Ancient ocean slowdown warns of future climate chaos

When it comes to the oceans’ response to global warming, we are not in completely uncharted waters. A UC Riverside study shows that extreme heat events in Earth’s past led to a decrease in water exchange from the surface to the deep ocean.

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

Using tiny fossilized shells recovered from ancient deep-sea sediments, the study published in the Proceedings of the National Academy of Sciences shows how the treadmill reacted around 50 million years ago.

At that time, Earth’s climate resembled conditions predicted for the end of this century, if significant steps are not taken to reduce carbon emissions.

The oceans play a crucial role in regulating the Earth’s climate. They move warm water from the equator to 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 linked to significant and abrupt climate change.

Additionally, oceans play a critical role in removing anthropogenic carbon dioxide from the atmosphere.

“The oceans are by far the largest reservoir of carbon 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 trillion tons of carbon, more than 40 times the amount of carbon present in the atmosphere. The oceans also absorb around a quarter of anthropogenic CO.₂ shows,” Kirtland Turner said. “If ocean circulation slows, carbon uptake in the ocean could also slow, amplifying the amount of CO₂ it stays in the atmosphere.”

Previous studies have measured changes in ocean circulation during Earth’s more recent geologic past, such as the emergence from the last ice age; however, these do not approach atmospheric CO levels₂ or the warming that affects the planet today. Other studies provide the first evidence that deep ocean circulation, 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 looked back to the beginning of the Eocene, approximately 49 to 53 million years ago. The Earth was then much warmer than today, and this high reference temperature was punctuated by CO peaks.₂ and temperature called hyperthermic.

During this period, the ocean depths were up to 12 degrees Celsius warmer than today. During hyperthermia, the oceans warmed by an additional 3 degrees Celsius.

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

By analyzing tiny fossil shells from different seafloors around the world, researchers reconstructed deep ocean circulation patterns during these hyperthermic events.

The shells come from microorganisms called foraminifera, which are found 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 these shells to globally reconstruct information about ancient ocean temperatures and circulation patterns,” said Kirtland Turner.

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

The researchers also looked at carbon isotopes in the shells, which reflect the age of the water where the shells were collected, or the length of time the water was isolated from the ocean surface. In this way, they can reconstruct the movement patterns of deep ocean waters.

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

“Conversely, 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 decompose. Thus, older water contains relatively more of carbon 12 compared to “young” water.

Today, scientists 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 also used models to simulate the ancient ocean’s response to warming. They then used analysis of the foraminiferal shells to test the results of their climate models.

During the Eocene, there were about 1,000 parts per million (ppm) of carbon dioxide in the atmosphere, which contributed to the high temperatures of that era. Today the atmosphere contains about 425 ppm.

However, humans emit nearly 37 billion tonnes of CO₂ in the atmosphere every year; if these emission levels continue, conditions similar to those of the early Eocene could occur by the end of this century.

Kirtland Turner therefore affirms that it is imperative to do everything possible to reduce emissions.

“It’s not an all-or-nothing situation,” she said. “Every small incremental change matters when it comes to carbon emissions. Even small CO reductions₂ correlate with fewer impacts, less loss of life, and less change in the natural world.