April – April Fools.
This 'underwater waterfall' near Le Morne Brabant, in the south of the island of Mauritius in the Indian Ocean (near Madagascar & Africa) has captured our imagination for centuries. But actually, April Fools, it's not an 'underwater waterfall' at all; it's sediment and sand, swept out to sea from behind a series of reefs.
However, there are underwater waterfalls occurring near the earths, southernmost, highest, driest, windiest, coldest, and iciest continent – Antarctica; 14 million square kilometres of ice at the end of the world. And due to Global Heating and Climate Change these waterfalls and life as we know it will change.
In a recent scientific paper published in the prestigious journal Nature – on the 30th of March, researchers (Li, England, & Hogg et al.) projected the fate of Antarctic currents. The team used a high-resolution computer model that could simulate high emissions, as well as the currents and changes in the world’s oceans over 50 years. The currents begin their life near the frozen continent, as the sea ice forms in winter. Sea ice does not contain salt, so this mineral becomes concentrated in the water below the ice.
Cold, salty, dense ‘Antarctic Bottom Water’ as its called, forms at four spots along the coast: about 250 trillion tonnes of liquid per year. Being so dense, it sinks to the bottom of the ocean and forms a layer. Sometimes the water is colder than 0°C, but is kept in a liquid state because of its high concentration of salt. It’s also high in dissolved oxygen. Yet this layer still has further to fall: once it reaches the edge of the Antarctic continental shelf it cascades into deep basins – like a deep-sea waterfall tumbling into a pool.
From there, it feeds into deep ocean currents heading north. One current hugs the east continental coast of Aotearoa, crosses the equator, and rises in the North Pacific. When it rises, it brings its oxygen with it to the multitude of ocean creatures. The current also pulls up important nutrients from the ocean floor, feeding marine plants – and everything great & small that eats them.
As the water rises, the Pacific current splits and turns southwards – feeding into a cycle that has been in a relatively stable state for thousands of years.
However, the very deep sea below 4,000m has warmed in recent years, according to measurements taken from ships. With our ongoing greenhouse gas emissions, we’re changing that system. Human-made emissions have heated the atmosphere by roughly 1.2°C. Antarctic sea ice has diminished in recent decades and the ice sheets – which lock away vast amounts of water on land – are melting.
If the world continues to produce high levels of greenhouse gas, the dense and salty Antarctic Bottom Water would slow to “just a trickle” by 2050. It’ll be replaced by something less dense and salty.
Consequently, the ocean depths in the Southern Hemisphere will warm – particularly from 2040. The Pacific’s northbound current would lose nearly half its strength and could collapse altogether, the modelling found. A collapse is likely to trigger more melting in Antarctica and could affect the amount of rain falling in the central Pacific Ocean.
The slowing supply of oxygen and nutrients would also hurt marine life and human communities. Without nutrients, the plants of the marine world – such as tiny algae – would not grow. With less oxygen, larger stretches of the ocean would become dead zones.
TL,DR:
The abyssal ocean circulation is a key component of the global meridional overturning circulation, cycling heat, carbon, oxygen, and nutrients throughout the world ocean. The strongest historical trend observed in the abyssal ocean is warming at high southern latitudes, but it is unclear what processes have driven this warming and whether this warming is linked to a slowdown in the ocean's overturning circulation. Future change remains uncertain, with the latest coordinated climate model projections not accounting for dynamic ice-sheet melt. This study (Li, England, & Hogg et al.) used a transient forced ocean–sea-ice model to show that under a high-emissions scenario, abyssal warming is set to accelerate over the next 30 years. The meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. This has implications for global ocean biogeochemistry and climate that could last for centuries.
Li, Q., England, M.H., Hogg, A.M., Rintoul, S.R., Morrison, A.K. (2023). Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater. Nature 615, 841–847. https://doi.org/10.1038/s41586-023-05762-w
Geography, digital geography has a role to play in the various outcomes of Climate Change, watch this space as new technologies and methodologies evolve in our ever-changing world.
- Matt Couldrey [geoid - digital geography]