A recent study reveals that the Atlantic Meridional Overturning Circulation (AMOC) is set to cease circulating within the next 75 years at the latest, causing a mini-ice age in Europe and potentially disrupting weather patterns across the entire planet.
The AMOC is an ocean current that carries warm water from the equator to the northern hemisphere, increasing average temperatures, especially in winter. However, large amounts of fresh water flowing into the sea as the ice cap melts threaten to stop the current, leading to significantly colder temperatures. Winter temperatures could drop to -30°C, making Berlin and London as cold as Siberia.
Additionally, the domino effects of changing ocean temperatures will affect precipitation patterns worldwide and could cause a cascade of environmental disasters across the planet.
“These AMOC-induced precipitation changes could severely disrupt the Amazon rainforest ecosystem and potentially lead to a cascading tipping point,” according to a Science Advances paper on physics-based early warning signals that show the AMOC is on a tipping path.
“The Northern Hemisphere shows cooler temperatures after the AMOC collapse, while the reverse is true for the Southern Hemisphere, although not all changes are significantly different (due to large interannual variability).”
But it is clear that the biggest changes will be in Northern Europe. Temperatures have already dropped in Europe during the industrial period due to a 1°C increase in temperatures that has already occurred. And the tipping point will be sudden when it occurs, because it is related to the difference in weight between salt water and fresh water: at some point, there is no longer enough energy to push the warm salt water and the flow simply stops.
It is not yet known when this will happen, but other studies have shown that there is a 95% chance that the AMOC will stop by 2095 if global warming is not stopped.
“Although AMOC collapses have been driven in complex global climate models by strong freshwater forcing, the processes of an AMOC tipping event have not been studied so far. Here we present the results of the first tipping event in the Community Earth System Model, including the important climate impacts of the collapse,” according to the authors of the paper.
The new study looks at the physics of the AMOC. It assumes that the AMOC has a strength of about 15 sverdrups (15 million cubic meters of salt water per second), which is equivalent to about 1 petawatt (PW) of heat transported northward each year, or 32 zettajoules (ZJ) of heat per year.
That’s a lot of energy. Total global energy consumption in 2021 was about 580 exajoules, which means that the AMOC transports northward the equivalent of 55 times the world’s total energy production each year, or the equivalent of 2.5 trillion household energy sources, or 500 million Hiroshima nuclear bomb explosions.
When the AMOC ceases, this immense heat transfer will stop abruptly. The impact of the change will be immediate, catastrophic and irreversible.
The study, along with previous research, suggests that a substantial influx of freshwater from increased precipitation and melting of Greenland’s freshwater ice could potentially disrupt the AMOC this century, and the AMOC’s flow is already visibly slowing.
“With the decrease in southerly heat transport, we would expect Antarctic sea ice to melt faster and Arctic sea ice to melt slower. That’s what the models predict and what we’re seeing,” says climate scientist Lee Simons.
As more freshwater is added in the form of meltwater and precipitation, which are also increasing, the speed of the AMOC slows, with the tipping point in the model occurring at about 0.5 sverdrups (500,000 cubic meters of water per second) of freshwater input.
That potential tipping point is within reach given the current volume of fresh water available and the amount of heat needed to melt the Greenland ice sheet to cause it, Simons calculates.
Greenland contains 2.85 million cubic kilometres of ice, capable of providing about 0.5 sverdrups of fresh water for about 170 years, which would cause sea levels to rise by more than seven metres. To melt this volume of ice for one year requires 5.3 ZJ of heat, or one-sixth of the energy that the AMOC transports north each year.
Since 1970, human-caused greenhouse gas (GHG) emissions have contributed an additional 450 ZJ of heat to the Earth system, about 90% of which has been absorbed by the oceans. The study highlights that while aerosols from coal-fired power plants and the burning of sulfur-rich fuels have reduced this heat buildup, the current trend of aerosol reduction is leading to increased heat buildup and climate models are therefore wrong, underestimating the rate of acceleration of the climate crisis. As a result, the North Atlantic region has seen a significant increase in absorbed sunlight in recent years, adding 4 ZJ of heat over 12 months – slightly less than is needed to melt glaciers.
This contributed to a record global Earth Energy Imbalance (EEI) – the difference between the amount of energy Earth radiates into space and the amount it absorbs from the Sun – of about 1.8 watts per square meter (W/m²) by 2023, adding 29 ZJ of heat to the Earth system each year.
At the 2015 Paris Climate Summit, all attention was focused on the global warming effects of GHGs, but enhanced warming from the sun is a new and underappreciated global factor that is becoming increasingly important over time due to the removal of aerosols (SO2) that were primarily emitted by ship fuel combustion until a cleanup that began in 2020.
“This heat buildup suggests that conditions are ripe to melt enough of Greenland’s ice to potentially close the AMOC. Additionally, increased precipitation is contributing to freshwater,” Simons says. “I’m sorry I can’t be more hopeful about this, but the numbers don’t lie.”
The study concludes that continued increases in GHG concentrations and reductions in aerosols only exacerbate climate forcing and the Earth’s energy imbalance, pushing the AMOC closer to its tipping point.