The issue is a percentage of a Celsius value is not that. For example, an increase from 1°C to 2°C is a "100% increase", but is only 1 percentage point from freezing to boiling.

You could say things like that with anything in percentages? 100% increase in your pension from 100k to 200k is only 10% (increase, to 20% total) of your target 1M, or whatever.

100k to 200k is a 100% increase in absolute, but a 10 percentage point increase to your target of 1M. The difference between the example you give and the one in the article is that 0 in the case of your pension meaningfully refers to its emptiness, but in the case of Celsius, it has no "emptiness" interpretation.

The equivalent would be saying that going from 600k to 700k was a 100% increase... compared to 500k.

It's not completely meaningless, to be fair. Saying 10°C to 20°C is a 100% increase has the meaning of "it's twice as far from freezing", which isn't totally meaningless (kind of like saying Everest is twice as high as Mont Blanc, which really means "its summit is twice as far from sea level").

But in your example, the 10% has nothing at all to do with the increase of 100%.

If your ”whatever” target instead was 50k, is the argument that going from 100k to 200k would be 400%?

Yes? I didn't see any link in comment I replied to between 100% and 100deg besides it happening to be the same number - I already changed that to 1M, changing it to 50k no different either.

If the argument was saying there's something special about 100% being 100 quantity then.. no? I don't really know where to go from there, what I said still holds with a 100k target, but I'm not going to be able to give 'another' example where the 100 quantity is meaningful because it isn't for degrees either. It's the freezing point at 0 that makes it work better for centigrade than Fahrenheit, imo.