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

Stable now: Unfortunate timing and rate of change may be enough to tip a climate system

Credit: TiPES / HP

Imagine sudden shifts of the tropical monsoons, reductions in precipitation in the northern hemisphere, and strengthening of the North Atlantic storm track within decades. These are some of the effects that climate scientists expect if the Atlantic Meridional Overturning Circulation (AMOC), which redistributes heat from equatorial regions to the northern hemisphere, suddenly collapses into a dormant state as a result of 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 from Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen, Denmark, now proposes AMOC, and potentially other climate subsystems approaching tipping points may tip long before expected due to speed-induced tip. The work, published today in PNAS is part of the TiPES project funded by EU Horizon 2020.

Time matters

There is a growing concern among climate scientists that more climatic subsystems may turn irreversibly and abruptly into a new state if atmospheric CO2Levels are pushed beyond still unknown thresholds. These subsystems include the ice sheets of Antarctica and Greenland, the Amazon rainforest, the Asia-Australian monsoon, the Arctic sea ice, and the AMOC.

In addition, it is still uncertain whether velocity-induced tipping effects can also occur. These effects manifest themselves as a rollover of the system to a new state, even before a theoretical threshold under the external conditions (such as the atmospheric CO2 levels) have been reached. For speed-induced tipping, the rate of change ̵

1; not the magnitude of the change – is the important factor. This is because tipping is easier when system conditions change quite quickly.

To study rate-induced deposition in the climate system, Drs. Johannes Lohmann phenomenon in a complex ocean model, Veros.

Inherently unpredictable

First, the model’s tipping threshold was identified at very slow rises in the North Atlantic freshwater input. Subsequently, a series of experiments were performed in which the supply of fresh water was increased at varying rates, but only to levels below the tipping threshold. The results clearly showed the properties of rate-induced deposition.

Specifically, when the ocean model was exposed to increases in freshwater supply to the North Atlantic, which simulated accelerating melting from the Greenland ice sheet over time scales of 10 to 150 years, the AMOC had a strong tendency to tilt to a dormant state before reaching its threshold.

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

Worrying if that’s right

The occurrence of velocity-induced deposition in a global ocean model provides important evidence that one or more climate subsystems may tip off being pushed too fast as a result of global warming. Whether this is really a reality remains to be seen across multiple models in the climate model hierarchy.

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

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

Setting up an alarm system in the Atlantic

More information:
Johannes Lohmann et al., “Risk of overturning the overturning circulation due to increasing speeds of ice melting,” PNAS (2021). www.pnas.org/cgi/doi/10.1073/pnas.2017989118

Provided by the University of Copenhagen

Citation: Unfortunate timing and rate of change may be enough to tip a climate system (2021, February 22) retrieved February 23, 2021 from https://phys.org/news/2021-02-unfortunate-climate.html

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