The UN's World Meteorological Organization said in a report that the world's water cycle was becoming ever more unpredictable, with shrinking glaciers, droughts, unbalanced river basins and severe floods wreaking havoc.

"The world's water resources are under pressure from growing demand, and at the same time, we are seeing more water-related hazards," WMO chief Celeste Saulo told reporters in Geneva.

The agency's annual State of Global Water Resources report "shows quite clearly that the water cycle has become increasingly erratic and extreme," she said, pointing to the "cascading impacts on infrastructure, agriculture, energy, health, and economic activities".

Last year was the hottest on record, leading to prolonged droughts in northern parts of South America, the Amazon Basin and southern Africa, the report said.

Meanwhile, parts of central Africa Europe and Asia dealt with wetter weather than usual, being hit with devastating floods or deadly storms, it pointed out.

Africa's tropical zone experienced unusually heavy rainfall in 2024, resulting in around 2,500 deaths and displacing some four million people.

Asia and the Pacific were meanwhile hit by record-breaking rainfall and tropical cyclones, killing more than 1,000 people.

And Europe saw its most extensive flooding in more than a decade, with a third of river networks exceeding high flood thresholds, the report found.

- 'Clear imbalance' -

WMO said 2024 was the sixth consecutive year globally where there had been a "clear imbalance" in the world's river basins.

"Two-thirds have too much or too little water -- reflecting the increasingly erratic hydrological cycle," it said.

The organisation also flagged how the water quality in major lakes was declining due to warmer weather, and glaciers shrank across all regions for the third year in a row.

It found that across the globe, a full 450 gigatonnes of ice was lost -- enough to fill 180 million Olympic swimming pools, WMO said.

The meltwater had added about 1.2 millimetres (0.05 inches) to the global sea level in a single year, contributing to the risk of flooding for hundreds of millions of people living in coastal zones, the report warned.

"From the 1970s until now, 9,000 gigatonnes have been lost, which is 25 millimetres of sea level rise," WMO scientific officer Sulagna Mishra told reporters.

According to the UN, 3.6 billion people worldwide currently have insufficient access to water at least one month per year, and this is expected to increase to more than five billion by 2050.

Stefan Uhlenbrook, head of WMO's hydrology, water and cryosphere division, stressed that "the total amount of water we have on this planet remains the same".

But with "increasingly melting glaciers and also with over-abstracting groundwater... we lose more water from the continents, (which) flows to the sea, contributing to sea level rise".

It is vital to properly manage underground water systems called aquifers and ensure that freshwater from melting glaciers is stored to prevent it from flowing into the oceans, Mishra said.

She also called for optimising water use in agriculture, which accounts for between 75 and 90 percent of all water abstracted.