Unfortunate timing and pace of change may be enough to tip a climate system

Imagine sudden shifts of the tropical monsoons, the decline in rainfall in the Northern Hemisphere and the strengthening of the North Atlantic storm tracks within decades. These are some of the effects that climate scientists expect as the Atlantic Meridional Overturning Circulation (AMOC), which redistributes heat from equatorial regions to the Northern Hemisphere, suddenly turns into a dormant state due to global warming. The consequences would drastically change the conditions for agriculture, biodiversity and the economy in large parts of the world.

A model study by Johannes Lohmann and Peter D. Ditlevsen of Physics of Ice, Climate, and Earth, The Niels Bohr Institute, University of Copenhagen, Denmark, now suggests that the AMOC, and possibly other climatic systems approaching tipping points . long before expected due to rate-induced seesaw. The work, which is in PNAS is part of the TiPES project funded by the EU Horizon 2020.

Time matters

Climate scientists are growing that various climate subsystems can irreversibly and suddenly tend to a new state as atmospheric CO₂Levels are pushed past still unknown thresholds. These subsystems include the Antarctic and Greenland ice sheets, the Amazon rainforest, the Asian-Australian monsoon, the Arctic Ocean sea ice, and the AMOC.

In addition, it is still uncertain whether rate-induced tilting effects can also occur. These effects manifest themselves as a tilting of the system to a new state, even before a theoretical threshold in the external conditions (such as the atmospheric CO₂ levels) are reached. In rate-induced tilting, the rate of change – not the amount of change – is the most important factor. This is because tilting occurs more easily when the conditions of the system change fairly quickly.

Dr. Johannes Lohmann investigated the phenomenon in an intricate ocean model, Veros, to study rate-induced tilting in the climate system.

Inherently unpredictable

First, the tipping threshold of the model was identified against very slow increases in North Atlantic freshwater inputs. Subsequently, a series of experiments were performed, where the fresh water supply was increased at varying doses, but only to below the tilt threshold. The results clearly showed the characteristics of rate-induced bounce.

Specifically, when the ocean model was subject to the increase in freshwater supply to the North Atlantic Ocean, which simulated the accelerating melting of the Greenland ice sheet to a time scale of 10 to 150 years, the AMOC had a strong tendency to a dormant state to tilt threshold has been reached.

It also appears that, due to the chaotic dynamics of the ocean model, the rate-induced tilt was highly sensitive to small changes in the initial conditions and the rate of change in the meltwater. This blurs the tilt threshold. Therefore, the qualitative fate of the ocean circulation, i.e. whether it collapses or, like the modern state, remains inherently unpredictable.

Worrying, if it really is

The occurrence of rate-induced tilting in a global ocean model provides important evidence that one or more climate subsystems may be pushing too fast due to global warming. Whether this is indeed a reality needs to be shown across more models in the climate model hierarchy.

However, the findings point to fundamental limitations in the predictability of climate and confirm the need to limit CO₂ emissions to stay away from dangerous and unpredictable bounces.

“This is worrying news. If it’s true, it reduces our safe operating space,” says Johannes Lohmann.