MOREHEAD CITY — Analysis of nearly 120 years of tropical storm and hurricane landing data and associated rainfall in coastal North Carolina shows climate change is creating a feedback loop that suggests increased flooding and flood damage is likely along the coast in the future.
That’s a general conclusion of research led by Dr. Hans Paerl, Kenan Professor of Marine and Environmental Sciences at the UNC-Chapel Hill Institute of Marine Sciences. The research findings and conclusions were published in a paper, “Recent increase in catastrophic tropical cyclone flooding in coastal North Carolina, USA: Long-term observations suggest a regime shift,” featured in the July 23 edition of Nature Research’s Scientific Reports.
The rainfall data, compiled by the National Oceanographic and Atmospheric Administration, show six of the seven highest precipitation events since 1898 have occurred within the last 20 years, lead author Dr. Paerl said in a recent interview.
Those events include hurricanes Floyd in 1999, Matthew in 2016 and Florence in 2018. Those three resulted in abnormally large floods, Dr. Paerl said, and the chance of that happening in such a short time frame is less than 2%.
As he wrote in the paper, “…either North Carolina has been very unlucky, or the historical record used to define the storm statistics is no longer representative of the present climatic regime.
“This analysis suggests that the occurrence of three extreme floods resulting from high rainfall tropical cyclone events in the past 20 years is a consequence of the increased moisture carrying capacity of tropical cyclones due to the warming climate.”
What are the ramifications for estuarine and coastal ecosystems? Dr. Paerl said receiving ecosystems are becoming overloaded by the frequency of rain events and the sheer amount of rainfall, as well as the nutrient- and organic matter-laden runoff from the watersheds of the Neuse River estuary and Pamlico Sound.
Although the study was not meant to be predictive, he said, the increased runoff means more organic, carbon-based materials enter from the watersheds and are broken down into carbon dioxide by aquatic microbes. As a result, he said, “more CO2 is generated and reenters the atmosphere,” adding to CO2 levels that already are in part responsible for heating the planet and resulting in the likelihood of stronger and more frequent tropical storms and other rainfall events.
That represents a major “feedback loop,” Dr. Paerl said, and since the effects can last for months, there is less time for ecosystems to recover.
These events, he said, can last weeks to months and can lead to massive finfish and shellfish kills, as well as an abrupt increase in fish disease.
“Evidence is accumulating that we may also be seeing changes to the ‘system state’ of coastal waters in terms of their ability to capture or release CO2,” Dr. Paerl wrote in the study. “Such changes caused by an increased frequency of extreme storm events are ostensibly reorganizing coastal carbon cycles.”
For example, he said, flood waters that have reached the inner shelf of the Gulf of Mexico have resulted in extensive degradation of terrestrial organic matter and the return of that carbon as CO2 to the atmosphere.
Floodwaters, he said, contain contaminants, and runoff from urban and agricultural land uses often lead to low oxygen conditions when they enter an estuary. That happened in the Neuse River estuary after each storm.
In addition, he said, “high freshwater inflows lead to an ‘oil and vinegar’ situation, with the relatively light freshwater sitting on top of the denser salt water.”
That reduces vertical mixing, and organic matter is “trapped” in the denser, salty bottom water, causing extensive low oxygen conditions, he said.
All of these things – fish and shellfish kills, floods and more frequent and stronger tropical events – pose serious threats to coastal residents and the economy, Dr. Paerl wrote.
“…Severe economic and societal implications for fisheries, tourism, and real-estate … have raised concerns about coastal resiliency and sustainability,” he added in the paper. “In North Carolina alone, Hurricane Floyd in 1999 caused fisheries losses of $6 million, and overall economic (tourism, property and business damage and losses, agriculture and silviculture) amounted to $2 billion.”
Analysis of Florence’s longer-term impacts are continuing, Dr. Paerl wrote, and “while the hydrologic, nutrient and carbon inputs attributable to Florence are yet to be fully tallied, the rainfall associated with this event was roughly equivalent to Matthew in 2016.
“Like Floyd and Matthew, Florence’s floodwaters led to ‘freshening’ and expanding hypoxic zones in the (Albemarle Pamlico System-APS), as well as massive pulses of carbon overflowing from the APS into coastal waters, as viewed from space, with effects that can linger for months after a storm,” he added.
He noted in the interview none of these storms were classified as major hurricanes – Category 3, 4 or 5 – when they made landfall and affected North Carolina. Floyd was a Category 2, while Matthew and Florence were Category 1. Yet they carried enough rainfall to cause massive flooding.
“Rather than attributing a particular event to global warming, we should consider whether a warming climate made these events more likely, which our records suggest is the case for coastal NC,” Dr. Paerl wrote in the report.
“For example, increased precipitation in other US coastal areas subject to tropical cyclones (e.g., coastal Texas from Hurricane Harvey in 2017) and increased hurricane activity since 1970 have been attributed to global warming. Factors potentially driving the increased precipitation include; (1) greater heat content of ocean waters, which not only fuels storm intensity but also increases precipitation, (2) a decrease in tropical cyclone forward movement, providing more opportunity for heavy precipitation over a particular area, (e.g. Harvey and Florence), (3) an observed poleward migration of tropical cyclones, perhaps making coastal NC more vulnerable than in the past, and (4) an increase in tropical cyclone intensity…”
He said in the interview that ocean temperatures in recent years “have broken all sorts of records” in the Atlantic and Pacific basins.
Additionally, the population of the world – and North Carolina – is growing, further increasing use of carbon-based fuels that contribute to climate change and increasing the amount of hardened surface on land, fomenting more runoff. For example, he noted, North Carolina’s population in 1990 was 6.6 million; by 2018, according to the U.S. Census Bureau, it was 10.3 million.
What to do?
So what, if anything, can be done?
Dr. Paerl said it has become increasingly evident that we need to reduce emissions of greenhouse gases. Also, as individuals, he said, we can reduce the harmful impacts of flooding by “not developing” properties “down to the water’s edge,” instead keeping or putting in vegetative buffers that help filter out pollutants before they reach creeks, rivers and estuaries.
Farmers, he said, have done a better job in recent years of maintaining riparian buffers, but city planners and state leaders need to increase their efforts along those lines as well. If we must use them, fertilizers need to be applied at “agronomic rates,” supporting crop production, while avoiding excess.
“We should minimize fertilizer application during hurricane season; one ‘wet’ storm can lead to major losses of fertilizer to downstream nutrient sensitive waters,” he said.
To limit property and economic damage when floods occur, he said, we should cease building in flood plains.
“There’s a reason flood plains are called flood plains,” he noted. The fact that they are called flood plains means they’re more likely to flood often, and rebuilding in flood-prone areas should be carefully considered and at least limited.
“Considering these extreme precipitation events and their hydrologic and bio-geochemical consequences in totality, it is clear that they are unparalleled in the past 120-plus years of recorded tropical cyclones in coastal North Carolina,” Dr. Paerl concluded in his published paper.
“The potential exists for receiving waters globally to undergo unprecedented perturbations to nutrient and carbon cycling, fisheries habitat and sustainability due to increasing frequency of extreme precipitation events; all of which are still to be determined.
“With roughly 40% of the world’s population within 100 kilometers of the coast, development inland, as well as along the coastline, will exacerbate the perturbations caused by this type of regime shift,” he added.
“We stress that stakeholders, state and federal governments need to better prepare for the acute as well as cumulative water quality, fisheries resource and overall socioeconomic effects of this recently-documented rise in catastrophic flooding associated with elevated tropical storm activity.”
Others listed as authors of the report are Nathan Hall, a research assistant professor at UNC-IMS; Alexandria Hounshell, a UNC-IMS graduate student; Dr. Rick Luettich, UNC-IMS director and professor; Karen Rossignol, a research specialist in the Paerl Lab; Dr. Christopher Osburn, a researcher and professor at N.C. State University; and Dr. Jerad Bales, chief scientist for water at the U.S. Geological Service.
Funding for the research came from the National Science Foundation, the N.C. Department of Environmental Quality, the Lower Neuse Basin Association, N.C. Sea Grant and the UNC Water Resources Institute.
This article first appeared in Coastal Review Online, the news service of the N.C. Coastal Federation. It is reprinted here as part of a reporting partnership.
Contact Brad Rich at 252-864-1532; email Brad@thenewstimes.com; or follow on Twitter @brichccnt.