Clues from the last ice age help predict oceans’ response to continued warming

A team of researchers, led by Yi Wang from Tulane University, has unveiled critical insights into the warming ocean’s role in climate change, specifically during the last ice age which concluded over 11,000 years ago.

Their research highlights the intricate relationship between ocean oxygen levels and atmospheric carbon dioxide (CO2) during this period.

Yi Wang, an assistant professor of Earth and Environmental Sciences at Tulane University, specializes in marine biogeochemistry and paleoceanography.

Alongside colleagues from the Woods Hole Oceanographic Institution (WHOI), a premier organization in ocean research, Wang embarked on a mission to unravel the mysteries of the ocean’s past.

Global ocean oxygen levels

The team’s focus was on analyzing the deep-sea sediments from the Arabian Sea. By examining isotopes of the metal thallium trapped in these sediments, they could reconstruct the average global ocean oxygen levels thousands of years ago.

Wang explained that this approach of studying metal isotopes on glacial-interglacial transitions was unprecedented.

The results were revelatory: during the last ice age, the global ocean overall lost oxygen compared to the current, warmer interglacial period.

Role of the Southern Ocean in global climate

Wang’s research reveals a striking correlation between global ocean oxygen content and atmospheric CO2, stretching from the last ice age to the present day.

“The research reveals the important role of the Southern Ocean in controlling the global ocean oxygen reservoir and carbon storage,” Wang said.

This correlation underscores the role of the oceans, particularly the Southern Ocean, in regulating atmospheric CO2 levels, a crucial component in understanding climate dynamics.

“This will have implications for understanding how the ocean, especially the Southern Ocean, will dynamically affect the atmospheric CO2 in the future,” she said.

Ocean oxygenation and warming

The study found that during abrupt warming in the Northern Hemisphere, the global ocean experienced significant deoxygenation, while periods of abrupt cooling led to an increase in oceanic oxygen. These fluctuations were closely tied to processes in the Southern Ocean.

Sune Nielsen, an associate scientist at WHOI and co-author of the research, highlighted the study’s significance.

He pointed out that this research presents a first-of-its-kind overview of how global ocean oxygen content evolved as the Earth transitioned from the last glacial period into today’s warmer climate. The findings highlight the Southern Ocean’s critical role in modulating atmospheric CO2.

“This study is the first to present an average picture of how the oxygen content of the global oceans evolved as Earth transitioned from the last glacial period into the warmer climate of the last 10,000 years,” Sune explained.

Nielsen also stressed the contemporary relevance of these findings. Given the profound impact of anthropogenic climate change, particularly in high-latitude regions, the research underscores the Southern Ocean’s outsized influence on atmospheric CO2 levels.

In summary, this study enhances our understanding of the oceans’ role in past climate change, while providing vital insights into how oceanic processes might influence future global warming.

As Wang succinctly put it, understanding the dynamic relationship between the ocean, particularly the Southern Ocean, and atmospheric CO2 is key to predicting how the ocean’s carbon cycles will respond to ongoing climate change.

More about ocean oxygenation

The ocean, covering more than 70% of the Earth’s surface, plays a pivotal role in sustaining life both within its depths and on land.

As discussed above, a key aspect of this role is ocean oxygenation, a process that maintains the delicate balance of oxygen levels in marine environments.

At the heart of ocean oxygenation lies the process of oxygen dissolving from the atmosphere into the surface waters of the ocean.

Winds and waves facilitate this exchange, ensuring a continual supply of oxygen to the upper layers of the ocean.

Moreover, photosynthesis by marine plants and phytoplankton contributes significantly to oxygen levels, particularly in sunlit surface waters.

Deep ocean oxygen supply

The deep ocean receives oxygen through a remarkable process called thermohaline circulation. This global conveyor belt moves oxygen-rich surface waters to the depths, and brings nutrient-rich water to the surface.

As surface waters cool at the poles, they become denser and sink, carrying oxygen to the deep ocean. This circulation is critical for sustaining life in the deep sea, where sunlight cannot penetrate.

Oxygen is vital for the survival of most marine organisms. Fish, marine mammals, and various invertebrates rely on dissolved oxygen for respiration.

Healthy oxygen levels support rich biodiversity, from coral reefs to deep-sea ecosystems.

Moreover, oxygenation plays a crucial role in the decomposition of organic matter, preventing the buildup of harmful substances.

Challenges and threats

Despite its critical importance, ocean oxygenation faces significant threats, primarily due to human activities.

Climate change, leading to ocean warming, reduces the solubility of oxygen in seawater. Additionally, increased nutrient runoff from agriculture leads to eutrophication, causing harmful algal blooms that deplete oxygen levels and create dead zones.

In summary, ocean oxygenation is an essential yet often overlooked component of our planet’s health. It supports a diverse array of marine life, contributes to global climate regulation, and is integral to the ocean’s overall productivity.

Protecting and preserving this vital process requires concerted global efforts to reduce greenhouse gas emissions, manage nutrient runoff, and understand the intricate dynamics of our oceans.

Only through such actions can we ensure the continued vitality of our marine ecosystems for generations to come.

The full study was published in the journal Science Advances.

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