Heat can enter a system without causing a temperature rise.
Extra heat can be used in ‘changing state’ instead of raising temperature.
A change of state could be …solid melting to liquid. Or liquid evaporating to gas.
So heat is needed to change ice at zero C to water at zero C.
And to change water into water vapour….. without raising the temperature.
The climate systems include atmosphere, water and ice.
So if ice is warming up, that will use heat from the air, and so that bit of heat is not warming the air.
If the oceans warm, that bit of heat is not heating the air.
So these bits of heat don’t show up in rising air temperatures.
The amount of heat that might be doing one or other of these things will change constantly, so the air temperature will not rise smoothly.