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Sea-level rise influences coastal carbon storage capacity

Coastal ecosystems are natural carbon storehouses, making them a valuable resource in the fight against climate change. However, the effectiveness of these ecosystems in capturing carbon dioxide from the atmosphere is dependent on various factors, including tidal range, wave climate, sediment supply, coastal slope, and changes in sea level.

Now, by using a recently developed computer model, a team of researchers led by the Virginia Institute of Marine Science (VIMS) has conducted a study to gain a better understanding of coastal carbon storage and to simulate how carbon moves among different coastal ecosystems as their boundaries and relative sizes shift due to rising seas. 

While previous studies of coastal or “blue” carbon had mostly focused on its fate within static habitats, the current approach is a more realistic modeling of marsh dynamics, revealing that the rate of sea-level rise plays a crucial role in determining whether blue carbon enters a long- or short-term coastal reservoir or “sink.” Long-term storage, which could span centuries or even millennia, is the most effective way to mitigate the worst effects of climate change.

“Our key takeaway is that the coastal system is resilient and can continue to store increasing amounts of carbon at moderate rates of sea-level rise. But if rise rates get too fast, the marsh is unable to keep pace, the marsh system collapses, and coastal carbon storage decreases,” said lead author Kendall Valentine, an ecogeomorphologist at the University of Washington, who conducted the study during a postdoctoral fellowship at VIMS.

“Our research shows that sea-level rise is a double-edged sword for coastal carbon sequestration. While moderate rates of sea-level rise enhance plant productivity and carbon preservation in soils, rapid sea-level rise changes the location of carbon storage – from relatively stable forests to more vulnerable marshes,” added co-author Matt Kirwan, an associate professor of Coastal Geomorphology and Ecology at VIMS.

By modeling the behavior of coastal ecosystems at rates of sea-level rise ranging from one to 15 millimeters per year, the scientists found that carbon storage more than doubles when the rate of sea-level rise increases from two to five mm/yr. However, past a 10 mm/year “tipping point,” carbon storage begins to decline as marsh replaces forest and the seaward edge of the marsh experiences faster erosion rates.

These findings have major implications for policies governing blue-carbon offsets, which – like those governing tree planting and other land-based or “green carbon” solutions – only count carbon that is captured within a particular ecosystem to avoid double-counting of carbon initially captured elsewhere. “Corroborating results from previous field experiments, we found that up to 50 percent of the carbon in marsh soils is created elsewhere,” Valentine said.

Better understanding the dynamics of coastal ecosystems in the context of rising sea levels could help policymakers develop more effective strategies to mitigate the effects of climate change.

The study is published in the journal Nature Communications.

By Andrei Ionescu, Staff Writer

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