Will climate change add more nutrient pollution to the Chesapeake Bay?

Climate change has already transformed the Chesapeake Bay: It’s brought higher water, warmer temperatures and even new species, such as shrimp that have migrated in from the South.

But is it also bringing more nutrient pollution into the Bay?

The official answer, for now, is yes.

The state-federal Chesapeake Bay Program estimates that more rain is increasing the amount of nutrients — nitrogen and phosphorus — reaching the estuary, creating a headwind that offsets a portion of recent cleanup work.

Those estimates come with a high amount of uncertainty, though. Figuring out how rain and temperature patterns will affect nutrient trends is one of the most complex and consequential questions facing efforts to improve Bay water quality.

It’s complex because many interrelated factors affect the amount of nutrients running off the land: More rain can cause more runoff, but warmer temperatures trigger processes that could act as a counterbalance.

Determining the outcomes from such competing factors is difficult. Some studies forecast conditions that could accelerate nutrient runoff significantly more than the Bay Program computer models currently estimate. On the other hand, a few suggest that conditions could reduce nutrient-laden runoff.

It’s consequential because the Bay Program’s computer models not only show that climate is already increasing nutrient runoff, but also predict that the rate of increase will sharply accelerate in coming decades. With the region already struggling to meet Bay nutrient reduction goals, that would be a huge additional challenge.

A recent analysis by Bay Program scientists, published in the Journal of the American Water Resources Association, highlights the importance of the issue. They found that nutrient increases triggered by climate change in the next three decades would be significantly greater than the nutrient impacts of development, population growth and economic activity combined.

The issue is slated for a closer look in coming years. The Bay Program is updating its computer models and aims to refine estimates of climate impacts over the next decade — and through the end of the century.

This May, the Bay Program Scientific and Technical Advisory Committee is conducting a workshop to consider how the models could be improved to better predict climate impacts.

“We’re at the edge of science here,” said Lewis Linker, modeling coordinator with the U.S. Environmental Protection Agency’s Bay Program Office. “That’s the challenge.”

The ultimate answer, he said, is important not only for the Bay, but for coastal waters around country, most of which also suffer from degraded water quality caused by increased nutrient runoff.

Potential for big impacts 

Understanding how climate will affect nutrients is important for Bay water quality. 

Nutrient pollution comes from a variety of sources, such as animal manure, crop and lawn fertilizer, sewage, and pet waste, which wash off surfaces and travel through groundwater to reach waterways.

Excess nutrients essentially fertilize the Bay, spurring algae blooms that cloud the water and kill underwater plants that provide important habitat. When the algae die, they decompose in a process that draws oxygen from the water, creating “dead zones” that are largely off limits to aquatic life.

To improve conditions for fish, shellfish and other aquatic life, the EPA in 2010 established the Chesapeake Bay Total Maximum Daily Load, which set nutrient limits for each state and major tributary. 

When established, the TMDL recognized that climate change could affect those numbers, but the models used at the time couldn’t estimate the impacts with confidence. The TMDL committed to adjusting the goals, if needed, when new models came online after 2017. 

When that happened, updated information showed that precipitation and streamflow had already increased from the mid-1990s period, which had been used to establish “average” climate conditions. 

That change was enough to show that the amount of nitrogen predicted to reach the Bay in 2025 was 2.3% more than what was originally estimated. That meant the region would have to achieve about 5 million additional pounds of nitrogen reductions to achieve the same water quality goals.

Put another way: Under the TMDL, states by 2025 needed to reduce the amount of nitrogen reaching the Bay each year by 71 million pounds. Through 2022, they had taken actions to achieve 31 million pounds of reductions. But already, climate change has essentially offset a fifth of that effort.

And the challenge accelerates in the future. The recent analysis shows that the rate of the climate-related nitrogen increase from 2025 to 2055 will be four times greater than during the last 30 years.

Similarly, the updated models estimate that the amount of phosphorus reaching the Bay in 2025 would be 4.5% greater than originally projected. But the rate of increase in the next 30 years will be six times higher than that.

Sorting out rain, heat effects

To be clear, there is not much uncertainty about climate change impacts as a whole: There is widespread scientific agreement that recent increases in temperature and precipitation will continue to grow, along with the intensity of storms.

But exactly how that affects the amount of nutrients delivered to the Bay is less certain.

More precipitation and more intense storms are generally associated with increased nutrients. Rain picks up nitrogen and phosphorus as it runs off the land and carries them into streams — more rain equals more nutrients.

But temperatures are also warming, and that means longer growing seasons and an increase in the rate that plants and trees absorb water and release it into the air. That process, known as evapotranspiration, reduces the amount of water that reaches streams. And it could reduce runoff.

Further, increased carbon in the atmosphere fuels more rapid plant growth, which also results in more water uptake and less runoff.

Other variables, such as the times of year that have greater or lesser amounts of precipitation, also influence nutrient runoff. Having more rain and less snow is also an important change. The list of variables goes on and on.

“It all depends on so many other things,” said Zach Easton, a Virginia Tech professor who worked on several reviews of Bay Program models. “And all those interactions are kind of tied together. The more complex the process, the more uncertainty you are going to have about any prediction.”

A team of outside scientists, including Easton, reviewed the studies that informed the Bay Program climate modeling and agreed that the underlying assumptions were reasonable.

But their review also found that most of the available studies dealing with climate change and nutrients focus on how the balance between increased precipitation and increased evapotranspiration might play out. Most suggest that increased rain would have greater weight, resulting in more runoff.

That contributed to the Bay models’ general findings that precipitation and increased streamflow would be the greatest factor in driving increased nutrient runoff.

Other outcomes possible

But some climate-related studies suggest that warming temperatures could also change the way nutrients are transformed, or cycled, on the landscape. 

That’s a much harder issue to assess, and there are far fewer studies that examine those processes. But some analyses suggest it could be important and potentially lead to less nitrogen reaching coastal waters.

For instance, warmer temperatures could greatly increase denitrification — the process by which bacteria convert nitrogen into a harmless gas — because warmer conditions accelerate microbial activity.

A computer modeling exercise by scientists at the U.S. Geological Survey found that over a 30-year period, if all other conditions remained the same, increasing temperatures would reduce the amount of nitrogen reaching the Chesapeake by about 6.5%.

Their paper suggested that increased denitrification is a likely suspect, along with some other heat-related factors.

Scott Ator, the study’s lead author, cautioned that “there are an awful lot of assumptions” behind those results. But “what it shows is that in warmer areas, all else being equal, we get a smaller percentage of nitrogen that is applied in the watershed reaching streams than we do in cooler areas.”

A study published last fall by researchers at Stanford University reached a similar conclusion for North America as a whole. 

It said that while increased precipitation has historically been closely related to the amount of nitrogen runoff to waterways, rapidly warming temperatures would tip the scales toward denitrification. Nitrogen loads, it noted, are typically less in warm regions and greater in cooler areas.

The study also acknowledged that sorting out the exact impact, especially for future conditions that will likely be different from anything observed in the past, is “extremely challenging.” 

James Galloway, professor emeritus at the University of Virginia who edited the Stanford team’s journal article — titled Warming may offset impact of precipitation changes on riverine nitrogen loading — cautioned that “a key word in the title of the paper is ‘may.’”

Galloway, an expert on nitrogen cycling, said he expected that the impact of climate change on nitrogen, and how precipitation and temperature interact, would be “quite variable” depending on local conditions.

Larry Band, a professor emeritus at the University of Virginia who has long studied hydrology and nitrogen movement in watersheds, said increased denitrification could be important, but is hard to estimate with confidence because so many factors are involved. 

The bacteria that perform denitrification require moist conditions. That means soil moisture, water table levels, the frequency and extent of wet and dry spells, all influence denitrification rates. Most denitrification takes place in relatively small “hot spots,” Band said, but those hotspots could still have a cumulative impact for the Bay. 

“If you have very heavy rainfalls and intense precipitation, that will mobilize more nitrogen, especially from urban and agricultural areas,” he said. “But you have to think about the offset from these other effects, and that’s where the interactions become more interesting — and more uncertain.”

A ‘horse race’

Other climate-related factors will affect the amount of nutrients reaching the Bay as well, potentially creating opportunities and challenges.

A significant amount of the Bay’s nutrient reductions in the last quarter century resulted from air pollution regulations that reduced emissions of nitrogen oxides, a byproduct of burning fossil fuels. When that material falls to the ground, it adds to the Bay’s nitrogen problem. 

But new policies to further reduce fossil fuel use could yield additional, unexpected benefits. Linker said an analysis is underway, and it appears the impact is “not trivial.”

“We’re going to get a little bit of a lift under our wings from the necessary decarbonization that’s taking place,” Linker said.

More uncertain is how actions taken to reduce runoff from farms and developed lands will be affected by climate. The Bay Program recognizes more than 200 types of “best management practices” — such as stormwater detention ponds, stream buffers and cover crops — as means of meeting nutrient reduction goals.

But how those will fare under future climate conditions is highly uncertain. Increasingly intense rains could overwhelm stormwater basins and other measures aimed at slowing runoff.

Longer growing seasons could boost the performance of vegetative practices, such as stream buffers. On the other hand, summertime droughts could reduce their effectiveness.

Linker said the EPA is funding research that could shed more light on the performance of best management practices — and could lead to revised recommendations about how some things, such as detention ponds, should be designed to withstand storms of the future.

Meanwhile, the Bay Program is working to update its computer models by 2027. In the next several years, it should become a bit clearer how those competing factors — increased precipitation, increased evapotranspiration and perhaps increased denitrification — will affect nutrient trends. Researchers might also provide insight on how those trends may be further influenced by policies and the effectiveness of pollution controls.

Linker characterized it as something of a “horse race” between competing variables. But for the Bay, the ultimate outcome of that race, he noted, “is a really big deal.”