It's unquestionably the most significant way in which the Earth warms and cools itself. Heat in any given region causes evaporation of surface water, lake water, ocean water, etc. That evaporation puts water vapor into the atmosphere. The moist, warm air rises until it hits cooler temperatures, and the water vapor condenses and forms clouds. Clouds help cool the earth below them. Further cooling causes those clouds to condense into rain or snow. The rain and snow further cools the surface of the Earth, starts evaporating, and the whole thing starts over again.

You could say it's like a giant air conditioning system.

There's also a large exchange of CO2 between the atmosphere and the pant world, so I think I'd call that part of the Earth's natural warming and cooling system too.

Man's activities would certainly have some effect on the overall situation, the question is "how much?".

These warming and cooling phases have been happening for millions of years, as is shown in the geological record and in ice core samples. There have been periods when there was far more CO2 in the atmosphere than now....and life flourished during a number of those periods. That makes me skeptical that an increase in CO2 is the apocalyptic scenario that we have been presented with.

I also can't see why there would have been a notable planetary cooling phase between 1940 and 1975 if our increasing level of industrial emissions were the key factor behind increased planetary warming. It just doesn't make sense in the light of that historically recorded cooling period (which led to scare stories in the press during the mid-70s about the threat of a new "ice age").

It seems more likely to me that changing solar output of energy to the Earth is the dominant factor.

I'm not saying that our activities have NO effect on planetary warming and cooling, but I think their effect has probably been exaggerated of late.