Modeling a Control Strategy to Reverse Sea-Ice Loss Near its Tipping Point

The Science

Many of the complex processes that make up Earth’s climate system, such as the formation and persistence of Arctic and Antarctic sea-ice, may be poised to undergo sudden irreversible changes or ‘tipping’ as critical warming thresholds are reached. These are known as tipping elements. This study sought to address the question of controllability and stabilization of polar sea-ice near its critical threshold using a simplified, process-specific (i.e. idealized) climate model. The results reveal that polar sea-ice loss can be reversed in the vicinity of its tipping threshold, with the most important takeaway being that preemptive control measures to prevent tipping are far less intrusive and costly compared to the restoration of sea-ice after the tipping threshold has been crossed.

The Impact

This work developed an optimal control strategy for stabilizing, and even reversing, a sea-ice tipping point in an idealized climate model. It emphasizes that preventive measures are less costly and less intrusive than post-tipping corrective interventions. Overshoots past the tipping threshold allow for a finite intervention window where the cost of requisite control scales linearly with delay, but past which there is a steep rise in costs. Despite being highly idealized, the proposed strategy can be adapted to more realistic models and applied to other tipping elements, like Atlantic meridional overturning circulation and the West Antarctic Ice Sheet.

Summary

Several Earth system components are at a high risk of undergoing rapid, irreversible qualitative changes or ‘tipping’ with increasing climate warming. It is necessary to investigate the feasibility of arresting, or even reversing, any crossing of these tipping thresholds. This study investigated feedback control of an idealized energy balance model for Earth’s climate. It exhibited a small icecap instability responsible for a rapid transition to an ice-free climate under increasing greenhouse gas forcing. Researchers developed an optimal control strategy for the energy balance model under different forcing scenarios to reverse sea-ice loss while minimizing costs. Control is achievable for this system, but the cost nearly quadruples once the system tips. While thermal inertia may delay tipping, leading to an overshoot of the critical forcing threshold, this leeway comes with a steep rise in requisite control once tipping occurs. Additionally, the optimal controls are localized in the polar regions.

PNNL Contacts

Funding

This study was supported by the Regional and Global Model Analysis component of the Earth and Environmental System Modeling program of the Department of Energy's Office of Science Biological and Environmental Research program, as a contribution to the High-Latitude Application and Testing of Earth System Models project. This research used resources of the National Energy Research Scientific Computing Center, a Department of Energy Office of Science user facility located at Lawrence Berkeley National Laboratory.