The unfortunate timing and rate of change may be enough to tilt a climate system

Credit: TiPES / HP

Imagine abrupt swings in tropical monsoons, reductions in Northern Hemisphere rainfall, and the strengthening of North Atlantic storm tracks over decades. These are some of the impacts that climate scientists expect if the Atlantic Southern Reversal Circulation (AMOC), which redistributes heat from the equatorial regions to the Northern Hemisphere, suddenly enters a dormant state as a result of global warming. The consequences would drastically alter the conditions of agriculture, biodiversity and the economy in much of the world.

A model study by Johannes Lohmann and Peter D. Ditlevsen of Physics of Ice, Climate, and Earth, The Niels Bohr Institute, the University of Copenhagen, Denmark, now suggests that the AMOC, and potentially other climate subsystems approaching points turning point, they could tip much earlier than expected due to fare-induced tips. The work, published today in PNAS It is part of the TiPES project funded by the EU Horizon 2020.

Time matters

There is growing concern among climate scientists that various climate subsystems could shift irreversibly and abruptly to a new state if atmospheric COtwo-Levels are pushed beyond as yet unknown thresholds. These subsystems include the Antarctic and Greenland ice sheets, the Amazon rainforest, the Australian and Asian monsoons, the Arctic Ocean sea ice, and the AMOC.

Furthermore, it is still uncertain whether fare-induced rollover effects could also occur. These effects manifest as a reversal of the system to a new state even before a theoretical threshold in external conditions (such as atmospheric COtwo levels) is reached. In rate-induced tipping, the rate of change, not the amount of change, is the important factor. This is because rollover occurs more easily when system conditions change fairly quickly.

To study rate-induced tilt in the climate system, Dr. Johannes Lohmann investigated the phenomenon in a complex oceanic model, Veros.

Inherently unpredictable

First, the tipping threshold of the model was identified at very slow increments of freshwater inflow from the North Atlantic. Then, a series of experiments were carried out, in which freshwater inflow increased at variable rates, but only at levels below the tipping threshold. The results clearly showed the characteristics of the fare-induced tip.

Specifically, when the ocean model was subjected to increases in freshwater inflow to the North Atlantic, which simulated accelerated melting of the Greenland ice sheet on timescales of 10 to 150 years, the AMOC had a strong tendency to tilt. to an inactive state before its threshold was reached.

It also appeared that due to the chaotic dynamics of the ocean model, the rate-induced capsize was very sensitive to minimal changes in initial conditions and the rate of change of the meltwater rise. This makes the rollover threshold blurry. Therefore, the qualitative fate of ocean circulation, that is, whether it will collapse or remain as the modern state, remains inherently unpredictable.

Worrying, if it’s real

The occurrence of a rate-induced capsize in a global ocean model provides important evidence that one or more climate subsystems may be tilted by being pushed too fast as a result of global warming. It remains to be demonstrated whether this is actually a reality in more models in the climate model hierarchy.

However, the findings point to fundamental limitations in climate predictability and corroborate the need to limit COtwo emissions to avoid dangerous and unpredictable spills.

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

Set up an alarm system in the Atlantic Ocean

More information:
Johannes Lohmann el al., “Risk of overturning circulation due to increased rates of ice melt”, PNAS (2021).

Provided by the University of Copenhagen

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