Updated 9/21/17: clarified lack of water on Venus and time line
The Venus Myth
One sees citations in the Green popular literature about Venus being a runaway greenhouse effect. There are two implications in this to scare us about global warming. One is that Venus once had a climate like the present Earth climate that was trashed by a runaway greenhouse effect. The other is that if Man continues to burn hydrocarbons for energy, Earth will suffer a runaway greenhouse effect and end up like Venus today. You won’t see this in refereed scientific journals because both implications are scientific impossibilities.
4.3 billion years ago, when their crusts stabilized, both Earth and Venus had similar climates. Both were essentially hot, volcanic hell-holes with completely toxic atmospheres for humans. All the water was in the form of steam in the atmospheres of both planets. Both planets began to cool, but that was when the similarities quickly ended. Venus cooled very little. That’s because Venus is smaller than Earth and closer to the Sun. The energy flux from the Sun for Venus is 661 watts/square meter while the energy flux for the Earth is 340 watts/square meter (adjusted for planet curvature). So Venus gets almost twice the heat flux from the Sun as Earth. The Earth has a larger surface area, so the total energy is somewhat greater for Earth. However, that energy is dissipated in the planetary volume and Earth has a vastly larger volume than Venus to heat up. In addition, Earth’s black body radiation is very much greater than Venus’ because of Earth’s larger radius (the black body radiation is in all directions, not just back towards the Sun, so there are no area adjustments for curvature). The bottom line is that Venus could never cool enough for liquid water oceans to form until much later than the Earth formed liquid water.
For Venus, there was another problem. Unlike Earth, Venus’ magnetic field is tiny and provides virtually no protection from solar radiation. Thus Venus’ upper atmosphere receives orders of magnitude more hard radiation than Earth’s. That radiation disassociated the steam molecules into hydrogen and oxygen. The hydrogen was blown away in the solar wind and the oxygen bonded with metallic ions from the violent volcanism to precipitate out of the atmosphere. The result was that Venus was dry as a bone by 4b years ago when Earth was just beginning to condense steam into oceans. Thus Venus never had liquid oceans on its surface.
Finally, the lack of water meant there was no lubrication for plate tectonics like on Earth. The tidal forces on Venus are much greater than on Earth, so that produces lots of fracturing and friction in the crust. That friction heat is the source of Venus’ massive and almost continuous volcanism that continues to put CO2 into the atmosphere to this day. Thus the “runaway greenhouse effect” was the result of lack of water.
Now let’s look the hypothesis that our present Earth could have a runaway greenhouse effect that would result in a Venus-like climate. That is not possible because on Earth we have prolific carbon-based life. Photosynthesis and respiration from plants and animals form a dynamic and self-correcting system that precludes any sort of runaway greenhouse. That’s because when CO2 increases in the atmosphere, plants evolve to do more photosynthesis while animals find it more difficult to cope in a CO2-rich atmosphere. Thus the major CO2 producers die off or evolve into smaller critters.
We can see this pattern repeated several times in the geologic record. Every time the Earth warms to the point of a tropical paradise where large animals with high respiration and methane production dominate, we soon get an Ice Age (assisted strongly by plate tectonics and atmospheric Hadley Cells). The reality is that extreme climates with temperatures above 30°C usually result in mass extinctions. Those mass extinctions kill off the larger animal species first, and those are the marginal producers of CO2 via respiration. The last to go are the algaes and single-celled organisms doing photosynthesis. So as long as the Earth has multi-cellular carbon-based life, there is no way that the climate can run away and produce massive amounts of CO2.
In fact, most major mass extinctions have been triggered by catastrophic events like asteroid hits, supervolcano eruptions, plateau basalts, and sublimation of methane hydrates. Those events dumped vast amounts of greenhouse gases into the atmosphere – often vastly more than the entire carbon cycle combined — in a very short time (geologically speaking) that temporarily overrode the self-correcting balance between photosynthesis and respiration. But such events also hastened extinctions that drove towards an excess of photosynthesis compared to respiration. Thus the planet almost immediately reacted to even catastrophic events by purging CO2 from the atmosphere.
Suppose the Earth gets hit by a gamma ray burst and all life is killed off. The amount of CO2 in the “goldilocks” zone that supports our climate is far too small to grow into a runaway greenhouse effect. Instead the Earth would simply revert to being an ice ball again, like 700 million years ago.