SciTech

Hurricane effect: A big case of ‘burping’ estuaries

It’s hurricane season, but so far we’ve been spared from the tropical weather that causes wind damage, flooding and storm surge. Beyond the regular dangers that come with big seasonal storms, a new threat has emerged: “burping estuaries.”

Hans Paerl, Kenan Professor of Marine and Environmental Sciences at UNC Chapel Hill’s Institute of Marine Sciences, explained: “When these hurricanes perturb the system and mix everything up and redistribute the sediments from the bottom into the water column, it essentially causes the system to ‘burp’ a huge amount of carbon dioxide into the air.”

Why is this a problem? It comes down to a surge of the greenhouse gas carbon dioxide suddenly being released into the environment.

Scientists already knew that large bodies of water such as the Pamlico Sound are great at absorbing CO2 like a sponge and storing it. But Paerl, along with Joey Crosswell, then a doctoral student who did most of the work for his dissertation, discovered that tropical storms and hurricanes cause carbon dioxide that has been building up for years to be released all at one time.

The paper was published in late August by the Association for the Sciences of Limnology and Oceanography.

The Institute of Marine Sciences is an off-campus research lab and teaching facility in Morehead City.

“What the paper basically has concluded is that one major storm – in this case Hurricane Irene, which came along at the end of August 2011 – can cause a system to give up as much carbon dioxide as it actually accumulates over several years,” Paerl said.

According to the study, Hurricane Irene-induced release of CO2 from the Neuse River estuary alone was at least four times the annual carbon input from the whole Neuse watershed – and seven times the annual uptake of CO2 by rooted plants and algae in the estuary.

Reinforced greenhouse effect

Carbon dioxide is introduced into the Pamlico Sound and other bodies of water in a variety of ways. Tree and plant material can break off during storms, get washed away and end up in the estuary. The vegetative material is converted by microbes into CO2 by decomposition. Carbon can also be produced by plankton and microscopic organisms, as well as sea grasses and other aquatic plants. Paerl said the largest contributor of carbon dioxide is manmade sources.

“Coastal environments act like a bank, in the sense that they collect organic matter and anything that flows into them from the watershed,” he said. “But also, it collects material that’s formed in the system itself. And if we think about carbon, the major sources of carbon that go into that sink are runoff from the watershed that delivers carbon that’s grown on land and then ultimately washes into the systems.”

When a tropical system passes over our area, it churns the water, causing the CO2 to be vented into the atmosphere. This release is what Paerl referred to as a “burp.”

While there’s no evidence that the excess CO2 affects air quality, he said it does augment the greenhouse effect.

“There’s sort of a boomerang effect there that will actually increase over time. These storms are increasing in intensity, and most of us in the scientific community think that’s due to warming, which is caused by an increase in carbon dioxide levels in the atmosphere. Those storms come along and then cause more carbon dioxide to actually be lost into the atmosphere.”

Like estuaries, marshes are good at storing carbon dioxide. Paerl said that with an uptick in storms because of climate change, marshes will erode more, and release more carbon into the air.

“It’s changing the function of these systems,” he said. “We know wetlands are really important in terms of storing up carbon. But if these perturbations come along and we get more of them over time, that will turn into more of a source. So it will really actually kind of exacerbate the CO2 problem that we’re experiencing. They will no longer be as good of a storage bank as they have been.”

Controlling the carbon

“Burping estuaries” are not a problem unique to Eastern North Carolina. It happens in other areas along the East Coast, the Gulf Coast and even Asia, which is vulnerable to tropical typhoons. Since it’s a global problem, “burping estuaries” could be a significant source of CO2 emissions into our atmosphere.

The good news is that the outflowing of CO2 from these systems can be controlled. Paerl said the only way we can lessen the effect is to reduce the amount of carbon dioxide that’s introduced into our environment. “And so that means controlling emissions, like agricultural emissions, for example of agricultural waste, which generate quite a bit of CO2, but also power plants. … We can reduce that by being able to trap CO2 before it leaves the smokestack. Or to have more fuel efficient and better conversion mechanisms in our vehicles, and to start using alternative fuels.”

As we reduce the amount of carbon input into the water system, Paerl said, vegetation in and around the estuaries that use CO2 will be able to keep up, stabilizing levels.

“I would think that we would see a reversal of the feedback loop,” he said. “The warming in the oceans will no longer occur at the rate they currently do. There will be cooling events, and I’m talking about long-term now, not talking about a single event. We’re thinking decades to centuries in terms of the impact of those types of management strategies.”

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