(Image: Earth live : circumpolar surface temps.)
- Just like us, the Earth has a circulatory system. Instead of blood, oceanic currents transport heat, oxygen, carbon dioxide and nutrients around the planet.
- The world’s oceans have absorbed around 93% of global warming and heating up 40% faster than the IPCC estimated in 2013.
- The Great Oceanic Conveyor Belt or AMOC is literally a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans. It’s now weaker than at any time in the past thousand years.
- El Niño brings warmer and windier conditions to New Zealand when sea surface temperatures in the tropical Pacific Ocean rise to above-normal levels for an extended period of time.
- The Antarctic Circumpolar Current (ACC) is so powerful it moves an equivalent area of the South Island one metre every second. It’s also warming faster than the global ocean as a whole.
- Ocean currents played a significant role in how the current Ice Age started, and why we still are in an Ice Age.
- Ocean currents are now changing due to warming temperatures and feedback effects.
“A conveyor belt of ocean water that loops the planet and regulates global temperatures could be heading for a tipping point.” – National Geographic science special, December 2019
Literally a conveyor belt for exchanging heat and nutrients across four of the five world’s major oceans, the AMOC (Atlantic Meridional Overturning Circulation) is such a key part of that conveyor belt that the two names are often used to describe the same current. A major part of this current includes the Gulf Stream, which keeps Europe warmer than the east coast of the United States. Below is a simple explanation from National Geographic. Videos 1 and 2 explain how it works and why it’s so important.
The current, which moves nearly 20 million cubic metres of water per second, 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. The scale of sea ice formation is so large that this sinking salty water is the world’s largest waterfall. However, less sea ice is forming in the Arctic every year. The Greenland ice sheet is also melting, 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? A key part of the mechanism that drives AMOC is disappearing. The current is now weaker than at any time in the past thousand years.
This current abruptly shut down when Earth warmed quickly at the end of the last glacial maximum, leading to equally abrupt cooling over (the ‘Younger Dryas’) over much of Europe, with an unstable climate and wild weather globally for several thousand years.
Today, the climate is warming far faster than it at the end of the last glacial maximum, so there is concern amongst key scientists that the current may soon reach a tipping point and shut down once more, with similar global consequences. For more details see the references at the bottom of this page and tipping points.
Video 1: 13 minutes; explains how one of the key oceanic currents transports heat and nutrients around the planet, and the implications if this current shuts down.
Video 2: 6 minutes; less detailed but covers the key points.
“We are 50 to 100 years ahead of schedule with the slowdown of this ocean circulation pattern relative to what the models predict. Why might that be true? Well one of the things that the models also aren’t capturing is the rate at which we’re losing ice from the ice sheets: the West Antarctic Ice Sheet and the Greenland Ice Sheet.” – Dr. Michael Mann, Pennsylvania State University (Video 2).
El Niño-Southern Oscillation (ENSO) and equatorial currents
While 2020 tied with 2016 as the warmest year on record for average global surface temperatures, there is one very big difference. Warming in 2016 was boosted by one of the largest El Niño events in the last century. Whereas La Niña helped cool 2020.
Probably best known for its episodic impact on seasonal weather rather than as oceanic currents. This highlights the interlinked relationship between the ocean and weather, especially for small island nations like New Zealand (Video 3). The Pacific basin covers one third of the planet, so changes in this area also have profound effects to east Asia and the western sides of the Americas, and there’s also a domino effect on other parts of the planet.
Why and how El Niño and its reverse current, La Niña flip back and forth is still unclear. As too are the effects that climate change will have on their frequency and intensity. Recent research suggests that the increasing loss of sea-ice around Antarctica is leading to more warming in the eastern equatorial Pacific, which is where ENSO patterns form. This could result in more frequent and stronger El Niños.
Video 3: NIWA video explaining how this system affects New Zealand’s weather, including drought in Canterbury.
Antarctic Circumpolar Current (ACC)
The strongest ocean current on Earth, it encircles Antarctica and extends from the surface to the bottom of the ocean. It carries an estimated 165 million to 182 million cubic metres of water every second (a unit called a ‘Sverdrup’) from west to east, more than 100 times 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. 1).
“The ACC also plays a part in the Meridional (or Global) Overturning Circulation, which brings deep waters formed in the North Atlantic southward into the Southern Ocean. Once there it becomes known as Circumpolar Deep Water, and is carried around Antarctica by the ACC. It slowly rises toward the surface south of the Polar Front.
“Once it surfaces, some of the water flows northward again and sinks north of the Subarctic Front. The remaining part flows toward Antarctica where it is transformed into the densest water in the ocean, sinking to the sea floor and flowing northward in the abyss as Antarctic Bottom Water. These pathways are the main way that the oceans absorb heat and carbon dioxide and sequester it in the deep ocean.” – Phillips et al
Video 3: Prof. Eric Rignot explains how the ACC is changing due to climate change, and how that’s melting marine glaciers and ice sheets along the coast of Antarctica.
Like the Arctic Ocean, the Southern Ocean (where the current flows) has become warmer (we are now experiencing marine heat waves). The warmest water is deep and it’s undercutting the marine ice sheets and ice shelves around Antarctica that hold back the massive ice sheets which hold ~30 million cubic kilometres of ice, the equivalent of 60-70m of sea level rise if it all melted.
As the climate warms, the deeper warmer waters of the ACC are being by pushed closer to the coast by the strengthening Westerly winds. This is speeding up the rate at which the water is eroding the base of the marine ice sheets, which in turn is accelerating the rate at which they’re melting and collapsing (Video 4: 10.07 – 12.23) (see also the page on Antarctica).
The term ‘climate forcing’ comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives, the planet cools (negative RF). Conversely, if less energy escapes than gets in, the planet warms (positive RF). This is due to the The Law of Conservation of Energy, a basic law of thermodynamics, which states that: ‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’
Different climate forcings each determine how much solar energy arrives and escapes.
- Natural Forcings are those that happen through natural changes.
- Anthropogenic Forcings are those due to human activities.
These are long events (millions of years) in geological time called Periods, when there’s at least one major ice sheet on the planet. An ice sheet is defined as an area 50,0002 km or more. As Greenland and Antarctica still have much larger ice sheets than this, we are still in an Ice Age called the Quaternary Period. Click here to see more (links to another page on this site).
These are shorter much colder Epochs (thousands of years) that happen during Ice Ages, when glaciers and ice sheets extend out over continents. The last glacial Epoch was the Pleistocene, which began 120,000 years ago. Because the coldest part of this Epoch (and therefore the maximum extent the ice sheets and glaciers reached) was from ~26,500-19,500 years ago, it’s called the last glacial maximum.
The Pleistocene Epoch ended ~14,500 years ago, when an Interglacial Epoch called the Holocene—the epoch we’re now living in—began. Click here to see more (links to another page on this site).
Thermo means temperature and haline means salt. Cold water is denser than warm water, so it sinks. Adding salt makes it even more dense, so it sinks faster.
References and further reading
- NIWA: What are El Niño and La Niña
- NIWA: The impact of El Niño and La Niña on New Zealand’s climate
- NSIDC (National Snow Ice Data Center): Quick Facts on Ice Sheets
- Marine Heat Waves: modelling and prediction
- 2021: Caesar et al; Current Atlantic Meridional Overturning Circulation weakest in last millennium, Nature Geoscience 6369
- Carbon Brief article explaining the research (open access)
- 2021: Hausfather; The State of the Climate; Carbon Brief
- 2020: Cai et al; Butterfly effect and a self-modulating El Niño response to global warming Nature 585 pp68–73
- 2020: McSweeny (Carbon Brief); Scientists shed light on human causes of North Atlantic’s ‘cold blob’
- 2020: Keil et al; Multiple drivers of the North Atlantic warming hole, Nature Climate Change 10, 667-671
- 2020: England et al; Tropical climate responses to projected Arctic and Antarctic sea-ice loss Nature Geoscience 13, 275–281
- 2020: Sigmond et al; Ongoing AMOC and related sea-level and temperature changes after achieving the Paris targets Nature Climate Change 10, 672-277
- 2020: Wu et al; North Pacific subtropical mode water is controlled by the Atlantic Multidecadal Variability Nature Climate Change 10, 238–243
- 2020: Zou et al; Density-compensated overturning in the Labrador Sea Nature Geoscience 13, 121–126
- 2019: Cheng et al; How fast are the oceans warming? Science 363/6423 pp128-129
- 2019 IPCC: The Ocean and Cryosphere in a Changing Climate: Summary for Policymakers
- 2019: Lozier et al; A sea change in our view of overturning in the subpolar North Atlantic Science 363/6426, 516-521
- 2018: Caesar, Rahmstorf et al; Observed fingerprint of a weakening Atlantic Ocean overturning circulation: Nature 556, 191–196
- Rahmstorf at RealClimate online discussion forum: ‘If you doubt that the amoc has weakened read this’
- 2017: Stieglitz; The Atlantic Meridional Overturning Circulation and Abrupt Climate Change Annual Review of Marine Science 9, 83-104
- 2017: Science ‘How Stuff Works’ The World’s Largest Waterfall Is Deep Underwater
- 2016: Donahue et al; Mean Antarctic Circumpolar Current transport measured in Drake Passage Geophysical Research Letters 43/22, 11,760-11,767
- 2015: Rahmstorf et al; Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change 5, 475–480
- 2013: Rahmstorf & Feulner: Chapter 2. Paleoclimate Ocean Circulation and Sea-level changes, in Ocean Circulation and Climate: a 21st century perspective; International Geophysics Series Vol 103, Elselvier Inc.
- 2011: NASA; Thermohaline Circulation using Improved Flow Field (animation)
- 2005: Schmidt; Gulf Stream slowdown? Realclimate (climate scientists’ blog)
- 2002: Rahmstorf; Ocean circulation and climate during the past 120,000 years: Nature 419, 207–214