Assuming that one of these targets will be achieved “because it must be to avoid catastrophic climate change,” as is common in climate risk discussions today, reflects a "false certainty."
The plausible range of potential climate change outcomes is actually much larger:
5oC is not out of the question given the known unknowns and likely unknown unknowns of the climate system.
3-5oC represents today’s most likely climate outcome by 2100, based on “stay the current course” climate policies and measures.
2oC scenario is today’s prevalent climate change target, but one that would require much more aggressive mitigation actions than we have yet seen.
1.5oC, roughly represented by the idea of returning CO2 concentrations in the atmosphere to 350 ppm, is being called for by scientists and activists around the world, but would require extraordinary mitigation efforts.
With Your Climate Change PhD you can explore the drivers and impacts associated with each of the scenarios introduced above, and in the case of the last two scenarios the policies and measures that would be required to achieve them. You’ll also see an assessment of the likely ramifications of each scenario, and have hyperlinked access more resources and curated information collections in the Climate Web.