A new study, out Thursday in the journal Science, suggests that increased rainfall in the coming decades may wash more agricultural nutrients and fertilizers — including nitrogen, a prime cause of toxic algae growth — into our waterways. If we fail to cut down on our carbon emissions, nitrogen pollution in U.S. watersheds could increase by a total of 19 percent across the continent by the end of the century, it concludes.

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The findings “confirm that climate change is worsening nitrogen pollution” and highlight the need to control the contamination, wrote biogeochemistry expert Sybil Seitzinger and science writer Leigh Phillips of the University of Victoria, who were not involved with the new research, in a comment also published Thursday in Science.

The study helps to challenge the idea that water quality is only affected by local factors such as land use and agriculture, said study co-author Anna Michalak, an earth scientist at the Carnegie Institution for Science.

“In reality, what happens to water quality is more about this interplay between what we as humans are doing locally — things like land use and land management — and what we as humans are doing globally, in terms of climate change,” she told The Washington Post.

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Scientists are already well aware that climate change may affect precipitation patterns in the future, causing more frequent or more intense rainfall in certain parts of the country. Michalak and her fellow researchers — Eva Sinha, a graduate student in Michalak’s lab, and Venkatramani Balaji of Princeton University — used a new model to investigate the way these changes might affect nitrogen runoff in the future under different climate scenarios. It’s an “almost embarrassingly simple model,” according to Michalak, who developed and published a paper on the technique with Sinha last year.

“If you take into account total precipitation, extreme springtime precipitation, factors related to land use and total nitrogen inputs, you can quite reliably reproduce the observed nitrogen loading historically in different regions across the U.S.,” she said. The same factors can be used to make predictions about what might happen in the future.

They used the model to investigate the effects of three different climate scenarios — a business-as-usual climate trajectory in which emissions remain at high levels into the future, and two others in which moderate or partial climate mitigation efforts take place. They examined the outcomes for both the near-future, around midcentury, and the far-future, or the end of the century.

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They found that nitrogen pollution generally increases at a countrywide scale in all of the scenarios. But the most pronounced effects occurred under the business-as-usual climate scenario at the end of the century. The most severely affected regions of the country included the Northeast, the upper Mississippi Atchafalaya River Basin and the Great Lakes Basin, all of which saw greater than 20 percent increases in nitrogen input.

This could make it a lot harder for officials to get pollution back down to safe levels in the future, the researchers note. The Environmental Protection Agency has set a goal of reducing nitrogen and phosphorus loading in the Mississippi/Atchafalaya River Basin by 20 percent, relative to its levels between 1980 and 1996, by the year 2025. But if precipitation causes the region’s nitrogen runoff to grow throughout the rest of the century, policymakers will have to work harder and harder just to offset those increases. In this way, the study serves as a reminder to policymakers that they should take future climate change into account when setting environmental goals.

That said, the study’s findings aren’t necessarily a concrete prediction of what the future will look like. For one thing, we aren’t necessarily locked into a business-as-usual climate trajectory. For another, the study assumes that human land use and the amount of nitrogen found on the land will remain constant through the rest of the century. The researchers made this assumption to cleanly investigate the question of how climate change will affect nitrogen runoff, without other factors creating too much noise. In real life, though, human land use and other agricultural practices will probably change.

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Exactly how remains to be seen. It’s possible that in the coming decades, agriculture will intensify and the use of nitrogen compounds will increase. In that case, climate change will only exacerbate a growing pollution problem. On the other hand, we could also succeed in developing more efficient and less nutrient-intensive agricultural practices. In that case, we’d need to be aware that our successes could be partially offset by the effects of climate change.

The good news is that scientists are already coming up with all kinds of ways societies can reduce their nitrogen footprints. In their comment, Seitzinger and Phillips pointed to a variety of techniques to improve nitrogen use efficiency, including improving agricultural practices, but also reducing food waste and recycling nitrogen-containing compounds.

“There is potentially an exciting optimistic story to be told about global nitrogen stewardship in the Anthropocene,” they write.

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And Michalak adds that the study shouldn’t be interpreted as “just a gloom-and-doom story.” There’s still time to make progress on reducing greenhouse gas emissions — and doing so can actually produce side benefits for water quality as well.

Burning fossil fuels often produces nitrogen compounds in addition to carbon dioxide, which may later settle out of the air and deposit into our land and our waterways. So cutting down on fossil fuels may inadvertently reduce a little bit of our nitrogen pollution as well. On the flip side, working to reduce nitrogen pollution, through improved agricultural practices or otherwise, may also benefit the climate, since nitrous oxide is a potent greenhouse gas.