What is a ‘black swan’ event and why are they key to the climate crisis?

A report by the Intergovernmental Panel on Climate Change (IPCC) has revealed the physical changes, both underway and looming, that are a result of climate change - from harsher extreme weather to rising sea levels.

The report, compiled by leading scientists and signed off by governments, confirms that the world is unlikely to meet the goals of the Paris Agreement on climate change and that holding global warming to 1.5C above preindustrial times is now "very challenging".

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At the request of governments, it looked at the growing possibility of ‘black swan’ events, such as irreversible melting of major ice sheets that could lead to huge increases in global sea levels.

A black swan event is one that was unprecedented and unexpected at the time it occurred.

"The fact we’re starting to see some of the impacts of climate change... really ought to be a wake-up call for global governments that this isn’t something they can ignore," said Emily Shuckburgh, a University of Cambridge climate scientist.

Where does the term ‘black swan’ come from?

The term black swan originates from the historical European belief that all swans are white because all swans observed and recorded up to a certain point had white feathers – and therefore a swan of any other colour was impossible.

However, in 1697 the Dutch explorer Willem de Vlamingh discovered black swans in Australia, upending the belief and transforming how we understand the natural world.

The term then became a metaphor for the idea that something that was previously thought of as impossible can arise.

And not only is it possible that this belief is proved wrong, it may also be inevitable after evaluating the surrounding context – in this case, the existence of other birds that have different coloured feathers but are the same species.

In other words, a black swan event cannot be predicted but may seem obvious in hindsight.

Black swan events were discussed by the writer and statistician Nassim Nicholas Taleb in his 2001 book about financial events called Fooled By Randomness.

The dotcom crash of 2001 is an example of an unforeseen and unpredictable event with catastrophic results.

In his 2007 book The Black Swan, Taleb goes further and states that almost all major scientific discoveries and historical events are black swans.

He says the rise of the personal computer, the First World War, the collapse of the Soviet Union, and the September 11 attacks are all examples of unforeseen black swans.

Taleb says three criteria make up a black swan: “First, it is an outlier, as it lies outside the realm of regular expectations because nothing in the past can convincingly point to its possibility.

“Second, it carries an extreme ‘impact’. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurrence after the fact, making it explainable and predictable.

He adds: “A small number of black swans explains almost everything in our world, from the success of ideas and religions, to the dynamics of historical events, to elements of our own personal lives.”

What are ‘black swan’ events in the climate crisis?

Natural disasters, such as the recent wildfires and destructive flooding that have swept the globe, are classed by many climate experts as black swans.

For instance, scientists said this month that record-smashing heatwaves in the US Northwest and Canada in late June and early July would have been "virtually impossible without human-caused climate change.”

World Weather Attribution, an international group of leading climate scientists, added: “The observed temperatures were so extreme that they lie far outside the range of historically observed temperatures.

“This makes it hard to quantify with confidence how rare the event was. In the most realistic statistical analysis the event is estimated to be about a 1 in 1,000-year event in today’s climate.”

Scientists are concerned economic models that claim that the costs of preventing climate change do not outweigh the projected damage are ignoring the possibilities of climate ‘black swans’.

"We are now observing climate change with our own eyes in ways we did not broadly before," said Corinne Le Quere, a climate scientist at the University of East Anglia and an author of the IPCC report.