My high school physics is rusty, but is the relationship between heat and temperature a linear one? The article seems to be talking about heat energy, rather than temperature in degrees:

> The results show heat increasing at an accelerating rate as greenhouse gases accumulate in the atmosphere. The rate from 1987 to 2019 is four and a half times faster than that from 1955 to 1986.

And the axis on the chart says:

> Change in heat content relative to 1981-2010 average in zettajoules