Drivers of Greenland Ice Sheet Mass Balance during the Medieval Warm Period and Little Ice Age

Cullum, Michael (2018) Drivers of Greenland Ice Sheet Mass Balance during the Medieval Warm Period and Little Ice Age. Doctoral thesis, Northumbria University.

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Abstract

The period between 850 and 1850 AD is defined by natural climate fluctuations in response to changes in volcanic and solar activity, unrelated to the increased influence of anthropogenic forcings which occur from 1850 AD onwards. These changes resulted in the Medieval Warm Period (MWP; 950 – 1250 AD) and Little Ice Age (LIA; 1450 – 1850 AD), where global temperatures varied by 0.5 - 1°C compared to present-day. However, the response of the Greenland Ice Sheet (GrIS) to these climate variations is largely unknown, as previous studies using salt marsh records of relative sea level (RSL) change, and glacier isostatic adjustment (GIA) indicate that individual ice streams across the GrIS reached their maximum extents anywhere from 1200 – 1800 AD, advancing by between 2 – 26 km ahead of their present-day position. This indicates that the response of the GrIS differed extensively both spatially and temporally, and that how the GrIS responded as a whole is uncertain. Current ice sheet models produce different results depending on how they are setup and initialised, with varying resolutions, forcings, and parameter ensembles resulting in different resultant ice volumes and extents. This issue is amplified by the lack of longterm observations encompassing the entire ice sheet, with current observations being limited to point-specific ice and sediment cores which fail to reflect the spatial variability in ice response at present-day, and therefore serve as inadequate constrains for use in ice sheet models.

This study addresses the uncertainties associated with the setup of an ice sheet model for this period, quantifying the influence of model initialisation, internal parameter perturbation and climate model forcing on the mass balance of the GrIS during 850-1850 AD. In this study, the Parallel Ice Sheet Model (PISM) is forced using climate model output from the last millennium ensemble of the Palaeoclimate Intercomparison Project (PMIP3) to produce GrIS mass balance scenarios for 850-1850 AD.

This thesis determined that the dominant controls on absolute mass balance of the GrIS during the MWP and LIA are the values chosen for surface mass balance, englacial and subglacial model parameters within PISM, rather than the applied climate forcing, which controls the mass balance inter-annual variability. Non-significant trends in Greenland annual temperature and precipitation forcing during LIA across the PMIP3 ensemble result in mass balance trends of -21.22 ± 64.69 km3/yr to 12.74 ± 108.84 km3/yr. Mass balance trends of up to 200.83 ± 196.22 km3/yr during the MWP are also simulated, but this is due to dynamic adjustments. These changes translate into long-term marginal thickness increases of up to 1000 m. However, isolated glaciers such as 79North and Helheim demonstrate numerous abrupt responses where thickness fluctuates by ~20 m or more, particularly during the early-MWP and late-LIA, where ice thickness shows decadal scale variations. The chosen model settings likely produce an overly stable GrIS, including coarse resolutions and extensive parameterisations, meaning results represent the lower end of GrIS responses. Despite these limitations, this study has shown that the GrIS is likely to have contributed -0.55 mm/yr to 0.059 mm/yr to global sea level change between 850-1850 AD, indicating that the mass balance of GrIS was relatively stable during the MWP and LIA.

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