Hidden in caves: Mineral overgrowths reveal 'unprecedented' sea level rise

The early 1900s were an exciting time across the world, with rapid advances in the steel, electric and automobile industries. The industrial changes also mark an inflection point in our climate. According to an international team of researchers led by the University of South Florida (USF), the sea level has risen 18 centimeters since the start of the 20th century.

The study, featured on the cover of the July 1 issue of Science Advances, works to identify preindustrial sea levels and examines the impact of modern greenhouse warming on sea-level rise.

The team, which includes USF graduate students, traveled to Mallorca, Spain -- home to more than 1,000 cave systems, some of which have deposits that formed millions of years ago. For this study, they focused on analyzing deposits from 4,000 years ago to present day.

The team found evidence of a previously unknown 20 centimeter sea-level rise that occurred nearly 3,200 years ago when ice caps melted naturally over the course of 400 years at a rate of 0.5 millimeters per year. Otherwise, despite major climatic events like Medieval Warm Period and the Little Ice Age, the sea level remained exceptionally stable until 1900.

"The results reported in our study are alarming," said lead author Bogdan P. Onac, geology professor at USF. "The sea-level rise since the 1900s is unprecedented when compared to the natural change in ice volumes over the last 4,000 years. This implies that if global temperatures continue to rise, sea levels could eventually reach higher levels than scientists previously estimated."

To create the timeline, the team gathered 13 samples from eight caves along the coastline of the Mediterranean Sea. The deposits are rare -- only forming near the coastline in cave passages that were repeatedly flooded by sea water, making them accurate markers of sea-level changes overtime. Each deposit holds valuable insight into both the past and future, helping researchers determine how quickly the sea level will rise in the coming decades and centuries.

The samples were taken to the University of New Mexico and University of Bern in Switzerland, where special instruments were used to determine their age by uranium-series method. Over time, uranium decays into other elements such as thorium and lead, allowing researchers to create a timeline of the sea level documented in each deposit.

A complex software at Harvard University helped generate predictions using various ice models and Earth's parameters to showcase an accurate history of the sea level. These predictions are essential because they allow researchers to estimate past global mean sea level, which is key in addressing future sea-level rise.

"If humans continue to be the main driver and the temperature increases 1.5 degrees in the near future, there will be irreversible damage," Onac said. "There will be no turning back from that point on."

Based on ice mass loss from the Antarctic and Greenland, the average sea-level rise since 2008 is 1.43 millimeters per year.

Permanent flooding from the rising sea level won't happen overnight, but Onac says it will be seen more and more during storm surges and hurricanes. With nearly 40 percent of the world's population living within 62 miles of a coast, the rising sea level could be catastrophic with substantial societal and economic impacts.

"Even if we stop right now, sea level will continue to rise for at least a couple of decades, if not centuries, simply because the system is warmed up."

In June, Onac received a new research grant from the National Science Foundation to continue his research to predict future sea-level rise due to global warming. The grant will allow Onac to expand the research further into history by 130,000 years and create a better understanding of sea level globally. Starting in September, Onac and his team will begin analyzing cave deposits from around the globe, including Italy, Greece, Mexico and Cuba.

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Effects of ancient carbon releases suggest possible scenarios for future climate

A massive release of greenhouse gases, likely triggered by volcanic activity, caused a period of extreme global warming known as the Paleocene-Eocene Thermal Maximum (PETM) about 56 million years ago. A new study now confirms that the PETM was preceded by a smaller episode of warming and ocean acidification caused by a shorter burst of carbon emissions.

The new findings, published March 16 in Science Advances, indicate that the amount of carbon released into the atmosphere during this precursor event was about the same as the current cumulative carbon emissions from the burning of fossil fuels and other human activities. As a result, the short-lived precursor event represents what might happen if current emissions can be shut down quickly, while the much more extreme global warming of the PETM shows the consequences of continuing to release carbon into the atmosphere at the current rate.

"It was a short-lived burp of carbon equivalent to what we've already released from anthropogenic emissions," said coauthor James Zachos, professor of Earth and planetary sciences and Ida Benson Lynn Chair of Ocean Health at UC Santa Cruz. "If we turned off emissions today, that carbon would eventually get mixed into the deep sea and its signal would disappear, because the deep-sea reservoir is so huge."

This process would take hundreds of years -- a long time by human standards, but short compared to the tens of thousands of years it took for Earth's climate system to recover from the more extreme PETM.

The new findings are based on an analysis of marine sediments that were deposited in shallow waters along the U.S. Atlantic coast and are now part of the Atlantic Coastal Plain. At the time of the PETM, sea levels were higher, and much of Maryland, Delaware, and New Jersey were under water. The U.S. Geological Survey (USGS) has drilled sediment cores from this region which the researchers used for the study.

The PETM is marked in marine sediments by a major shift in carbon isotope composition and other evidence of dramatic changes in ocean chemistry as a result of the ocean absorbing large amounts of carbon dioxide from the atmosphere. The marine sediments contain the microscopic shells of tiny sea creatures called foraminifera that lived in the surface waters of the ocean. The chemical composition of these shells records the environmental conditions in which they formed and reveals evidence of warmer surface water temperatures and ocean acidification.

First author Tali Babila began the study as a postdoctoral fellow working with Zachos at UC Santa Cruz and is now at the University of Southampton, U.K. Novel analytical methods developed at Southampton enabled the researchers to analyze the boron isotope composition of individual foraminifera to reconstruct a detailed record of ocean acidification. This was part of a suite of geochemical analyses they used to reconstruct environmental changes during the precursor event and the main PETM.

"Previously, thousands of foraminifera fossil shells were needed for boron isotope measurement. Now we are able to analyze a single shell that's only the size of a grain of sand," Babila said.

Evidence of a precursor warming event had been identified previously in sediments from the continental section at Big Horn Basin in Wyoming and a few other sites. Whether it was a global signal remained unclear, however, as it was absent from deep-sea sediment cores. Zachos said this makes sense because sedimentation rates in the deep ocean are slow, and the signal from a short-lived event would be lost due to mixing of sediments by bottom-dwelling marine life.

"The best hope for seeing the signal would be in shallow marine basins where sedimentation rates are higher," he said. "The problem there is that deposition is episodic and erosion is more likely. So there's not a high likelihood of capturing it."

The USGS and others have drilled numerous sediment cores (or sections) along the Atlantic Coastal Plain. The researchers found that the PETM is present in all of those sections, and several also capture the precursor event. Two sections from Maryland (at South Dover Bridge and Cambridge-Dover Airport) are the focus of the new study.

"Here we have the full signal, and a couple of other locations capture part of it. We believe it's the same event they found in the Bighorn Basin," Zachos said.

Based on their analyses, the team concluded that the precursor signal in the Maryland sections represents a global event that probably lasted for a few centuries, or possibly several millennia at most.

The two carbon pulses -- the short-lived precursor and the much larger and more prolonged carbon emissions that drove the PETM -- led to profoundly different mechanisms and time scales for the recovery of the Earth's carbon cycle and climate system. The carbon absorbed by the surface waters during the precursor event got mixed into the deep ocean within a thousand years or so. The carbon emissions during the PETM, however, exceeded the buffering capacity of the ocean, and removal of the excess carbon depended on much slower processes such as the weathering of silicate rocks over tens of thousands of years.

Zachos noted that there are important differences between Earth's climate system today and during the Paleocene -- notably the presence of polar ice sheets today, which increase the sensitivity of the climate to greenhouse warming.

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