Ancient ocean slowdown warns of future climate chaos

We are not in completely uncharted waters when it comes to the ocean’s response to global warming. A UC Riverside study shows that episodes of extreme heat in Earth’s past caused a decrease in water exchange from the surface to the deep ocean.

This system has been described as a “global conveyor belt” because it redistributes heat around the globe through the movement of ocean waters, making large parts of the planet habitable.

The study emerged using small, fossilized shells recovered from ancient deep-sea sediments Proceedings of the National Academy of Sciences shows how the conveyor belt reacted about 50 million years ago.

At that time, Earth’s climate resembled conditions predicted by the end of this century unless significant action is taken to reduce carbon emissions.

Oceans play a vital role in regulating Earth’s climate. They move warm water from the equator toward the north and south poles, balancing the planet’s temperatures.

Without this circulation system, the tropics would be much warmer and the poles much colder. Changes in this system are associated with significant and sudden climate change.

In addition, the oceans play a vital role in removing anthropogenic carbon dioxide from the atmosphere.

“The oceans are by far the largest stationary carbon reservoir on Earth’s surface today,” said Sandra Kirtland Turner, vice chair of UCR’s Department of Earth and Planetary Sciences and first author of the study.

“Today, the oceans contain nearly 40,000 billion tons of carbon—more than 40 times the amount of carbon in the atmosphere. The oceans also absorb about a quarter of anthropogenic CO₂ emissions,” said Kirtland Turner. “If ocean circulation slows, the absorption of carbon into the ocean may also slow, amplifying the amount of CO₂ which remains in the atmosphere.”

Previous studies have measured changes in ocean circulation in Earth’s more recent geologic past, such as the arrival of the last ice age; however, these do not approach atmospheric CO levels₂ or the warming happening on the planet today. Other studies provide the first evidence that circulation in the deep oceans, particularly in the North Atlantic, is already beginning to slow.

To better predict how ocean circulation responds to global warming caused by greenhouse gases, the research team focused on the early Eocene epoch, roughly 49 to 53 million years ago. Earth was much warmer then than it is today, and this baseline of high temperatures was punctuated by jumps in CO₂ and a temperature called hyperthermal.

During this period, the deep ocean was up to 12 degrees Celsius warmer than today. During hyperthermia, the oceans warmed by another 3 degrees Celsius.

“Although the exact cause of hyperthermal events is debated and occurred long before humans existed, these hyperthermal events are the best analogs we have for future climate change,” said Kirtland Turner.

By analyzing tiny fossil shells from different seafloor locations around the world, scientists have reconstructed patterns of deep ocean circulation during these hyperthermal events.

The boxes come from microorganisms called foraminifera, which can be found living throughout the world’s oceans, both on the surface and on the sea floor. They are about the size of a period at the end of a sentence.

“As creatures build their shells, they incorporate elements from the oceans, and we can measure differences in the chemistry of those shells to broadly reconstruct information about ancient ocean temperatures and circulation patterns,” Kirtland Turner said.

The shells themselves are made of calcium carbonate. Oxygen isotopes in calcium carbonate are indicators of the temperatures in the water in which the organisms grew and the amount of ice on the planet at the time.

The researchers also examined carbon isotopes in the shells, which reflect the age of the water where the shells were collected, or how long the water was isolated from the ocean’s surface. In this way, they can reconstruct patterns of water movement in the deep ocean.

Foraminifera cannot photosynthesize, but their shells indicate the impact of photosynthesis by other organisms in the environment, such as phytoplankton. “Photosynthesis only occurs in the surface ocean, so water that was recently at the surface has a carbon-13-rich signal that is reflected in the shells as the water sinks into the ocean depths,” Kirtland Turner said.

“In contrast, water that has been isolated from the surface for a long time has accumulated relatively more carbon-12 as the remains of photosynthetic organisms sink and decay. So older water has relatively more carbon-12 compared to ‘young’ water.” “

Scientists today often make predictions about ocean circulation using computer climate models. They use these models to answer the question, “How will the ocean change as the planet continues to warm?” This team similarly used models to simulate the ancient ocean’s response to warming. They then used foraminifera shell analysis to help test the results from their climate models.

During the Eocene, there was about 1,000 parts per million (ppm) of carbon dioxide in the atmosphere, which contributed to the high temperatures of the time. Today, the atmosphere hovers around 425 ppm.

However, humans emit nearly 37 billion tons of CO₂ into the atmosphere each year; if these emission levels continue, early Eocene-like conditions could occur by the end of this century.

Kirtland Turner therefore argues that it is essential to make every effort to reduce emissions.

“It’s not an all or nothing situation,” she said. “Every small change is important when it comes to carbon emissions. Even a small reduction in CO₂ they correlate with less impact, less loss of life, and less change to the natural world.”