Cleaning up human sewage is an international issue. As growing populations grapple with how to properly process and dispose of human waste, a significant amount of sewage ends up in waterways around the world. More than 2 billion people worldwide lack any sanitation, which means that millions of tons of human waste enters into the environment  untreated every year. And in wealthier countries like the US, most treated sewage is placed directly into rivers and oceans. Human waste also introduces too much nitrogen in our waterways, which causes watersheds to experience dead zones that used to house marine life. 

To map this issue, postgraduate researcher Cascade Tuholske, who is now affiliated with the Columbia Climate School, and his colleagues at UC Santa Barbara mapped out the ways that  human sewage has introduced extra nitrogen and pathogens into watersheds. The team used data sets to detail whether populations were urban or rural, how much protein was consumed per person, how much nitrogen was then excreted, and how much the waste was sanitized and how, says Tuholkse. The more meat a person eats, the more nitrogen is found in their waste, which means more nitrogen ends up in the waterways near them. This happens because nitrogen comes from amino acids which make up protein. The breaking down of those proteins during digestion creates the nitrogen that ends up in wastewater. 

“We estimate that 25 watersheds contribute approximately 46 percent of global nitrogen inputs from wastewater into the ocean,” Tuholske said in a release. “Nearly half as much nitrogen comes from wastewater as agricultural runoff globally, which is a huge fraction.” 

[Related: It’s not just methane—meat production fills the air with noxious particulate matter too.]

More than 130,000 watersheds with a fixed point on the coastline were analyzed across the world. The study also models the journey of nitrogen into the ocean to get a rough idea of where nitrogen is building up coastal ecosystems, which can hurt marine life. Tuholske tells Popular Science the uptick of nitrogen in rapidly “developing” nations, such as some countries in Asia, may be attributed to the increase of animal protein consumption as those countries become wealthier—the OECD-FAO Agricultural Outlook for the next decade is expecting a 0.8 percent increase in meat consumption every year compared to 0.24 percent in developed nations. 

The researchers found the most worrisome watersheds are in Korea, India and in China, with the Chang Jiang (Yangtze) River in China contributing 11 percent of the global total of human-sewage nitrogen. Hotspots for areas where coral reefs and seagrass was overly exposed to the sewage and nitrogen are in various countries including China, Haiti, Ghana, and Nigeria. Seagrass and coral reefs are especially impacted by increased nitrogen because nitrogen runoff can lead to eutrophication. Eutrophication triggers “dead zones,” making room for harmful algal blooms, coastal dead zones, and a decline in fishery that can devastate human and marine populations alike, Tuholske says.  

Both seagrass and coral have a special importance in a variety of ecosystems, especially since both feed or house endangered species like some species of turtles, seahorses, and manatees. This also means that in some areas, there isn’t enough oxygen in the water to sustain a healthy environment for fish, which hurts fisheries. 

[Related: What is the ‘dead zone’ in the Gulf of Mexico, and why is it super-sized this year?]

“The whole ecosystem can tip into a highly degraded state when nutrient levels are too high,” co-author Ben Halpern, a marine science professor at UC Santa Barbara, said in a release. “Coral reefs can be converted into fields of algae that overgrow and kill the corals below them.”

Tuholske hopes that the research will encourage more research on how human waste contributes to excessive nitrogen in our watersheds. Currently, most research about this issue often focuses on agricultural runoff instead of sewage. 

So while agriculture takes the cake for the majority of runoff-induced issues, there’s a lot that can be done for both an ecological and public health benefit when it comes to our own poo. Tuholske suggests that eating less animal products could help, but thinks that this should be approached in a way that can address the needs of different populations of people. 

“Food is culture, it’s really important and I’m never one to prescribe my dietary beliefs on anyone else,” he says. “ [With enough research] we can create culturally appropriate food systems with appropriate protein consumption.”