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Model Forecasts for a Warming Watershed

By Erica Goldman

When it comes to using models to predict the impact of future climate change on the Chesapeake, scientists face a two-pronged challenge. First, they must try to predict how climate warming will affect temperature and precipitation on this regional, rather than global scale. Second, they must take these predictions and apply them to ask specific questions about the Bay itself — questions such as: How much coastal land will become submerged by the end of the century? Will the estuary become more hypoxic? Will existing nutrient loads increase or decrease?

To forecast scenarios for changes in temperature and precipitation on a regional basis, scientists must refine models that were designed to make global predictions. Roughly 20 so-called "coupled general circulation models" currently operate out of 14 supercomputing centers around the world, part of the fourth major assessment by the Intergovernmental Panel on Climate Change (IPCC). These models link atmosphere, ocean, and land interactions to reconstruct climate history of the past and to project scenarios for the future. Current model runs will recreate "controls" such as the climate of pre-industrial times and the present day. They will also forecast climate scenarios under different amounts of CO2 emission in the future — such as 100-year predictions under conditions of low, middle, and high CO2 emissions.

In a report soon to be published by the Pew Center on Global Climate Change, researchers at the University of Maryland Center for Environmental Science (UMCES) compare two of these general circulation models to predict the impact of climate change for Chesapeake Bay hypoxia — a depletion of dissolved oxygen in the bottom waters that occurs each summer and can be devastating for fish, crustaceans, and mollusks.

Precipitation, river runoff, sea level, and temperature interact with each other to affect seasonal hypoxia. All of these variables will respond to climate warming. "When you think about all the things that can change with climate and how they interact with things that we are trying to manage in the Bay, there are a lot of moving parts that we have to understand," says Donald Boesch, first author of the upcoming Pew report and president of UMCES.

At the high and middle levels of the CO2 emissions scenarios used by the IPCC, the two models predict precipitation increases in the Bay region of up to 30% in some months and decreases greater than 10% in the fall by the end of the century — including more events with extreme rainfall. Both models predict temperature increases ranging from 3.5 to 6.5°C, clustered in the summer months. But the global models do a better job of predicting temperature than precipitation at the regional scale, cautions mathematical modeler and study co-author Victoria Coles.

"We know that climate change is going to happen.
Step one is to quantify the effects we might see in this region."

But even with some uncertainty surrounding the precipitation changes under climate warming scenarios, researchers can make some overall predictions related to the Bay's hypoxia. The team anticipates that increasing streamflow, increasing summertime temperatures, plus an increased depth of the Bay due to sea level rise, would reduce the exchange between warmer surface waters and cooler deeper waters (enhancing stratification). This change would spread hypoxia into shallower areas of the Bay.

Although this study is preliminary and the researchers are currently looking for funding to do a more in-depth analysis, their findings carry a clear warning message for restoration efforts. "Given the long lag times, both in terms of implementation of nutrient control strategies and in the responses of the ecosystem," they write, "it is not too early to begin assessing the implications of climate change on management objectives for hypoxia and for Chesapeake Bay restoration."

The Environmental Protection Agency's Chesapeake Bay Program has already begun to factor climate warming predictions into their modeling scenarios for the next two-year period of decision-making. The Bay Program's models are used for management purposes — primarily to track nutrient loads and to evaluate progress towards reaching water quality goals.

As part of their 2008-2010 assessment, modelers will for the first time be able to include predictions for future changes in temperature and precipitation into a set of scenarios run through the year 2030, explains Lewis Linker, coordinator of the Bay Program's modeling subcommittee. Although the model runs will not yet be able to factor in projected sea level rise or changes in the Bay's depth (bathymetry), this new climate assessment tool will allow managers to assess how climate change may interact with progress towards restoration goals.

"We know that climate change is going to happen," Linker says, "but we don't know what it will do with respect to the flow of the rivers, or with respect to [nutrient] loads. Step one is to quantify the effects we might see in this region."

When it comes to policy decisions, these model scenarios provide a base of information. "We'll be able to make all these runs," says Linker. "How they will get used will be up to the decision makers in the Bay Program."

Chesapeake Bay Models

   Air Quality Model: Depicts air deposition of nitrate and ammonia
Air Quality Model
Depicts air deposition
of nitrate and ammonia
   Watershed Model: Depicts land use and changing best management practices (BMPs)
Watershed Model
Depicts land use and changing best management practices (BMPs)
   Estuary Model: Depicts sediment, algal blooms, and the effects of filter feeders
Estuary Model
Depicts sediment, algal
blooms, and the effects
of filter feeders

Models use mathematical representations of the real world to estimate the effects of complex and varying environmental events and conditions. The Watershed Model (above center), for example, estimates the delivery of nutrients and sediment to the Bay by simulating hydrologic and nutrient cycles, using inputs such as atmospheric nutrient deposition, precipitation, fertilizer application, and land cover or land use. As the Chesapeake Bay Program prepares to re-evaluate its current set of benchmarks for 2010 and to set new goals for 2030, new model scenarios will incorporate predictions for warming temperatures and changing precipitation patterns.

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