CHAPEL HILL – A “regime shift” defined as “large, persistent changes in the structure and function of social-ecological systems, with substantive impacts on the suite of ecosystem services provided by these systems” appears to be underway for coastal North Carolina with precipitation levels in recent storms being “off the charts,” say researchers at UNC-Chapel Hill. Driving the change is climate change, they conclude.

Citing data accumulated since 1898, the researchers note that six of the largest “highest precipitation events” such as hurricanes and tropical storms have taken place over the past 20 years. And three of those – hurricanes Floyd, Matthew and Florence – produced large flooding. Floyd occurred in 1999, Matthew in 2016 and Florence in 2018.

These events have caused “catastrophic human impacts,” the study points out.

“North Carolina has one of the highest impact zones of tropical cyclones in the world, and we have these carefully kept records that shows us that the last 20 years of precipitation events have been off the charts,” said Hans Paerl, Kenan Professor of Marine and Environmental Sciences at the UNC-Chapel Hill Institute of Marine Sciences and lead author of the study published today in Nature Research’s Scientific Reports.

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“The probability of these three flooding events occurring in such a short time period is 2%,” UNC reports. Paerl describes wetter storms as the “new normal.”

The study examines that point in detail – why?

“With less than a 2% chance of three such events occurring in a twenty-year period, 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,” the study reads.

“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.”

The researchers say “our observations are consistent with observations elsewhere and with predicted changes in a warming climate. Moreover, 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. ”

Talking with UNC’s news service, Paerl added:

“We are in part responsible for what’s going on in the context of fossil fuel combustion emissions that are leading to global warming. The ocean is a huge reservoir that is absorbing heat and seeing more evaporation. With more evaporation comes more rainfall.”

Flooding’s widespread impact

The abstract of the study makes for worrisome reading with the impact of flooding being much more than in the loss of lives and homes.

“Coastal North Carolina, USA, has experienced three extreme tropical cyclone-driven flood events since 1999, causing catastrophic human impacts from flooding and leading to major alterations of water quality, biogeochemistry, and ecological conditions. The apparent increased frequency and magnitudes of such events led us to question whether this is just coincidence or whether we are witnessing a regime shift in tropical cyclone flooding and associated ecosystem impacts,” it reads.

“Examination of continuous rainfall records for coastal NC since 1898 reveals a period of unprecedentedly high precipitation since the late-1990’s, and a trend toward increasingly high precipitation associated with tropical cyclones over the last 120 years.

“We posit that this trend, which is consistent with observations elsewhere, represents a recent regime shift with major ramifications for hydrology, carbon and nutrient cycling, water and habitat quality and resourcefulness of Mid-Atlantic and possibly other USA coastal regions.”

Defining a regime shift

What exactly is a regime shift? The Regime Shifts website offers this definition:

“We define regime shifts as large, persistent changes in the structure and function of social-ecological systems, with substantive impacts on the suite of ecosystem services provided by these systems. Better understanding of regime shifts is important as they may have substantial impacts on human economies, societies and well-being, and are often difficult to anticipate and costly to reverse.”

What can be done?

“We can help minimize the harmful effects of a ‘new normal’ of wetter storm events,” Paerl told UNC. “Curbing losses of organic matter and nutrients by vegetative buffers around farmlands and developed areas prone to storm water runoff, minimizing development in floodplains and avoiding fertilizer applications during hurricane season, and reducing greenhouse gas emissions are positive steps which we can all contribute to.”

The report in Nature can be found online.