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Record low Antarctic sea ice is another alarming sign the ocean’s role as climate regulator is changing


A changing climate is imminent, with more frequent terrestrial and marine heat waves, wildfires, atmospheric rivers and floods. For some it is the background of everyday life, but for a growing number of people it is a life-changing reality.

It is now more remarkable when a year is not the hottest since our species began developing civilizations.

Whenever we experience extreme climate events, it can be difficult to understand the concept that they are minor outliers in the planetary experiment we are conducting. But the main act takes place elsewhere in the oceansthat absorb more than 90% of the excess heat energy.

We wind a clockwork spring without knowing exactly when, how fast and how it will unwind. The ocean warming isn’t so much a canary in a coal mine as a booming shark we accidentally (at least initially) hoisted into our fishing boat.

A bonfire of records

A decline in sea ice coverage, both in the Arctic and more recently in Antarctica, is one of the latest record-breaking changes. These floating expanses of frozen seawater are central to how our world works. They regulate how much light our planet reflects, help ventilate the oceans and host important ecosystems in the form of algal meadows at their undersides.

But now, with ocean warming, we have the lowest sea ice area ever recorded.

Ocean scientists are not used to thinking about rapid changes, but the trajectory of the global average temperature on the ocean’s surface has now entered unknown territory – And fast.

This graph shows global ocean surface temperatures, with the 2023 trajectory at the top.
climateanalyzer.org, CC BY-ND

We know the extent of this thanks to satellite technology that can detect small temperature changes at the ocean’s surface.

Read more: How much will our oceans warm and sea level rise this century? We just improved our estimate

This surface data is just that: the temperature of the ocean’s skin itself. To get a sense of warming in the deeper ocean, we use measurements on ships and a fleet of underwater robots, known as Argo.

A ship deploys instruments to measure the temperature just below the ocean's surface.
What satellites measure can differ from temperatures just below the surface or in the deeper ocean.
Author provided, CC BY-ND

global ocean heat transport

The deep ocean is clearly changing. This is because polar sea ice acts as a link between the atmosphere, the surface of the ocean and deeper waters. With less sea ice, less cold, salty, oxygen-rich water sinks to the deep ocean.

These icy coastal waters of Antarctica are a crucial engine room for global currents that transport energy across the planet – and this ocean transport mechanism is now changing.

Read more: Deluges of Antarctic meltwater are slowing the currents that cause our vital ocean to ‘topple over’ – and threaten to collapse

One of the unknowns of ocean warming is how the oceans will adapt and store all the heat. Warming the ocean surface makes the headwaters more stable. This, in turn, changes the way the upper ocean absorbs carbon dioxide.

The difficulty for researchers in determining how best to respond is that the processes that move and mix this heat operate on a very small scale. It’s beyond even our most powerful climate simulators to model exactly what the heat is like scattermaking predictions less certain.

Even if our models could operate at very large and very small scales at the same time, they would have little data for validation. This is because very little of the ocean’s mixing has been directly observed.

While we can predict some of this mixing, the ocean is full of surprises. The Drake Passage has recently been shown to be even more of a mixing hot spot than previously thought.

An ocean turbulence sensor deployed in rough conditions.
An ocean turbulence sensor deployed in rough conditions.
Author provided, CC BY-ND

A warming Pacific Ocean

A map showing surface temperatures around Aotearoa for May 1, 2023, with dotted areas marking marine heat waves.
Surface temperatures around Aotearoa, NEW Zealand for May 1, 2023. The dotted areas mark the marine heat wave.

Despite the connected ocean, the separate basins have their own characteristics and contributions to the climate. Aotearoa New Zealand is located in the southwest corner of the Pacific Ocean, which covers about one-third of the Earth’s surface.

The Pacific Ocean is so large that it has its own internal cycles, such as the El Niño Southern Oscillation (ENSO). We need to untangle these to understand long-term changes.

While El Niño conditions can bring marine heat waves to some parts of the Pacific, the oceans around Aotearoa New Zealand, especially in the south, are already experiencing near-constant marine heat waves.

Read more: It may be the world’s largest ocean, but the mighty Pacific is in danger

The scale of the oceanic contribution to heat storage means that any small change in the way this has worked over the past millennia could have very large consequences. It is impossible to overestimate the urgency with which we need to reduce greenhouse gas emissions.

Technologies that can capture carbon dioxide already emitted have many advocates, but they should not come at the expense of efforts to eliminate emission sources. Without eliminating the emitters, these emergency measures will only delay the inevitable. As the Intergovernmental Panel on Climate Change put it in its final report:

There is a rapidly closing opportunity to secure a livable and sustainable future for all.

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