What Past Oceans Tell Us About Our Climate’s Future

How the Oceans Held the Key to Earth’s Changing Climate

Oceanic Ecosystems was one of the most feared units at the university where I did my Bachelor of Science in Biology—and for a reason. Most signed up expecting a semester of coral reefs, whales, and sharks. However, the unit was more about the physical aspects of ecosystems, such as ocean circulation and temperature exchanges—hardcore physics, in other words.

At least our professor was amazingly good at presenting the topics and would use something we were all familiar with to introduce each topic.

Do you know when you are swimming in a seemingly warm ocean and, all of a sudden, get hit with a body of cold water — he mentioned — well, that showcases the power of oceans as energy stores.

I found the concept fascinating, so I’ve been reading about it for a while, especially as related to the ongoing climate change we are experiencing. Luckily, the latest research on the topic brings us good news.

Here’s the thing: the oceans are our planet’s largest heat sink, absorbing more than 90% of the excess energy produced by global warming. But what if the oceans once stored heat far more efficiently than they do today?

A recent study, led by a group of scientists from the U.S. and China, reveals that during the last deglaciation — when massive ice sheets retreated — the ocean’s ability to store heat was significantly higher than in the modern era. The important bit here? This could have important implications for understanding the role of oceans in regulating climate over long periods.

Published in Science Advances, the research provides a detailed look at how ocean heat storage efficiency (HSE) during the last deglaciation increased to levels much higher than those we observe today. The team discovered that heat was primarily stored in intermediate-depth waters (300 to 1,500 meters), a pattern quite different from modern times, where heat is mostly confined to the upper 500 meters.

Remembering what I learned from Oceanic Ecosystems, this is, per se, a great discovery. But there’s more. Let’s first look at how they performed the study.

To explore how efficiently the ocean stored heat thousands of years ago, the researchers used a combination of advanced climate simulations and data from proxy reconstructions. These proxies include sediment cores and ice cores, which give a glimpse into past ocean temperatures.

What Are Proxy Data?

In paleoclimatology, the study of past climates, scientists use “proxy” data to reconstruct past climate conditions.

The researchers ran their simulations using the Community Earth System Model (CESM), a powerful tool for simulating complex climate interactions. The model was combined with data on changes in greenhouse gas concentrations, ice sheets, and meltwater during the deglaciation period.

By comparing these simulations with real-world proxy data, the team could track how ocean temperatures evolved across various depths and regions, providing a three-dimensional view of the ocean’s heat storage during the last deglaciation.

Cool, right, but what did they find?

The results were surprising: heat storage efficiency in the ocean during the last deglaciation was far greater than what we see today. In fact, the ocean’s HSE during this period was nearly 10 times higher. This increased efficiency was due to the unique way heat was distributed, primarily warming intermediate waters rather than just the surface or deep ocean.

Dr. Chenyu Zhu, the study’s co-first author, explained that “the three-dimensional ocean warming during the last deglaciation was strongly nonuniform, with the strongest warming occurring at intermediate depths.” This contrasts to what happens today, where most warming occurs in the upper 500 meters. Thus, the study’s findings suggest that surface warming, driven by greenhouse gases and ice sheet retreat, was ventilated into intermediate waters, allowing the ocean to store more heat.

In addition to surface warming, the study also highlights the role of meltwater from retreating ice sheets. The Atlantic Meridional Overturning Circulation (AMOC), a key ocean current, weakened during the deglaciation due to this meltwater. This weakening allowed more heat to be stored in the intermediate layers of the ocean, further enhancing the ocean’s heat storage efficiency.

Read more about the AMOC in the link below.

https://www.ncei.noaa.gov/news/what-are-proxy-data

The study’s findings offer an important glimpse into how Earth’s climate system has worked in the past — and what it might mean for the future. The fact that the ocean stored heat more efficiently during the last deglaciation suggests that the oceans can play an even bigger role in regulating global temperatures over long periods than we previously thought. And that’s good news, that’s what we want.

As Prof. Zhengyu Liu of The Ohio State University pointed out, “The unique ocean warming structure facilitates a large ocean heat storage efficiency.” In other words, when strong surface warming and strong ventilation (the process of heat being transferred to deeper waters) are aligned, the ocean can absorb more heat.

This process could potentially slow down atmospheric warming, at least temporarily, by trapping heat in the ocean’s intermediate depths.

However, the study also raises important questions about how this process might unfold in today’s warming climate. The patterns of surface warming are different now, with most of the heating happening at subtropical latitudes, where the ocean’s ability to transfer heat to deeper layers is limited.

This could mean that today’s oceans are less efficient at storing heat than they were in the past. If that’s the case, we might expect the atmosphere to warm more quickly because the oceans absorb less heat. This is not ideal.

The study provides a timely reminder of Earth’s complex and dynamic climate system. However, we must remember that while the past offers clues about how oceans store heat, there’s still much to learn about how these processes will play out in the near future.

In the end, understanding how efficiently the oceans store heat is key to predicting how our climate will change. The oceans have always acted as a buffer, absorbing heat and slowing the pace of global warming. But how long can they continue to perform this critical function?

Studies like this one are helping to provide the answers, and they may point the way toward better climate models — and, ultimately, better climate solutions. Once again, the past is key to inform us about the future.