I begin this post with a fairly lengthy science-y preamble, whose purpose will become clear by the end (but before the footnote!)

The planets of our solar system can be broadly divided into two groups, the small rocky inner planets (Mercury, Venus, Earth, Mars) and the four gas giants (Jupiter, Saturn, Uranus, Neptune). I omit Pluto from this discussion, as its position as a planet versus one of the largest of many large distant icy rocks of the Kuiper belt is at best ambiguous.

Here I focus on the four rocky inner planets, and in particular, what specific aspects of Earth's geology have made it hospitable for our kind of life (carbon based, using photosynthesis as the ultimate energy source, primarily oxygen-burning, made up of proteins and nucleic acids among other biomolecules, and so on). Some comparisons with the other three rocky inner planets are important, especially with regard to the ongoing and potentially catastrophic climate change.

The first and most important point to note is the relative distances of the planets from the sun. The amount of energy received by each planet relatively decreases as the square of distance, due to the inverse square law (a consequence of our universe having three dimensions, and therefore surfaces having two). Most significantly for this discussion, a square inch of Earth (93 million miles from the Sun on average) receives about half the solar energy of a square inch of Venus (67 million miles from the sun on average).

The surface temperature of a planet depends on more than just the energy it receives from the sun, it also depends on how much energy is reflected back into space immediately by the upper atmosphere (if any), also internal geological processes such as radioactive decay. But given that Venus receives about twice as much energy as Earth per square inch (at the top of the atmosphere that is), why are their surfaces so different? Venus' atmosphere is a hellish 700 degrees C or so, hot enough to melt lead. It's mostly carbon dioxide and water vapor, with nitrogen and argon as on earth. No free oxygen to speak of. Incessant winds of hundreds of miles an hour. Sulfuric acid rain. Nothing like our present day Earth, low pressure atmosphere mostly nitrogen and oxygen, rest mostly argon, bit of water vapor and of course growing CO2 (now at 0.04% see my previous post).

There is not one simple answer to this question, however some geological and venerological facts offer suggestions. There is actually about the same amount of nitrogen in Venus' atmosphere as in Earth's, but because of the much greater amount of CO2, nitrogen makes up a smaller proportion of Venus'. The key difference is the amount of CO2. The fascinating thing I recently learned was that the amount of carbon in Venus' atmospheric CO2 is fairly comparable (maybe not exactly the same) as the amount of carbon believed to exist on earth, where it is mostly in living organisms or in fossil fuel and other rocky deposits beneath the surface (such as graphite and diamond). In other words, Venus is what Earth could become if a lot more of its carbon was oxidized (i.e. burned) to CO2 and released to the air. Oxygen is more complicated, as even on earth most oxygen is tied up in rocky ores. But it seems to me that the carbon balance is the most relevant.

It's accepted that Venus' atmosphere has evolved over the past billions of years, just as Earth's has. It's hard to know whether it was always so inhospitable, although some scientists have proposed possible scenarios in which long ago Venus could have had a similar atmosphere to Earth's of that time (not today). They conjecture that tremendous volcanic activity, together with the intrinsically greater amount of solar energy absorbed at Venus' distance from the sun, caused much of its carbon to be oxidized to CO2, which built up in the atmosphere to the current tremendous levels of heat and pressure. Also at an early stage the higher general temperature meant that the atmosphere could hold more water vapor than Earth's, which also contributed to a greenhouse effect.

The point of this extended discussion of solar system -ology is that things change, and not necessarily for the better as far as our form of carbon/DNA-based life is concerned. Even though Earth receives half the solar energy as Venus, there is no obvious geological obstacle to Earth's atmosphere becoming extremely, it not as, inhospitable as Venus'. We just need to burn a lot more carbon, enough to initiate a positive feedback loop including water vapor. And since burning carbon is how our society works, endless economic growth implies more burning carbon. When some politician talks about jobs, just show him some molten lead and ask if he would enjoy watching his grandchildren swimming around in it. That is where we are headed if we don't either burn less carbon, figure out how to sequester most of the CO2 released, or both.

Incidentally, we can offset as much of this as we like with sustainable energy, but all processes in the universe generate heat, so even a completely "green" industrial economy has a problem with how to radiate the excess heat generated back into space. Burning carbon speeds our problem up, a lot, but isn't the sole source of the problem. For the moment however, increased CO2 production is the most pressing problem. It'll be a while before we need to think about moving Earth further from the sun (if that is even possible, see Larry Niven's Ringworld for the idea).