In a recent commentary published on 27 November in the journal Nature, leading climate scientists warn the planet may have already surpassed several tipping points (1). “We are in a state of planetary emergency,” they write, calling for urgent international action.
The climate system is at the mercy of several tipping points, thresholds beyond which an abrupt shift might accelerate the collapse of ecosystems. For example, unprecedented permafrost thawing and runaway melting of glacial ice sheets could release huge amounts of carbon back to the atmosphere leading to catastrophic effects. Lenton and colleagues highlight the potential risk as “an existential threat to civilisation”.
Once a tipping point is reached, changes become self-perpetuating via so-called positive feedbacks and the effects may be long-lasting and difficult to reverse, which could have extreme impacts on biodiversity, global warming, and humanity. Moreover, a cascade of interrelated climate tipping points is possible and wholly plausible, according to the authors.
‘To err on the side of danger is not a responsible option’
Tipping points were previously assumed to have low probability — only likely if global warming exceeded 5 degrees Celcius above pre-industrial levels. However, “evidence is mounting that these events could be more likely”. Two recent International Panel on Climate Change (IPCC) reports suggest several tipping points could, in fact, be reached if global warming exceeds 1–2 degrees Celcius. And could have long-term irreversible impacts.
In their commentary, the authors suggest nine tipping points may have already been reached. For instance, the West Antarctic ice sheet may be currently in irreversible retreat and total collapse would cause metres of sea-level rise; Thawing of permafrost, which releases large amounts of methane and carbon, may now be unstoppable; Melting of Greenland’s ice sheet is accelerating.
Since 1970, the Amazon rainforest has shrunk by 17 per cent, according to scientists. At what point will forest lead to drying out, turning the Amazon from a living breathing rainforest into a dry savannah? Frighteningly, some experts suggest this tipping point could occur at 20-40 per cent forest loss.
We must act now
The science of tipping points is complex and uncertainties still remain. Nonetheless, the potential adverse effects are grave. “The stability and resilience of our planet are in peril. International action – not just words – must reflect this”, the authors write.
The findings are not all doom and gloom, though. Although we are ‘too close for comfort’, according to the authors, curbing greenhouse gas emissions could still dampen the potential impacts and we may still be able to at least slow the rate — and indeed, the risk — posed by tipping points to some extent. Even so, it could take another three decades to reach net-zero emissions.
But current international pledges are not enough to keep global temperature rises well below the 2 degrees Celcius specified in the 2015 Paris Climate Agreement. To keep limit warning to 1.5 degrees Celcius will require an ‘emergency response’.
(1) Lenton, T.M. et al. Climate tipping points — too risky to bet against. The growing threat of abrupt and irreversible climate changes must compel political and economic action on emissions. Nature (2019). DOI: 10.1038/d41586-019-03595-0