by Daniel Brouse and Sidd Mukherjee
August 9, 2025
Greenland Subglacial Flood Sheds Light on a Dangerous Climate Feedback
If you've been following the giant feedback loop example involving Sudden Sea Level Pulses and Cork Release events, there's now a new paper documenting one in action -- an outburst of 23 billion gallons of water in just ten days. That's the equivalent of nine Niagara Falls roaring beneath the ice, warping and fracturing the once-pristine sheet into a chaotic mess.
The Earth's climate system is a tightly woven network of interdependent processes. Disturb one, and you risk setting off a cascade of reinforcing feedback loops. Consider just one example: the potential collapse of the Atlantic Meridional Overturning Circulation (AMOC).
When the AMOC slows, tropical waters grow hotter while the Arctic warms even faster. This accelerates polar ice melt, raising global sea levels more quickly and injecting vast amounts of freshwater into the North Atlantic. The added freshwater disrupts ocean salinity and density, further weakening the AMOC in a dangerous feedback cycle.
Meanwhile, elsewhere in the system, Amazon droughts intensify under the altered climate, pushing the rainforest toward dieback and eventual desertification. This reduces the Amazon's ability to recycle rainfall and sequester carbon, further amplifying global warming--and thus accelerating ice melt, sea level rise, and AMOC destabilization.
One of the most powerful feedbacks in the polar regions is the albedo effect. As bright, reflective ice melts, it reveals darker land or ocean surfaces that absorb far more solar energy. This speeds up further melting. While melting sea ice mainly changes heat balance without directly raising sea levels, the melting of land-based ice--especially from Greenland and Antarctica--not only raises global seas but also changes ocean salinity and temperature, further destabilizing circulation systems like the AMOC.
These ice sheets hold vast "corks" of land ice restraining enormous reservoirs of meltwater. When these corks break, sudden sea level rise pulses--sometimes 1-3 feet per year for multiple consecutive years--could occur. The impacts on coastlines, global weather, and ocean currents would be both severe and unpredictable.
Recent research has identified a startling example of this process. In the paper Outburst of a subglacial flood from the surface of the Greenland Ice Sheet (2025), scientists documented a 90-million-cubic-meter flood that forced its way upward through the ice sheet, bursting out at the surface. This was caused by the rapid drainage of a subglacial lake in a region where the bed was thought to be frozen solid--an event that current ice sheet models do not account for.
The flood's upward path fractured the ice sheet, disrupting the downstream marine-terminating glacier and altering its flow. This bi-directional coupling between surface and basal hydrology highlights just how complex--and poorly understood--ice sheet dynamics truly are.
Over the last three decades, Greenland has lost roughly 169 billion tons of ice per year on average, contributing about 14 mm to global sea level rise. Roughly half of this loss comes from surface melting and runoff, which are projected to increase sharply as Arctic warming intensifies.
A massive upstream basin of rainwater and snowmelt, dammed by Alaska's Mendenhall Glacier, began releasing in August of 2025, prompting officials to urge residents in parts of Juneau to evacuate ahead of a potentially dangerous surge of floodwater.
A glacial outburst flood occurs when meltwater or rainwater accumulates behind a natural ice dam, creating a substantial reservoir of water under pressure. In the case of the Mendenhall Glacier, snowmelt and rainfall from the upstream basin -- ironically named Suicide Basin -- accumulate behind the glacier, which acts as a solid barrier, trapping the water in depressions known as proglacial lakes or subglacial reservoirs. As the water volume increases, hydrostatic pressure builds against the ice dam. Ice behaves like a viscoelastic material--it can deform slowly under pressure but can fracture if stress exceeds its strength. The weight of the water eventually exceeds the ice's ability to hold it, particularly if crevasses or melt channels weaken the glacier structure. Once the pressure exceeds the strength of the ice or underlying bedrock, cracks propagate rapidly, and water can exploit subglacial channels, forcing its way beneath or through the ice, a process known as hydraulic fracturing. When the dam fails, the water stored in the basin rushes downstream in a high-energy flood, converting potential energy into kinetic energy, generating destructive flow speeds and forces that can erode soil, uproot trees, damage infrastructure, and rapidly raise river levels. Warming temperatures increase surface melt and rainfall, filling these basins faster, while ice thinning and increased meltwater lubricate the glacier bed, reducing friction and making outbursts more likely. In essence, a glacial outburst results from the buildup of pressure from trapped water, ice weakening or cracking, and the sudden release of gravitational energy, producing a high-speed, destructive flood downstream.
Before-and-after shots of Suicide Basin "popping its cork." In the first, a small, fractured section of glacier holds back millions of gallons of water, both behind and beneath it. In the next, it's gone.
The National Weather Service (NWS) Juneau office issued a flood warning for areas along the Mendenhall River near Auke Bay. The released water from this glacial outburst is flowing downstream, putting riverside homes and properties at immediate risk. As of Tuesday afternoon local time, river levels were measured at 9.85 feet, with major flooding classified at 14 feet. Officials expect the river to crest Wednesday afternoon at near-record levels of 16.3 to 16.8 feet, setting a new historic high. NWS meteorologist Nicole Ferrin stated, "This will be a new record, based on all of the information that we have." The City and Borough of Juneau issued a public advisory confirming that the glacial outburst originated from Suicide Basin. Flooding is expected to continue along Mendenhall Lake and River from late Tuesday through Wednesday. Residents in areas at risk are strongly encouraged to evacuate immediately. A Red Cross shelter is open at Floyd Dryden Gymnasium (3800 Mendenhall Loop Road). Important notices for pet owners: The Red Cross shelter cannot accommodate pets. Evacuated animals should be taken to Juneau Animal Rescue at (907) 789-6997. Safety warning: Do not approach the river. Floodwaters are extremely dangerous, and entering the area endangers both residents and first responders. Stay away from the river to allow safe evacuations and emergency response efforts.
The Mendenhall River crested at a record-setting 16.65 feet deep as of 7:15 a.m. Alaska time (12:15 Eastern).
If hydrofracture events like this outburst become more frequent, the world could face abrupt, multi-foot-per-year sea level jumps--not the gradual rise most models currently project. This would leave little time for adaptation in coastal cities and could unleash profound economic, humanitarian, and ecological consequences.
Current ice sheet models typically treat meltwater movement as predictable and gradual. The Greenland event shows that under certain conditions, trapped subglacial water can build enough pressure to fracture ice and erupt at the surface--what could be called a "cork release" event. These sudden failures are not fully understood, but they could represent one of the most dangerous tipping points in the cryosphere.
Understanding and integrating these processes into predictive models is urgent. The more we learn, the more it becomes clear that the climate system is capable of abrupt, nonlinear shifts--far faster than human infrastructure, economies, or governance can adapt.
In particular, Sidd said: "Yes, I saw that. Under-ice hydrology is hard to observe, but there have been efforts with maps made of Greenland and Antarctica -- probably incomplete. I still think Greenland will melt largely in place; Antarctica is the big one.
* Our climate model -- which incorporates complex social-ecological feedback loops within a dynamic, non-linear system -- projects that global temperatures could rise by up to 9°C (16.2°F) within this century. This far exceeds earlier estimates of a 4°C rise over the next thousand years, signaling a dramatic acceleration of warming.
We analyze how human activities (such as deforestation, fossil fuel use, and land development) interact with ecological processes (including carbon cycling, water availability, and biodiversity loss) in ways that amplify one another. These interactions do not follow simple cause-and-effect patterns; instead, they create cascading, interconnected impacts that can rapidly accelerate system-wide change, sometimes abruptly. Understanding these dynamics is essential for assessing risks and designing effective climate adaptation and mitigation strategies.
What Can I Do?
There are numerous actions you can take to contribute to saving the planet. Each person bears the responsibility to minimize pollution, discontinue the use of fossil fuels, reduce consumption, and foster a culture of love and care. The Butterfly Effect illustrates that a small change in one area can lead to significant alterations in conditions anywhere on the globe. Hence, the frequently heard statement that a fluttering butterfly in China can cause a hurricane in the Atlantic. Be a butterfly and affect the world.
Ignite a Domino Effect: Albedo, Brown Carbon, AMOC, Permafrost, Amazon Rainforest Dieback, Sea Level Rise Pulses, Hydroclimate Whiplash, and Arctic Sea Ice Brouse and Mukherjee (2025)