So often in enhanced greenhouse discussions we still hear how carbon dioxide emissions risk Earth suffering "runaway greenhouse" "just like Venus". The question is, does Venus really suffer from extraordinary greenhouse effect?
In recent months we have been showing you calculations (and scripted calculators) for Earth's expected temperature given solar output and our distance from the Sun. To compare Venus then, all we need is the planet's atmospheric pressure/temperature profile and we see that at an altitude of about 50Km the "morning star" has an atmospheric pressure similar to Earth's and a temperature of about 300 K (about 27 °C), slightly warmer than Earth's 288 K (15 °C).
Aha! Some will say, that's because Venus is closer to the Sun!
Well, yes and no. Let's look at the calculation a little more closely.
Venus is closer to the Sun at 0.723332 AU (1 Astronomical Unit = Earth's average distance from the Sun) which means a simple blackbody calculation (for that altitude) would be higher due to an insolation of ~2615 W/m2 compared with Earth's 1367 W/m2 resolving to an equilibrium temperature of 328 K (55 °C). Taking into account the higher albedo of the bright little planet (about 75% compared with Earth's ~30%) drops that to 232 K (-41 °C) and requires a feedback (greenhouse effect) of 65% to yield the observed ~300 K (27 °C). Of course, some or all of this albedo could come from below 50 Km, in which case our greenhouse feedback is anything from a little to a lot too high -- we'll just assume it's sourced at or above 50 Km for the sake of these calculations.
Told you! Say the enhanced greenhouse fraternity, it's hotter due to greenhouse.
Actually, we wouldn't get just too excited about that. Venus has an atmosphere somewhat different from Earth's with 96.5% CO2 (965,000 ppmv) rather than our 0.039% (385 ppmv). This suggests 2500 times the CO2 concentration delivers at most a trivial 1.6 times the greenhouse effect (Earth's feedback from GHE is thought to be about 40% or about six-tenths that of Venus).
So why has Venus got a surface temperature of about 735 K? Pressure. Venus has a massively dense atmosphere and a surface pressure a little over 90 times that of Earth. Its listed adiabatic lapse rate, dry, is 10.468 K/Km, so 50 Km deeper into the atmosphere delivers an expected warming of 50 x 10.468 = ~525 K, meaning it is actually about 100 K cooler than expected at the surface. In fact Earth's environmental lapse rate is about 6.5 K/Km rather than the listed dry rate of 9.76 too, so the probe-measured rate of ~8 K/Km for Venus down to 10 Km altitude makes more sense.
Because Earthlings are somewhat ethnocentric, let's look at what would happen if we added 50 Km to Earth's atmosphere:
50 x 6.5 = 325 + 288 = 613 K (340 °C). Always provided the lower environmental lapse rate held in a massively increased atmosphere that's a little cooler than Venus but allowing for the difference in solar irradiation not by much. Of course, if the lapse rates of similarly massive atmospheres aligned then there'd be just the difference accounted for by solar proximity, meaning Venus is not experiencing any enhanced greenhouse effect from all that CO2 at all.
Where does that leave us? For one thing it means we are certain Earth is not like Venus and humans cannot make it so by tweaking trace atmospheric constituents. To make Earth another Venus we'd need to add another 90 atmospheres (all CO2) and get rid of all the water on the planet. Venus is not Earth's "evil twin" and does not appear to be significantly warmed by enhanced greenhouse effect. In fact there's the possibility (probability?) some or all of the morning star's albedo is sourced below 50 Km, making its greenhouse effect even smaller than calculated above.