Substantial Risk of 21st Century AMOC Tipping even under Moderate Climate Change (2407.19909v1)

Abstract: The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate system and considered to be a tipping element. There is still a large uncertainty on the critical global warming level at which the AMOC will start to collapse. Here we analyse targeted climate model simulations, together with observations, reanalysis products and a suite of state-of-the-art climate model results to reassess this critical global warming level. We find a critical threshold of +3C global mean surface temperature increase compared to pre-industrial with a lower bound of +2.2C (10%-Cl). Such global mean surface temperature anomalies are expected to be reached after 2050. This means that the AMOC is more likely than not (> 50%) to tip within the 21st century under a middle-of-the-road climate change scenario and very likely (> 90%) to tip under a high emissions scenario. The AMOC collapse induced cooling is shown to be offset by the regional warming over Northwestern Europe during the 21st century, but will still induce severe impacts on society.

• The paper finds a substantial risk of Atlantic Meridional Overturning Circulation (AMOC) tipping within the 21st century, predicting a critical collapse threshold around +3 C global warming.
• Researchers integrated climate models, observations, and reanalysis to determine that AMOC collapse is likely ( 50%) this century under moderate emissions scenarios.
• AMOC collapse carries profound implications for regional climates, particularly in Northwestern Europe, though potential local cooling may not fully offset broader climate change impacts.

Substantial Risk of 21st Century AMOC Tipping under Moderate Climate Change

The paper addresses the critical question of the potential tipping of the Atlantic Meridional Overturning Circulation (AMOC) under various climate change scenarios in the 21st century. The AMOC is a crucial component of the Earth's climate system, often regarded as a tipping element due to its significant impact on the global climate, particularly concerning its heat and salt transport mechanisms.

Key Findings

The paper integrates a diverse array of tools, including targeted climate model simulations, observational data, reanalysis products, and results from state-of-the-art climate models. The central finding suggests that the AMOC has a critical collapse threshold at a global mean surface temperature increase of +3°C compared to pre-industrial levels, with a lower bound at +2.2°C (10%-Cl). This scenario is anticipated to be reached post-2050, implying that an AMOC tipping is more than likely (≥50%) within the 21st century under moderate climate change scenarios and very likely (≥90%) under high emissions scenarios.

Methodology

The researchers employed the Community Earth System Model (CESM) under quasi-equilibrium hosing scenarios to simulate freshwater flux impacts on the AMOC. By combining these simulations with Coupled Model Intercomparison Project (CMIP6) outputs and observational data, the team assessed the likelihood of AMOC collapse. The analysis indicates a significant weakening of the AMOC by 5-9 Sverdrups under RCP4.5 and RCP8.5 scenarios, accompanied by a decline in AMOC’s stabilizing freshwater transport at 34°S (F_{ovS}).

Implications and Impact

The implications of an AMOC collapse are profound, impacting regional climates, particularly in Northwestern Europe, and potentially attenuating regional warming trends due to AMOC-induced cooling. However, such cooling may not mitigate the severe societal impacts of climate change fully. The AMOC dynamics hinge on surface buoyancy changes over its isopycnal outcropping regions, driven largely by heat flux variations under climate change scenarios, contrasting with the salinity-driven destabilization observed in hosing simulations.

Future Directions

The paper emphasizes further exploration into AMOC dynamics using CMIP phase 7 models to better capture long-term effects beyond 2100. Given the AMOC’s sensitivity to both freshwater and heat fluxes, future studies should prioritize reducing biases in model projections of these parameters, especially under transient climate conditions.

Conclusion

This research highlights a substantial risk of AMOC tipping within the current century under moderate climate warming, necessitating urgent climate action to avert or delay such outcomes. It underscores the need for precise modeling and monitoring of AMOC-relevant processes to ensure robust predictions and effective climate mitigation strategies. The challenge remains to reconcile model predictions with observational constraints to refine estimates of critical temperature thresholds for AMOC stability.