Consequences of the Collapse of the Atlantic Meridional Overturning Circulation for Europe's Forests Would be More Severe Than Those of a ‘Normal’ Climate Change

Abstract

In the blockbuster film ‘The Day After Tomorrow’ (2004), a fictitious collapse of the Atlantic Meridional Overturning Circulation (AMOC; often colloquially referred to as the ‘disruption of the Gulf Stream’), triggered by climate change, caused an abrupt drop in air temperature followed by the start of a new ice age. Director R. Emmerich took for his film the much-discussed theory of the collapse of the AMOC (e.g., Rahmstorf and Ganopolski 1999) that recently gained new attention (van Westen et al. 2024). Then and now, the most pressing question is, whether an abrupt decline of AMOC is possible. According to IPCC (2023), a shutdown of the AMOC must be seen as a real possibility and a consequence of climate change. However, the AMOC has been stable for the last 8000 years, and while there is high confidence in its 21st-century decline, the confidence is still medium that there will not be an abrupt collapse before 2100. Only few studies have attempted to estimate the climatic consequences of an AMOC collapse for Europe in a geographically differentiated way (Kuhlbrodt et al. 2009; Jackson et al. 2015; Liu et al. 2017). The study with the highest spatial resolution was conducted by British climate researcher L. Jackson and colleagues, who used complex climate assumptions to create monthly temperature and precipitation maps (Jackson et al. 2015). Heubel et al. 2025 now made use of these maps to create model scenarios for the AMOC collapse by 2071–2100, which roughly correspond to the date of 2060 for a recently anticipated tipping point (Ditlevsen and Ditlevsen 2023). In this scenario, the climate would become significantly cooler throughout Europe, in summer by about 2°C–4°C and in winter by 2°C–8°C. Precipitation would generally decrease, with substantial seasonal variations: in the Mediterranean, the summer months June, July, August would turn slightly wetter, while large parts of Eastern Europe would become drier. Winter precipitation in the months December, January and February would fall more often as snow, resulting in 2–3 months of snow cover in western France and 6–8 months in Scandinavia.

While several spatial projections have been published on the effects of climate change on future climatic suitabilities of the principal European tree species under the assumption of an intact AMOC (e.g., Buras and Menzel 2019), comparable estimates of the impact of an inactive AMOC are lacking. Sina Heubel, Anja Rammig and Allan Buras (Heubel et al. 2025) have now ventured to fill this knowledge gap. In their unique modelling study, they compare the change in the potential occurrence of 24 European tree species under a future climate assuming an active or inactive AMOC, while looking closer at the four main tree species in Europe. In total, the authors calculated three AMOC-collapse scenarios based on the SSP1-2.6, SSP2-4.5 and SSP5-8.5 projections for the period 2071–2100 using the scenario by Jackson et al. (2015) described above. The paper focuses on the SSP2-4.5 scenario, but the others can be visualised with a shiny app that comes with the publication (http://app.forestmonitoringhub.eu/). Based on the climate envelope modelling of climatic habitat suitability, the paper aims to answer four questions.

For Question 1—‘How are the simulated occurrence probability and spatial distribution of Europe's currently most abundant tree species affected?’—the distributions of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) H. Karst.), European beech (Fagus sylvatica L.) and common oak (Quercus robur L.) were projected, and changes compared to the present were analysed, assuming AMOC persisted or not. The study shows that unabated global warming for the whole of Europe with an intact AMOC would fundamentally reduce the occurrence of these tree species across Europe and lead to northward shifts (Figure 1). Also, under an inactive AMOC, the probability of occurrence for all four tree species would decrease but with more varied responses. Mainly, spruce and beech would be severely affected, with the former becoming restricted to the Alps and the latter to parts of Ireland, Great Britain, Northern Spain and the Baltic states. For Question 2—‘How is the current appearance of Europe's forests—represented by locally dominant species—projected to change?’ the authors provide the following answer: With an inactive AMOC, Europe's forests will likely experience a decline in historically dominant tree species across most regions, causing local extinctions in Northern Scandinavia and eastern Baltic areas. In contrast, an active AMOC would increase the extinction of locally abundant species mostly in Southwestern Europe. With an active AMOC, several other tree species would replace the climate-limited principal tree species, leaving only some parts of Southern Spain treeless. By contrast, an inactive AMOC resulted in drastic consequences for the occurrence of tree species, turning large areas of Scandinavia into treeless terrain and causing the extinction of today's common tree species in Eastern Europe.

Question 3 raised in the paper was about which species might replace locally extinct species in the future. Here, the authors found that under an active AMOC, the substituting species mainly consisted of Mediterranean, drought-tolerant species, while the discontinuation of AMOC would lead to a higher abundance of cold-tolerant, more boreal species.

Lastly, the authors addressed their question 4, how, in the different scenarios, the potential tree species diversity in European regions would be affected. Overall, they found tree species diversity to decline independent of AMOC. Still, while with an active AMOC, biodiversity would increase in Northern Europe, it would be a coldspot of biodiversity with absent AMOC, and the opposite pattern is seen for the Iberian peninsula.

The study by Heubel et al. (2025) enables a discussion about which variant of unabated climate change would be worse for Europe. Increasing evidence for the AMOC being en route to a tipping point has been published (Ditlevsen and Ditlevsen 2023; van Westen et al. 2024). For the first time, it becomes clear that an AMOC collapse would have a greater impact on forest cover in Europe than under the previous assumptions of the ‘usual’ climate change scenarios ignoring the AMOC state and assuming generally increasing temperatures. As if it were not already sufficiently worrying that the current forest types would change regionally and their areas would shift, further consequences of this altered tree species suitability can be derived.

Climatic tipping points such as the AMOC collapse bring further uncertainty into forest management planning, climate-smart forestry activities, assisted migration and sustainable maintenance of ecosystem services. Forests have very long rotation times; the generation times of trees impede evolutionary adaptation (Alberto et al. 2013), and migration speed is too low to track climate change already in a continually changing climate. Even when not accounting for any AMOC change, this will already lead to substantial tree species bottlenecks for forest management in large areas of Europe. The reason for this is that trees that are planted or promoted now have to cope with the current climate and the climate when they are fully grown. An AMOC collapse would jeopardise even the slightest remnant of predictability for forest management. On top of this uncertainty, the collapse of the AMOC would probably bring timber production in Northern Scandinavia to a standstill.

The Northern Hemisphere will not freeze over in the event of a collapsing AMOC, as staged in the disaster movie mentioned at the beginning. However, the consequences of such an event for forest vegetation would be significantly worse than under ‘usual’ climate change effect projections by the year 2100. Heubel et al. (2025) therefore conclude to ‘mitigate the realisation of such a scenario’. Their study is beyond alarmism but a careful, long-overdue forecast of the consequences of an AMOC collapse, which is not a completely unrealistic scenario.

Thomas Wohlgemuth: conceptualization, writing – original draft, writing – review and editing. Arthur Gessler: conceptualization, writing – review and editing.

The authors declare no conflicts of interest.

This article is a Commentary regarding (Heubel et al, https://doi.org/10.1111/gcb.70185).