The Southern Ocean absorbs more heat than any other ocean

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Posted on September 11, 2022 8:36 PM by

Maritime Executive

[By Maurice Huguenin, Matthew England and Ryan Holmes]

For the past 50 years, the oceans have been working too hard to slow global warming, absorbing about 40 percent of carbon dioxide emissions and more than 90 percent of the excess heat entrapped in the atmosphere.

but we have Research Published last week in Nature Communications found that some oceans work harder than others.

We used a computational global ocean circulation model to examine how ocean warming has occurred over the past 50 years. We found that the Southern Ocean dominated the global absorption of heat. In fact, Southern Ocean heat uptake causes nearly all of the warming of the planet’s ocean, and thus controls the rate of climate change.

This warming of the Southern Ocean and its associated effects are effectively irreversible on human time scales, because it takes thousands of years for heat trapped in the depths of the ocean to be released back into the atmosphere.

This means that the changes happening now will still be felt for future generations – and those changes will only get worse, unless we can stop CO2 emissions and achieve net zero.

It is important but difficult to measure ocean heating

A warming ocean protects against the worst effects of climate change, but it is not without a cost. Sea levels are rising because heat causes water to expand and ice to melt. Marine ecosystems are experiencing unprecedented heat stress, and the frequency and intensity of extreme weather events are also changing.

However, we still don’t know enough about when, where and how ocean warming occurs. This is due to three factors.

First, temperature changes at the surface of the ocean and in the atmosphere closely over each other’s path. This makes it difficult to know exactly where the excess heat is entering the ocean.

Second, we don’t have measurements to track temperatures across the ocean. In particular, we have very sparse observations in the deep ocean, in remote locations around Antarctica and under sea ice.

Finally, the notes we have do not go back in time. Reliable data from depths of more than 700 meters were virtually non-existent prior to the 1990s, apart from observations along specific cruise paths.

Our modeling approach

To learn the intricacies of how ocean warming occurs, we first ran an ocean model with atmospheric conditions permanently stuck into the 1960s, before any significant human-caused climate change.

Next, we allowed each ocean basin individually to go forward in time and experience climate change, while all other basins were put off to experiment with the 1960s climate.

We also separated the effects of atmospheric warming from changes from surface winds to see how each factor contributed to the observed ocean warming.

By taking this modeling approach, we can isolate that the Southern Ocean is the most important absorber of this heat, even though it covers only about 15% of the total ocean surface area.

In fact, the Southern Ocean alone could be responsible for nearly all of the global ocean heat uptake, as the Pacific and Atlantic basins lose any heat that returns to the atmosphere.

An important environmental impact of the strong warming of the Southern Ocean is Antarctic krill. When ocean warming occurs beyond the temperatures they can tolerate, krill’s habitat shrinks and they move south to cooler waters.

Since krill is an essential component of the food web, this would also change the distribution and population of larger predators, such as commercially viable tooth and ice fish. It would also increase pressure on penguins and whales already threatened today.

The warming of the Southern Ocean is affecting the habitat of the krill. AAP photo, Brett Wilkes/Australian Antarctic Department

So why does the Southern Ocean absorb so much heat?

This is largely due to the geographical configuration of the region, with strong westerly winds surrounding Antarctica exerting their influence on an ocean unbroken by land masses.

This means that the winds of the Southern Ocean blow over a vast distance, constantly bringing large amounts of cold water to the surface. Cold water is pushed north, to easily absorb vast amounts of heat from the warmer atmosphere, before pumping excess heat into the ocean about 45-55°S (the latitude strip south of Tasmania, New Zealand, and just south of South America).

This warming uptake is facilitated by both the warmer atmosphere caused by greenhouse gas emissions, as well as the wind-driven circulation which is important for getting heat into the ocean.

And when we combine the effects of warming and winds on the Southern Ocean only, with the remaining oceans dating back to the 1960s climate, we can explain nearly all of the global ocean heat absorption.

But this does not mean that other ocean basins are not warming. Hmm, only that the heat they gain locally from the atmosphere cannot explain this warming. Rather, it is the massive heat uptake in the Southern Ocean that has led to changes in the total ocean heat content worldwide over the past half century.

We have a lot to learn

While this discovery sheds new light on the Southern Ocean as a major driver of global ocean warming, we still have a lot to learn, particularly about the 50-year ocean warming that we highlight in our study. All future forecasts, including the most optimistic scenarios, predict a warmer ocean ahead.

And if the Southern Ocean continues to account for the vast majority of ocean heat uptake until the year 2100, we could see its heat content increase by up to seven times what we’ve already seen until today.

This would have massive impacts worldwide: including more disruptions to the Southern Ocean food web, rapid melting of Antarctic ice shelves, and changes in the ocean transport belt.

To capture all of these changes, it is imperative that we continue and expand our observations of the Southern Ocean.

One of the most important new data streams will be new ocean buoys that can measure deeper ocean temperatures, as well as small temperature sensors on elephant seals, which provide us with key data on winter ocean conditions beneath Antarctic sea ice.

Even more important is the recognition that the fewer carbon dioxide emissions, the less the ocean will change. This will ultimately reduce the disruption to the livelihoods of the billions of people who live near the coast around the world.

Maurice Hugginen is a PhD candidate, University of New South Wales, Sydney.

Matthew England is Saintia Professor and Deputy Director of ARC’s Australian Center for Excellence in Antarctic Science (ACEAS), University of New South Wales in Sydney.

Ryan Holmes is a research fellow at the University of Sydney.

This article appears with permission from The Conversation and can be found in its original form over here.

The opinions expressed here are those of the author and not necessarily those of The Maritime Executive.