(CNN) — The climate crisis is touching nearly every region of the world. But perhaps one of the most visible indicators of its impact is its effect on Earth's iconic glaciers, a major source of freshwater supply. Glaciers have been melting at a breakneck pace in recent decades, leading to around 20% of global sea level rise since 2000.
Now researchers at the UN Educational, Scientific and Cultural Organization have found that glaciers in one-third of the planet's most beautiful parks and protected areas are set to disappear by 2050 -- whether or not global warming is slowed.
Among the glaciers on the brink of vanishing at World Heritage sites are those in two of the most visited and most beloved parks in the United States -- Yellowstone National Park, which saw unprecedented flooding earlier this year, and Yosemite National Park.
The list also includes some of the largest and most iconic glaciers in Central Asia and Europe as well as the last remaining glaciers in Africa, namely Mount Kenya and Mount Kilimanjaro.
Glaciers at World Heritage sites shed around 58 billion tons of ice each year, UNESCO reports, which is equivalent to the total volume of water used annually in France and Spain combined. And these glaciers have already contributed nearly 5% of global sea level rise in the last 20 years.
The study provides the first global assessment of both the current and future scenario of glaciers in World Heritage sites, according to Tales Carvalho Resende, project officer at UNESCO's natural heritage unit and author of the report.
"This report brings a very powerful message in the sense that World Heritage Sites are iconic places -- places that are extremely important for humanity, but especially for local communities and Indigenous peoples," Resende told CNN. "Ice loss and glacial retreat is accelerating, so this sends an alarming message."
Only by limiting global warming to 1.5 degrees Celsius above preindustrial levels can we save glaciers at the other two-thirds of these parks, scientists report -- a climate target that recent reports say the world is far from achieving. The global average temperature has already risen around 1.2 degrees since the industrial revolution.
Glaciers cover around 10% of land, providing fresh water supply for households, agriculture and industry downstream. Under normal conditions, they take as long as a millennium to fully form; each year, they gain mass through snow or rain, and lose mass by melting in the summer.
Melting glaciers may seem like a faraway problem, but Resende said it's a serious global issue that can hit downstream communities hard. He highlighted Pakistan's deadly floods this year, which left nearly one-third of the country underwater. Reports say the multiweek floods were likely triggered by a combination of heavier than usual monsoon rains and several glacial lake outbursts due to melting that followed the recent extreme heat that enveloped the region.
"As water melts, this water will accumulate in what we call glacial lakes; and as water comes, these glacial lakes might burst," he said. "And this outburst can create catastrophic floods, which is something we can see very recently in Pakistan."
Thomas Slater, a glaciologist at the University of Leeds in London, noted that these glaciers are contributing a small fraction of sea level rise compared to the amount of ice loss the Antarctic and Greenland ice sheets could generate. Researchers like Slater have already found those ice sheets to be the major contributors to global sea-level rise this century.
"While it's sad to hear some of these glaciers could be lost, we should take hope in the fact that reducing emissions can save the majority of them and avoid disruption to the water supply of millions of people worldwide who live downstream," Slater, who is not involved with the UN report, told CNN.
With the rate at which the climate crisis is accelerating, more water will be released from glaciers. In drought-stricken areas like the Western US, an increase in meltwater may be a good thing, but Resende said it is only temporary.
Once a glacier's peak water -- the maximum meltwater it contributes to the system -- has been reached, annual runoff decreases as the glacier shrinks to the point where it's no longer able to produce water supply.
According to the report, many small glaciers in the Andes, Central Europe and Western Canada either have already reached peak water or are expected to in the coming years. Meanwhile, in the Himalayas, annual glacier runoff is forecast to jump around 2050, before it plunges steadily afterward.
If countries fail to keep global warming under 1.5 degrees, or even 2 degrees, glaciers will only continue to recede, the report shows. In that future, places would see significant glacier runoff during the wet periods, with little to no flow to quench drier and hotter conditions.
"This is a hot topic currently in the research community -- to see what will be the landscape after glacier melting," Resende said. "Unfortunately, glaciers will keep melting because there's always a delay. Even if we stop or drastically cut our emissions today, they will keep retreating because there's this inertia -- and it is extremely important that we manage to set up adaptation measures."
The report comes as world leaders gather in Sharm el-Sheikh, Egypt, next week for the UN-brokered international climate negotiations, where the focus will be on getting countries to commit to stronger fossil fuel cuts that would limit global warming to 1.5 degrees. They will also discuss plans to adapt to worsening extreme weather events including heat waves, floods and storms.
"We need to really unite ourselves, to make as much as possible this 1.5 objective feasible," Resende said. "The impacts might be irreversible, so this is really a pledge to take urgent action."
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