Originally Posted by
Leewei
That 30 degrees Celcius is fair to use -- in fact, it's probably understating the heat our planet radiates into space by a fair amount. Consider the temperature drop each night following sunset.
That said, ground temperature is far more constant, and solid matter holds far more heat than a gas at a similar temperature. According to Google, Venus is 10,000 times the mass of its atmosphere. Venus's atmosphere is about 100 times as massive as Earth's, for that matter. Surface area is similar to Earth, but the total amount of heat stored on this pizza oven of a planet is staggeringly high.
If you only need to radiate off 1% of Venus's total heat (very dubious), you'd need 10,000 years to drop 30° K. (1% cancels out the x100 atmospheric size, leaving 10,000 times the mass.) Assuming you can make the planet somehow livable at 600° K, it would need to radiate for 100,000 years. 200,000 years gets you to a comfortable 300° K.
This is just spitballing, of course. Hotter bodies radiate heat faster, it likely will take more than 1% of the mass cooling to make Venus's surface livable, and the heat radiated annually from Earth is probably more than 30° K. Still, this illustrates an optimistic time frame of many multiple thousands of years for Venus to become habitable.