Mainz University is a major contributor to the latest report by the World Meteorological Organization

The global water cycle is becoming increasingly unbalanced. Extreme droughts and floods are the result. This is shown in this year's report by the World Meteorological Organization (WMO) on the state of global water resources, "State of Global Water Resources 2025," which was published today. "The total volume of water stored on the continents is steadily decreasing," said Professor Robert Reinecke of the Department of Geography at Johannes Gutenberg University Mainz (JGU), who has played a key role in compiling the report, which has been published annually since 2022, from the very beginning. This is evident, for example, in the rivers: "Since 2021, the total volume of water in rivers worldwide has consistently been lower than during the comparable period from 1991 to 2020. Last year was among the driest of the past 35 years. In 2025, rivers carried less water in 36 percent of the world's basin areas than during the comparison period."

The decline in water availability on the continents is also evident in groundwater levels: In 2025, these levels were lower than in the reference period at 65 percent of all monitoring stations included in the report. A significant decline continues to be observed in glaciers as well: Last year, glaciers lost approximately 400 gigatons of mass. The potential consequences of this became clear during the flood disaster in Nepal and Tibet a few weeks ago, which was triggered by a glacier collapse and resulted in the deaths of thousands of people. In Africa, meanwhile, heavy rains led to flooding that claimed numerous lives last year as well.

The causes lie in climate change and groundwater use

Reinecke attributes these developments to climate change and human consumption of groundwater: "A warmer atmosphere can hold more water; glaciers are melting, and at the same time, evaporation is increasing. And we humans are transferring vast amounts of groundwater into the oceans and the atmosphere; for example, by using it for irrigation." The combination of climate change and groundwater consumption is drying out the continents. Although the water is not lost, it is increasingly found in parts of the water cycle where it is of less use to humanity and many ecosystems.

The current report is more comprehensive than any previous reports

According to Reinecke, the latest WMO report on global water resources contains more data than any of the previous reports. For example, data on water temperature were analyzed for the first time. According to the report, 2025 will also be characterized by significantly higher water temperatures, which poses problems for both ecosystems and water quality.

Together with researchers from Goethe University Frankfurt am Main and the Global Runoff Database Center in Koblenz, which operates under the auspices of the WMO, Reinecke and his research group contributed model results and data to the report. In addition, the group helped develop the methodology for analyzing groundwater data for the report and played a leading role in further developing one of the global water models used. Using innovative water models that also simulate groundwater, the group is investigating the effects of climate change on global groundwater resources.