More than five billion people could have difficulty accessing water in 2050, the United Nations warned Tuesday, urging leaders to seize the initiative at the COP26 summit.

Already in 2018, 3.6 billion people had inadequate access to water for at least one month per year, said a new report from the UN's World Meteorological Organization.

"We need to wake up to the looming water crisis," said WMO chief Petteri Taalas.

"The State of Climate Services 2021: Water" report comes just weeks before COP26 -- the UN Climate Change Conference being held in Glasgow from October 31 to November 12.

The WMO stressed that over the last 20 years, the levels of water stored on land -- on the surface, in the subsurface, in snow and ice -- had dropped at a rate of one centimetre per year.

The biggest losses are in Antarctica and Greenland, but many highly-populated lower latitude locations are experiencing significant water losses in areas that traditionally provide water supply, said the WMO.

The agency said there were major ramifications for water security, as only 0.5 percent of water on Earth is useable and available fresh water.

"Increasing temperatures are resulting in global and regional precipitation changes, leading to shifts in rainfall patterns and agricultural seasons, with a major impact on food security and human health and well-being," said Taalas.

We cannot wait

Meanwhile water-related hazards have increased in frequency over the past 20 years.

Since 2000, flood-related disasters have risen by 134 percent compared with the previous two decades. 

"We have seven percent more humidity in the atmosphere because of the current warming and that's also contributing to the flooding," Taalas told a press conference.

Most of the flood-related deaths and economic losses were recorded in Asia, where river flood warning systems require strengthening, said the WMO.

At the same time, there has been around a 30 percent increase in the amount and duration of drought events since 2000, with Africa the worst-affected continent.

Taalas urged countries at COP26 to raise their game.

He said most world leaders were talking about climate change as a major risk to the welfare of mankind, but their actions were not matching their words.

"We cannot wait for decades to start acting," he said.

"That's also a message for countries like China which has said that they would like to become carbon neutral by 2060 but they don't have a concrete plan for the coming decade."

He said the top priority at COP26 was stepping up ambition levels in climate mitigation, but more work was also needed on climate adaptations, as the negative trend in weather patterns will continue for the coming decades -- and the coming centuries when it comes to the melting of glaciers and sea levels rising.