BetterAsked where the world's fresh water comes from, most people answer rivers, rain, or groundwater. Those are places water passes through.
The UN's 2025 water report is about where it comes from.
Mountains supply 55-60% of global annual freshwater flowsfrom 24% of the Earth's land surface.
They do it by “storing water in the form of ice and snow during cold seasons, releasing it during warmer seasons”. A store, not a pipe.
Two billion people depend on mountain waters to live, most of them downstream.
The mechanismWhat a water tower actually does
The metaphor is doing real work here, so it is worth taking literally.
A quarter of the land producing close to two-thirds of the flow. That ratio is the whole reason the report exists, and it comes from what mountains do to time, not just to volume.
And it goes further than the water itself. The report notes that mountains “play a unique and critical role in the global water cycle, and they affect atmospheric circulation, which drives weather and precipitation patterns”. They do not only hold water. They help decide where rain falls.
TimingThe problem is not only quantity
Here is the sentence worth reading twice, because it does not say what people expect it to say.
“Climate change is accelerating glacier melt, decreasing snow cover, increasing permafrost thaw, and causing more extreme rainfall events and natural hazards leading to more variable, erratic and uncertain water flows.”
Not less water. Less predictable water. A store that releases on schedule is what makes a dry season survivable; a store that releases early, or all at once, is a different kind of failure, and it can arrive as a flood rather than a shortage.
This is the distinction most coverage of glaciers misses. The headline is usually the ice disappearing. The report's operative words are about what happens to the schedule long before the ice is gone.
The rangeThe figure that needs its bracket
One number from this report will circulate on its own. It should not.
“26-41% of mountain glacier mass is at risk by 2100 with 1.5°C - 4°C global warming.”
26% goes with 1.5°C. 41% goes with 4°C. Quoting 41% alone turns a conditional projection into a forecast, and quoting 26% alone turns a worst case into a floor.
The range is not vagueness. It is the answer to a question that has not been settled yet, and what settles it is how much warming happens.
DownstreamWho this is about
A report on mountains sounds like a report about mountain people. The numbers say otherwise.
- Two billion people depend on mountain waters to live, the report counts those downstream, not only those at altitude.
- 2.2 billion people had no access to safely managed drinking water in 2022.
- 3.5 billion lacked access to safely managed sanitation, also in 2022.
Those last two figures carry their year for a reason: they are the most recent the report gives, and they are already several years old. A number about access to water without its date is a number you cannot check.
Which is the honest end of a first article on water. More than half of it starts as ice and snow on a quarter of the land, two billion lives are timed to when that melts, and the measurable change so far is less in the total than in the timing.
QuestionsFrequently asked questions
What share of the world's fresh water comes from mountains?
Between 55 and 60 per cent of global annual freshwater flows, according to the UN World Water Development Report 2025. Mountain regions cover 24 per cent of the Earth's land surface, excluding Antarctica.
Why are they called “water towers”?
Because they store and release, rather than simply letting water pass. The report puts it this way: mountains “store water in the form of ice and snow during cold seasons, releasing it during warmer seasons”. It is not a reservoir, it is a schedule.
How many people depend on them?
Two billion people depend on mountain waters to live, according to the report. Most of them live downstream, not at altitude.
How much mountain glacier mass is at risk?
Between 26 and 41 per cent of mountain glacier mass by 2100, but the range depends on warming: 26 per cent corresponds to 1.5°C, 41 per cent to 4°C. The figure means nothing without its temperature bracket.
Is the problem that there will be less water?
Not only, and this is the report's most useful point. It describes freshwater flows becoming “more variable, erratic and uncertain”. A water tower that releases at the wrong time is a different problem from one that runs dry.
How many people lack safely managed drinking water?
2.2 billion had no access to safely managed drinking water in 2022, and 3.5 billion lacked access to safely managed sanitation the same year. Both figures are for 2022, not for today.
Read nextRelated
Sources
- UN World Water Development Report 2025, “Mountains and Glaciers: Water towers”, UNESCO and UN-Water, 2025
- Full report, summary and “Facts, figures and action examples”, UNESCO
About this article. General and educational information. All figures come from the UN World Water Development Report 2025, published by UNESCO for UN-Water, official page consulted in both languages. The water and sanitation access data are for 2022, and the 26-41% range is a conditional projection to 2100 that should not be quoted without its warming bracket.
Spotted an error or an out-of-date figure? Write to us: we will correct it and re-date the article. Our texts are written with the help of artificial intelligence and checked against official sources before publication, but a figure or a definition can change without us catching it. If anything here does not match what an official body tells you, they are right.