In an analysis of the best available Earth systems models, researchers found that by the turn of the next century, 850 million people will feel the effects of declining runoff from the world’s major rivers.

That is more than three times the number estimated by previous analysis of Earth system models, says Puja Das, a post-doctoral research fellow at AI for Climate and Sustainability within the Institute for Experiential AI at Northeastern University.

“What (Das) found is that some of the more skillful models seem to project the worst conditions in terms of the impact of water scarcity,” says Northeastern professor Auroop Ganguly, who served as co-author for the paper Das published recently in the journal npj Climate and Atmospheric Science.

Population estimates are important because they give policymakers an idea of what to expect in terms of the availability of food, water and energy, since river runoff recharges water supplies, enriches agricultural soil and generates hydropower, Das says.

Her research shows that the five most skillful models project 40% of the world’s 30 major rivers will exhibit decreased runoff by 2100, affecting a population 100 times the size of New York City, as opposed to earlier estimates of 260 million.

“We chose the 30 biggest river basins around the world, including the Amazon, Congo, Ganges, Brahmaputra and Nile rivers,” she says. “We were trying to see how the runoff in those river basins, or water availability in those river basins, are presented in climate models.”

Professor Auroop Ganguly says the more skillful models of Earth systems show worse outcomes in terms of water scarcity from declining river runoff. Photo by Matthew Modoono/Northeastern University

“We know that climate models use different equations and parameterizations to estimate these variables. We are trying to see how good they are,” she says.

The researchers also ran the models against five different carbon emission scenarios.

“We saw that if there is a greener world, the water availability will be higher and fewer people will be impacted because of the decrease in water availability,” Das says.

With lower carbon emissions, she says, “We found that 500 million people (would be affected) instead of 900 million people, but water availability will still decrease in certain parts of the world.”

Das says the research is important for two communities: policymakers and water resources managers who use Earth systems model results for understanding impacts and informing adaptation, as well as natural scientists, data scientists and computational modelers who build the Earth systems models and analyse the results.