A study published recently in the journal Science Advances warns that marshes in the Mississippi Delta are likely to be submerged by sea-level rise within the current context of climate change. The study uses sediment data from the last thirty years to monitor sea-level rise over the past 8,500 years, documenting the longest-known marsh record in the Delta.
"Previous investigations have suggested that marshes can keep up with rates of sea-level rise as high as half an inch per year (10 mm/yr), but those studies were based on observations over very short time windows, typically a few decades or less," said lead author Torbjörn Törnqvist, who is the Vokes Geology Professor in the Tulane Department of Earth and Environmental Sciences.
"We have taken a much longer view by examining marsh response more than 7,000 years ago when global rates of sea-level rise were very rapid but within the range of what is expected later this century."
This long-view showed the researchers that, contrary to previous estimates, once the rate of sea-level rise exceeds about one-tenth of an inch per year (3 mm/yr), marshes in the Mississippi delta “drown” in a few hundred years. But when that rate increases to a quarter of an inch per year (7.5 mm/yr), marshes drown in about fifty years. Törnqvist refers to this phenomenon as a tipping point for coastal marshes.
"The scary thing is that the present-day rate of global sea-level rise, due to climate change, has already exceeded the initial tipping point for marsh drowning," Törnqvist said. "And as things stand right now, the rate of sea-level rise will continue to accelerate and put us on track for marshes to disappear even faster in the future."
The findings from the study estimate that the remaining 15,000 square kilometers of marshland in coastal Louisiana will “inevitably drown,” although pinpointing when exactly is challenging because of varying topography in the delta. Over the past century, 5,000 square kilometers of wetlands in coastal Louisiana have already been lost.
Despite this grim news, Törnqvist says curbing greenhouse gases and implementing major river diversions could be meaningful actions in order to prevent the worst possible outcomes. Though to be effective, both require fast action.