Intensifying rainfall linked with a warmer and wetter atmosphere is increasing nitrogen pollution in rivers and oceans, exacerbating algae growth and expanding dead zones in coastal areas.
A new study in the journal Science shows how that intensifying rainfall will affect the nitrogen cycle as the planet warms.
If countries continue to pump out greenhouse gases at a high rate, the amount of nitrogen going into American rivers could surge 19 percent by the end of the century—with water quality in the Northeast, Upper Mississippi River Basin and Great Lakes suffering the most, the researchers found. Even if the goals of the Paris climate agreement are met and emissions are reduced, nitrogen levels will still rise, the authors found.
Nitrogen that washes into rivers and coastal zones comes mostly from fertilizers, feedlots and sewage, but also from car and truck exhaust, power plants and industrial manufacturing. It's essential to plant life, but in excessive amounts, it becomes a harmful pollutant. In a process called eutrophication, it feeds explosive algae growth; and when bacteria decompose the dead algae, they use up all the oxygen, causing dead zones like those found in the Gulf of Mexico and Chesapeake Bay.
"The types of precipitation changes we're expecting are substantial enough to make a big difference in terms of nitrogen runoff," said Carnegie Institution for Science ecologist Anna Michalak, co-author of the new study with Stanford University graduate student Eva Sinha.
"There are impacts to fisheries, and the nuisance aspect of the green mats washing ashore near people's homes. There are many manifestations of this problem, but they are all interlinked in one way or another," Michalak said.
Some species of algae produce toxins that can kill fish and threaten human health. During the 2008 Beijing Summer Olympics, sailing teams had to cope with a thick, smelly sludge of algae at their venue, giving a worldwide television audience a good look at eutrophication. In 2014, a toxic algae bloom in Lake Erie shut down part Toledo's water supply, leaving half a million people without water.
Many climate studies have concluded that the Midwest is already seeing an increase in heavy rains because of global warming. The downpours are washing nitrogen off the fields, feeding algae that are already thriving in warmer air and water.
For now, the most immediate impact is in coastal regions, especially estuaries, where rivers empty into the ocean, said University of Amsterdam environmental scientist Jan Willem Erisman. The number of dead zones worldwide has increased from just nine in the 1960s to more than 500 today, he said.
"You see the hotspots growing. In Central America, eastern China, India, there are major (nitrogen) losses to the air and water. "Almost all the major rivers in the world experience hypoxic zones," he said.
Warming Increases the Nitrogen Challenge
Michalak and Sinha were able to estimate increases in nitrogen runoff on a national scale in the U.S. by calculating how seasonal and annual precipitation, as well as land-use patterns, affect the nitrogen cycle.
They compared three different greenhouse gas emissions scenarios and found that nitrogen loading would increase in all cases. Under the worst-case scenario, without cuts to greenhouse gas emissions, nitrogen runoff would increase 19 percent in the U.S.
That means it would be much harder to reach targets for addressing existing impacts from nitrogen pollution. For the Mississippi River, which carries pollution fueling the giant Gulf of Mexico dead zone, the EPA, partnering with 12 states, set a goal a few years ago of cutting the nitrogen input by 20 percent to improve water quality.
The increased rainfall spurred by global warming will wash so much more nitrogen into the river that the reduction would have to be far greater—62 percent—to reach those water quality targets, Michalak said.
Strategies for Reducing Nitrogen Runoff
In a related article in the same edition of Science, University of Victoria researchers Sybil Seitzinger and Leigh Phillips describe potential strategies for reducing nitrogen pollution, including genetic modification of grains to reduce their nitrogen levels, switching to lab-produced meat, and more basic measures like improved sewage treatment and more efficient application of fertilizers.
Solutions to eutrophication and climate change are connected and will require "efficient nitrogen management in agriculture, maximizing nitrogen recycling and reuse, changes in diet, and development and adoption of new technologies," they wrote.
"If we can increase the efficiency with using nitrogen, we can also go a long way toward addressing the eutrophication issue," Seitzinger said.
That would include better management of manure on the many giant cattle feedlots, which are a substantial source of nitrogen as well as planet-warming methane. The industry has fiercely lobbied against regulation.
There are some success stories. Denmark and the Netherlands both halved nitrogen emissions after the European Union set limits on nitrogen in 1991. Agricultural production increased at the same time in both countries. In the Netherlands, farmers went from using more than 2,200 pounds per 2.5 acres to less than 375 pounds, but now there's less low-hanging fruit for more cuts, Erisman said.
One of the best ways to prevent even more disruption to the nitrogen cycle would be through an international framework, similar to the UN climate talks and other global efforts to limit mercury and ozone-destroying pollution, said environmental physicist Mark Sutton, with the UK's National Environment Research Council Center for Ecology and Hydrology.
"Emissions regulations in the U.S. have decreased nitrous oxide from autos, factories and power plants, but emissions from livestock feedlots are soaring," he said. "There's been very little regulation compelling farmers to reduce those emissions. There have been some conversations about regulatory measures but no real enforcement."