> If the assertion is increasing temperature, why wouldn't you set 1901 as baseline and map difference from there?

Choosing the very best baseline is difficult.

Do we choose the warming period since the end of the "Little Ice Age" about 300 years ago?
No, that would have a lot of warming that is inconveniently before the build up of manmade CO2.

Maybe the cooling trend that was seen from about 1940 to 1975?
No, that is not right either.

Or perhaps the "Medieval Warm Period" when the Vikings were growing grapes in Greenland where it is too cold to grow grapes today?
No, that won't do either, too warm, and not enough manmade CO2.
And it doesn't explain the little ice age that ended the warm period.

How about near the end of the last ice age 10 or 12 thousand years ago?
Nope too cold and too much sudden warming before mankind began reshaping the planet.

I will admit, I am confused by all of it.
It is obvious we are changing the planet, but the planet seems to change a lot by itself.