Ancient ocean slowdown warns of coming climate chaos

When it comes to the ocean’s response to global warming, we’re not in completely uncharted waters. A UC Riverside study shows that episodes of extreme heat in Earth’s past caused the exchange of water from the surface to the deep ocean to decline.

This system has been described as a “global conveyor belt” because it redistributes heat across 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 appeared in Proceedings of the National Academy of Sciences shows how the conveyor belt responded 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 crucial 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 hotter and the poles much colder. Changes in this system are associated with significant and sudden climate changes.

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

“The oceans are by far the largest carbon pool 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 about 40,000 billion tons of carbon, more than 40 times the amount of carbon in the atmosphere. The oceans also take up about a quarter of anthropogenic CO.₂ emissions,” said Kirtland Turner. “If ocean circulation slows down, the uptake of carbon into the ocean can also slow down, amplifying the amount of CO₂ that rests in the atmosphere”.

Previous studies have measured changes in ocean circulation in Earth’s more recent geologic past, such as outflows from the last ice age; however, they do not approximate atmospheric CO levels₂ or the warming happening to the planet today. Other studies provide the first evidence that deep ocean circulation, particularly in the North Atlantic, has already begun to slow.

To better predict how ocean circulation responds to global warming driven by greenhouse gases, the research team looked at the early Eocene epoch, roughly 49 and 53 million years ago. The Earth at that time was much warmer than it is today, and that high-heat baseline was indicated by CO drops₂ and temperature called hyperthermic.

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

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

By analyzing small fossil shells from different seafloor locations around the globe, the researchers reconstructed deep ocean circulation patterns during these hyperthermal events.

Shells are from microorganisms called foraminifera, which can be found throughout the world’s oceans, both on the surface and on the seabed. They are roughly the size of a period at the end of a sentence.

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

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 deposited, or how long the water has been isolated from the ocean’s surface. In this way, they can reconstruct the movement patterns of deep ocean water.

Foraminifera cannot photosynthesize, but their shells show the impact of the photosynthesis of 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 when that water sinks into the deep ocean,” Kirtland Turner said.

“On the other hand, water that has been isolated from the surface for a long time has built up relatively more carbon-12 as the remains of photosynthetic organisms sink and decay. So old water has relatively more carbon-12 compared to “new” water.

Scientists often make predictions about ocean circulation today 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 response of the ancient ocean 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 that era. Today, the atmosphere contains about 425 ppm.

However, humans emit about 37 billion tons of CO₂ in the atmosphere every year; if these emission levels continue, Early Eocene-like conditions could occur by the end of this century.

Therefore, Kirtland Turner argues that it is imperative to make every effort to reduce emissions.

“It’s not an all or nothing situation,” she said. “Every little bit of change in growth is important when it comes to carbon emissions. Even small reductions in CO₂ associated with fewer impacts, less loss of life, and less change in the natural world.”