Rising Ocean Temperatures Threaten Carbon-Storing Sea Grass

sea grass
Sea grass wafting in an underwater landscape. Credit: MorganLeeAlain/iStock/Getty Images Plus/Getty Images

A new model predicts that as ocean temperatures rise, carbon-storing sea grass may disappear and even go extinct in some ecosystems

January 17, 2018 — Sea grasses are part of a team of coastal vegetation, including mangroves and salt marshes, that store up to 100 times more carbon than tropical forests at 12 times the speed. Vast prairies of sea grasses stretch for kilometers along the seafloor, storing enough carbon to rival the world’s forests.

If rising ocean temperatures cause these sea grass ecosystems to fail, the loss will only expedite the global warming that did them in, scientists say. So, how exactly will the world’s sea grasses fare in the face of climate change? Thanks to a newly made model, researchers now have answers.

We can see that the coasts of Australia, Polynesia, and Hudson Bay will lose sea grass if ocean temperatures rise 1.5°C.

“We can see that the coasts of Australia, Polynesia, and Hudson Bay will lose sea grass if ocean temperatures rise 1.5°C,” said Orhun Aydin, a spatial statistician and product engineer at the Environmental Systems Research Institute (ESRI) in California. “The species Zostera marina only grows in these areas and will become extinct.”

Aydin and his coauthor Kevin Butler, a product engineer at ESRI, developed their model from publicly available data on sea grasses and their environments from the U.S. Marine Cadastre. They identified key environmental conditions involved in sea grass abundance and modeled how these would change with increased temperatures. Then they scaled up their model to encompass the global ocean, using the Ecological Marine Units data set, which provides 3-D maps of ocean ecosystems around the world.

Aydin presented the team’s predictions for the fate of sea grass last month at the American Geophysical Union’s 2017 Fall Meeting in New Orleans, La.

Rising Temperatures and Rising Concerns

The researchers looked at five environmental conditions affected by rising ocean temperatures: salinity, dissolved oxygen, nitrate, phosphate, and silicate concentrations. They compared ocean ecosystems using these parameters and grouped similar environments. They then cranked up the model’s thermostat and predicted how each ecosystem type would likely change with each 0.1°C increase in ocean temperature.

“We found an increase of 1°C was the tipping point,” said Aydin. Changing patterns in sea grass occurrence reveal themselves at 1°C and are exacerbated at 2°C and beyond, he explained.

For example, the Gulf of Mexico, a current sea grass hotbed, will be preserved as a haven for underwater meadows. But some places, such as Australia and Polynesia, will become increasingly unsuitable for sea grass.

Other places will become more suitable for growth, Aydin continued. For instance, if ocean temperatures rise 1.5°C, the frigid Arctic Ocean off the north coast of Siberia could become suitable for sea grass.

However, “Just because sea grass might be able to grow in new places doesn’t mean it will,” said Aydin. “The seeds still need to get there.”

A map shows how, over the course of a 2°C increase in ocean temperature, the suitability of areas to serve as sea grass habitats will likely change. To build the map, scientists began with today’s ocean conditions, then simulated sea grass ecosystem health every tenth of a degree as ocean temperatures rose. Aggregating these simulations through this 2°C temperature change yields a map that shows the trends of sea grasses at any given point. “Recently” refers to a switch in state; for example, ocean around southern South America started off as unsuitable and switched to being suitable for sea grasses over the course of the temperature change. “Increasingly” and “decreasingly” refers to an amplification of the trend; for example, areas in the Caribbean started off suitable and became more so over the 2°C temperature increase. “Consistently” means that the trend continued unchanged; for example, areas in the North Sea were consistently suitable for sea grasses over the 2°C temperature increase. “Improving” and “declining” mean that locations were trending toward being good or bad habitats. For example, areas north of Siberia are “recently improving”: They started off as slightly unfavorable to sea grasses and switched to being favorable as the simulation progressed but are still not prime locations for sea grasses to grow. “Sporadically” means that areas that started one way oscillated between conditions throughout the simulation. For example, regions north of Canada started as unsuitable and oscillated between suitable and unsuitable as the simulated temperatures rose. Credit: Orhun Aydin, ESRI

The Future of Sea Grass

The researchers note that the model does not take into account polar ice sheet melting, which would affect ocean salinity and thus environmental hospitableness for sea grass. The model also does not account for how changing levels of atmospheric carbon dioxide will increase the acidity of oceans.

Global warming is actively destroying mechanisms for storing carbon dioxide.

Acidity matters. According to a report from the Intergovernmental Panel on Climate Change, “changes in salinity and temperature and increased sea level, atmospheric CO2 [carbon dioxide], storm activity and ultraviolet irradiance alter sea grass distribution, productivity and community composition.”

Altered salinity and acidity would likely lower the threshold at which rising temperatures change distribution of sea grass, Aydin noted. The researchers’ next steps involve incorporating the compounding conditions into their model.

“Global warming is actively destroying mechanisms for storing carbon dioxide,” said Aydin. “This means increasing temperature will not be a linear process; intuitively, I’d say it will be exponential.”

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