What scientists say about tipping points, cascades and losing control

"There is now clear evidence that changes in the Earth's climate may be sudden rather than gradual… we play Russian Roulette with climate [and] no one knows what lies in the active chamber of the gun”, writing in1987.
     Unpleasant surprises in the greenhouse

Prof. TIMOTHY LENTON and colleagues
“The evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute […] If damaging tipping cascades can occur and a global tipping point cannot be ruled out, then this is an existential threat to civilisation.”
      Climate tipping points — too risky to bet against

Prof. EYSTEIN JANSEN and colleagues
The Arctic “is currently experiencing an abrupt climate change event… climate models underestimate the abruptness of the recent changes observed in the Arctic [and] underestimate this ongoing warming”.
      Past perspectives on the present era of abrupt Arctic climate change

“If we go beyond 2°C it’s very likely that we have caused so many tipping points that you have probably added another degree just through self-reinforcing changes… The moment that the Earth system flips over from being self-cooling — which it still is — to self-warming, that is the moment that we lose control.”
      COP26: Why The UN Climate Conference Matters Like Never Before

"Glacier retreat has knocked the dynamics of the whole [Greenland] ice sheet into a constant state of loss...  Even if the climate was to stay the same or even get a little colder, the ice sheet would still be losing mass."
      Warming Greenland ice sheet passes point of no return

Prof. JAMES HANSEN and colleagues
“Could the Greenland ice sheet survive if the Arctic were ice-free in summer and fall?... It has been argued that not only is ice sheet survival unlikely, but its disintegration would be a wet process that can proceed rapidly.”
      Dangerous human-made interference with climate: a GISS modelE study

Prof. NICO WUNDERLING and colleagues
“We show that the risk for some tipping events could increase very substantially under certain global warming overshoot scenarios… Even if we would manage to limit global warming to 1.5 degrees after an overshoot of more than two degrees, this would not be enough as the risk of triggering one or more global tipping points would still be more than 50 percent. With more warming in the long-term, the risks increase dramatically.”
      Overshooting climate targets could significantly increase risk for tipping cascades

Bifurcation: A fork in the road

“In some cases, there is evidence that once the system has jumped to a different state, then if you remove the climate forcing, the climate system doesn’t just jump back to the original state – it stays in its changed state for some considerable time, or possibly even permanently.” [This is known as “hysteresis”. It occurs when a system undergoes a “bifurcation” – which means to divide or fork into two branches – and it is subsequently difficult, if not impossible, for the system to revert to its previous state.]
      Nine “tipping points” that could be triggered by climate change

“The current trajectory is accelerating the [Earth] system towards [a point of] bifurcation, with the increasing risk that our pressures will push the system onto the ‘Hothouse Earth’ trajectory. The critical point here is that there is a point beyond which we lose control of the system and its own internal feedbacks drive it past a global threshold and irreversibly into a much hotter state.”
      The Earth System, the Great Acceleration and the Anthropocene

Losing control: Hothouse Earth

Prof. WILL STEFFEN and colleagues
“We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a “Hothouse Earth” pathway even as human emissions are reduced.”
“Even if the Paris Accord target of a 1.5°C to 2°C rise in temperature is met, we cannot exclude the risk that a cascade of feedbacks could push the Earth System irreversibly onto a ‘Hothouse Earth’ pathway.”
      Trajectories of the Earth System in the Anthropocene

“If the (climate system) tipping elements interact and cascades develop, then the heating could become independent (i.e. self sustaining) at 2°C. Whether that is the case is perhaps the most important question of science right now because it would mean the end of our civilisation.”
      Video of presentation in German,