Small icebergs add a lot of Greenland fjord freshwater
High-resolution UNet mapping finds far more small icebergs than Sentinel-2 thresholding, raising Sermilik meltwater estimates toward ~2000 m³ s−1.
Source
Significant contribution of small icebergs to the freshwater budget in Greenland fjords
Study at a glance
- Design
- Other — UNet mapping of icebergs in Planet imagery vs Sentinel-2 thresholding in Greenland fjords
- N
- N=10000 · ~10,000 manual iceberg labels used to train the UNet
- Population
- Icebergs in Sermilik and Kangerlussuaq fjords, East Greenland
- Outcome
- Higher small-iceberg area and meltwater discharge estimates (~1300–2000 m³ s−1 Sermilik)
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
Authors trained a UNet on ~10,000 manual iceberg labels in ~3 m Planet imagery of Sermilik and Kangerlussuaq fjords, compared with same-day Sentinel-2 TOA thresholding, fit size distributions, and converted area to meltwater with empirical keel/width relations.
What they found
P-UNet mapped 60.4 km² of icebergs in Sermilik, ~50% more than S-TOA, with ~5× more small bergs. Size distributions follow a power law only above ~300 m². Sermilik discharge is ~1310 m³ s−1 along-fjord and ~2020 m³ s−1 in a June–July comparison, exceeding some prior melt estimates; small bergs melt near the surface.
The limits
What it doesn't show
Two fjords and a few summer images do not give an annual cycle. Melt rates ignore fjord stratification and deep keels. Power-law extrapolation of small-berg contribution year-round is an assumption.
Key terms
- P-UNet
- UNet convolutional network trained on Planet imagery to segment icebergs.
- S-TOA
- Sentinel-2 near-infrared reflectance threshold used to pick icebergs.
- Power-law size distribution
- Fractal iceberg area-frequency relation that fails below ~300 m².
- Keel depth
- Submerged iceberg thickness estimated from surface length.
- Subglacial discharge
- Meltwater exiting at the glacier grounding line, compared here with iceberg melt.
Flashcards
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Common questions
Why miss small icebergs in 10 m imagery?
They occupy few pixels and are too dark for simple reflectance thresholds.
Do small bergs change freshwater location?
They melt near the surface, unlike deep-keeled large bergs.
Is iceberg melt bigger than subglacial discharge?
On an annual scale in Sermilik, net iceberg freshwater exceeds subglacial discharge.
What size breaks the power law?
About 300 m².
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