Why climate tipping points may not be tipping points after all

New research suggests environmental damage is reversible if thresholds are crossed only briefly

Greenland ice sheet disintegration – Rising temperatures causing the retreat of the ice sheet, which would cause sea levels to rise.

Amazon rainforest dieback – Large-scale dieback of the forest, via increased temperatures and drying, or direct deforestation, would amplify global warming.

Permafrost loss – Thawing of carbon-rich soils, which releases greenhouse gas into the atmosphere.

Atlantic meridional overturning circulation breakdown – An increased amount of freshwater in the Northern Atlantic disrupting the system of currents.

Boreal forest shift – Warming causes dieback in the south of the forests, and expansion into the tundra in the north, which would cause regional warming.

West Antarctic ice sheet disintegration – The melting of major ice sheets would lead to significant increases in sea level.

West African monsoon shift – A change in the monsoon season would lead to agricultural disruption and effect the ecosystem.

Indian monsoon shift – An increase in the planetary albedo (increases in the atmospheric brown cloud haze over India) has the capability of switching off the monsoon, which is crucial for the local economy, as well as being important for agriculture.

Coral reef die-off – Exposure to increased sea temperatures can kill off reefs, which has a serious effect on ecosystems and local economies.

The disastrous consequences of climate tipping points could be averted if global warming were reversed quickly enough, new research suggests.

Crossing these limits was considered a point of no return, but a University of Exeter study concludes that thresholds could be "temporarily exceeded" without causing permanent shifts.

Tipping points are defined as small changes within a complex system causing disproportionately large changes to the system. They can be compared to the game of Jenga where bricks are removed from a tower one by one until the 'tipping brick' causes the whole edifice to crumble.

Climate tipping points are a little more nuanced and can be grouped into fast-onset and slow-onset categories.

The study focused on four examples: Amazon rainforest dieback – fast onset; Indian summer monsoon – fast onset; Atlantic Meridional Overturning Circulation – slow onset; and ice sheet melt – slow onset.

For the study, the researchers calculated "time to act"  as the time taken to reverse warming and stabilise at 1.5°C above pre-industrial levels.

The window available to recover the damage spawned by reaching these tipping points depended on whether they were fast or slow onset and the level of global warming at the time.

"The more extreme the warming, the less time we would have to prevent tipping points," said lead author Dr Paul Ritchie, of the university's Global Systems Institute and the Department of Mathematics.

"This is especially true for fast-onset tipping points like Amazon forest dieback and disruption to monsoons, where irreversible change could take place in a matter of decades.

"Slow-onset tipping points take place over a timescale of many centuries and, depending on the level of warming, this would give us more time to act."

Climate tipping points and thresholds 

Dr Ritchie told The National that it was "important to highlight the difference between tipping points and tipping point thresholds" which are "frequently overlooked in the literature".

"The tipping point threshold is the critical level of forcing (i.e. global mean warming) at which tipping would first be triggered if the forcing stayed constant at that level for a sufficiently long period of time," he said.

"The tipping point itself is the point at which we actually observe the transition. So if the forcing were to continue to rise after crossing the threshold, then the tipping point would be later than the threshold.

Need to act on climate change still urgent

Even if there is a little more leeway on some climate tipping points than others, Dr Ritchie said it shouldn't lead to complacency among global policymakers.

"Action still needs to be taken now to reduce global warming because ultimately the safest thing to do is not to cross these thresholds," he said.

"However, if we are to cross a threshold then the key message is to not give up, there is still the hope that we can avoid the disastrous consequences of a climate tipping point if global warming can be reversed quickly."

The four climate tipping points in the study

1. Amazon rainforest dieback - fast onset

The Amazon rainforest is one of the largest natural carbon sinks. Large-scale dieback of the forest, via increased temperatures and drying, or direct deforestation, would further amplify global warming.

2. Indian summer monsoon - fast onset

The Indian monsoon is crucial for the local economy, as well as being important for agriculture, river systems and water stock. However, an increase in the planetary albedo (increases in the atmospheric brown cloud haze over India) has the capability of switching off the monsoon.

3. Atlantic Meridional Overturning Circulation – slow onset

The Amoc is a large-scale circulation current in the Atlantic Ocean, which is responsible for giving western Europe its relatively mild climate when compared to other locations at a similar latitude (such as Siberia). Adding freshwater to the North Atlantic could disrupt the downwelling that occurs in this region leading to a collapse of the large-scale current.

4. Ice sheet melt – slow onset

The melting of major ice sheets would lead to significant increases in sea level. Under increased warming ice starts to melt, which results in less incoming radiation being reflected and more being absorbed. This in turn leads to hotter temperatures and more ice melting, possibly generating a feedback loop to an ice-free state – but crucially over many centuries.

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