Circumpolar surface temperatures image: Earth live (click for live updates)
Summary
Just like us, the Earth has a circulatory system. Instead of blood, oceanic currents transport heat, oxygen, carbon dioxide, and nutrients around the planet (Figures 1 & 2).
Ocean currents played a significant role in how our current Ice Age began. These currents are now changing due to warming temperatures and feedback effects.
The Copernicus Marine Service Ocean Climate Portal is an interactive user-friendly hub to help users understand the impact of climate change on our oceans.
An term of smaller currents that affect Aotearoa, the increasing frequency and intensification of upwelling in the Agulhas Current and East Australian Current is resulting in mass mortalities of fish, sharks, and other marine animals:
Since 2006, strong, full-depth ocean warming has occurred south of the Chatham Islands at around 5x the global rate because of the ocean currents are moving 120km west. – NIWA, 2024
What’s covered on this page:
- The Antarctic Circumpolar Current (ACC)
- The Atlantic Meridional Overturning Circulation (AMOC) Thermohaline Current
The Antarctic Circumpolar Current (ACC)
The Southern Ocean is the formation site for much of the dense water that fills the deep ocean, sequesters the majority of anthropogenic heat and carbon, and controls the flux of heat to Antarctica. – Bennetts et al 2024
The strongest ocean current on Earth, the ACC in the Southern Ocean encircles Antarctica and extends from the surface all the way to the ocean floor. The current carries an estimated 165 million to 182 million cubic metres of water every second (a unit called a ‘Sverdrup’) from west to east. That’s more than 100x the flow of all the rivers on Earth, or the equivalent of pushing the entire South Island of New Zealand one metre every second. It helps to act as a planetary thermostat, keeping Antarctica cool (Fig. 3).
But, like the Arctic Ocean the Southern Ocean is now warming faster than the global ocean as a whole, leading to marine heat waves. The warmest water is deep and it’s undercutting the marine ice sheets and ice shelves around Antarctica. These shelves hold back ~30 million cubic kilometres of ice.
Recent modelling and observations shows that ACC is also slowing down. This is threatening the world’s largest ice sheet and disrupting global ocean currents, which in turn affect global weather systems.
As Antarctica melts, more freshwater flows into the oceans. This disrupts the sinking of cold, salty, oxygen-rich water to the bottom of the ocean. From there this water normally spreads northwards to ventilate the far reaches of the deep Indian, Pacific and Atlantic Oceans. But that could all come to an end soon. In our lifetimes. – England et al, 2023
Our new study has revealed that the Southern Ocean is changing, but in a different way to what we expected. We may have passed a tipping point and entered a new state defined by persistent sea ice decline, sustained by a newly discovered feedback loop…Antarctica is no longer the stable, frozen continent we once believed it to be. It is changing rapidly, and in ways that current climate models didn’t foresee. Until recently, those models assumed a warming world would increase precipitation and ice-melting, freshening surface waters and helping keep Antarctic sea ice relatively stable. That assumption no longer holds. – Silvano, July 2025
If the Antarctic overturning slows down, nutrient-rich seawater will build up on the seafloor, five kilometres below the surface. These nutrients will be lost to marine ecosystems at or near the surface, damaging fisheries.
Changes in the overturning circulation could also mean more heat gets to the ice, particularly around West Antarctica, the area with the greatest rate of ice mass loss over the past few decades. This would accelerate global sea-level rise.
An overturning slowdown would also reduce the ocean’s ability to take up carbon dioxide, leaving more greenhouse gas emissions in the atmosphere. And more greenhouse gases means more warming, making matters worse.
Put simply, a slowing or collapse of the overturning circulation would change our climate and marine environment in profound and potentially irreversible ways.
The signs of melting around the edges of Antarctica are very clear, with increasingly large volumes of freshwater flowing into the ocean and making nearby waters less salty and therefore less dense. And that’s all that’s needed to slow the overturning circulation. Denser water sinks, lighter water does not….
…We ran three different experiments, one where conditions remained unchanged from the 1990s; a second forced by projected changes in temperature and wind; and a third run also including projected changes in meltwater from Antarctica and Greenland.
In this way we could separate the effects of changes in winds and warming, from changes due to ice melt.
The findings were striking. The model projects the overturning circulation around Antarctica will slow by more than 40% over the next three decades, driven almost entirely by pulses of meltwater. – England et al
Atlantic Meridional Overturning Circulation (AMOC) Thermohaline Current
For updated monitoring and research, see the RAPID project.
The AMOC is a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans. The term thermo means temperature and haline means salt.
Moving nearly 20 million cubic metres of water per second, the AMOC is driven in part by the formation of Arctic sea ice each year. When ocean water freezes, it leaves salt behind, making the surrounding water denser and heavier, so it sinks. Cold water is denser than warm water, so it also aids the sinking process.
The scale of sea ice formation was so large that until recently, the sinking salty water is one of the the world’s largest waterfalls (the largest is the ACC around Antarctica, discussed above). However, less sea ice is forming in the Arctic every year. The Greenland ice sheet is also melting at a record-breaking pace, along with glaciers and permafrost in the lands surrounding the Arctic Ocean. Together, this is disgorging ever-increasing amounts of freshwater into the ocean, so less salt and more freshwater is being added to the Arctic ocean The result? The mechanism that drives AMOC is disappearing. The current is now weaker than at any time in the past thousand years.
The last time the current shut down was at the end of the Last Glacial Maximum (LGM) leading to abrupt cooling across much of Europe (the ‘Younger Dryas’) with an unstable climate and wild weather globally for several thousand years. The climate is now warming far faster than at the end of the LGM.
In 2025, global insurance companies began to take this very seriously.
And what happens in the Atlantic doesn’t stay there; 2024 research reveals the impacts on Aotearoa.
The transfer of heat from ocean to atmosphere is at the heart of one of the most important components of Earth’s climate — a system of currents that snakes across the entire length of the Atlantic Ocean and transports warm water from the tropics to high northern latitudes. Known by the unwieldy name of the Atlantic Meridional Overturning Circulation (AMOC), this network of currents affects weather conditions for billions of people around the world. It’s the reason that north-western Europe is relatively mild in the winter and much warmer than Labrador in Canada, which is at a similar latitude. – Kalvelage, June 2025
A full AMOC collapse would be a massive, planetary-scale disaster. We really want to prevent this from happening. In other words: we are talking about risk analysis and disaster prevention. This is not about being 100% or even just 50% sure that the AMOC will pass its tipping point this century; the issue is that we’d like to be 100% sure that it won’t. That the IPCC only has “medium confidence” that it will not happen this century is anything but reassuring, and the studies discussed here, which came after the 2021 IPCC report, point to a much larger risk than previously thought. – Rhamstorf, April 2025
Climate models suggest a reduction in AMOC strength of 32 ± 37% by 2100 (90% probability, Shared Socioeconomic Pathways 2-4.5 scenario. – Portman et al 2026
Scientists have found that the AMOC may be far more vulnerable to rapid warming than to temperature alone. When warming happens slowly, the ocean can adapt, but at faster rates similar to today’s, the massive Atlantic circulation could reach a tipping point at much lower temperatures. – Science News, August 2026
April 2026: Carbon Brief published an excellent storymap: Is global warming tipping key Atlantic ocean currents towards ‘collapse’?
Videos 3 & 4 explains how the current works and why it’s so important. Video 5 goes into more technical detail.
