Last year, slimy green and foul-smelling algae took over Florida’s beaches, releasing toxins that killed fish and shellfish and sickened people. The algal bloom prompted the Florida governor to declare a state of emergency and likely caused widespread economic damage. If climate change goes unchecked, we could see more of these algal blooms along our coasts and in lakes, according to new research. That means that climate change won’t just affect the quantity of our water supply — causing drought, for instance — but it will also affect its quality.

A study published today in Science shows that, in the future, more rain and more extreme storms will wash out increasing amounts of nutrients like nitrogen into rivers and coastal waters. Nitrogen is food for tiny algae, called phytoplankton — and when it’s washed ashore, it can feed algal blooms like the ones in Florida. (Warming ocean waters are also to blame.) Using several climate models and projections, researchers showed that nitrogen runoff could increase by nearly 20 percent in the continental US by the end of the century — with the upper Mississippi Atchafalaya River Basin and the Great Lakes seeing the largest increases.

Nitrogen is food for tiny algae

Nitrogen leaches into waterways from a variety of sources: farmers use it to fertilize crops; animals and humans produce it naturally in their poop; and nitrogen compounds are produced when we burn fossil fuels. Whenever it rains, this excess nitrogen (and other nutrients like phosphorus) are washed up from the soil and air into rivers, lakes, and ground water, and eventually into the sea. Here, nitrogen can do great damage: not only can it cause algal blooms, but when those algae die, they sink to the bottom and decay, using up oxygen in the process. This creates areas of low oxygen where fish and shellfish can’t survive. Algae blooms can disrupt fisheries, causing millions of dollars in lost revenue, and cut drinking water supplies.

“There are huge impacts that go much beyond what you would think of, oh well, it’s just nitrogen in the water,” study co-author Anna Michalak, a professor of global ecology at the Carnegie Institution for Science at Stanford University, tells The Verge.

Algal blooms and so-called dead zones — like the one in the Gulf of Mexico or Chesapeake Bay — are on the rise, and not just in the US. The world’s population, with all its nitrogen-rich waste, is increasing; and agriculture is spreading and becoming more intensified. As a result, we’re pumping more and more nitrogen into the environment. But what’s going to happen in the future? For this study, researchers at Stanford and Princeton wanted to understand how climate change will affect nitrogen runoff — and as a consequence, water quality.

Michalak and her colleagues analyzed 21 different climate models to see how rainfall is likely to change by mid-century and by 2100. If we do nothing to address climate change, there’s likely going to be more rain almost anywhere in the US, except in the Texas area. There are likely also going to be more extreme storms, especially in the Midwest and Northeastern US. Even if the amount of nitrogen we put in the environment doesn’t increase, and we don’t do anything to address the problem, the increased rain will carry more nitrogen into waterways, the study found. The Northeast could see an almost 30 percent increase in nitrogen runoff, the upper Mississippi Atchafalaya River Basin a 24 percent increase, and the Great Lakes a 21 percent increase, the study says.

“The one-sentence takeaway would be that we already have a nitrogen problem in the US, as many places in the world. [And] it’s only going to get worse because precipitation is going to increase,” says Ellen Douglas, an associate professor of hydrology at the University of Massachusetts Boston, who did not take part in the study. There are other factors that can make the effects of this nitrogen overload even worse — like rising temperatures. Algae, for instance, grow faster when it’s warmer. But this study is only taking rain into consideration. The projections also assume that the amount of nitrogen we’re dumping into the environment will stay the same, but we know that it’s likely going to increase given our booming population and rising food demands, Douglas says.

Yet, the study is “the most comprehensive examination” of how climate change and rain will affect nitrogen runoff, says Denise Breitburg, a senior scientist at the Smithsonian Environmental Research Center, who did not take part in the study. The results also show that water quality isn’t just a local issue, it’s affected by how humans are changing the climate globally, Michalak says.

water quality isn’t just a local issue

Michalak hopes that policymakers will use the results to inform strategies to reduce how much nitrogen we produce. Local governments in the US are already trying to address the problem — by implementing better septic systems, for instance, or working with famers to determine how much fertilizer is applied, when, and where. In the Mississippi Atchafalaya River Basin, farmers are already trying to reduce nitrogen input by 20 percent compared to 1980–1996 levels, according to a mandate by the Environmental Protection Agency. But to meet that target in the future, considering climate change, they should reduce nitrogen input by over 60 percent, the study says.

There’s always the option of reducing greenhouse gas emissions, of course. But keeping nitrogen at bay is also key. “We can’t keep dumping our waste into the environment, whether agricultural or human-based waste,” Douglas says. “We need to find ways to reduce that flux of nitrogen.”