Oceanographers study the Pacific Ocean's 'dead zones "dead zones" in 2009, which appear each summer off the Oregon and Washington coast. (National Science Foundation)

Three years ago, the Chesapeake Bay was hit by an unusually large “dead zone,” a stretch of oxygen-depleted water that killed fish from the Baltimore Harbor to the mid-channel of the Potomac River and beyond, about a third of the bay.

Another giant dead zone returned last summer, smaller than the first but big enough to rank as the estuary’s eighth largest since state natural resources officials in Virginia and Maryland started recording them in the 1990s.

In a future of climate change, those behemoths might not seem so unusual, according to a new report by the Smithsonian. As the global temperatures warm, they will create conditions such as rain, wind and sea-level rise that will cause dead zones throughout the world to intensify and grow, the report says.

Ninety-four percent of places where dead zones have been recorded are areas where average temperatures are expected to rise by about 4 degrees Fahrenheit by the turn of the century. In addition to the Chesapeake Bay region, that includes the Black and Baltic seas and the Gulf of Mexico, where a dead zone equal to the size of Connecticut took shape in August.

Coastal dead zones will be exacerbated by global warming.

“Over 40 percent of the world’s population lives in coastal areas,” said Keryn B. Gedan, codirector of a conservation program at the University of Maryland and a researcher at the Smithsonian Environmental Research Center in Cambridge, Md. “We depend on these resources. No one wants to see a fish kill or harmful algal bloom at their local beach.”

Gedan was a co-author of the study with Andrew H. Altieri of the Smithsonian Tropical Research Institute in Panama. They found that the number of dead zone events have doubled each decade since the 1950s and that humans have likely contributed to their growth in intensity and size.

“We just don’t know how much of this doubling is due to climate change or nutrient runoff,” Gedan said. More studies with more “sophisticated modeling” are needed to determine that, she said.

Dead zones are summer plagues that happen when waters warm. As the water temperatures increase, three key events pave the way for a catastrophe that kills any fish, crab, oyster and shrimp that relies on oxygen.

Piles of mussels washed onto a beach after a dead zone event in Narragansett Bay, R.I. Besides providing food and habitat for other creatures, mussels can also filter water. When mussels die, the bay loses its ability to clear water of phytoplankton, increasing the risk of future dead zones. (Andrew Altieri/Smithsonian)

The metabolism of animals in the water revs up, turning them into hungry eaters that use more oxygen as they search and feed on algae. Algae that feeds on nutrient pollution that runs off farms in rains and pours out of overflowing sewers bloom and perish in a rapid and enormous death spiral. Microbes feed on the dead algae in a frenzy that sucks out oxygen to a point where it can no longer sustain life.

In a warming world, this process, which currently starts around May, will likely start sooner unless steps are taken to reduce the overabundance of nitrogen, phosphorous and other pollutants that flow into water.

Gedan said the Chesapeake Bay cleanup plan is one example of how government can act to mitigate climate driven impacts that create dead zones. Previous research supports that assertion.

A study of the bay’s water quality by researchers for Johns Hopkins University and the University of Maryland Center for Environmental Science found that dead zones have been reduced since pollution limits were first implemented in the 1980s.

Close-up of a bloom of Scripsiella phytoplankton in the Rhode River, Md., a tributary of the Chesapeake Bay. (Smithsonian Environmental Research Center)

Microscopic Coscinodiscus diatoms are one of many phytoplankton species in the Chesapeake Bay. (Smithsonian Environmental Research Center)

They studied water-quality data for the Chesapeake from 1949 to 2009 and found “evidence that cutting back on the nutrient pollutants pouring into the bay can make a difference,” the study’s lead author, Rebecca R. Murphy, a doctoral student at Johns Hopkins, said when the study was released in 2011.

There is no certainty that cleanup plans such as the bay’s so-called pollution diet, which calls for the governments in six states and the District to reduce sewer overflows and nutrient runoff from farms, can counteract changing climate and its accompanying ill effects.

Downpours, wind storms and sea-level rise are a lot to overcome. Downpours cause sewers to overflow, wind storms wipe fertilizers off yards and push them down drains and sea-level rise threatens to drown wetlands that block pollution’s path to rivers that feed bays and oceans.

Unlike crabs and bivalves such as oysters, striped bass, an iconic Maryland and Virginia fish, could swim to the shallows to escape deeper oxygen depleted water. But as the weather warms and raises the temperature in shallow water, that refuge could go away.

High temperatures in shallows cause “thermally induced hypoxia,” the report said. “This combination is predicted to reduce habitat for striped bass. In extreme situations, the temperatures of shallow water may exceed thermal tolerance of organisms, leaving them with the dilemma of choosing death by hypoxia at depth or by thermal stress in the shallows.”

A handful of dead soft shell clams were stranded on a beach following a dead zone event. The soft-shell clam is a popular food source for humans, as well as a valuable water filter. (Andrew Altieri/Smithsonian)