The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in regulating global and especially European climate, as it efficiently transports heat from the Southern Hemisphere to the Northern Hemisphere. New simulations using the model Community Earth System Model (CESM) suggests that a potential AMOC tipping point could cause cooling Europe by several degreesHowever, it is important to note that global warming is likely to limit this AMOC-induced cooling response. Future temperatures in Europe are therefore dependent on both the strength of the AMOC and the emissions scenario.

Research quantifies European temperature responses under different AMOC regimes and climate change scenarios. If the AMOC collapses completely without the impact of global warming (under pre-industrial conditions), winter temperature extremes in northwestern Europe become more intense. Temperatures can drop below -50°C in Scandinavia, and for a place like De Bilt in the Netherlands, extreme minimum temperatures that occur once every 10 years (1:10-year) could range from -34.3°C to -37.6°C. These more intense cold extremes are related to the presence of sea ice near northwestern Europe. Cold extremes in De Bilt are often associated with northwesterly winds in this scenario, as opposed to easterly winds in the steady state AMOC.

If we consider significantly reduced AMOC state under conditions of moderate global warming (≤+2°C, RCP4.5 scenario), sea ice retreats further north and temperature impacts are smaller but still significant. Despite warming, the 1:10-year daily minimum extreme at De Bilt could still reach around -20.0°C.

The key factor is extent of sea ice in the North Atlantic, which strongly controls the intensity of European cold extremes. Sea ice significantly increases the albedo of the relatively dark ocean surface, contributing to the cooling caused by reduced ocean heat transport. The largest temperature responses are observed during the winter months, and these responses are strongly influenced by the extent of sea ice.

In addition to temperature changes, it is expected that Winter storms will intensify and will lead to significantly larger diurnal temperature variations under a significantly weaker AMOC. The larger meridional temperature gradient in the lower atmosphere under a collapsed AMOC leads to a strengthening of the jet stream and an increase in baroclinic instabilities, resulting in greater storm activity. For example, in De Bilt, diurnal temperature variations can increase by up to 14 times in an AMOC collapse scenario. It is important to note that the impacts of the AMOC are primarily felt in winter, while warm extremes are only marginally affected by the AMOC collapse and rather increase due to global warming.

Although the 2021 IPCC report assessed the risk of AMOC collapse due to climate change as low, there is growing evidence that the AMOC is more sensitive than previously thought. Discussions about whether the AMOC is approaching its tipping point remain a matter of debate. The findings presented here provide a quantitative assessment of steady-state European temperatures after the AMOC has stabilized at a new state, conditions that are expected to persist beyond 2200. Spring

Glossary of key terms

• Atlantic Meridional Overturning Circulation (AMOC): A large system of ocean currents in the Atlantic Ocean that transports heat from the tropics to the North Atlantic. It is crucial for regulating climate, especially in Europe.
• AMOC collapse: A hypothetical scenario in which the AMOC significantly weakens or stops, which could lead to widespread climate change.
• Community Earth System Model (CESM): A fully coupled global climate model used to simulate and predict climate change. It includes interactions between the atmosphere, ocean, sea ice, and land.
• Representative Concentration Pathways (RCP): These are greenhouse gas emission scenarios adopted by the Intergovernmental Panel on Climate Change (IPCC) that describe different possible future concentrations of greenhouse gases. The study uses RCP4.5 (moderate warming) and RCP8.5 (high emissions).
• Pre-industrial (PI) conditions: Climatic conditions that existed before large-scale anthropogenic influences, typically before 1850. They are used as a reference point for comparison.
• Hysteresis experiments: Climate model experiments where parameters (e.g. freshwater flux) are gradually varied up and down to explore multiple system stabilities and potential threshold points.
• Sea ice: Ice, which forms when seawater freezes and floats on the ocean surface, is a key factor affecting albedo and heat transfer.
• Albedo: The reflectivity of a surface. Surfaces with high albedo (e.g. sea ice) reflect more sunlight, while surfaces with low albedo (e.g. open ocean) absorb more heat.
• Tipping point: A critical threshold beyond which a small change leads to dramatic and often irreversible changes in the system.
• Meridional temperature gradient: The difference in temperature between the equator and the poles. Significant changes in this gradient can affect atmospheric circulation, such as jet streams.
• Jet stream: A fast, narrow, air current that is found high in the Earth's atmosphere and influences weather patterns.
• Storm track activity: The intensity and frequency of pressure systems and associated storms along specific paths. It is influenced by baroclinic instabilities in the atmosphere.
• Daily temperature fluctuations: Short-term, daily temperature changes, often caused by the passage of atmospheric systems such as storms.
• Generalized Extreme Value Distribution (GEV): A statistical method used to model the behavior of extreme events (e.g., the coldest or warmest days of the year).
• Freshwater flow strength (F_H): A constant force that is applied to simulate a decrease in AMOC strength. A larger F_H makes the AMOC more prone to transitioning to a weaker or collapsing state.
• Equilibration: A state in which a modeled climate system reaches a steady state, where its statistical properties (e.g., temperature, flow) do not change significantly over time.
• Winter months: In the study, they are specified as December, January, and February.