This means source of variation. Nitrogen and oxygen in the atmosphere are not a source of variation. Greenhouse gasses are not a significant source of variation, producing parts per trillion effect.

Tilt of the earth never changes over a moderate time range. Some scientists assume change in the earth's orbital plane could be the cause of ice ages (Milankovich cycles), because it has a 100,000 year element to it. But there never is a change in yearly average effects from orbital variations; so it's hard to see how such minor shifts affect climate.

The 100,000 year parallel of Milankovich cycles appears to be a coincidence. There are many components to the earth's orbital variations including a 40,000 year cycle and a 25,000 year cycle. So it's not surprising that there is a 100,000 year cycle. Correlation is not causation.

Oceans, however, store heat and release it over short and moderate time scales as well as releasing water vapor based on surface temperature. These are the primary factors which determine variations in climate.

Solar radiation can be stored in the oceans much more effectively than in the atmosphere for three major reasons. One is that the oceans have a heat capacity of a thousand times that of the atmosphere. Two, a liquid state does not promote escape through radiation like a gas. And three, solar radiation goes some distance into the oceans allowing it to stay and circulate, unlike the earth's surface where the heat cannot penetrate.

The oceans also should be expected to pick up a significant amount of heat from within the earth. Heat constantly flows out from within the earth. Most people don't understand this. Deep wells are warm because of the heat flowing up from within the earth. This is the source of geothermal energy which is often harnessed for warming or electrical generation. When this heat enters the oceans, it would accumulate for millennia before circulating to the surface and being released into the atmosphere.

The heat within the oceans is released into the atmosphere in highly erratic and unpredictable ways creating not just the primary affect on climate but almost the entire cause of variation in climate.