Greenland ice sheet surface mass loss: recent developments in observation and modeling

Surface processes currently dominate Greenland ice sheet (GrIS) mass loss. We review recent developments in the observation and modelling of GrIS surface mass balance (SMB), published after the July 2012 deadline for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). Since IPCC AR5 our understanding of GrIS SMB has further improved, but new observational and model studies have also revealed that temporal and spatial variability of many processes are still poorly quantified and understood, e.g. bio-albedo, the formation of ice lenses and their impact on lateral meltwater transport, heterogeneous vertical meltwater transport (‘piping’), the impact of atmospheric circulation changes and mixed-phase clouds on the surface energy balance and the magnitude of turbulent heat exchange over rough ice surfaces. As a result, these processes are only schematically or not at all included in models that are currently used to assess and predict future GrIS surface mass loss

1,500-year cycle in the Arctic Oscillation identified in Holocene Arctic sea-ice drift

Weather and climate in the Northern Hemisphere is profoundly affected by the Arctic Oscillation, a quasi-periodic fluctuation in atmospheric pressure that occurs on interannual to interdecadal timescales. Reconstructions of the Arctic Oscillation over longer timescales have suggested additional centennial- to millennial-scale variations in the phase of the oscillation, but often with conflicting results. Here we assess patterns of sea-ice drift in the Arctic Ocean over the past 8,000 years by geochemically determining the source of ice-rafted iron grains in a sediment core off the coast of Alaska. We identify pulses of sediment carried by sea ice from the Kara Sea, which can reach the coast of Alaska only during a strongly positive Arctic Oscillation. On the basis of these observations, we construct a record of the Arctic Oscillation phase, and identify a 1,500-year periodicity similar to that found in Holocene records of ice-rafted debrisin the North Atlantic, distinct from a 1,000-year cycle that has been found in total solar irradiance. We conclude that the 1,500-year cycle in the Arctic Oscillation arises from either internal variability of the climate system or as an indirect response to low-latitude solar forcing.</p