(CN) — A new study conducted by scientists at Rutgers University reaffirms that modern sea-level rise is in fact linked to human activities, and not changes in Earth’s orbit as it once was millions of years ago.
According to their paper, published Friday in the journal Science Advances, Earth has undergone nearly ice-free conditions with carbon dioxide levels not much higher than today, as well as glacial periods in times that were previously believed to be ice-free, over the span of the last 66 million years.
“Our team showed that the Earth’s history of glaciation was more complex than previously thought,” said lead author Kenneth G. Miller, a distinguished professor in the Department of Earth and Planetary Sciences in the School of Arts and Sciences at Rutgers University in New Brunswick, New Jersey.
“Although carbon dioxide levels had an important influence on ice-free periods, minor variations in the Earth’s orbit were the dominant factor in terms of ice volume and sea-level changes — until modern times,” he continued.
Sea level has been increasingly rising at an accelerated rate in recent decades, with about a third of the total increase since 1880 coming about within the last two and a half decades. Millions of years ago during the Pliocene era, minor changes in Earth’s orbit were largely responsible for sea level variations as it affected distribution of solar energy, but today it is mostly due to meltwater from glaciers and ice sheets and thermal expansion from increased temperatures.
The sea-level crisis threatens to permanently submerge some densely populated coastal cities and communities, affecting an estimated 250 million people in places including several Asian countries and parts in the U.S. along the Gulf Coast. It could severely damage low-lying lands and costly infrastructure by the year 2100 and poses a very real threat to ecosystems and economies, as many coastal areas depend on agriculture for income and sustenance.
Not to mention as coastal cities continue to develop, it greatly increases the environment’s vulnerability to sea-level rise and decreases its ability to naturally cope with flooding. Take Louisiana for example: they have lost approximately 2,000 square miles of wetlands in recent decades to human activity and alterations of the Mississippi River’s sediment system, causing the land to sink and the wetlands to lose their functionality as a buffer.
In their paper, the scientists were able to successfully reconstruct the history of sea levels and glaciation since the age that marked the end of the dinosaurs. They used deep sea geochemistry data to compare estimates of the global average sea level with the continental margin records. These continental margins include the relatively shallow ocean waters over a continental shelf and can extend over hundreds of miles from the coast.
What they found showed that Earth has undergone periods, such as 17 million to 13 million years ago, of nearly ice-free conditions, occurring when the concentration of atmospheric carbon dioxide was not much higher than today. On the other hand, they also found that Earth had some glacial periods occurring when it was thought to be ice-free, such as from 48 million to 34 million years ago.
“We demonstrate that although atmospheric carbon dioxide had an important influence on ice-free periods on Earth, ice volume and sea-level changes prior to human influences were linked primarily to minor variations in the Earth’s orbit and distance from the sun,” Miller said.
Global sea levels experienced the largest declining event about 20,000 years ago during the last glacial period, during which the water level dropped by about 400 feet. This drop off was then followed by a rapid increase at about a foot per decade rise in sea level, which then slowed from 10,000 to 2,000 years ago. Since then, sea level rise remained at a standstill until around 1900 when rates began rising as human activities began influencing the climate and effectively contributing to global warming.
The authors note that further research to reconstruct the history of sea-level changes before 48 million years ago will be needed to determine unclear factors, including the times when the Earth was entirely ice-free, the role of atmospheric carbon dioxide in glaciation and the cause of the natural fall in atmospheric carbon dioxide before humans.
The Rutgers study co-authors include Professor James V. Browning, doctoral student W. John Schmelz and professors Robert E. Kopp, Gregory S. Mountain and James D. Wright, the senior author of the study, which was funded by the National Science Foundation.