A landscape reshaped after the last Ice Age
When the last Ice Age ended around 11,700 years ago and the climate warmed, the world entered the geological epoch called the Holocene in which we are living today. This led to the retreat of vast ice masses in Greenland, triggering a tightly coupled response between ice loss, land uplift, and sea-level change in both directions.
The study identifies three distinct phases in this development based on the sediment samples from the Isortoq Fjord:
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Early Holocene – falling relative sea-level
Rising temperatures led to the continued retreat and thinning of the Greenland Ice Sheet from its maximum extent during the last Ice Age. As the weight of the ice on the Earth’s crust began to decrease, crustal rebound led to local uplift in Greenland, causing the sea to retreat locally relative to the land. Over just a few thousand years, the coastline in Isortoq Fjord retreated from its highest level to approximately the level we know today.
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Middle Holocene – lowest relative sea level
Temperatures reached its maximum during the Middle-Holocene, the Greenland Ice Sheet were at its minimum extent and local relative sea level was significantly lower than today – by up to ~20 metres.
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Late Holocene – rising relative sea-levels
The pattern shifted again. Temperatures became cooler, the ice sheet regrew and crustal uplift was reversed in Greenland. Crustal reloading led to relative sea level gradually rising again in Greenland, returning to approximately present-day levels within the last millennium.
Together, the results paint a picture of a landscape in constant motion. As ice melts away, the land surface rises – and this can cause local sea level to fall, even in a world where global sea level is rising.
Challenges existing models
When the researchers compare their observations from Isortoq Fjord with existing models of glacial isostatic adjustment (GIA) – which describe how the Earth’s crust responds to ice loading and unloading and how this affects relative sea level – a clear mismatch emerges. The observed changes are larger than those predicted by the models.
“Our results show that current models underestimate both how fast and how strongly the relative sea-level responds when ice melts. If models fail to reproduce past changes correctly, we also risk misjudging future sea-level change – which highlights the need for better data and more refined models,” says Gregor Luetzenburg.
This insight is not only of historical interest. More accurate models are essential if data from Greenland are to be used to improve projections of future sea-level change in a rapidly warming climate.