It is my understanding that the main way that heat (well, energy) enters the Earth (including its atmosphere) is as radiation from the sun. It is also my understanding that pretty much the only way that heat can leave the Earth is in the form of some sort of radiation (electromagnetic waves are radiation, right?). Does anyone here know if the rate at which energy is entering the Earth is greater or lesser than the rate at which it is leaving? Was it at some sort of equilibrium (I assume it would kinda disrupted now by the increasing of the greenhouse effect)?
Yes, the earth is in equilibrium. Since the temperature has been more or less stable throughout the recorded history of mankind, and probably much longer, it has to be in equilibrium. The earth receives energy in the form of visible light from the sun. It radiates this heat away as infrared radiation. If either the energy absorption increases or the readiation decreases, the temperature drops to a point where they are once again in equilibrium. So greenhouse effects and all that don’t really “disrupt” the equilibrium, it just changes the temperature at which this equilibrium is established.
In addition to solar radiation, the Earth’s mantle is a nice heat source. The Earth reflects some of the radiation from the sun. It also emits heat to space through radiation (warm Earth, cold space, heat transfer via radiation toward thermodynamic equilibrium).
The incoming solar radiation/energy would be balanced by what is reflected, what is used up on Earth (sunk into weather, photosynthesis, etc.) plus what is re-emitted back to space. (other factors?) This maintains a nice “equilibrium” (not absolute of course, but we do have general climates that can be counted on). Periodically, climates change, so there is a transition period to a new equilibrium.
Greenhouse gases (water vapor, CO2, CH4, etc.), hold back some of the radiation that would normally be re-emitted to space. Thus, if global warming is happening (FWIW, I tend to think it is, or soon will), then it would be a transition period to a hotter equilibrium.
Venus found a nice equilibrium at 900 F due to it’s level of CO2 in the atmosphere. 
I’ve been under the impression that Earth and Venus are very similar in astronomical terms, so why is it that Venus has such a higher temperature? Is it just that Venus retains more heat throught the greenhouse effect, or are there other contributions, like increased internal sources of heat or a greater input of heat from the sun (I guess with Venus closer to the sun, it must receive more heat, but it also has more reflective cloud cover than Earth)?
If Venus is just retaining more heat from the sun, is it still heating up?
While Phobos’ answer is interesting, it’s a lot simpler than that. When I was a kid I would sometimes go outside and not close the door. My mom would say “You’re letting the heat out. Are you trying to heat the whole world?” My guess is that, since the population is growing steadily, more people are leaving the door open and they ARE heating the whole world.
I have other crackpot theories but this is my favorite.
No, this effect is cancelled out by all the people leaving their refrigerator doors open.
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*Originally posted by tcburnett *
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Not at all. The open refrigerator doors cause the refrigeration units to stan on constantly, using extra electricity, requiring the power plants to burn more fuel, releasing more heat into the atmosphere.
Pretty soon we will reach my theory about the Ozone layer.
(similar to the Bozone layer, but without the clowns)
Ahhhhhhhhhhh…the atmospere…Ahhhhhhhhhhh.
Venus has a major Greenhouse Effect going on. It’s hotter than Mercury (which is, of course, even closer to the sun). Venus does receive more sunlight than Earth, and Venus also seems to have a lot of ongoing volcanic activity, but I think it’s atmosphere is the main source of it’s high temp. Lots of greenhouse gases & a very thick atmosphere. I have not heard anything about whether Venus is getting any hotter…I would assume it has cycles like the Earth.