Figure 1: The Amazon rainforest to reach irreversible tipping point. Image: Al’fred / Shutterstock.com
Loading the atmosphere with five million tonnes of CO2 every hour has pushed the Earth dangerously close to a no-return threshold, beyond which lies an inhabitable hothouse world. There are 15 known tipping points in the planet's complex climate system, and nine of them-including permafrost, the Amazon rainforest, the Greenland ice sheet, Arctic sea ice, and the Atlantic Ocean's circulation - are drifting alarmingly, as reported in the journal Nature. The authors of this journal persist that it is an existential threat to civilization, implying that the Earth is in a state of planetary emergency.
Tipping points are reached when particular impacts of global heating become unstoppable, such as the runaway loss of ice sheets or forests. In the past, extreme heating of 5 degC was thought necessary to pass tipping points, but the latest evidence suggests that this could happen between 1 degC and 2 degC.The planet has already heated by 1 degC and the temperature is expected to rise further, due to past emissions and increasing greenhouse gases. The authors further warned that one tipping point, such as the release of methane from thawing permafrost, may fuel others, leading to a cascade. However, the researchers who wrote in a commentary article in the journal Nature acknowledged that the complex science of tipping points means great uncertainty remains. But they say the potential damage from the tipping points is so big and the time to act so short, that to misapprehend this situation is not a responsible option. They call for urgent international action.
Prof Tim Lenton at the University of Exeter, the lead author of the article, said that the threshold for a cascade of interrelated tipping points has already been crossed. Phil Williamson at the University of East Anglia, who did not contribute to the article, also agreed on the prognosis made by Tim Lenton and his colleagues and said that we might have already lost control of the Earth’s climate. This new article comes as the United Nation warns action is very far from stopping global temperature rise, with the world currently on track for 3-4 degC. The commentary lists nine tipping points that may have been activated as shown in Figure 2.
Figure 2: Schematic diagram illustrating nine tipping points that may have been activated.
All signals indicate that part of the west Antarctic ice sheet may be in irreversible retreat as well as the Wilkes basin in east Antarctica. The collapse of these ice sheets would eventually raise sea level by many meters. The massive Greenland ice sheet was melting at an accelerating rate while the Arctic sea ice is shrinking fast. Alarming evidence indicates that the Permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane. According to a few studies, the Gulf Stream current in the Atlantic which brings heat to Europe has also slowed down by 15% since the mid-20th century. The scientists report that 17% of the Amazon rainforest has been lost since 1970 and the tipping point, where loss of forest leads to it drying out, could lie in the range 20%-40%. In temperate forests, especially in North America, heating has triggered more fires and pest outbreaks, potentially turning some regions from a sink for carbon to a source. In the tropics, corals are predicted to be wiped out by an increase in sea surface temperature of 2 degC.
A cascade of tipping points could occur because the melting of Arctic sea ice amplifies heating by exposing dark ocean that absorbs more sunlight. That may increase the melting of Greenland ice and permafrost areas. Multiple risks can interact, with one change reinforcing another, and therefore, warming of just a degree or two is sufficient to result in dramatic cascading effects. Prof Martin Siegert, at Imperial College London, said: “The new work is valuable. They are being a little speculative, but maybe you need to be.” He also pointed out that the extremely rapid rate at which CO2 was being pumped into the atmosphere was unlikely to have ever occurred on Earth before. “It may mean that tipping points can occur in unexpected ways as there is no geological precedent for this rate of CO2 change.”
The article reports that preliminary results from the latest climate models suggest global heating will be greater than expected, increasing the risk of tipping points. Prof Piers Forster, at the University of Leeds, disagreed on that point. However, he added: “I completely endorse their call for action. Although possibly low probability, the risks they identify are real.” Lenton said action would still have real benefits, by slowing the impacts and giving more time for people to adapt. He said: “This article is not meant to be a counsel of despair. If we want to avoid the worst of these bad climate tipping points, we need to activate some positive social and economic tipping points [such as renewable energy] towards what should ultimately be a happier, flourishing, sustainable future for the generations to come.”