Age-old ocean slowdown warns of future 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 periods of extreme heat in Earth’s past reduced the exchange of water from the surface to the deep ocean.

This system has been called the ‘global conveyor belt’ because it redistributes heat around the world through the movement of ocean water, making large parts of the planet habitable.

Using small, fossilized shells recovered from ancient deep-sea sediments, the study appears in the Proceedings of the National Academy of Sciences shows how the conveyor belt responded about 50 million years ago.

At the time, Earth’s climate resembled the conditions predicted by the end of this century if significant action was not taken to reduce CO2 emissions.

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

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

Furthermore, the oceans play a crucial role in removing anthropogenic carbon dioxide from the atmosphere.

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

‘Today, the oceans contain almost 40 trillion tonnes of carbon – more than 40 times the amount of carbon in the atmosphere. Oceans also absorb about a quarter of anthropogenic CO2.₂ emissions,” said Kirtland Turner. “If ocean circulation slows, the absorption of carbon in the ocean can also slow, increasing the amount of CO2.₂ that remains in the atmosphere.”

Previous studies have measured changes in ocean circulation in Earth’s more recent geological past, such as the emergence of the last ice age; however, these do not approach the levels of CO in the atmosphere₂ or global warming today. Other studies provide the first evidence that circulation in the deep oceans, especially in the North Atlantic, has already begun to slow.

To better predict how ocean circulation responds to global warming caused by greenhouse gases, the research team looked to the early Eocene, between about 49 and 53 million years ago. The Earth was much warmer then than it is today, and that high temperature baseline was interrupted by spikes in CO₂ and temperature called hyperthermia.

During that period, the deep ocean was up to 12 degrees Celsius warmer than it is now. During the hyperthermia, the oceans warmed another 3 degrees Celsius.

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

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

The shells come from microorganisms called foraminifera, which are found in the world’s oceans, both on the surface and on the seabed. They are about the size of a period at the end of a sentence.

“As the creatures build their shells, they absorb elements from the oceans, and we can measure the differences 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 the calcium carbonate are indicators of the temperature 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 in which the shells were collected, or how long water has been isolated from the ocean’s surface. In this way they can reconstruct patterns of the movement of deep ocean water.

Foraminifera cannot photosynthesize, but their shells indicate the impact of photosynthesis from other organisms nearby, such as phytoplankton. “Photosynthesis only occurs in the surface 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 to the deep ocean,” said Kirtland Turner.

‘Conversely, 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. Older water therefore contains relatively more carbon-12 compared to ‘young’ water. “

Nowadays 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 similarly used models to simulate the ancient ocean’s response to warming. They then used foraminifera scale analysis to test the results of their climate models.

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

However, humans emit almost 37 billion tons of CO₂ in the atmosphere every year; If these emission levels continue, similar conditions to the early Eocene could emerge by the end of this century.

That’s why Kirtland Turner argues that it is imperative to do everything we can to reduce emissions.

“It’s not an all-or-nothing situation,” she says. “Every bit of change is important when it comes to CO2 emissions. Even small CO2 reductions₂ correlate with less impact, less loss of life and less change in the natural world.”