The UN weather agency is reporting that 2023 was the driest year in more than three decades for the world’s rivers, as the record-hot year underpinned a drying up of water flows and contributed to prolonged droughts in some places.

The World Meteorological Organization also says glaciers that feed rivers in many countries suffered the largest loss of mass in the last five decades, warning that ice melt can threaten long-term water security for millions of people globally.

“Water is the canary in the coalmine of climate change. We receive distress signals in the form of increasingly extreme rainfall, floods and droughts which wreak a heavy toll on lives, ecosystems and economies,” said WMO Secretary-General Celeste Saulo, releasing the report on Monday.

She said rising temperatures had in part led the hydrological cycle to become “more erratic and unpredictable” in ways that can produce “either too much or too little water” through both droughts and floods.

The “State of Global Water Resources 2023” report covers rivers and also lakes, reservoirs, groundwater, soil moisture, terrestrial water storage, snow cover and glaciers, and the evaporation of water from land and plants.

The weather agency, citing figures from UN Water, says some 3.6 billion people face inadequate access to water for at least one month a year — and that figure is expected to rise to 5 billion by 2050. WMO says 70 percent of all the water that humans draw from the hydrological systems goes into agriculture.

The world faced the hottest year on record in 2023, and the summer of this year was also the hottest summer ever — raising warning signs for a possible new annual record in 2024.

“In the (last) 33 years of data, we had never such a large area around the world which was under such dry conditions,” said Stefan Uhlenbrook, director of hydrology, water and cryosphere at WMO.

The report said the southern United States, Central America and South American countries Argentina, Brazil, Peru and Uruguay faced widespread drought conditions and “the lowest water levels ever observed in Amazon and in Lake Titicaca,” on the border between Peru and Bolivia.

The Mississippi River basin also experienced record-low water levels, the report said. WMO said half of the world faced dry river flow conditions last year.

The data for 2024 isn’t in yet, but Uhlenbrook said the extremely hot summer is “very likely” to translate into low river flows this year, and “in many parts of the world, we expect more water scarcity.”

Low-water conditions have had an impact on river navigation in places like Brazil and a food crisis in Zimbabwe and other parts of southern Africa this year.

WMO called for improvements in data collection and sharing to help clear up the real picture for water resources and help countries and communities take action in response.