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The review focused on two major classes of instabilities in the Earth system, caused by changes in ice and oceans, and variability in rain and snowfall.

Past abrupt climate changes provide 'early warning signals' of cascading tipping points

Past tipping points triggered cascades of global climate, ecological and societal change, researchers say.

An international team of natural and social scientists examined abrupt changes over the last 30,000 years.

Their review, published in Nature Geoscience, shows how these tipping points were preceded by "early warning signals" – and similar signs can help inform our response to the current climate crisis.

Tipping points can trigger rapid change, challenging "the capacity of human societies to adapt to environmental pressures", the researchers say.

"For humans, it is crucial to anticipate the future – we need to know what surprises are ahead," said lead author Professor Victor Brovkin, of the Max Planck Institute for Meteorology.

"It sounds counterintuitive, but to foresee the future we may need to look into the past.

"The chance to detect abrupt changes and tipping points – when small changes lead to big impacts – increases with the length of observations.

"This is why analysis of abrupt changes and their cascades recorded in geological archives is of enormous importance.”

The review focused on two major classes of instabilities in the Earth system, caused by changes in ice and oceans, and variability in rain and snowfall.

In particular, it looked into cascading impacts during the onset of Bølling-Allerød warming about 14,700 years ago, and the termination of the African humid period from 6,000 to 5,000 years ago that led to regional changes in ecosystems and pre-historical human societies.

"Earth’s recent past shows us how abrupt changes in the Earth system triggered cascading impacts on ecosystems and human societies, as they struggled to adapt," said Professor Tim Lenton, Director of the Global Systems Institute at the University of Exeter.

"We face the risk of cascading tipping points again now – but this time it is of our own making, and the impacts will be global.

"Faced with that risk, we could do with some early warning signals.

"What examples from the past show is that different climate, ecological, or social systems all become slower at recovering from perturbations before they reach a tipping point – where they fail to recover at all."

Commenting on the concept of early warning signals, Dr Sebastian Bathiany, from the Helmholtz Center Hereon, said: "There are useful statistical indicators that can be interpreted as precursors of abrupt changes.

"Those include so-called slowing down before abrupt changes in oceanic circulation, or increased spatial variance of vegetation cover before the end of African humid period.

"At the same time, one needs to be cautious as some abrupt changes, such as the Black Sea flooding about 9,500 years ago, cannot not be detected with such methods."

Abrupt changes in the Earth system are not limited to one particular domain but can cascade through space and time.

Dr Jonathan Donges, from the Potsdam Institute for Climate Impact Research, said: “Ice-ocean interactions, for example, at onset of the Bølling-Allerød warming during the transition from the last ice age to the current Holocene warm period, lead to cascading impacts in ocean hypoxia (low oxygen), vegetation cover and atmospheric carbon dioxide and methane concentrations.

"These changes could also interact with and amplify each other, and propagate among different spatial scales, to eventually affect human hunter-gatherer societies at that time."

The paper is entitled: "Past abrupt changes, tipping points and cascading impacts in the Earth system."

Tipping points can also refer to positive changes in human societies, and the University of Exeter is working to identify positive tipping points in numerous areas.

Date: 29 July 2021

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