The world is approaching a “point of no return” in the climate crisis, beyond which runaway global heating may become unstoppable, scientists have warned. Continued warming could trigger a cascade of climate tipping points, locking the planet into a “hothouse Earth” scenario far exceeding current projections of 2-3°C of warming, and fundamentally different from the stable conditions that have characterized the last 11,000 years of human civilization.
The assessment, published in the journal One Earth, synthesizes recent scientific findings on climate feedback loops and 16 key Earth systems, including ice sheets, oceans, forests, and permafrost. Researchers found that several of these systems may be closer to destabilizing than previously believed, raising the specter of irreversible changes even if greenhouse gas emissions are eventually reduced.
Already, the planet has warmed by 1.3°C, resulting in increasingly frequent and intense extreme weather events globally. Scientists warn that a warming of 3-4°C would likely lead to the collapse of economic and social systems as they are currently understood. However, a “hothouse Earth” scenario would represent a far more severe outcome.
“Crossing even some of the thresholds could commit the planet to a hothouse trajectory,” said Dr. Christopher Wolf, a scientist at Terrestrial Ecosystems Research Associates in the US, and a member of the study team. “Policymakers and the public remain largely unaware of the risks posed by what would effectively be a point-of-no-return transition.”
The study highlights the potential for self-reinforcing feedback loops. For example, the death of large portions of the Amazon rainforest and tropical coral reefs would reduce the planet’s capacity to absorb carbon dioxide, accelerating warming. Similarly, thawing permafrost releases potent greenhouse gases, further exacerbating the problem. The Atlantic Meridional Overturning Circulation (Amoc), a crucial system of ocean currents, is also identified as a key tipping element, with potential for significant disruption to global climate patterns.
Tipping points may already have been crossed in Greenland and West Antarctica, with the Amazon rainforest and mountain glaciers appearing to be on the verge of collapse. The Amoc is also showing signs of weakening, potentially increasing the risk of Amazon dieback, according to researchers.
“Research shows that several Earth system components may be closer to destabilizing than once believed,” the scientists concluded. “While the exact risk is uncertain, current climate action commitments are insufficient.”
The prospect of a “hothouse Earth” was previously outlined in 2018, with scientists warning of a scenario where global temperatures remain significantly above 4°C of warming for millennia, leading to substantial sea level rise and the inundation of coastal cities. The impacts on human societies would be “massive, sometimes abrupt, and undoubtedly disruptive,” they cautioned.
Professor William Ripple, of Oregon State University, who led the analysis, emphasized the urgency of the situation. “The Amoc is already showing signs of weakening, and this could increase the risk of Amazon dieback. Carbon released by an Amazon dieback would further amplify global warming and interact with other feedback loops. We need to act quickly on our rapidly dwindling opportunities to prevent dangerous and unmanageable climate outcomes.”
Professor Tim Lenton, an expert on tipping points at the University of Exeter in the UK, cautioned that even avoiding a full “hothouse Earth” scenario does not guarantee safety. “We know we are running profound risks on the current climate trajectory, which we can’t rule out could turn into a trajectory towards a much less habitable state of the climate for us. However, we don’t need to be heading towards a hothouse Earth for there to be profound risks to humanity and our societies – these will already be upon us if we continue to 3C global warming.”
The study underscores the accelerating pace of climate change, with processes that historically unfolded over thousands of years now occurring within decades. Scientists note that current global temperatures are likely as warm as, or warmer than, at any point in the last 125,000 years, and that carbon dioxide levels are the highest they have been in at least 2 million years.
The researchers acknowledge the difficulty in predicting precisely when climate tipping points will be triggered, but stress that the potential consequences are so severe that a precautionary approach is essential. The warning comes as global efforts to reduce greenhouse gas emissions continue to fall short of the targets needed to limit warming to 1.5°C, as outlined in the Paris Agreement.
