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Sea-Level Rise Will Continue To Fuel Destructive Coastal Flooding

December 5, 2013
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April Flowers for redOrbit.com – Your Universe Online

Two new literature review studies, published in Nature, reveal changing coastal ocean functions.

The first study, “Coastal flooding by tropical cyclones and sea-level rise,” reveals that the clamor about whether climate change will cause increasingly destructive tropical storms may be overshadowing a more unrelenting threat to coastal property: sea-level rise.

Jonathan D. Woodruff, an assistant professor of sedimentology and coastal processes at the University of Massachusetts Amherst, collaborated with Jennifer Irish, associate professor of civil and environmental engineering with the Virginia Tech College of Engineering, and Suzana Camargo, a Lamont research professor at the Lamont-Doherty Earth Observatory of Columbia University, to review nearly 100 research studies. Their analysis revealed that accelerated sea-level rise certainly will increase the flooding and property damage triggered by tropical cyclones — commonly known as hurricanes in the Atlantic and Northern Pacific — but predicting where, how often, and how powerful these storms will be when they make landfall is full of uncertainty.

“The potential for sea-level rise to dramatically change the landscape is an understudied aspect of coastal flooding,” said Irish. “For example, shoreline erosion, barrier-island degradation, and new tidal inlet formation — these sedimentary changes could lead to catastrophic changes in hurricane flood risk in some areas.”

Regardless of changes in storm activity, the researchers say, rising sea levels will become the dominant driver of flooding and coastal damage.

The research team cited information from the International Disaster Database of the Center for Research on the Epidemiology of Disasters (CRED) that indicates that, since 1970, more than 60 percent of all economic losses — about $400 billion — occurred in the North Atlantic, even though it is one of the least active basins for hurricanes.

A holistic approach to manage coastal systems is needed, the researchers stress, especially in the context of almost certain flooding from tropical cyclones because of rising sea levels.

“Sea-level rise, severe storms, changing climate, erosion, and policy issues are just some of the factors to assess in order to understand future risk,” Irish said. “We reviewed just three of the physical factors — tropical cyclone climatology, sea-level rise, and shoreline change. If we look at them separately, we don’t see how they are interconnected. But if we pull back to look at the whole picture, we stand a better chance of protecting our homes, roadways, energy and water networks, and the most critical and expensive infrastructure along the coastlines.”

The data suggests that it is practical to focus on approaches that integrate vertical and landward retreat. This means that planners should consider elevated structures and building farther inland. These approaches should be combined with other engineering and management measures, including sediment management.

By 2100, global sea level is expected to rise by one meter (3.25 feet). Simulation studies reviewed by the researchers suggest that flooding that is currently considered to be a 100-year event in New York City could become a three- to 20-year event.

“It is widely accepted that sea level will rise. We just don’t know how much,” Irish said. “We need to consider the full range of sea-level estimates and plan our engineering strategies from that, designing for moderate protection now in a way that these designs can be modified in the future if necessary. The Dutch have been dealing with this problem for centuries, so it can be done.”

COASTAL CARBON CYCLE

The second study, “A changing carbon cycle of the coastal ocean,” from the University of Delaware’s School of Marine Science and Policy, demonstrates that carbon dioxide pumped into the air since the Industrial Revolution appears to have changed the way the coastal ocean functions.

The research team, led by Wei-Jun Cai, professor of oceanography, performed a comprehensive review of research on carbon cycling in rivers, estuaries and continental shelves, suggesting that collectively this coastal zone now takes in more carbon dioxide than it releases. Global models of carbon’s flow through the environment and future predictions related to climate change could be impacted by the shift.

“We need to better understand the role of the coastal ocean in carbon dioxide exchange between the atmosphere and the ocean,” said Cai. “That will give us a much better capacity to predict future global carbon budgets and fluxes due to climate change and other anthropogenic factors.”

Cai and other environmental scientists have been investigating the complex dynamics that move forms of carbon through coastal waters. How much carbon is present in water at any given time can be influenced by numerous variables, from rainfall, to temperature, to plant photosynthesis.

“Carbon is not stationary,” Cai said. “It flows and changes among its different forms.”

Studying coastal carbon is challenging because of the multiple sources and processes at play, however. Cai said that coastal carbon is traditionally overlooked in global carbon budget calculations. For example, the annual estimate of how much anthropogenically-released carbon dioxide is trapped by land has been determined by subtracting the amount taken up by the ocean from the amount put into the air.

“If there is another reservoir — the coastal ocean — that also takes up carbon dioxide, then that changes the balance,” Cai said.

Researchers point out that although the coastal zone is relatively small compared to the open ocean, it represents a disproportionately large amount of the carbon dioxide exchanged between air and water—suggesting that the coastal ocean may have its own mechanism for holding carbon dioxide.

Cai first suspected the presence of this mechanism during a cruise off the coast of Georgia in 2005. Even though there were significantly greater amounts of the greenhouse gas in the atmosphere, he was surprised to see that sea surface carbon dioxide levels were about the same as 10 years prior.

Sea surface carbon dioxide should rise in tandem with levels in the atmosphere, according to conventional wisdom, as is the case in most of the ocean basin.

“However, if the coastal ocean has its own way to hold sea surface carbon dioxide and atmospheric carbon dioxide keeps increasing, that makes the coastal ocean more important as a carbon dioxide sink in the future — as the rate of carbon dioxide uptake by the ocean is determined by the concentration difference between the atmosphere and the ocean, which is increasing,” Cai said. “The global carbon cycling model should take this additional carbon dioxide sink into account.”

In the new study, the researchers suggest that an increased physical uptake of atmospheric carbon dioxide explains the continental shelf switching from a carbon dioxide source to a sink — or repository — over the industrial age, as well as providing a mechanism to explain the slower carbon dioxide increase in the coastal ocean. Other studies have theorized that agricultural fertilizers feeding extra nutrients into water caused the shift.

Scientists are able to generate new best estimates of carbon cycling in coastal areas through new instrumentation. Cai and his colleagues used the latest measures available to create a model estimating that coastal areas released, on average, about 150 million metric tons of carbon per year a century ago. Currently, these same waters are estimated to absorb approximately 250 million metric tons of carbon each year.

The research team calls for further observations and field studies to better understand the complicated dynamics in coastal systems, including additional human-caused changes such as land-use modification, waterway construction and wetland degradation. The study findings have implications for future predictions of ocean acidification, global warming and climate change.

“Compared to the open ocean, we know less about the coastal ocean’s carbon cycle even though it’s right in front of us,” said James Bauer, professor of evolution, ecology and organismal biology in Ohio State University’s College of Arts and Sciences and lead author of the paper. “We just have to commit to increasing the number and types of coastal regions being studied.”




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