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North Greenland mass loss

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Greenland meltwater runoff

The Greenland ice sheet is the second largest ice mass on Earth. Totally melted, it would rise sea level by 7.4 meters. 

Since the early 1990s, the ice sheet has lost mass at an accelerating rate due to enhanced meltwater runoff flowing into the ocean. 

Although runoff increased all over Greenland after 1991, we show that this increase is much stronger in the North than in the South.
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Along the ice sheet margin lies the ablation zone (red), where summer meltwater runoff exceeds winter snowfall resulting in a net mass loss at the surface.

There, the snow cover deposited in winter melts away to expose darker bare ice at the surface in summer.

At the end of summer, the snowline marks the ablation zone area. Above the snowline lies the accumulation zone (blue), that is covered by a permanent snow layer.

The widest ablation zone is found in the southwest and contributes one third to the ice sheet runoff total, while northern Greenland is responsible for about one tenth.





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Rapid snowline retreat

In this research we use a high-resolution regional climate model (RACMO2) to reconstruct the runoff production of Greenland since 1958.

Runoff remained low before the 1990s, after which it rapidly increased over the entire ice sheet. 

We find that runoff increased twice as much in the North (blue; +65%) as in the South (red; +34%) after 1991.


As a result, the relative contribution of the North to the ice sheet runoff total substantially increased.
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Before/after view

Rapid ablation zone expansion in North Greenland

Ablation zone extent (red) in North Greenland before (right) and after (left) 1991. 

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At the same time, the extent of the ablation zone in the North grew two times faster (+46%) than in the South (+25%).

This regional contrast is triggered by a rapid snowline retreat in early summer, exposing dark bare ice at the surface and causing high runoff rates in the North.
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North Greenland is dry and only a shallow snow cover forms in winter. Following a ~1ºC warming after 1991, the winter snow cover rapidly melts in early summer to expose bare ice at the surface. 

The fast snowline retreat in the North is also captured by MODIS satellite imagery. Watch how the bare ice zone progressively expands in summer 2019 (video).

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The role of clouds

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Before/after view

Role of summer clouds

High cloudiness in early summer (right) and sketch explaining the cloud warming effect. Low cloudiness in late summer (left) and sketch explaining runoff amplification over bare ice. 

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Summer clouds play a critical role in the rapid snowline retreat in North Greenland.

In summer, a persistent high-pressure system forms more frequently in western Greenland, driving warm and moist air toward the North. This increases the early summer cloud cover that acts as a blanket and warms the snow by preventing surface heat loss (right). 


Cloud warming rapidly melts the shallow winter snow cover, exposing bare ice in late summer. The fast snowline retreat amplifies northern runoff as dark ice absorbs more solar energy than bright snow (left).

This mechanism is unique to North Greenland as cloudiness decreased elsewhere in summer.


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Conclusions

In brief, increased summer clouds and strong atmospheric warming in the North triggered an early melt of the winter snow cover, expanding the northern ablation zone twice as fast as in the South. 

This regional contrast is responsible for the enhanced contribution of the North to the total Greenland mass loss. 

At the current rate, the extent of the North ablation zone could equal that of the South in another 45 years. 
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