It's been a slow steady increase in CO2 since industrialisation.

The atmosphere has become increasingly better insulated in the thermal energy spectrum .. albeit still losing a lot of heat to the outer layers and to space.

Basic back of the envelope thermodynamics tells the story - more trapped energy at the surface layer - land, sea, and near surface air becomes warmer across the globe and that warmth cascades through energy transfers.

For some it's confusing that warmth -> rising air -> inrushing colder air -> circulating air cells -> freezing conditions (just as fridges / freezers heat pump via air pressure).

The first significant paper on this was

Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity (1967)

https://journals.ametsoc.org/view/journals/atsc/24/3/1520-04...

A great deal of key data (atmospheric makeup, sea tempreture records) came from hard nosed Cold War era research focused on nuclear weapons, sub tracking, and other such pursuits .. much of it "disguised" as environmental research (we listen to whales!) but not at all driven by a 'need' to invent and justify an AGW agenda (as some have claimed).