Runaway global warming could be just decades away, say scientists

Feedback mechanisms acting “like a row of dominoes” will then spin the world into a “Hothouse Earth” state of uncontrollable climate change.

Limiting greenhouse gas emissions may not be enough on its own to control global warming, the study suggests. (John Giles/PA)

Long term, the Hothouse Earth climate will stabilise at a global average of 4C-5C above pre-industrial levels, the study shows.

If that happened, swathes of the planet around the equator will become uninhabitable, with sea levels up to 60 metres (197ft) higher than they are today threatening coastal cities.

A Hothouse Earth would pose “severe risks for health, economies, political stability, and ultimately, the habitability of the planet for humans”, the international scientists wrote in the journal Proceedings of the National Academy of Sciences.

The research highlighted 10 feedback processes that were predicted to kick in at around 2C of global warming.

The “tipping elements” could turn natural carbon storage systems or “sinks” into powerful greenhouse gas emitters.

Professor Johan Rockstrom, a leading member of the team from the University of Stockholm, Sweden, said: “These tipping elements can potentially act like a row of dominoes. 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.

“Places on Earth will become uninhabitable if Hothouse Earth becomes the reality.”

The tipping point dangers were identified as thawing permafrost, the release of methane trapped on the ocean floor, weakening land and ocean carbon sinks, increased carbon dioxide production by ocean bacteria, Amazon rainforest die-back, coniferous forest die-back, reduced northern hemisphere snow cover, loss of Arctic summer sea ice, reduced Antarctic sea ice and melting polar ice sheets.

Scientists have warned of catastrophic climate change (Julien Behal/PA)

The scientists wrote: “Our analysis suggests that the Earth system may be approaching a planetary threshold that could lock in a continuing rapid pathway toward much hotter conditions – Hothouse Earth. This pathway would be propelled by strong, intrinsic, biogeophysical feedbacks difficult to influence by human actions, a pathway that could not be reversed, steered or substantially slowed.

“Where such a threshold might be is uncertain, but it could be only decades ahead at a temperature rise of (around) 2C above pre-industrial.”

Avoiding a Hothouse Earth would require “deep cuts” in greenhouse gas emissions as well as concerted efforts to remove carbon dioxide from the atmosphere, both by preserving natural carbon sinks and using technology, said the researchers.

Commenting on the findings, climate researcher Dr Phil Williamson, from the University of East Anglia, said: “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.”

Chris Rapley, Professor of Climate Science at University College London said: “Previous research has shown that an increase in the mean global temperature of 11-12C would make more than half of the land area currently occupied by humans uninhabitable. So, a ‘runaway’ warming to a new and uncontrollable hot state would represent an existential threat to humanity and the majority of existing species.”