While most of the world's glaciers are retreating as the climate warms, a small but significant population behaves very differently - and the consequences can be severe.
A team of international scientists, led by the University of Portsmouth, has carried out a comprehensive global analysis of surging glaciers, examining the hazards they cause and how climate change is fundamentally altering when and where these dramatic events occur.
Glacier surges - when a glacier suddenly moves much faster than normal - rapidly transport ice to the glacier front and often cause advances. These events typically last for several years, with many glaciers experiencing repeated surges separated by decades of relative inactivity.
The study, published in Nature Reviews Earth and Environment , compiled a database of over 3,100 glaciers that have surged. Rather than being scattered globally, these glaciers cluster in dense groupings across the Arctic, High Mountain Asia, and the Andes.
It includes the key features and basic processes controlling glacier surges, characterises the wide range of surging behaviours, and maps the global locations of surge-type glaciers and the climate conditions that cause them to cluster in specific areas.
"Surge-type glaciers are very unusual and can be troublesome," said lead author Dr Harold Lovell , Senior Lecturer and glaciologist from the University of Portsmouth's School of the Environment and Life Sciences. "As a friend and fellow glaciologist once put it, they save up ice like a savings account and then spend it all very quickly like a Black Friday event. But while they only represent 1 per cent of all glaciers worldwide, they affect just under one-fifth of global glacier area, and their behaviour can result in serious and sometimes catastrophic natural disasters that affect thousands of people."
Contrary to what you might expect, the researchers found surging makes glaciers more vulnerable to climate change. When they surge, these glaciers are responsible for a large proportion of ice mass loss in some regions.
Six major hazards identified
The research identified six main types of hazards caused by glacier surges that affect communities in high mountain regions, which can result in damage to infrastructure and in some cases loss of life:
Glacier advance - ice overruns buildings, roads and farmland
River blockages - surging glaciers dam rivers, creating dangerous lakes that can release devastating floods
Meltwater outbursts from beneath the glacier - these also release potentially devastating floods
Sudden detachments of glaciers - cause large ice and rock avalanches
Widespread crevassing - high ice velocities fracture glacier surfaces, making travel extremely hazardous in regions where glaciers serve as highways between settlements and are used for tourism activities, and affecting climbing routes where glaciers provide access to mountain peaks
Iceberg hazards - when glaciers surge into the sea, they release numerous icebergs in a short time, creating risks for shipping and marine tourism
Using this data, the research team identified 81 glaciers that pose the greatest danger when they surge. The majority are in the Karakoram Mountains in High Mountain Asia, where populated valleys and critical infrastructure lie directly below surging glaciers. These glaciers are large, close to communities, and most have histories of repeated surging behaviour.
Climate change is making surges increasingly unpredictable
Perhaps most concerning is the growing evidence that climate warming is fundamentally changing how glacier surges behave - making them harder to predict just when better forecasting is most needed for hazard management.
"By drawing on previous studies, we have been able to piece together the growing body of evidence that shows how climate change is affecting glacier surges, including where and how often they happen," Dr Lovell said. "This includes instances of extreme weather such as heavy rainfall events or very warm summers triggering earlier than expected surges, suggesting an increasing unpredictability in their behaviour."
The picture emerging from the research is complex and regionally varied. Some glaciers are now surging more frequently than they did several decades ago, while in other regions surge activity is declining. There is evidence that some glaciers have thinned so much they can no longer build sufficient ice to surge in the future.
Surge behaviour may also shift to new regions entirely. It is known that surging glaciers are currently concentrated in the Arctic and sub-Arctic (48 per cent) and High Mountain Asia (50 per cent), where specific climate conditions make surges more likely. But ongoing climate warming may change this distribution dramatically.
Surges might largely stop in places like Iceland, where glaciers are shrinking rapidly and struggling to accumulate enough ice. Meanwhile, they could become more frequent in parts of High Mountain Asia and in the Canadian and Russian Arctic due to warmer temperatures and increased meltwater. It's even possible that glaciers might begin surging in the Antarctic Peninsula and other areas where surge-type glaciers are not currently known to exist.
Co-author Professor Gwenn Flowers , from Simon Fraser University in Canada, said: "The challenge we face is that just as we're starting to develop a more comprehensive understanding of the mechanisms behind glacier surges, climate change is rewriting the rules. Extreme weather events that might have been rare even 50 years ago could become triggers for unexpected surges. Given that surges cause hazards in some settings, this makes protecting vulnerable communities much more difficult."
Dr Lovell added: "This research is extremely important because understanding which regions have concentrations of surging glaciers helps us plan monitoring efforts and understand future behaviour. Knowing which specific glaciers pose the greatest risks can help protect communities, especially those most at risk. But the increasing unpredictability means we need much better surveillance and forecasting capabilities."
The findings emphasise the urgent need for continued satellite monitoring, expanded on-the-ground observations of conditions during surges, improved computer models of surge processes, and projections of how surging glaciers will respond to ongoing climate warming.
