Hofer Stefan

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  • Article
    Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet
    (Nature Research, 2021-05-24) Hawkings, Jon ; Linhoff, Benjamin S. ; Wadham, Jemma L. ; Stibal, Marek ; Lamborg, Carl H. ; Carling, Gregory T. ; Lamarche-Gagnon, Guillaume ; Kohler, Tyler J. ; Ward, Rachael ; Hendry, Katharine R. ; Falteisek, Lukáš ; Kellerman, Anne M. ; Cameron, Karen A. ; Hatton, Jade E. ; Tingey, Sarah ; Holt, Amy D. ; Vinšová, Petra ; Hofer, Stefan ; Bulínová, Marie ; Větrovský, Tomáš ; Meire, Lorenz ; Spencer, Robert G. M.
    The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km−2 year−1) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km−2 year−1). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year−1), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year−1). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow and ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming.
  • Article
    Importance of Orography for Greenland cloud and melt response to atmospheric blocking
    (American Meteorological Society, 2020-04-16) Hahn, Lily ; Storelvmo, Trude ; Hofer, Stefan ; Parfitt, Rhys ; Ummenhofer, Caroline C.
    More frequent high pressure conditions associated with atmospheric blocking episodes over Greenland in recent decades have been suggested to enhance melt through large-scale subsidence and cloud dissipation, which allows more solar radiation to reach the ice sheet surface. Here we investigate mechanisms linking high pressure circulation anomalies to Greenland cloud changes and resulting cloud radiative effects, with a focus on the previously neglected role of topography. Using reanalysis and satellite data in addition to a regional climate model, we show that anticyclonic circulation anomalies over Greenland during recent extreme blocking summers produce cloud changes dependent on orographic lift and descent. The resulting increased cloud cover over northern Greenland promotes surface longwave warming, while reduced cloud cover in southern and marginal Greenland favors surface shortwave warming. Comparison with an idealized model simulation with flattened topography reveals that orographic effects were necessary to produce area-averaged decreasing cloud cover since the mid-1990s and the extreme melt observed in the summer of 2012. This demonstrates a key role for Greenland topography in mediating the cloud and melt response to large-scale circulation variability. These results suggest that future melt will depend on the pattern of circulation anomalies as well as the shape of the Greenland Ice Sheet.