It's a marketing copy thing. If instead of 1 degree, they said 1.4 million TWh, which is the amount of energy needed to heat the Earth by 1 degree, it would hit better extreme number emotional buttons, but I'm not in charge of things.

Frankly I think expressing it in Celsius has made Americans think it’s less of a problem than it is. It took me awhile to realize that it’s basically 4°F, which is enormous.