.12% energy increase from .025% atmospheric composition change. That's a big delta from a tiny change.
Is there a reason you feel that way? The explanation is pretty simple but the physics to calculate the magnitude of the effects are non-intuitive so you shouldn't expect a 1:1 relationship.
Let’s go over an example that might help you accept that fact. Sulfur dioxide is a chemical where a tiny amount can have a huge effect on climate. It will take a little bit of explaining to show why.
People intuitively know that the closer a planet is to the sun the more energy it receives and the warmer it is.
Mars is farther away than the Earth is so it’s colder while Mercury and Venus are closer so they are much warmer.
Now there’s a simple formula called the Stefan-Boltzmann Law that can be used to calculate how much energy the sun produces based on its surface temperature. The sun produces 3.846x10^26 watts. Divide that by the suns surface area and you get 63.15MW/m^2.
Using the fact that physics says energy is neither created nor destroyed we know if there are 3.846x10^26 watts on a sphere around the surface of the sun there are the same number of watts on a sphere with a radius equal to the distance from Mercury to the sun, from Venus to the sun, etc.
Since the spheres are much larger the energy per area must drop. Do some geometry and you can figure out about how much energy reaches each planet.
The following table shows how much solar energy per meter squared on average (S ave) reaches each of the inner planets.
So Mercury gets 6.7 times as much energy as we do. Venus gets 90% more and Mars only gets 42% as much as Earth.
Now since you are on Anandtech you are likely aware of how computers behave when they are put under load. At idle pulling 15w the CPU is at say 30C. When you run a benchmark and it pulls 100w the temperature ramps up to say 65C and stays there. That’s called
steady state. When the benchmark ends and the power drops back to idle the temperature drops back down to a lower steady state.
At steady state, energy in equals energy out. What temperature you reach depends not only on the amount of energy but also the type of system. I’m sure you are familiar with incandescent bulbs. A 100w bulb may pull the same energy as our computer example but the temperature of the filament is over 4000F.
All thermal systems including planets follow this basic behavior.
Since we know how much energy reaches each planet, we known thermal systems will try and reach steady-state where energy in = energy out, and the only way for energy to leave a planet is via radiation. We can again use the Stefan–Boltzmann law to try and predict the average steady-state temperature of each planet.
If we try with Mercury we find that our predicted temperature is too high by about 10%. That leads me to explain another concept called a “black body”. The Stefan–Boltzmann law assumes the object in question is a black body. A black body is an object that absorbs all frequencies of light and radiates in all frequencies of light. A cold black body would look black in visible light, hence the term.
The sun very closely radiates as a black body. Mercury on the other hand does not. While being dark and absorbing most light from sun it does reflect about 10% of the light from the sun. If it didn’t we wouldn’t be able to see it in visible light. The light lost to reflection is called “albedo”. It’s the “alpha” on the chart above”
When we redo the calculation taking into account Mercury’s 10% albedo using only 90% of the energy coming from the sun we get an average planetary temperature of 437K (Tp above) which is very close to the observed temperature of ~440K (Tobs). That’s pretty good!
However when we redo those calculations for the other planets it under predicts the temperature by a little for Mars, a bit more for Earth, and is wildly wrong for Venus.
Venus gets less than a third of the energy of the sun that Mercury gets yet its almost 2 times as hot. Not only that but it’s albedo is an extremely high 75% meaning it actually receives less energy at the surface than Earth does!
If you’ve ever looked at Venus in the night sky it shouldn’t be a surprise that it has such a high albedo. Venus is very bright due to all the reflected sunlight.
So why does Venus reflect so much light?
It’s from the all the light colored clouds. Which is where sulfur dioxide ( SO2) comes in. Those clouds are made from SO2 which forms droplets of sulfuric acid.
This next table includes the atmospheric pressures and compositions for each of the planets
Venus’s atmosphere is 96.5% carbon dioxide, 3.5% nitrogen and the rest are trace gases. According to the inter webs SO2 makes up 150PPM or 0.015% of the atmosphere.
So SO2 at 0.015% of the atmosphere can block 75% of the incoming solar energy.
Venus also shows what high pressure CO2 atmosphere can do. With significantly less energy reaching the surface than Earth the surface temperature is hot enough to melt lead.
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