Looking again at the IPCC definition of "exceptionally unlikely" as a less than 5% chance, I wonder if the term is really appropriate.
If I was told on boarding plane that it was "exceptionally unlikely" to crash, I would think not think they meant there was a 1 in 20 chance that it would crash.
How about "very unlikely" for a less than 1% chance?
Still I wouldn't board an aircraft I believed had an up to 1% chance of crashing.
Here's a possible revision:
• "More likely than not" (more than 50%) [same]
• "up to one in three chance" (less than 33%)
• "a one in ten chance" (less than 10%)
• "a one in twenty chance" (less than 5%)