After a week of cool science at AGU, these give me headaches, but here's a short history of Venus.
Once upon a time, there was a planet, pretty comparable to Earth in size. It could of had a nice climate for a bit...depending on things like topography and rotation rate. A slow spin exposes its dayside to the sun for almost two months at a time, which sounds bad but...
...if you tell a General Circulation model about that rotation state, you can make the sun-lit part of the planet overcast for many months, shading it, so you can retain temperate conditions for some billions of years. It wouldn't work for Earth rotation. Depends on other things.
The distance to the Sun doesn't matter too much. A bare rock's temperature is only a square root function of distance. Venus' average temperature would be kind of like Earth's tropics, all else equal. Not a big deal, but when you start hitting a planet with a bit too much Sun...
Eventually you make a wet stratosphere and lose water to space. So at temperatures a few tens of Kelvin higher than Earth, you can't really maintain an Earth-sized ocean over 4 billion years anymore. That's not good, even if Steve is content that the actual rock still exists.
Eventually, at temperature just a tiny but warmer than Earth's tropics, you'd start to make the upper atmosphere wet enough to have some issues. Earth's stratosphere is really dry, but once the stratospheric mixing ratio is ~10^-3, you'd have sufficient photolysis to lose ...
...lose an Earth's worth of ocean to space in less than the age of the Earth. Losing an ocean worth of water to the cosmos is not fun for anything with a pulse, even if Steve is content that the actual rock still exists.
Now, once you lose all that water, CO2 comes in and says "hehe, no one can weather me out of the atmosphere and into the rocks anymore." That's where basically all the carbon on Earth is. In rocks. But on Venus, it took a vacation into the atmosphere and isn't going home.
Now we've had more CO2 in the atmosphere in the past, but typically 1) when we were also getting less sunlight 2) There were usually crocodiles in the Arctic or no ice at the poles
Now turning New York into George is enough for sensible people to care...like the military, farmers, insurance companies. Turning the Arctic into Georgia and more people care. Different people have different standards for existential threats, even if we don't kill everyone.
Oh, but back to Venus....so yea, CO2 is still on vacation in the atmosphere. 90 Earth atmospheres worth of it. And lead would melt there. That turns out to also not be fun for anyone else.
So Venus could have been habitable. In fact, maybe the first habitable planet in the solar system. then it became Mad Max. Then it became hell.
Blame the plant food.
/End Venus story.
Blame the plant food.
/End Venus story.
