Analysis of climate events from the Ice Age using three models suggests another role for the Atlantic current system: When it is strong, more heat is released from the Earth; when it is weak, the oceans accumulate heat. The researchers emphasize that the study does not predict an imminent collapse of the system
The weakening of the Atlantic gyre system may cool the North Atlantic and the areas surrounding it, but at the same time increase the amount of heat remaining in the global climate system. This is the main conclusion of New research published in the journal Nature Geoscience.
The system, known by the acronym I LOVE C – Atlantic Meridional Overturning Circulation – moves warm water in the upper layers of the ocean northward and returns cold, dense water southward at depth. It affects temperatures in the North Atlantic, the climate of Europe and North America, sea level, and the transport of oxygen and nutrients inoceans.
Until now, the AMOC's influence has been presented mainly in terms of the distribution of heat between different regions. The new study suggests adding a broader role to the picture: the system acts as a kind of "heat valve" that affects not only where the heat is located, but also the total amount of energy that the Earth and the oceans absorb or lose.
“When you look at the entire Earth, the total amount of heat actually increases during periods when the AMOC is weak,” explained Christo Boisset, a paleoclimatologist at Oregon State University and the lead author of the study, In the university's announcement.
How does the Atlantic "heat valve" work?
Most of the solar radiation that warms the oceans is absorbed in the tropics. Some of the heat is carried north by ocean currents. When the AMOC is strong, warm, salty water reaches the North Atlantic, cools, becomes denser, and sinks to the depths.
In this process, a large amount of heat is released from the ocean into the atmosphere. The warming of the ocean surface and atmosphere in the North Atlantic also changes the ice cover, cloudiness, and the radiation balance at the top of the atmosphere. Some of the energy is eventually emitted into space.
When the AMOC weakens, less heat reaches the sea surface in the North Atlantic sunset regions. The upper layer in this region may cool, but the heat that is not transported north and not efficiently dissipated accumulates within the ocean.
According to the study, only a relatively thin layer at the surface of the North Atlantic Ocean is cooling, while larger parts of the ocean, including deeper layers, are warming. The result is an increase in the overall heat content of the oceans and the amount of energy remaining in the climate system.
Therefore, there is no contradiction between the possibility of regional cooling in the North Atlantic Ocean and further warming of the Earth as a whole.
Not just a "swing" between North and South
The researchers examined abrupt climate changes that occurred during ice ages, particularly events known as the Dansgaard-Osgar events. These events, documented in ice cores from Greenland and Antarctica, included rapid warming and cooling in the North Atlantic Ocean and parallel changes in other regions.
The conventional explanation described the process as a “bipolar thermal seesaw”: as the North Atlantic cooled due to the weakening of the AMOC, more heat remained in the Southern Hemisphere. As the North warmed, the South responded in the opposite direction.
According to the framework presented by the new study, it’s not just about transferring the same amount of heat from one side of the Earth to the other. When the AMOC weakens, the oceans and the entire Earth gain more energy; and when it strengthens, the rate of heat loss increases.
To test this, the researchers used simulations of abrupt changes in the AMOC in three different climate models: CCSM4, MIROC4m, and COSMOS. They tracked the movement of heat between ocean basins, the exchange of energy between the sea and the atmosphere, and the radiation balance at the top of the atmosphere.
The emergence of the same basic pattern in all three models has strengthened the conclusion that AMOC strength affects the global energy balance. However, this research is based primarily on models and reconstructions of ice age events, rather than a direct measurement of the future response to current warming.
An effect equivalent to about 25 parts per million of carbon dioxide
The researchers estimated that the temperature changes that accompanied the AMOC weakening events during the ice age were similar in their impact to the warming that would result today from an addition of about 25 parts per million to the concentration of carbon dioxide in the atmosphere.
According to them, this is an order of magnitude equivalent to about ten years of the increase in carbon dioxide concentration caused by human activity.
This does not mean that the weakening of the AMOC adds carbon dioxide to the atmosphere, or that any future weakening will produce warming equivalent to exactly 25 parts per million. This is a comparison intended to illustrate the order of magnitude of the energy balance shift found in simulations of past events.
Warming caused by greenhouse gas emissions remains the main driver of current climate change. The study suggests another possible feedback mechanism: that same warming is expected to weaken the AMOC, and the weakening of the system may in turn allow the Earth to accumulate more heat.
Weakening is not necessarily collapse
The study does not state that the AMOC is about to collapse, nor does it provide a date for a possible collapse. It is also important to distinguish between a gradual weakening, which many climate models predict, and a sudden transition to a very weak state or a near-complete cessation of part of the system.
The IPCC estimates that the AMOC is expected to weaken during the 21st century. However, the Sixth Assessment Report states that a sudden collapse before 2100 is not the expected outcome in the models tested. Uncertainty increases when trying to estimate developments beyond the end of the century and the impact of meltwater from Greenland.
The new study also suggests that in a warmer world, the AMOC may be more stable than the sharp transitions that characterized ice ages. In the simulations, Dansgaard-Osgar events appeared primarily under certain glacial climates, where the system struggled to balance heat gain in the oceans with heat loss in the North Atlantic.
According to Boisset, it is therefore possible that the AMOC will weaken due to climate change but could recover, without necessarily undergoing an irreversible collapse. However, he stressed that more research is needed to understand the stability of the system in a warming world.
This conclusion does not eliminate the risks associated with weakening. Even without a full collapse, a change in the strength of the AMOC could affect rainfall patterns, storms, temperatures in Europe, sea levels along the East Coast of the United States, and marine ecosystems.
Ice ages are not a replica of the present
Researchers warn that the natural changes that occurred during the ice ages are not a perfect parallel to the rapid warming caused today by greenhouse gas emissions.
Ice ages had much larger ice sheets, lower carbon dioxide concentrations, and different sea ice, winds, and ocean currents. Past events provide a natural laboratory for understanding the mechanisms of the climate system, but the rate or magnitude of change in the 21st century should not be directly extrapolated from them.
The study also does not replace long-term ocean observations. Direct measurements of the AMOC only exist for a few decades, a short time relative to the system's natural fluctuations. A combination of measurement arrays, ancient climate reconstructions, and improved models will be needed to determine how the system will actually evolve.
The main innovation in the study is therefore a change in perspective: the AMOC is not just a conveyor belt that transfers heat between the tropics and the North Atlantic, but also a mechanism that affects the entire Earth's ability to emit energy into space. If the conclusion is confirmed in further studies, climate projections will have to take into account not only the regional cooling caused by the weakening of the system, but also the possible additional heat in the ocean depths and at the global level.
Questions and Answers
What is the AMOC system? A system of currents in the Atlantic Ocean that transports warm water northward near the surface and cold, dense water southward at depth. It is part of the global ocean circulation system.
Is the AMOC The golf stream? No. The Gulf Stream is one of the surface currents associated with the system, but the AMOC is broader and also includes a North Atlantic subduction and a deep southward flow.
How can a weakening system cool Europe and yet warm the planet? Less heat reaches the sea surface in the North Atlantic, so the region may cool. At the same time, more heat remains in the rest of the ocean and less energy is emitted from the Earth into space.
Does the study predict an imminent collapse of the AMOC? No. It examines the effect of the system's strength on the energy balance. The models indicate a possible future weakening, but the study does not determine that the system will collapse or when that might happen.
Source/Original article
- Planetary energy budget during abrupt glacial climate events set by Atlantic Ocean heat valve – Nature Geoscience
- Oregon State University Announcement
- NOAA's explanation of the AMOC system
More on the subject on the science website
- A central Atlantic current has been weakening for almost 20 years, and the impact on weather could be global
- Deep currents in the Atlantic Ocean continued to operate during the last ice age.
- Will the Climate Crisis Shut Down the Powerful Gulf Stream?
- Hot, Hotter, Frozen: The Surprising Reason for the "Little Ice Age"
- Climate tipping points could bring unstoppable changes
For the scientific article: Opening the scientific article
