Actually, there is no set routine on when the earth warms or cools. And they can and do determine this through analysis of soils, setiments and glacial ice around the globe.
Valcanos have heated the earth in the past and have caused mass extinctions.
They can also determine with reasonable accuracy how long it took the earth to warm or cool when it has. This is the reason as you put it, that theories as to why the globe is warming right now that are not linked to man kind get vilified. The reason for that is the speed at which we are releasing CO2 into the atmosphere and the speed in which the globe appears to be warming.