significantly) with simulated runoff for the upper part of both the main fjord and southern fjord arm. Also, at the tidewater glacier margins in the northern and southern arm of Ilulissat Icefjord, salinity changes in the upper water column (upper 50 m) are significant after temporal spikes in runoff during late summer, while small-amplitude runoff variability during the recession of runoff did not create a clear signal in observed salinity variability. Also, in the southern arm near the glacier margin (between 100- and 150-m depth), the heterogeneous distribution in salinity could be because of the mixing of meltwater going upward from passing the grounding line. The effect of runoff spikes on observed salinity is less pronounced near the ice margin of Jakobshavn Isbrae than in the north and south arms.

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Jason E. Box
,
David H. Bromwich
,
Bruce A. Veenhuis
,
Le-Sheng Bai
,
Julienne C. Stroeve
,
Jeffrey C. Rogers
,
Konrad Steffen
,
T. Haran
, and
Sheng-Hung Wang

Abstract

Regional climate model runs using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesocale Model modified for use in polar regions (Polar MM5), calibrated by independent in situ observations, demonstrate coherent regional patterns of Greenland ice sheet surface mass balance (SMB) change over a 17-yr period characterized by warming (1988–2004). Both accumulation and melt rates increased, partly counteracting each other for an overall negligible SMB trend. However, a 30% increase in meltwater runoff over this period suggests that the overall ice sheet mass balance has been increasingly negative, given observed meltwater-induced flow acceleration. SMB temporal variability of the whole ice sheet is best represented by ablation zone variability, suggesting that increased melting dominates over increased accumulation in a warming scenario. The melt season grew in duration over nearly the entire ablation zone by up to 40 days, 10 days on average. Accumulation area ratio decreased by 3%. Albedo reductions are apparent in five years of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived data (2000–04). The Advanced Very High Resolution Radiometer (AVHRR)-derived albedo changes (1988–99) were less consistent spatially. A conservative assumption as to glacier discharge and basal melting suggests an ice sheet mass loss over this period greater than 100 km3 yr−1, framing the Greenland ice sheet as the largest single glacial contributor to recent global sea level rise. Surface mass balance uncertainty, quantified from residual random error between model and independent observations, suggests two things: 1) changes smaller than approximately 200 km3 yr−1 would not satisfy conservative statistical significance thresholds (i.e., two standard deviations) and 2) although natural variability and model uncertainty were separated in this analysis, the magnitude of each were roughly equivalent. Therefore, improvements in model accuracy and analysis of longer periods (assuming larger changes) are both needed for definitive mass balance change assessments.

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Sarah J. Doherty
,
Stephan Bojinski
,
Ann Henderson-Sellers
,
Kevin Noone
,
David Goodrich
,
Nathaniel L. Bindoff
,
John A. Church
,
Kathy A. Hibbard
,
Thomas R. Karl
,
Lucka Kajfez-Bogataj
,
Amanda H. Lynch
,
David E. Parker
,
I. Colin Prentice
,
Venkatachalam Ramaswamy
,
Roger W. Saunders
,
Mark Stafford Smith
,
Konrad Steffen
,
Thomas F. Stocker
,
Peter W. Thorne
,
Kevin E. Trenberth
,
Michel M. Verstraete
, and
Francis W. Zwiers

The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) concluded that global warming is “unequivocal” and that most of the observed increase since the mid-twentieth century is very likely due to the increase in anthropogenic greenhouse gas concentrations, with discernible human influences on ocean warming, continental-average temperatures, temperature extremes, wind patterns, and other physical and biological indicators, impacting both socioeconomic and ecological systems. It is now clear that we are committed to some level of global climate change, and it is imperative that this be considered when planning future climate research and observational strategies. The Global Climate Observing System program (GCOS), the World Climate Research Programme (WCRP), and the International Geosphere-Biosphere Programme (IGBP) therefore initiated a process to summarize the lessons learned through AR4 Working Groups I and II and to identify a set of high-priority modeling and observational needs. Two classes of recommendations emerged. First is the need to improve climate models, observational and climate monitoring systems, and our understanding of key processes. Second, the framework for climate research and observations must be extended to document impacts and to guide adaptation and mitigation efforts. Research and observational strategies specifically aimed at improving our ability to predict and understand impacts, adaptive capacity, and societal and ecosystem vulnerabilities will serve both purposes and are the subject of the specific recommendations made in this paper.

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