Last month, scientists predicted the oxygen-deprived "dead zone" that appears every year in the Gulf of Mexico could grow over the course of this summer to be the size of Vermont. That makes for the largest such dead zone since researchers began monitoring the phenomenon 32 years ago. Humans are, of course, largely to blame for the growth of this low-oxygen zone: Fertilizers contain nutrients like nitrogen and phosphorus that run into rivers such as the Mississippi and the Atchafalaya, which in turn empty into the Gulf, fueling harmful algal blooms and dead zones.
Well, here's some more bad news: Climate change-related increases in precipitation could grow nitrogen runoff by 18 percent in the region by the end of the century, according to a new study published in Science.
To find out how rainfall levels influenced runoff, a team of researchers from Stanford University and Princeton University developed a model that determined the relationship between "nitrogen loading"—that is, the transmission of nutrients away from agricultural settings by waterways—and precipitation. They then looked at data from 21 different climate models of future precipitation under three scenarios with differing levels of climate action—"business as usual," "stabilization," and "mitigation"—to get a sense of future precipitation levels (and thus nitrogen loading levels).
The study found that, across the continental United States, increased precipitation could drive up nitrogen runoff by nearly 20 percent by 2100 under the business-as-usual scenario—the very scenario that we are currently on track for, according to Anna Michalak, a researcher at the Carnegie Institution for Science and a study co-author.
Climate change is not just something that's melting the polar ice caps thousands of miles away, it's also shaping the environment in our own back yards.
"There's no shortage of motivating reasons to take action on climate change," she says. "This is yet another one." For Americans, it can bring the issue of climate change closer to home; climate change is not just something that's melting the polar ice caps thousands of miles away, it's also shaping the environment in our own back yards.
But more immediately, the study has important implications for land management practices. The Gulf's summertime dead zone has been increasing in size since the 1990s, and state and federal agencies have recently created plans to shrink the dead zone by reducing the amount of nitrogen runoff by 20 percent—roughly equivalent to the nearly 20 percent increase in nitrogen runoff in the Gulf and across the U.S. predicted based on changes to precipitation patterns.
"For me it was very interesting, as well as a little worrisome, to see that the changes in precipitation patterns can completely offset those reductions," says Eva Sinha, a Ph.D. student at Stanford and lead author on the study.
This problem won't be contained to the U.S. The analysis revealed that India and eastern China, for example, are also at risk for increased precipitation in the future, and thus increased nitrogen pollution.
"It's really a cautionary tale," Michalak says. "In thinking about how to manage water quality, we really need to be thinking not just about land management and land use locally, but also taking into account the impact of global changes due to things like fossil fuel burning."