If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . Abstract Climate scientists have raised concerns about the weakening of the Atlantic Meridional Overturning Circulation (AMOC) or even its potential collapse in the future. Their messages should not hinder urgent adaptation to climate risks; rather, they underscore the growing need for adaptive planning across a range of possible futures, including high-impact, low-likelihood AMOC scenarios. There are five ways to consider the consequences of AMOC weakening or collapse in adaptation planning: (1) broaden the set of future adaptation scenarios considered; (2) develop adaptation pathways beyond the most likely range of possible outcomes; (3) create robustness and redundancy in adaptation portfolios; (4) expand the solution space, attuned to path dependencies and their implications; and (5) monitor emerging, weak signals of AMOC changes to inform adaptation planning. We argue that closer collaboration between climate scientists and the adaptation planning community is needed to generate timely, policy-relevant insights that can guide proactive and effective adaptation action. Some studies note that adaptation responses will face challenges and limits under the rate and severity of impacts resulting from strong AMOC weakening or collapse (van Westen et al. 2024 ). Although there remain deep uncertainties in the scientific debates about AMOC, the large-scale potential impacts raise questions about what this means for current and future adaptation action to climate risks. We point to five areas where strong weakening of AMOC or even collapse can be considered in adaptation planning and decision-making. Broaden the set of futures for adaptation action Planning for deeply uncertain and high-impact events requires expanding the range of future scenarios considered and then stress-testing adaptation strategies. AMOC dynamics, including some weakening, have been included already in several CMIP6 Earth System Models, particularly those that model ocean–atmosphere interactions. However, climate impact and risk studies have infrequently examined strong weakening or potential collapse. Yet climate tipping points are considered ‘too risky to bet against’ (Lenton et al. 2019 ) because of the substantial potential impacts, costs and time needed to take action. Better insights into the magnitude and rate of the impacts of strong AMOC weakening or collapse, and how they might affect people and ecosystems across regions, will be important. This is particularly important in early planning stages, in order to assess the potential relevance of AMOC for long-term adaptation planning and decision-making, for example the time frames that would be necessary to adjust and implement adaptation plans. Future climate impact studies could also more explicitly explore the potential resulting climatic conditions that fundamentally differ from trends currently considered in adaptation planning (e.g. the substantial possible cooling that could occur), along with impacts beyond the ‘likely’ ranges normally considered (e.g. for sea level rise and droughts). With such robust climate impact studies currently lacking (Lenton and Ciscar 2013 ), exploring AMOC-induced changes in more qualitative ways still offers possibilities to explore deeply uncertain futures. Each adaptation pathway represents a set of adaptation measures and a sequence of decisions which allow for switching between pathways or adjusting strategies as new information becomes available or as conditions evolve. Historically, these approaches have focussed on estimated ‘likely’ climate scenarios. However, given the potential for more drastic climate shifts as result of strong weakening or collapse of AMOC, it is important to expand the scope of these pathways to incorporate more extreme scenarios. Using these tools, adaptation planning can become more robust, identifying pivotal decision moments in the future where adjustments in approach may be needed. Some adaptation options or whole adaptation pathways that anticipate AMOC collapse scenarios may fall beyond the current As we have highlighted here, there are several approaches in adaptation planning that can help anticipate possible changes, manage deep uncertainties associated with AMOC and prevent it from becoming a ‘predictable surprise’. Some have raised concerns that emphasizing the potential risks of AMOC collapse—or other climate tipping points—could foster fatalism, fuel misguided fear and anxiety, divert attention from urgent short-term climate actions, or even be misused to justify inaction (Kopp et al. 2025 ).

Forest tree species are expected to experience a substantial redistribution due to climate change. While previous work has emphasized the effects of a warmer and drier climate on European tree‐species distributions, to date no study has investigated the potential impact of a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Here, we deploy climate‐envelope models to quantile mapped, high‐resolution (1km2) CMIP6 climate projections and compare tree‐species distributions under an active AMOC vs. an inactive AMOC scenario. Across Europe, our tree‐species projections indicate contrasting impacts of the two scenarios. In Scandinavia, many of the currently abundant tree species were projected a dramatic decline and partial disappearance due to the strong cooling under an inactive AMOC. In Central and Southern Europe, however, some of the currently abundant species suffered less under an inactive AMOC compared to an active AMOC scenario while others—such as the economically important species of Norway spruce—almost went extinct. As opposed to the classic climate‐change scenario supporting Mediterranean species in Central Europe, projected European tree‐species portfolios consisted of a higher share of boreal, cold‐tolerant species in the inactive AMOC scenario. Finally, tree‐species diversity was projected to decline even stronger under an inactive vs. an active AMOC scenario. Altogether, while an AMOC collapse may locally result in more favorable conditions for specific species in comparison to a classic climate‐change scenario, the dramatic economic and ecological consequences suggested by our projections indicate the urgent need for climate‐change mitigation to lower the likelihood of an AMOC collapse. This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. ABSTRACT Forest tree species are expected to experience a substantial redistribution due to climate change. While previous work has emphasized the effects of a warmer and drier climate on European tree‐species distributions, to date no study has investigated the potential impact of a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Altogether, while an AMOC collapse may locally result in more favorable conditions for specific species in comparison to a classic climate‐change scenario, the dramatic economic and ecological consequences suggested by our projections indicate the urgent need for climate‐change mitigation to lower the likelihood of an AMOC collapse. Keywords: climate envelope models, CMIP6, quantile mapping, thermohaline circulation, tipping point, tree‐species richness This study examines the effects of a collapsing Atlantic Meridional Overturning Circulation (AMOC) in comparison to a common climate‐change scenario. It shows that an AMOC collapse would result in contrasting tree‐species portfolios across Europe in comparison to an active AMOC scenario and suggests catastrophic effects on forest ecosystems in Scandinavia. status released display-pdf yes is-in-collection-domain yes is-olf no is-manuscript no is-preprint no is-journal-matter no is-scanned no is-retracted no Revised 2025 Mar 4; Received 2024 Nov 19; Accepted 2025 Mar 6; Issue date 2025 Apr. 1. Introduction The climate‐change induced increasing frequency and magnitude of so‐called hotter droughts has resulted in widespread forest decline and tree dieback (Allen et al. 2015 ; Anderegg et al. 2015 ; Buras et al. 2020 , 2018 ; Cailleret et al. Since this would directly affect the realization of climate envelopes, tree‐species distributions and the resulting portfolios are likely to differ from the tree‐species projections provided thus far (e.g., Buras and Menzel 2019 ; Chakraborty et al. 2021 ; Dyderski et al. 2018 ; Martes et al. 2024 ; Mauri et al. 2022 ). Yet, no study has provided insights into the potential shifts in tree‐species distributions under an AMOC collapse, and it remains an open question whether an AMOC collapse may have more positive or negative effects on Europe's forests given a partial mitigation of warming vs. a reduction of precipitation. In addition to supporting previous findings at a high spatial resolution of 1 km 2 and deploying the latest ensemble of climate simulations (CMIP6), our study provides first insights into the effects of an AMOC collapse on European tree‐species distributions. 4.1. Effects of an AMOC Collapse on Europe's Forests In comparison to standard climate‐change projections, a collapsing AMOC would—in dependence of the region and species—partially dampen the effects of climate change due to a local cooling. Norway spruce and Scots pine constitute the economically most important tree species for However, under an AMOC collapse, this was only the case for parts of Western Europe, the Alps, and the Mediterranean, while for Eastern Europe and large parts of Scandinavia, the anticipated most suitable tree species featured a much lower occurrence probability than today (Figure 4b ). This underscores the detrimental effects an AMOC collapse would have on European forests caused by a cooling‐induced south‐ and downward shift of the thermal tree line (Körner 1998 ) and a locally drier climate due to reduced precipitation sums. The fact that the economically most important tree species of Norway spruce was heavily affected by an AMOC collapse (Figure 1f ) as well as the devastating effects on tree‐species diversity in Scandinavia and Eastern Europe (Figure 6b ) highlights the urgency to mitigate the realization of such a scenario. Since the main source of an AMOC collapse—enhanced freshwater input in the North Atlantic due to accelerated ice‐sheet melting (Rahmstorf 2006 ; Rahmstorf and Ganopolski 1999 ) – is caused by climate change, effective climate‐change mitigation is the only means to lower the risk of an AMOC collapse.

This paper investigates scientific research using Rumsfeld’s typology to grasp uncertainties in an era of ‘catastrophic climate change and the risk landscape of a ‘2°C-3°C world’. The study highlights the importance of 'unknown unknowns' and the emergence of 'Black Swan' climate events, such as frequent 500-year droughts and discusses how adaptive strategies can be crafted today, building on the “unknown knowns” of the past, such as the Great American Dust Bowl. It emphasizes the need for a structured approach to uncertainty in water resource management, relevant for understanding climate change’s significant impact on water cycles, evident in phenomena like cloudbursts and prolonged droughts. Additionally, research into abrupt climate change, such as the potential mid-century collapse of the Atlantic Meridional Overturning Circulation (AMOC) is analyzed using the Rumsfeld matrix to understand research gaps and generate insights for further study. Adapted from the post- 9/11 world that was 2002, this classification of risks is highly relevant given today’s climate risks, and the climate system's Journal of Ecology and Natural Resources 2 Hald-Mortensen C. Applying the Rumsfeld Matrix: Unknown Unknown Climate Risks in an AMOC Collapse Scenario. J Ecol & Nat Resour 2024, 8(1): 000364. Copyright© Hald-Mortensen C. known tipping points that require enhanced modelling to ensure better prediction. Moreover, the paper contributes to the growing field of ‘catastrophic climate change’ research, a research field that analyzes impacts and strategies for a 2°C-3°C world [3]. Roughly 12,000 years ago, there was an abrupt decrease in temperatures around Greenland by about -7.7 degrees Celsius or 18 degrees Fahrenheit, and this temperature drop came about over two decades, as the AMOC went from an "on" state to an "off" state. Abrupt climate change is no longer a distant threat; it could arrive our doorstep, sooner than expected, and an AMOC collapse is full of unanticipated effects. Recall that the North Atlantic saw a colder period persisting for 1,600 years, a climate shift that came about literally in two decades, due to the collapse of the AMOC. We recall the dramatization of the Hollywood movie “The Day After Tomorrow” . With the weakest state in 1,600 years, diligent research, projections, and investments are vital to address potential abrupt regional climate shifts affecting agriculture, water cycles, and coastal temperatures [13]. The Ditlevsens formulated their estimates using data spanning the past 150 years, leading to some inherent uncertainty. This uncertainty predominantly arises from the short duration of a key segment of the dataset, commonly referred to as the “fingerprint” of the AMOC. This fingerprint has only been directly monitored for 20 years and may not fully represent the balance between global warming effects and natural fluctuations. The Dansgaard-Oeschger Events 11,500 years ago – An Unknown-Known Relevant Today? The core message of the Ditlevsen study is clear and urgent: the AMOC’s collapse may be imminent, even in the near future. Winter temperatures in Europe could drop by as much as 10 degrees Celcius or 50 degrees Fahrenheit. Such abrupt changes could cause significant temperature fluctuations, with the AMOC already showing weakening signs since 2004 [14]. An unknown-known can in fact give some guidance; the so-called Dansgaard-Oeschger events represent a series of rapid and intense climate fluctuations during the last ice age. Specific to AMOC: Climate effects could lead to secondary collapses or unforeseen societal chain reactions such as the Arab Spring uprising, due to climate impacts on food security due to price spikes and climate induced migration. Extreme temperature risks could challenge predictive models in the financial sector. Table 1: Applied Rumsfeld Uncertainty Matrix to the Potential Collapse of the AMOC. Journal of Ecology and Natural Resources 6 Hald-Mortensen C. Applying the Rumsfeld Matrix: Unknown Unknown Climate Risks in an AMOC Collapse Scenario. J Ecol & Nat Resour 2024, 8(1): 000364. Copyright© Hald-Mortensen C. (Table 1) showcases the Applied Rumsfeld Matrix, providing a structured analysis of the potential risks associated with a potential AMOC collapse. The following can be derived: With AMOC at its weakest in a millennium, abrupt climate change carry with it a new risk landscape, it may bring new unpredictability and new unknowns, as it may lead to a range of major physical impacts, yielding uncertain global implications. Predicting the exact impacts of a collapse of the AMOC is difficult, given their complex and evolving aspects, but with significant progress being made on understanding the AMOC, the research gaps highlighted below in the matrix may soon be closed. In West Africa, a failure of the monsoon - one of the OECD report's tipping points - could lead to prolonged droughts and economic instability that induce migrations as populations seek better opportunities. This may cause competition for resources and lead to social and political instability in the Sahel Region impacted by these migrations, Journal of Ecology and Natural Resources 7 Hald-Mortensen C. Applying the Rumsfeld Matrix: Unknown Unknown Climate Risks in an AMOC Collapse Scenario. J Ecol & Nat Resour 2024, 8(1): 000364. Copyright© Hald-Mortensen C. as observed during events like the Arab Spring (from 2010- 2012.). Given this, it is advised that we consider revising our risk frameworks to account for catastrophic climate events. In essence, our financial models must now incorporate significant outlier risks and the potential knock-on effects, like the potential collapse of the AMOC. In short, we must integrate new “fat tail risks” into financial models. An interdisciplinary research agenda to tackle unknown-unknown climate risks related to AMOC collapse integrates climate science and finance.

Freshwater discharge from ice sheets induces surface atmospheric cooling and subsurface ocean warming, which are associated with negative and positive feedbacks respectively. However, uncertainties persist regarding these feedbacks' relative strength and combined effect. Here we assess associated feedbacks in a coupled ice sheet-climate model, and show that for the Antarctic Ice Sheet the positive feedback dominates in moderate future warming scenarios and in the early stage of ice sheet retreat, but is overwhelmed by the negative feedback in intensive warming scenarios when the West Antarctic Ice Sheet undergoes catastrophic collapse. The Atlantic Meridional Overturning Circulation is affected by freshwater discharge from both the Greenland and the Antarctic ice sheets and, as an interhemispheric teleconnection bridge, exacerbates the opposing ice sheet's retreat via the Bipolar Seesaw. These results highlight the crucial role of ice sheet-climate interactions via freshwater flux in future ice sheet retreat and associated sea-level rise.

August 2025 study concluded that the collapse of AMOC could start as early as the 2060s. Some climate models indicate that the deep convection in Labrador-Irminger In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patt In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patterns in the ocean, in ecosystems, and the atmosphere. Examples of tipping points include thawing permafrost, which will release methane, a powerful greenhouse gas, or melting ice sheets and glaciers reducing Earth's albedo, which would warm the planet faster. Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere. Tipping points are often, but not necessarily, abrupt. For example, with average global warming somewhere between 0.8 °C (1.4 °F) and 3 °C (5.4 °F), the Greenland ice sheet passes a tipping point and melts, but it would take place over millennia. Tipping points are possible at today's global warming of just over 1 °C (1.8 °F) above preindustrial times, and highly probable above 2 °C (3.6 °F) of global warming. It is possible that some tipping points are close to being crossed or have already been crossed, like those of the West Antarctic and Greenland ice sheets, the Amazon rainforest and warm-water coral reefs. A 2022 study published in Science found that exceeding 1.5 °C of global warming could trigger multiple tipping points, including the collapse of major ice sheets, abrupt thawing of permafrost, and coral reef die-off, with potential for cascading system effects. A danger is that if the tipping point in one system is crossed, this could cause a cascade of other tipping points, leading to severe, potentially catastrophic, impacts. Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. For example, ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such…