Why is Venus much hotter than Earth?

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I once read a comment in a Global Warming blog that flatly stated that the reason why Venus is hotter than Earth is the Greenhouse Effect.  There was even a comment to the effect that Venus once had life and a carbon cycle, but something went horribly wrong.  Now I hear Bill Nye “The Science Guy” telling Joe Bastardi that Venus is as hot as it is because of CO2.  Penn and Teller, did you hear that?

It’s nonsense.  It is true that all planets that with atmospheres have greenhouse effects that depend on the nature of the atmospheric gases.  That is why Mercury is cooler than Venus – Mercury has almost no atmosphere.  It is also true that Venus has a runaway Greenhouse Effect that exacerbates its temperature.  That is because it simply is too darn hot to sustain a carbon cycle and could never be cool enough to sustain one.  The high CO2 level and strong Greenhouse Effect are the effects of the heat, not the initial causes of it.  Keep reading, and think about it.

Irradiation from a point source diminishes proportionally to the square of the distance from that point.  Earth is one Astronomical Unit from the sun; Venus 0.725 AU.

In AUs, the square of Earth’s distance from the sun is 1.0; the square of Venus’s 0.525625.  Thus (in the absence of other factors) solar irradiation per sq metre at Venus is 1.902497 times that on Earth.  Yep, that’s nearly twice the intensity.  So one could expect Venus to be much hotter than Earth.

The average surface temperature on Earth is 290 K; the average on Venus is 737 K,, 2.541379 times that of Earth.  OK, that is two and a half times Earth’s temperature, more than you would expect.  Earth sustains a carbon cycle.  Venus has a C02-rich atmosphere with a surface pressure 90 times greater than the Earth’s, and a runaway greenhouse effect.  But that is not the prime cause of the heat.

Earth and Venus have similar diameters and masses.  They have very different atmospheres, rotations and inclinations.  Earth has approximately 365 solar days per year.  The Venus year is 224 Earth days, and its solar day is more that half of that, about 118 Earth days.  The daylight hours on Venus last 59 Earth days.  Venus has an inclination of only about 3 degrees, so does not have Earth’s seasonal variations. Much of Earth is covered by oceans, which form a vast carbon sink.  Venus has no oceans.

If a clone of the Earth was placed in the orbit of Venus, and had Venus’s  rotation and inclination, what would you expect to happen to the surface temperature?  Would it remain cool enough to sustain its carbon cycle?   Nope.  The carbon cycle has a snowball’s chance in hell, and hell is what Clone-Earth would become.  What would happen is something like the speculation that follows.  It may not happen over-night (Clone-Earth night).  I am unsure of how long the refrigeration effect of the evaporating oceans and the resulting rainfall would mitigate the temperature rise.  But it would happen:

Clone Earth Day Zero, First Half:

On the sunrise side (which is now in the west) all life, plant and animal, and all of the oceans on the daylight side face the first Clone-Earth day.  This is a day fifty-nine Earth-days long, with continuous heat from twice as much solar energy per square metre as on Earth.  Trees begin to die and dry up; animals expire.  The ocean surfaces and the air above heat up strongly, generating the first wave of a powerful greenhouse gas –  huge clouds of water vapour.  The increasing heat and the water vapour give rise to cyclonic winds and torrential rains.  The cooling effect of the evaporation and the rains mitigates the temperature rise, but not for long.  The rains very quickly evaporate in the heat, and as the temperature climbs above 373 K (100 degrees Celsius, the boiling point of water at standard atmospheric pressure), rain ceases.  The water vapour turns to steam.  The boiling oceans are now releasing the second wave of greenhouse gases – gigatons of carbon dioxide.

On the sunset side (the east), all life faces fifty-nine continuous Earth days worth of darkness and bitter cold.  No subsequent night on Clone-Earth will ever be that cold again.  By the end of that long night, all of the trees (including the conifers) are frostbitten to death, and most if not all of the animals.

Clone-Earth Day Zero, Second Half:

On the sunrise side, the dead trees and animals begin to dry out very quickly.  Fires start up.  On the sunset side, the fires are carried into the night.  By now, they are of sufficient intensity to continue burning in the cold.  Besides, the temperature on the night side is no longer so cold, as strong hot winds are blowing around the globe, and the atmosphere already has a heavy blanket of water vapour and carbon dioxide, trapping in the heat.  From this point onward, the temperatures on the night side will steadily rise, night after long night.

Clone-Earth Day One Onward:

Then, the forest fires really begin to take hold.  By the end of the second Clone-Earth day, there are forest fires fanned by very strong winds.  The carbon cycle is over – that is, life is extinguished.  The Green house Effect is running away.  As long as there is free oxygen in the atmosphere, all carbon, sulphur and combustible material exposed on the surface will burn, until CO2 and SO2 levels reach saturation.  After that, all carbon dioxide, sulphur dioxide and hydrogen sulphide released from the ongoing volcanic events over millions of years will remain in the atmosphere, and the percentage of CO2 in the atmosphere will climb inexorably to the saturation level on the Venus of the present day.  So will the atmospheric pressure, as the dense C02 settles.  And so will the temperature.  As the atmospheric pressure increases toward the present-day levels on Venus, raising the dew-point, the rains will return.  But they will no longer be cooling water.  They will be super-heated acid.  In the end. there will be little difference between the day and night-time temperatures.

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