Editor’s note: This article is part of a series examining issues related to the Chesapeake Bay Program’s Midpoint Assessment of Bay cleanup efforts.
Rising temperatures and sea levels, as well as increased precipitation, are all expected for the Chesapeake Bay region as the Earth’s climate changes, but no one has to wait until the end of the century to feel their impacts. For Bay cleanup efforts, the future is now.
Scientists and state and federal officials are already trying to determine the extent to which climate change will affect the region’s ability to achieve its 2025 cleanup goals.
Globally, temperatures are predicted to increase at least 2.7 degrees Fahrenheit, and could rise by three times that much by 2100. Sea levels during that period are expected to increase 1–4 feet, and global average precipitation is also expected to increase.
Right now, the impact of those large-scale changes on the Bay cleanup appears small, though still measurable. “We know in 2025 we’re not going to fall off the cliff,” said Lewis Linker, modeling coordinator with the U.S. Environmental Protection Agency’s Bay Program office in Annapolis. “We know December 2025 is going to look a lot like December today.”
That said, preliminary computer modeling does suggest that climate change will make the job of meeting water quality goals slightly more difficult. The estimates, which will be refined in coming months, predict that climate-related changes could increase the amount of nitrogen and phosphorus reaching the Bay in 2025 by about 2 percent above what was projected in 2010, when cleanup goals were set.
And, climate change will continue to make the job more difficult beyond that, as an upward trend in precipitation continues, storms intensify, and rising sea levels drown nutrient-absorbing tidal marshes.
Besides increasing nutrient runoff, climate change may also reduce the effectiveness of pollution control practices. More frequent intense storms could overwhelm some stormwater controls. Increasing — and more severe — droughts may reduce the effectiveness of vegetative buffers, while stream restorations might be challenged by flooding from more intense storms.
Current nutrient and sediment goals were established in 2010 when the EPA issued the Chesapeake Bay Total Maximum Daily Load, or pollution diet, which set nutrient and sediment caps for each state and river. The resulting pollution reductions were intended to reduce algal blooms, improve water clarity and enhance oxygen levels to sustain fish, crabs, oysters and other aquatic life.
Actions to achieve those caps were to be fully implemented by 2025. This year, the state-federal Bay Program is completing a “midpoint assessment” of the TMDL goals to determine what course corrections, if any, are needed to meet that deadline. While climate change will impact everything from forests in the Bay’s headwater streams to the makeup of fish communities in the Chesapeake itself, the assessment is focused on the changes that could impact the ability of cleanup efforts to ultimately meet the Bay’s clean water standards — things like rainfall, river flows, rising temperatures and sea level rise.
Predicting impacts a challenge
But predicting the exact impact of those changes, particularly things like rainfall and river flow, is challenging.
“Part of the problem of projecting climate into the future is that climate cycles naturally, anyway,” said Mark Bennett, director of the U.S. Geological Survey’s West Virginia and Virginia Water Science Center. Regional climate drivers, such as the North Atlantic Oscillation, can cause shifts in temperature and rainfall lasting a decade or so that are then reversed.
To sort short-term fluctuations from long-term trends, the Bay Program — at the advice of its Scientific and Technical Advisory Committee — is using two techniques. It is using records of rainfall and river flow, which date back almost a century, to determine long-term trends and project them to 2025. Separately, it is trying to parse out regional data from a suite of global climate models to predict changes for 2050 and thereafter.
That exercise revealed that climate change isn’t just a future problem — it’s already affecting the Bay and its watershed.
The amount of rainfall that lands on the watershed and flows into the river has a huge impact on the amount of nutrients and sediment washed off the land and, ultimately, into the Chesapeake. In making nutrient runoff estimates for the TMDL, the Bay Program used rainfall estimates from the 1990s to simulate nutrient runoff from the watershed. That baseline has been used ever since.
“All of our thinking on climate change was relatively static up to this point,” Linker said. But an examination of the long-term precipitation record shows that the region’s rainfall has been increasing over the last century. When that trend was applied to the 30-year period from the mid-1990s to the cleanup deadline, it showed that the rainfall predicted for 2025 was being underestimated by more than 3 percent.
“For the first time, our eyes are open to the kind of change that is really taking place, based on our observations,” Linker said.
Not all that rainfall makes it to the Bay, though. As temperatures warm, it has also increased evapotranspiration — the evaporation of water from the ground and transpiration of water into the air from plants. As a result, while river flows have also been increasing, they have not risen at the same rate as precipitation.
What that means for 2025 and beyond isn’t totally clear. More heat could increase the evapotranspiration, which would reduce the amount of rainfall that actually reaches a stream. On the other hand, some scientists believe that more carbon in the atmosphere will make plants more efficient and reduce transpiration — meaning more rainfall and nutrients would reach waterways.
How the tug-of-war plays out will have big impacts on whether more nutrients will be washed into the Bay.
“It’s a horse race, but our best assessment is that precipitation is winning by a nose,” Linker said.
Right now, preliminary computer model estimates indicate that nitrogen and phosphorus runoff could increase about 2 percent. But that grows over time; by midcentury, continued increases in precipitation and stream flows would drive nutrient runoff up by more than 5 percent.
Drier summers, wetter winters
When and where rain arrives is also changing. On average, summers are getting drier and winters wetter, a trend expected to continue in the future. But precipitation increases are not spread evenly throughout the watershed. While projections to 2025 show increasing rainfall everywhere, it increases slightly more in the northern part of the watershed — the Susquehanna basin — than in areas to the south.
That is consistent with larger-scale climate model projections that generally predict the Northeast becoming wetter and the Southeast a bit drier, Bennett said. “The mid-Atlantic is kind of a hinge point,” he said.
How precipitation and river flow patterns affect Bay water quality remains to be seen, as the Chesapeake is also being altered by climate change.
Bay water temperatures, and water levels, have been rising for decades and will continue to do so. Warmer water holds less oxygen than cool water, which could make it more difficult to meet dissolved oxygen water-quality goals for the Chesapeake. But at least through 2025, that impact might be offset by rising sea levels which — at least initially — may help improve oxygen concentrations in some places. Rising water levels draw more salty ocean water farther up into the Bay and increases the exchange of Bay and ocean waters. In the short term, computer models predict that rising sea levels will improve water mixing in some of the most problematic areas of the Upper Bay, helping to break up the oxygen-starved dead zones that form on the bottom.
“In those areas, we would have a better ventilation of the bottom,” Linker said.
At the same time, rising seas will reduce oxygen levels in parts of the Lower Bay, but not by enough to keep them from meeting water quality goals.
Loss of marshes
But any water quality benefits from sea level rise are relatively short-lived, in part because they also drown tidal marshes. Marsh losses between now and 2025 are expected to be minimal, but by 2050 and especially thereafter, marsh losses are expected to be substantial. Lost with them is their ability to absorb large amounts of nutrients and to slow water flow, giving sediments a chance to settle.
Initial projections suggest that 40 percent of Bay’s tidal marshes could be lost by 2100, which would increase the amount of nitrogen reaching the Bay by about 10 percent.
In fact, scientists generally expect to see an acceleration of temperature and sea level trends as time goes by, along with potentially even greater changes in precipitation — which could also pose problems for the Bay. Also, they consider it likely that more precipitation will come in heavy downpours rather than gentle rain. Storms tend to move more sediment and phosphorus — much of which binds to sediment — than a slow, soaking rain.
Linker said some of that increased intensity is already being seen in data. As a result, the computer models predict the rate of increase for phosphorus is slightly more than that of nitrogen.
In the coming months, those estimates will be improved as models are refined and scientists analyze additional information to better estimate what impact climate change will have on Bay water quality goals.
Environmental groups are pressing for assurances that revised cleanup plans to be written in 2018 will address any nutrient increases caused by climate change. “Numerous peer-reviewed and rigorous scientific studies show that climate change-induced impacts in the Bay are already occurring and will worsen over the coming decades,” said David Flores, climate adaptation policy analyst with the Center for Progressive Reform, in a letter to the Bay Program that was signed by several other groups.
Flores added: “To our knowledge, there is no credible legal, scientific, or policy rationale” to delay taking actions to offset additional pollution stemming from climate change.
At recent meetings, state officials have said they would first like to see updated computer model projections, expected in the spring, that will reflect not only climate change, but also the impacts of more nutrients stemming from the filling of the Conowingo Dam reservoir, land use changes throughout the watershed and other factors that may affect cleanup goals.
But Bay Program participants and the environmental groups generally agree that climate change needs to be taken into account when implementing future runoff control practices by prioritizing those expected to be most effective under changing conditions.
In addition, Bay Program leaders recently adopted “guiding principles” for addressing climate change in future cleanup plans that call for states to report every two years to the EPA about how they are using new information about climate change to adjust their programs, policies and on-the-ground actions.
Impact on existing BMPs unknown
But figuring out how climate change may affect the dozens of runoff control best management practices being deployed around the Bay watershed to reduce runoff will be difficult, said Zoe Johnson, coordinator for the Bay Program Climate Resiliency Workgroup. On one hand, warming temperatures may allow farmers in northern parts of the watershed a greater chance to plant nutrient-absorbing cover crops before the first frost. On the other, warmer temperatures could make vegetated BMPs more vulnerable to drought, erosion, insect damage or disease.
“It’s easier to understand how climate can affect the pollution load than its affect on the performance of BMPs,” Johnson said.
But in general, she said, practices that slow water flow and help retain it on the landscape for longer periods of time would help. That includes things like forested stream buffers, restored wetlands or stream restoration projects that reconnect streams to floodplains. In tidal areas, she said, states may want to ensure that, where possible, upland areas provide routes for marshes to migrate as water levels rise.
For some structural BMPs, such as stormwater retention ponds, adjustments might be straightforward — new ones can be designed to handle greater, more intense storms, Johnson said. Existing ones might be enlarged, or supplemented with green infrastructure practices, such as rain gardens, which could intercept some of the additional rainfall before it reaches the detention pond.
One thing is certain: The challenge of dealing with climate impacts won’t be finished in 2025, as carbon dioxide levels in the atmosphere, temperatures and sea levels are all predicted to continue rising for decades — and cleanup efforts will require constant adjustments.
“What good is it to meet the water quality standard for one day in 2025 only to have climate change drive you away from that standard as you move beyond that?” asked Bennett, who is also co-chair of the Bay Program Climate Resiliency Workgroup. “They really need to keep their eye further out when taking these other things into consideration.”
In perhaps a sign of things to come, the Earth System Science Center at the University of Alabama in Huntsville in January reported that 2016 edged out 1998 to become the warmest year on record in its 38-year satellite temperature record.