1.5 degrees is treated as a line that must not be crossed, but a tenth of a degree either side cannot be a physical boundary. Nothing in the climate changes discontinuously at exactly that point.
How was it arrived at, and what happens if it is passed?
Accepted Answer
It is a negotiated number informed by science, not a threshold found in the physics, and both halves of that matter.
The 2 degree figure came first, in policy discussions from the 1990s onwards. 1.5 entered through the Paris Agreement in 2015, pushed hard by small island states for whom the difference between 1.5 and 2 is existential rather than incremental.
The IPCC special report in 2018 then examined that difference rather than inventing it, and found it was substantial: roughly twice the species losing half their range, and coral reefs going from severe decline to near total loss.
So the number is a political choice about acceptable risk, resting on a real and measurable difference between two futures.
And on your second question, the framing of a cliff edge is misleading in a way worth correcting.
Nothing special happens at 1.50 that does not happen at 1.49. Damage rises continuously, and every tenth of a degree avoided is worth avoiding whether or not the target is met.
The genuine worry about thresholds is different: specific tipping elements such as ice sheet collapse or permafrost carbon release have their own ranges, mostly uncertain and mostly above 1.5. Those could be irreversible. But they are not located at the target, and treating the target as the cliff makes passing it look like a reason to stop trying, which is the opposite of what follows.
Also worth keeping straight: the target refers to a long term average relative to pre-industrial, not to any single year. A single year above 1.5 is not the same as crossing it.
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