Observing Greenland from a helicopter, the main problem is one of comprehending scale. I have thought we were skimming low over the waves of a fjord, before noticing the tiny shadow of a seabird far below and realizing what I suspected were floating shards of ice were in fact icebergs the size of office blocks. I have thought we were hovering high in the sky over a featureless icy plane below, before bumping down gently onto ice only a few meters below us.
Crevassescracks in the surface of glaciersare the epitome of this baffling range of scales. Formed by stresses at the surface, their direction and size tell us how the ice sheet is flowing toward the ocean. Inland, far away from the fast-flowing glaciers that discharge hundreds of gigatonnes of icebergs a year into fjords, crevasses can be tiny cracks only millimeters wide.
[Photo: Tom Chudley]
As the ice speeds up, they can be meters in diameter, sometimes covered by deceptive snow bridges that require suitable safety equipment and rescue training to traverse. Finally, where the ice meets the ocean and no scientist would ever dare to stand, they can be monsters more than 100 meters from wall to wall. And across Greenland, they are growing.
It shouldnt be particularly surprising to scientists that crevasses are getting larger across Greenland. As the ocean warms, the ice sheet has sped up in response, increasing the stresses acting upon its surface. However, observations from satellites and in-person fieldwork are so poor that to date, we had no idea how extensively or quickly this process has been occurring.
Cracks you could fly a helicopter through [Photo: Tom Chudley]
Mapping cracks
In a new study, my colleagues and I mapped crevasses across the entirety of the Greenland ice sheet in 2016 and 2021. To do this, we used the ArcticDEM: three-dimensional surface maps of the polar regions based on high-resolution satellite images. By applying image-processing techniques to more than 8,000 maps, we could estimate how much water, snow or air would be needed to fill each crevasse across the ice sheet. This enabled us to calculate their depth and volume, and examine how they evolved.
We found that from 2016 to 2021, there were significant increases in crevasse volume across fast-flowing sectors of the Greenland ice sheet. In the southeast of the ice sheet, an area that has been particularly vulnerable to ocean-induced acceleration and retreat in the past few years, crevasse volume increased by over 25%.
In most Greenland glaciers that flow into the ocean, scientists found crevasses are increasing in size and depth. [Image: Chudley et al./Nature Geoscience]
However, against our expectations, crevasse volume across the whole ice sheet increased by only 4.3%. Thats much closer to an overall balance than the extremes observed in certain sectors. What had happened? In fact, the significant increases elsewhere were being offset by a single source: an outlet glacier known as Sermeq Kujalleq (Danish: Jakobshavn Isbr).
Sermeq Kujalleq is the fastest-flowing glacier on the planet, reaching speeds of nearly 50 meters a day and providing an outsized proportion of Greenlands total sea-level rise contribution. In 2016, responding to an influx of cold water from the north Atlantic ocean, the glacier slowed and thickened. As it did this, the crevasses on the surface began to closeoffsetting increases across the rest of the ice sheet.
This slowdown was short-lived. Since 2018, Sermeq Kujalleq has once again reverted to acceleration and thinning in response to ongoing warming. We wont be able to rely on it to offset ice-sheet-wide increases in crevassing in the future.
Cracks grow into icebergs
Crevasses play an integral part in the life cycle of glaciers, and as they grow they hold the potential to further accelerate ice-sheet loss. They deliver surface meltwater into the belly of the ice sheet: once inside, water can act to warm the ice or lubricate the bed that the glacier slides over, both of which can make the ice sheet flow faster into the ocean. Meanwhile, where the ice meets the sea, crevasses form the initial fractures from which icebergs can break off, increasing the output of icebergs into the ocean.
In short, crevasses underpin the dynamic processes that occur across Greenland and Antarctica. However, these processes are very poorly understood, and their future evolution is the single largest uncertainty in our predictions of sea-level rise. Together, the increased discharge of ice holds the potential to add up to 10 meters of additional sea-level rise by 2300 (75% of all cities with more than 5 million inhabitants exist less than 10m above sea level). We need to better understand these processesincluding crevassesso that informed sea-level projections can form the basis of our responses to the global challenges thatclimate change presents.
Since 2023, an international coalition of polar scientists has been urging the world to limit warming to 1.5 degrees Celsius to avoid the most catastrophic melt scenarios for global glaciers and ice sheets. Last month, the EUs Copernicus Climate Change Service confirmed that 2024 was the first year in which average global temperatures exceeded this threshold.
Every fraction of a degree matters. We may still be able to save ourselves from the worst of the damage the climate change will bringbut we are desperately running out of time.
Tom Chudley is a Leverhulme Early Career Fellow at the Department of Geography at Durham University.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
