"2. The planets Venus and Mars have atmospheres that are almost entirely CO2 (97%) yet they have no runaway greenhouse heating effect. Their temperatures are stable."
Yes Venus has experienced a runaway greenhouse effect, that is why it now has a surface temperature of over 460 ?C. That is why its atmosphere is hotter than Mercury's, which is closer to the sun. It has reached an equilibrium now, but if you don't call that a runaway greenhouse effect then I have no idea what you call it. The fact that temperature is stable means that it has reached a point of equilibrium. You could argue the earth's atmosphere is not stable or in equilibrium right now, because human activity is changing the composition of it. I would have thought this was obvious, but clearly it is not.
From Wikipedia:
"Atmosphere and climate
Venus has an extremely dense atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The atmospheric mass is 93 times that of Earth's atmosphere while the pressure at the planet's surface is about 92 times that at Earth's surface?a pressure equivalent to that at a depth of nearly 1 kilometer under Earth's oceans. The density at the surface is 65 kg/m? (6.5% that of water). The CO2-rich atmosphere, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect in the Solar System, creating surface temperatures of over 460 ?C (860 ?F).[36] This makes the Venusian surface hotter than Mercury's which has a minimum surface temperature of −220 ?C and maximum surface temperature of 420 ?C,[37] even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. The surface of Venus is often said to resemble Hell.[38]
Studies have suggested that several billion years ago the Venusian atmosphere was much more like Earth's than it is now, and that there were probably substantial quantities of liquid water on the surface, but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.[39]
Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of the Venusian surface does not vary significantly between the night and day sides, despite the planet's extremely slow rotation. Winds at the surface are slow, moving at a few kilometers per hour, but because of the high density of the atmosphere at the Venusian surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even if the heat and lack of oxygen were not a problem.[40]
Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.[41][42] These clouds reflect about 60% of the sunlight that falls on them back into space, and prevent the direct observation of the Venusian surface in visible light. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well lit. Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth days.[43] Venusian winds move at up to 60 times the speed of the planet's rotation, while Earth's fastest winds are only 10% to 20% rotation speed.[44]
The surface of Venus is effectively isothermal; it retains a constant temperature not only between day and night but between the equator and the poles.[1][45] The planet's minute axial tilt (less than three degrees, compared with 23 degrees for Earth), also minimizes seasonal temperature variation.[46] The only appreciable variation in temperature occurs with altitude. In 1995, the Magellan probe imaged a highly reflective substance at the tops of the highest mountain peaks which bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide (galena).[47]
The clouds of Venus are capable of producing lightning much like the clouds on Earth.[48] The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. However, in 2006?07 Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.[48] In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of the planet.[49][50]"
