It's "watershed" because that was the number chosen at Paris. There's nothing scientifically significant about 1.5 or 2.0. They're just numbers that sit along the continuous path from here to whatever hellscape sits at the bottom of this human-created extinction event we've set into motion. I guess the people at Paris were all very optimistic and thought humanity could actually do something to avoid hitting 1.5. They were wrong.
More likely, we hit it, look around and see that nothing has change, and keep going.
Second most likely, we hit it, look around and see the devastation that has been wrought, and the people living in relatively safe zones institute military-backed authoritarian governments who are charged with keeping migrates from effected regions from coming in, and seizing what little available resources remain from weaker, neighboring states.
The article mentions that 1.5°C was the attainable target and 2°C was the hard limit. At this rate we'll blow through them both.
> The 1.5°C mark was established as the desirable target for all the countries of the world who signed up to the Paris Agreement to limit temperature rises, in order to prevent permanent changes that threaten the wellbeing of all life on earth. The agreement calls for limiting rises to 2°C or below.
1.5ºC is the lower bound for the estimated target.
In any case, the 2ºC nominal target was mostly arbitrary.
For those unaware, the problem with measuring global temperatures is that we can measure variation (relative values) with great accuracy (e.g. 0.5ºC increase), but the error bars on the absolute measurement are way to big to make useful statements on what will happen at XºC.
