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The titles indicate the baseline for each graph. e.g. "Global mean temperature difference from 1850-1900"

They are plotting the anomalies which tend to be much more useful for climate studies than the absolute values. e.g. [0,1]

[0] https://www.ncdc.noaa.gov/monitoring-references/dyk/anomalie...

[1] https://data.giss.nasa.gov/gistemp/faq/abs_temp.html

Right, so what is plotted is the deviation from the mean reading in the time range given? Thanks for the links, those are insightful.

Absolute sea level (from the bottom of the ocean?) is meaningless for people concerned about coastal flooding. Absolute temperature is also meaningless in deg C because the zero is arbitrary anyway. Even if they used kelvin, nobody cares how far we are from 0K.

All of these metrics have subtleties regarding the methodology, which makes the long term relative shifts more important than the absolute values.