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Imagine sudden tropical monsoon shifts, declines in rainfall in the Northern Hemisphere, and decades of reinforcing of North Atlantic storm tracks. This are some of the effects that climate scientists predict if, as a consequence of global warming, the Atlantic Meridional Overturning Circulation (AMOC), which redistributes heat from equatorial regions to the Northern Hemisphere, abruptly tips into a dormant state. For agriculture, wildlife, and the economy in vast areas of the world, the effects will radically alter circumstances.
A model analysis by Johannes Lohmann and Peter D. Ditlevsen of Ice, Atmosphere, and Earth Sciences, The Niels Bohr Institute, University of Copenhagen, Denmark, now indicates that due to rate-induced tipping, the AMOC and potentially other climate sub-systems approaching tipping points may tip long before expected. The work, published in PNAS, is part of the Horizon 2020 EU-funded TiPES project.

Time matters
There is a growing fear among climate scientists that if atmospheric CO2 levels are forced above yet uncertain thresholds, many climate sub-systems could tip irreversibly and suddenly to a new environment. These sub-systems include the Antarctic and Greenland ice caps, the rainforest of the Amazon, the Asian-Australian monsoon, the Arctic Ocean sea ice, and the AMOC.
Furthermore, whether rate-induced tipping effects may even exist is also unclear. And before a statistical threshold of environmental conditions (such as ambient CO2 levels) is met, these results manifest as a tipping of the system into a new state. In rate-induced tipping, the key factor is the rate of change-not the volume of change. This is because tipping happens more readily as the circumstances of the environment shift very rapidly.

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Dr. Johannes Lohmann studied the phenomenon in a dynamic ocean model, Veros, to study rate-induced tipping in the climatic environment.

Inherently unpredictable
First, the model’s tipping threshold at very slow increases of North Atlantic freshwater input was identified. Then, a series of experiments were carried out, where the freshwater input was increased at varying rates, but only to levels below the tipping threshold. The results clearly showed the characteristics of rate-induced tipping.
Specifically, the AMOC had a clear propensity to tip to a dormant state until its threshold was reached, while the ocean model was exposed to changes in freshwater supply to the North Atlantic that simulated accelerating melt from the Greenland Ice sheet over time scales of 10 to 150 years.

Due to the turbulent dynamics of the ocean model, it also appears that the rate-induced tipping was extremely susceptible to minute changes in the initial conditions and the rise in the rate of meltwater adjustment. This makes the threshold for tipping blurry. Therefore, the qualitative fate of ocean circulation remains fundamentally uncertain, i.e. whether it will crumble or remain like the current state.

Worrying, if real
The occurrence of rate-induced tipping in a global ocean model gives important evidence that one or more climate sub-systems may tip from being pushed too quickly as a result of global warming. Whether this is indeed a reality remains to be shown across more models in the climate model hierarchy.

However, the findings point to fundamental limitations in climate predictability and corroborate the need to limit CO2 emissions in order to stay away from dangerous and unpredictable tipping.

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“It is worrying news. Because if this is true, it reduces our safe operating space,” says Johannes Lohmann.

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