Nine crucial tipping points in Earth’s climate are now “active” and in danger of being crossed thanks to warming global temperatures caused by human activity, warn scientists in a commentary published on Wednesday in Nature.
These climate thresholds, such as the decline of ice sheets and loss of biodiverse habitats, could cumulatively trigger a global tipping point that would be “an existential threat to civilization,” cautioned the authors of the article.
“Evidence is mounting that these events could be more likely than was thought, have high impacts, and are interconnected across different biophysical systems, potentially committing the world to longterm irreversible changes,” said corresponding author Tim Lenton, director of the Global Systems Institute at the University of Exeter, in an email.
The nine tipping points that Lenton and his colleagues emphasize fall into two main categories: the loss of the planet’s icy cryosphere and key parts of its living biosphere. The cryosphere thresholds include the melting of Arctic sea ice, the Greenland ice sheet, the West Antarctic ice sheet, and regions of East Antarctica. The biosphere tipping points involve the devastation of boreal forests, the Amazon rainforest, warm-water coral reefs, and the thawing of frozen soil known as permafrost.
The most dangerous tipping point, in Lenton’s view, is the potential collapse of the Atlantic Meridional Overturning Circulation (AMOC), which is its own category of catastrophe.
This critical flow of ocean currents “has large direct effects on climate in heavily populated regions like western Europe, and also it couples together lots of other tipping points,” Lenton explained. “For example, weakening of the AMOC in the past was linked to serious disruption of the West African monsoon and the monsoons in India and South America.”
The Intergovernmental Panel on Climate Change (IPCC) identified some of these tipping points in its 2001 report, and Lenton led a team that expanded on the list in a 2008 study in Proceedings of the National Academy of Sciences.
But the IPCC initially underestimated the risk of large-scale discontinuities in Earth’s natural processes. Nearly two decades ago, the panel estimated that it would take an average global temperature rise of about 5°C above pre-industrial levels to activate irreversible processes.
The most recent IPCC reports, in contrast, conclude that those thresholds could be breached with increases of just 1.5 or 2°C. That range of elevated temperatures is extremely likely to be reached by 2050, even if nations participating in the Paris Climate Agreement meet their emissions goals.
While Lenton and his colleagues acknowledge there is a high degree of uncertainty about the activation and effects of climate tipping points, they stress that even the speculative implications demand urgent action.
“It is our position that, given its huge impact and irreversible nature, any serious risk assessment must consider the evidence, however limited our understanding might still be,” the team said in the article. “To err on the side of danger is not a responsible option.”
To that point, it is critical for governments, scientists, and citizens to continue monitoring known tipping points and looking out for new potential thresholds that could set off irreversible chain reactions. We are at an important inflection point at this time, as the Trump administration has been hostile to the global effort to decarbonize and has pulled the U.S. out of the Paris Climate Agreement.
While robust investment in climate research is incredibly important to Lenton and other climate scientists, it is even more essential for people of all backgrounds to demand efforts to avert, or even simply delay, the looming emergency.
“To be honest, I think we know enough climate science to act, yet we are not acting decisively, so we need to put resources into action,” Lenton said. That goal will necessitate not only a better understanding of climate thresholds, he said, but also “finding and triggering social tipping points towards a more sustainable future.”