Humans have been trying to wrangle Earth’s freshwater since the dawn of civilization. Case in point: the 3,000-year-old Sadd Al-Kafra embankment dam in Egypt. Things like dams and irrigation obviously affect local waterways, but it’s much harder to figure out how those local changes influence freshwater supplies worldwide.

Now, an analysis of water basins shows that the global impact of humans’ water consumption is much larger than initially thought. That's because local attempts to divert and control water actually increase global consumption by ramping up evapotranspiration, or the process by which water cycles from the land into the atmosphere.

Physical geologist Fernando Jaramillo of the University of Gothenburg and hydrologist Georgia Destouni of the University of Stockholm embarked on the study after linking local dams in Sweden to surprising spikes in regional evapotranspiration. To take the research global, they decided to abandon complex modeling in favor of a formula inspired by the water basins themselves.

“We knew we could use the data in a simpler way,” Destouni says. At the most basic level, a hydrological basin is a closed system, she explains—precipitation comes in, and runoff and water withdraws go out. Any difference between the input and the output must leave the basin via evapotranspiration.

But the work was made more complicated by a dearth of accessible, accurate data. The team collected public data for nearly 3,000 water basins, but found complete data sets on only 100. Still, using that sample they were able to analyze each basin over two periods covering the years 1901 to 2008.

Though the team suspected a strong link between water infrastructure and evapotranspiration, they first had to rule out other possible factors. “You have to differentiate the direct effects of humans,” Jaramillo says, adding that he was skeptical that his team could find that particular footprint among the deafening noise.

“Okay, we have deforestation, we have non-irrigated agriculture, urbanization, melting glaciers, permafrost thawing, climate change,” laughs Jaramillo. But when the team corrected for things like temperature and climate change and looked at evapotranspiration rates over time, they always ended up with the same result.

“These were landscape drivers—things changing the landscape itself," says Jaramillo. "The signal was just so clean and clear.”

Based on their analysis, published today in Science, they calculate that reservoirs, dams and irrigation are responsible for increasing evapotranspiration so much that humans’ overall water consumption is 18 percent higher than the most recent estimate.

In effect, we are using an average of 4,370 cubic kilometers of water every year, if you’re counting—and perhaps you should be. In 2011, environmental scientist Johan Rockström defined 4,000 cubic kilometers of annual global freshwater use as a critical planetary boundary that, if crossed, could spell irreversible environmental changes.

Experts suggest that when the world tips into a full-scale freshwater crisis, it will spur food shortages and civil unrest. Studies like this one suggest that dams—which are linked to between 12 and 16 percent of global food production—could ultimately drive crises that threaten the world’s food supply.

Destouni, who says she becomes nearly physically ill when she watches people waste water, says the study will be useless if it doesn’t spur more research. The team’s next hurdle is to look at the impacts of climate change on global freshwater supply—and push for more nuanced analyses of how human activities affect water worldwide.

“There are things I can do myself, but as a society we need to keep track of how we use our resources,” says Destouni. After all, she notes, humans control freshwater resources for a reason—to generate power, provide reliable sources of drinking water and grow food. “We need to know how much we use so we can know how much we can change before compromising our own existence.”