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Venus is a runaway greenhouse planet, so it won't let any of the heat absorbed from the Sun out by radiation, yet it will still take rays of energy from the Sun. So what will happen to Venus? Will it overheat? And if it will overheat, what will actually happen to it? I couldn't find any speculations over the internet...

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    $\begingroup$ Well, Venusian atmosphere has reached an equilibrium; absorption and radiation keep the balance. In order to change (heat further), the absorption of energy from outside or from internal heat flow must rise. Also, if i where you i'd also post this question in the astronomy department, the guys there are more up to date on extraterrestrial stuff :-) $\endgroup$ – user18607 Jan 5 at 15:55
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    $\begingroup$ @ebv the latest astronomical news say that venus have had volcanic activity during the las ten years space.com/venus-may-have-active-volcanoes-new-evidence.html $\endgroup$ – trond hansen Jan 5 at 16:48
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    $\begingroup$ @trondhansen Cool, thanks ! That indirectly leads to the question: does it (she ?) net loose or still accumulate atmopshere ? But i leave that to somebody else, in the astronomy department :-) $\endgroup$ – user18607 Jan 5 at 17:10
  • $\begingroup$ Most of us would say Venus is already overheated :-) But your question is based on a false premise: conditions on Venus have just raised the equilibrium temperature. $\endgroup$ – jamesqf Jan 7 at 19:19
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Venus does radiate away heat, but not enough to prevent the surface and lower atmosphere heating to 460C. Too late to prevent overheating, it has already overheated. In the long term, its atmosphere will gradually leak away into space, reducing the present pressure of 90 Earth atmospheres. Exactly how far this process will have gone in 5 billion years' time when the sun swells up to become a red giant is impossible to say. When the sun becomes a red giant, Venus will be inside its tenuous outer layers and therefore scorched by much higher temperatures than it has now, probably reduced to a ball of molten rock spiralling inwards towards the sun's interior.

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I make no claim to be an expert on Venus, and I welcome correction from those who are, but I think there's a misunderstanding in the question.

"Venus... won't let any of the heat absorbed from the Sun out by radiation" is false. It must be, due to fairly fundamental physical laws.

Now, I'm not sure whether any energy emitted from the surface of Venus reaches space. I imagine that some fraction probably does, but I could be wrong - it could be that the atmosphere is entirely opaque to the wavelengths radiated from the ground. However, the atmosphere itself is warm, and hence the atmosphere itself radiates, partly into space. This is different to the way things mostly work on Earth, where the atmosphere is mostly (but decreasingly) transparent to the wavelengths radiated from the ground.

At least on short timescales, then, there's a balance between incoming and outgoing energy, and Venus is in equilibrium. It won't keep getting warmer forever, unless something changes to change the temperature of that equilibrium.

By analogy, Earth was in equilibrium until the industrial revolution. Since then things have been increasingly disrupted by a massive increase in greenhouse gas concentrations. Once we stop increasing GHG concentrations, it will warm until it reaches a new equilibrium at a higher temperature and then stop. It won't keep getting warmer forever.

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  • $\begingroup$ The last part in your answer is technically wrong: We did not change the equilibrium temperature, we keep changing it. Earth's temperature has a very low optical depth for most relevant wavelengths, so the timescale to establish a new radiative equilibrium is about 10-20 days. If we removed all excess $CO_2$ from the atmosphere NOW, things would normalize quickly. On Venus, this timescale is about 30 years, as it is massively optically thick at all wavelengths, and energy transport takes more time. $\endgroup$ – AtmosphericPrisonEscape Jan 9 at 22:32
  • $\begingroup$ @AtmosphericPrisonEscape Thanks! I've edited - is it better now? $\endgroup$ – Semidiurnal Simon Jan 10 at 7:27

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