Table 1.

Excerpts from definition of the events of crossing a tipping point, and distribution of responses from experts

Reorganisation of the Atlantic Meridional Overturning Circulation (CMOC) ″that involves a permanent shutdown of convection in the Labrador Sea and a drastic reduction in deep water overflow across the Greenland-Scotland ridge by at least 80%.″
(A) 16 (B) 4 [Reasons: remote; model results inconclusive] (C) 2 [local cooling overwhelmed by overall warming trend]
Melt of the Greenland ice sheet (MGIS) ″… an alternative state that is largely ice-free.″
(A) 13 (B) 1 [Reason: too far into the future] (C) 1 [fastest melt is 600 years, too slow to be dangerous]
Disintegration of the West Antarctic ice sheet (DAIS) ″… in which West Antarctica becomes an archipelago … ″
(A) 14 (B) 1 [Reason: uncertainty about time scale of disintegration; possibility of collapse due to glacial readjustments only]
Dieback of the Amazon rainforest (AMAZ) ″… in which at least half of its current area is converted to raingreen forest, savannah or grassland. Besides climate change, a second driver … is land use change from human activity … factor out this driver by assuming … that not more than 20 percent of the current rainforest will be deforested by human activity in the long run.″
(A) 10 (B) 1 [Reason: vegetation change inconclusive for assessing feedbacks on climate] (C) 3 [Global effects limited; CO2 sink to source conversion not a dangerous feedback on the climate]
Shift to a more persistent El Niño regime (NINO) ″… a shift of the ENSO mean state towards El Niño like conditions.″
(A) 10 (A*) 1 (B) 3 [Reasons: original definition remote; Model results, paleo record inconclusive] (C) 1 [impacts of El Niño superimposed on a warmer world uncertain]
  • Option A: event will lead to potentially dangerous climate change, and willing to answer questions about its probability; option B: elicitation of probabilities not appropriate; option C: event will not lead to potentially dangerous climate change. See Table S1 for more details of the expert response.

  • Definition was changed during the final phase of the elicitation during which participants were allowed to revise their statements (see Table S1).

  • Expert specified probabilities only for the original definition of the event. The response is not included here.