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More than 500 million people, or nearly 7 percent of the human population, live on a delta. The fragile balance between sea level rise and sediment accumulation that deltas rely on is in peril, according to new research published in the Journal of Coastal Research. The world’s deltas formed prehistorically when the rate of sea level rise slowed and wetlands were built through the process in which sediment accumulates called accretion. However, today sea level rise is outpacing accretion.

‘If the past is prologue, and deltas formed when sea level rise slowed, then it appears that the reverse will occur. Deltas will not be able to keep up with a rising ocean,’ said Eugene Turner, Boyd Professor in the LSU College of the Coast & Environment. Turner and colleagues Michael Kearney at the University of Maryland and Randall W. Parkinson at Florida International University looked at data collected by the Smithsonian Institution decades ago on 36 of the world’s largest river deltas and analyzed peat, a brown, soil-like material, from each of these deltas collected to determine the age of each delta. The median age of the world’s largest deltas is 7,967 years old. They found that these deltas formed about the time that the vast, ancient Lake Agassiz, which spanned parts of Canada down to what is now North Dakota and Minnesota, drained after the ice dam broke about 8,000 years ago, releasing water mainly into the Atlantic Ocean. The oceans then cooled and ocean circulation changed causing sea level rise to slow; deltas simultaneously started to form throughout the world.

Today, sea level is rising at about the same rate when deltas formed.

Read the full article for more on sea level rise.