Earth at risk of entering ‘hothouse’ state from which there is no return, scientists warn

'In the context of the summer of 2018, this is definitely not a case of crying wolf... the wolves are now in sight'

In a summer marked by global heatwaves, wildfires and drought, scientists have warned that things could get considerably worse under a future scenario dubbed “hothouse Earth”.

Even if greenhouse gas emissions are reduced, there is a chance human-induced global warming could trigger other processes which will lead to uncontrollable warming, the team at the Stockholm Resilience Centre said.

As Amazon rainforest is destroyed, Arctic permafrost thaws and Antarctic sea ice melts, these natural feedback mechanisms that currently help store Earth’s carbon will instead begin emitting it, scientists at the Swedish institute warned.

While it is unclear how likely this scenario is, experts agree that were it to happen the runaway warming after this tipping point would be an existential threat to humanity.

“These tipping elements can potentially act like a row of dominoes,” said Professor Johan Rockstrom, Executive Director of the Stockholm Resilience Centre. “Once one is pushed over, it pushes Earth towards another. It may be very difficult or impossible to stop the whole row of dominoes from tumbling over.”

The prospect of such a situation has been laid out by Professor Rockstrom and his colleagues in the journal Proceedings of the National Academy of Sciences (PNAS).

“We address tipping elements in the planetary machinery that might, once a certain stress level has been passed, one by one change fundamentally, rapidly, and perhaps irreversibly,” explained Professor Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research.

“This cascade of events may tip the entire Earth system into a new mode of operation.”

Global average temperatures are currently 1C above pre-industrial levels and under the Paris climate agreement world governments have agreed to keep total warming below 2C.

In the worst-case scenario, the researchers predict the Earth’s climate would stabilise at around 4-5C higher – hotter than any point for 1.2 million years – and with sea level increase of up to 60m.

“Places on Earth will become uninhabitable if ‘hothouse Earth’ becomes the reality,” Professor Rockstrom said.

Other scientists acknowledged the situation laid out in the new PNAS paper as uncertain, as it is somewhat speculative and not covered by most existing climate change predictions, but they nonetheless admitted it was plausible.

“In the context of the summer of 2018, this is definitely not a case of crying wolf, raising a false alarm: the wolves are now in sight,” said Dr Phil Williamson, a climate researcher at the University of East Anglia who was not involved in the work.

Pointing out that evidence from geology shows Earth’s climate system is “inherently nervy”, he said human processes to the mix could well exacerbate this.

To avoid catastrophe, the researchers behind the new work said there was a need to move “from exploitation to stewardship” of the Earth, and not only reduce emissions but create new carbon stores – by planting forests, conserving biodiversity and creating technologies to remove carbon dioxide form the air.

However, they noted that while their runaway threshold may well be within the Paris target of 2C, this could be a point beyond which the risk of hothouse Earth will increase sharply.

As for whether staying below this target and maintaining a “stabilised Earth” is possible, climate scientists Professor Chris Rapley of University College London, who was also not involved in the study, did not have much hope.

He said in the face of “right wing populism” and climate change denial, drastically tackling the problem in the ways described seemed highly unlikely.

“The future habitability of the planet thus appears to rest on chance,” he said. “That the sensitivity of the climate system to greenhouse gas emissions and other human disruptions is fortuitously very low – or that some other global scale social calamity dramatically reduces human emissions before any runaway planetary threshold is breached. The latter offers cold comfort.”