ERC project TAOC group
Jelle Soons, René van Westen, Emma Smolders, Amber Boot, Valérian Jacques-Dumas, Elian Vanderborght and Henk Dijkstra (PI).
The Atlantic Ocean Circulation plays an important role in regulating Earth’s climate by redistributing heat through the global ocean. This large-scale ocean circulation consists of a northward transport of relatively warm surface waters from the tropics to the North Atlantic Ocean. It provides Western Europe with a relatively mild climate compared to other regions just as far north on the globe, such as Canada and Alaska. These relatively warm surface waters eventually reach the subpolar regions around Greenland and Iceland, where they are cooled by the atmosphere. As cold water is heavier than warm water, the surface waters sink to depths of 2000 to 3000 metres before returning southward as a cold, deep undercurrent. This Atlantic Ocean Circulation, schematically depicted in Figure 1, is crucial for Europe’s climate, but it also plays an essential role in the ocean’s ability to absorb CO2 and supply oxygen, as well as in regulating rainfall patterns in the tropics. Therefore, it is of global importance.
Tipping of the Atlantic Ocean Circulation
The current and potential future states of the Atlantic Ocean Circulation are actively investigated by the research community. Since 2004, its strength has been closely monitored along the 26N parallel. Unfortunately, this observational record is too short to detect any long-term trend. However, climate reconstructions indicate that the circulation’s strength has fallen by 15 per cent since 1950 (Caesar et al., 2021). The so-called ‘cold blob’ over the North Atlantic Ocean is a related sign of the weakening of circulation. It is the sole region on Earth that has experienced a cooling trend rather than a warming trend since the start of the last century—a sign consistent with declining transport by the Atlantic Ocean Circulation. Moreover, climate model-based projections of the circulation show it weakening in the remainder of the 21st century, as the sinking of the water in the north is inhibited by global warming (Weijer et al., 2020).
The Atlantic Ocean Circulation has been classified as a potential tipping element in the present-day climate (Armstrong McKay et al., 2022). A tipping element is a system that can (rapidly) shift from one state to another state as the result of a small change in an external force. Stommel (1961) was the first to realise that the Atlantic Ocean Circulation has two stable states and that transitions between these states are possible, i.e. the circulation can tip. Using a highly idealised model representing the Atlantic Ocean Circulation, Stommel identified the feedback loop that causes the circulation to collapse. A freshwater anomaly near the sinking regions will inhibit the sinking as freshwater is lighter than saline water. This, in turn, reduces the northward transport of salinity to these regions, which freshens the regions even more. As a response, this again decreases the circulation strength, and so forth. When this feedback is strong enough, the circulation will make a transition to a different stable state: the collapsed state.
Of course, Stommel’s model was highly idealised and captured few relevant feedback mechanisms in the actual climate system. However, since then, the tipping of the Atlantic Ocean Circulation has been simulated using increasingly sophisticated models. Moreover, another line of evidence for the circulation’s potential to tip is from sediment records. These show that abrupt transitions in the Atlantic Ocean Circulation have occurred multiple times in the geological past (Ganopolski and Rahmsdorf, 2001). Still, in complex modern climate models in which most relevant feedback is included, this kind of tipping behaviour has not yet been simulated.
The theoretical potential for a collapse of the Atlantic Ocean Circulation—in combination with reconstructions of it slowing down—presents a worrying possibility and uncertainty for the future of the Earth’s climate. Our TAOC project aims to determine the probability of a collapse of the present-day Atlantic Ocean Circulation before the end of this
century. We use a range of models, from conceptual to highly complex climate models. The former provides us insight into the core physical dynamics of the system and requires little computational time, while the latter includes most physical mechanisms and yields a result as close to reality as modern models can provide.
The collapsed state
In 2023 and 2024, we performed a groundbreaking climate model simulation, which is an important milestone in the TAOC project. Using a state-of-the-art climate model, the Community Earth System Model (CESM), a complete collapse of the Atlantic Ocean Circulation has been simulated under pre-industrial forcing conditions (van Westen et al., 2024). This answers a long-standing problem within climate science: an Atlantic Ocean Circulation collapse is indeed possible within a complex modern climate model. This was demonstrated by conducting a so-called hosing experiment where freshwater is slowly added to the North Atlantic surface while freshwater is removed from the ocean’s remaining surfaces to conserve the total salinity content of the ocean. Over time, this freshwater forcing reaches a critical level at which the previously mentioned positive feedback loop causes the collapse of the circulation (black curve, Figure 2).
The modelled climate impacts of an abrupt circulation collapse are severe. For example, they include a cooling of the Northern Hemisphere, in particular for Northwestern Europe, where yearly average temperatures drop by by 10 °C to 20 °C, see Figure 3. This effect is especially apparent in the winter months, with London and Bergen seeing a drop in their average February temperatures of 15 °C and 35 °C, respectively. These strong temperature decreases are related to the vast expansion of the Arctic sea-ice pack, which has an average increase of roughly 9 million km2 in March, meaning that the sea-ice cover will reach as far south as the Strait of Dover.
While the Northern Hemisphere cools, the opposite is true for the Southern Hemisphere. Here, the temperature changes are smaller, with an increase of about 1 °C. The Antarctic sea-ice extent retreats under these higher temperatures and decreases by 4 million km2 for September. Also, the tropics are affected due to changing precipitation patterns. In the Amazon Rainforest, the dry season becomes the wet season and vice versa. This would dramatically disrupt its ecosystem and could potentially lead to the rainforest’s collapse, as it is also a tipping element (Armstrong McKay et al., 2022).
All in all, this study is bad news for the climate and for humanity as it reveals that a tipping of the Atlantic Ocean Circulation is not just a theoretical concept as was proposed by Stommel (1961), but a drastic change that can happen in a modern complex climate model.
Hysteresis behaviour
When we reduce the freshwater forcing again (red curve in Figure 2), the circulation recovers at a much smaller value compared to the collapse, giving rise to so-called hysteresis behaviour (van Westen and Dijkstra, 2023). This behaviour was found in previous climate models and is now demonstrated in a state-of-the-art climate model.
The dynamics of this behaviour are largely explained by the extended Arctic sea-ice pack under a collapsed circulation. In the collapsed state, the North Atlantic is largely covered by sea-ice, which prevents cooling of the surface waters by the overhead atmosphere and hence limits the sinking of these waters around Greenland. This sea-ice is stabilising the collapsed state and inhibits a recovery, which only occurs at relatively low freshwater forcing values. The implication is that once the Atlantic Ocean Circulation collapses, it will be hard to return to its original state, making it irreversible on human timescales.
Predicting the tipping point
These two TAOC studies are important steps towards the goal of the probability estimate of an abrupt collapse of the Atlantic Ocean Circulation before the end of this century. As we now know that these transitions are possible within a complex climate model, we can now develop early warning indicators of a potential collapse in the near future. In van Westen et al. (2024), an early warning signal was already developed, which suggested that the present-day Atlantic Ocean Circulation is heading towards collapse. However, we have not yet been able to give an estimate of the tipping time. Our latest research focuses on obtaining this estimate, which will bring us nearer to estimating the probability that the Atlantic Ocean Circulation will tip within the next few decades.
References
Armstrong McKay, D. I., Staal, A., Abrams, J. F., Winkelmann, R., Sakschewski, B., Loriani, S., Fetzer, I., Cornell, S. E., Rockström, J. and Lenton, T. M. (2022) ‘Exceeding 1.5 °C global warming could trigger multiple climate tipping points’, Science, 377(6611). doi: 10.1126/science.abn795.
Caesar, L., McCarthy, G.D., Thornalley, D. J. R., Cahill, N. and Rahmstorf, S. (2021) ‘Current Atlantic meridional overturning circulation weakest in last millennium’, Nature Geoscience, 14(3), pp. 118–120. doi: 10.1038/s41561-021-00699-z.
Ganopolski, A. and Rahmstorf, S. (2001) ‘Rapid changes of glacial climate simulated in a coupled climate model’, Nature, 409(6817), pp. 153–158. doi: 10.1038/35051500.
Stommel, H. (1961) ‘Thermohaline convection with two stable regimes of flow’, Tellus, 13(2), pp. 224–230. doi: 10.1111/j.2153-3490.1961.tb00079.x.
Weijer, W., Cheng, W., Garuba, O. A., Hu, A. and Nadiga, B. T. (2020) ‘CMIP6 models predict significant 21st century decline of the Atlantic meridional overturning circulation’, Geophysical Research Letters, 47(12). doi: 10.1029/2019GL086075.
van Westen, R. M. and Dijkstra, H. A. (2023) ‘Asymmetry of AMOC Hysteresis in a State-Of-The-Art Global Climate Model’, Geophysical Research Letters, 50(22). doi: 10.1029/2023GL106088.
van Westen, R. M., Kliphuis, M. and Dijkstra, H. A. (2024) ‘Physics-based early warning signal shows that AMOC is on tipping course’, Science Advances, 10(6). doi: 10.1126/sciadv.adk1189.
Figure legends
Figure 1: Sketch of the present-day Atlantic Ocean Circulation, with warm surface waters flowing northward (red), eventually sinking near Greenland and Iceland before returning as a deep cold flow (blue).
Figure 2: The strength of the Atlantic Ocean Circulation under increased freshwater forcing (black) and when the forcing is reversed (red), with the freshwater forcing amount on the horizontal axis. The blue region on the globe indicates the region where the freshwater forcing is applied.
Figure 3: The difference in average yearly temperatures in Europe before and after the collapse of the Atlantic Ocean Circulation.
PROJECT NAME
TAOC (Tipping of the Atlantic Ocean Circulation)
PROJECT SUMMARY
The TAOC project aims to determine the probability that the Atlantic Ocean circulation will collapse under climate change before the year 2100. A hierarchy of ocean-climate models will be used to which we will apply modern rare event techniques.
PROJECT PARTNERS
We collaborate with many other groups in other (H2020 and Horizon Europe) projects.
PROJECT LEAD PROFILE
Dr Henk Dijkstra secured a PhD in mathematics from the University of Groningen (the Netherlands) and is currently Professor of Dynamical Oceanography at the Institute for Marine and Atmospheric Research Utrecht (IMAU), Department of Physics, Utrecht University, Utrecht, the Netherlands. Dijkstra holds a PIONIER award from the Dutch Science Foundation and a Lewis Fry Richardson Medal from the European Geosciences Union for his outstanding work in developing the nonlinear dynamical systems approach to oceanography and for his study of the role of ocean circulation in (palaeo)climate.
PROJECT CONTACTS
Dr Henk Dijkstra
https://webspace.science.uu.nl/dijks101
FUNDING
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101055096.
