Mass balance of the Greenland Ice Sheet from 1992 to 2018.

The Imbie Team

doi:10.1038/s41586-019-1855-2

Abstract

The Greenland Ice Sheet has been a major contributor to global sea-level rise in recent decades1,2, and it is expected to continue to be so3. Although increases in glacier flow4-6 and surface melting7-9 have been driven by oceanic10-12 and atmospheric13,14 warming, the magnitude and trajectory of the ice sheet's mass imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet's volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. The ice sheet was close to a state of balance in the 1990s, but annual losses have risen since then, peaking at 345±66billion tonnes per year in 2011. In all, Greenland lost 3,902±342 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.8±0.9 millimetres. Using three regional climate models, we show that the reduced surface mass balance has driven 1,964±565 billion tonnes (50.3 per cent) of the ice loss owing to increased meltwater runoff. The remaining 1,938±541 billion tonnes (49.7 per cent) of ice loss was due to increased glacier dynamical imbalance, which rose from 46±37 billion tonnes per year in the 1990s to 87±25 billion tonnes per year since then. The total rate of ice loss slowed to 222±30 billion tonnes per year between 2013 and 2017, on average, as atmospheric circulation favoured cooler conditions15 and ocean temperatures fell at the terminus of Jakobshavn Isbræ16. Cumulative ice losses from Greenland as a whole have been close to the rates predicted by the Intergovernmental Panel on Climate Change for their high-end climate warming scenario17, which forecast an additional 70 to 130 millimetres of global sea-level rise by 2100 compared with their central estimate.

Highlights

We find no significant reduction in SMB accuracy in the pre-satellite era, prior to 1978, for Box and Hanna values
[9] Ice discharge, D, for reference year 1996 or 2000, is calculated as the flux, F, at the flux gate plus the reference SMB for the area in between the flux gate and the calving front or grounding line. This assumes that the glacier lower elevations are in balance with the reference SMB on that reference year
North of basin 25, we assume no change in speed since these glaciers did not accelerate in the 2000s

Summary

Introduction

[2] Airborne altimetry measurements collected in the 1990s showed that the Greenland ice sheet was thinning along its periphery and slightly thickening in the interior [Krabill et al, 1999]. Enhanced melt reduced the annual input of mass at the ice surface and thinned the glaciers in their frontal regions, which caused them to unground and accelerate toward the sea by 150 – 210% [Thomas, 2004; Howat et al, 2007; Rignot and Kanagaratnam, 2006; Luckman et al, 2006]. In comparison, bed lubrication accelerates outlet glaciers by only 8 – 10% during months of peak melting [Rignot and Kanagaratnam, 2006; Joughin et al, 2008].

Data and Methodology

Discussion

Findings

Conclusions