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We've been researching how much CO2 is pulled from the air by a nice green lawn.  This is important for High Frontier since your colonies start out with quite a lot of grass, unless you either chose not to include soil, or have a water level so close to the surface that you get sand instead of grass.

We found this claim that one acre of well-managed grass (lawn) stores about 920 lbs of carbon (not CO2) per year.  We can convert this to CO2 by multiplying by 44 (the weight of CO2) / 12 (the weight of C), for 3400 lbs of CO2 per acre per year.  In modern units, that's 0.38 kg per m^2 per year.

Curiously, that works out to almost exactly 1 g per square meter per day!

An active person exhales about 1 kg of CO2 per day.  So, you would need 1000 square meters of grass to absorb one person's CO2.  That's a square about 32 meters on a side, or in archaic units, a quarter acre.  A one-square-kilometer park could support 1000 people, just with the grass.

More fun comparisons: New York's Central Park, which is 3.41 km^2, could support 3410 people with just the grass.  But there are also about 20,000 trees in that park.  Every 16 trees or so can absorb the CO2 of one person, so when you put it all together, Central Park could support over 4600 people indefinitely (at least, as far as respiration goes).

Now you see why so many of the buildings in High Frontier have grass on the roof!  Grass is photosynthesis, in a really convenient package.
Well, it took longer than expected, but we finally got the effect of grass worked into the simulation.  We sample your colony and see where we find grass, sand, water, or some building or path.  From those samples (and things like soil depth) we calculate the "grass fraction" — the portion of your livable land area covered in grass.

One thing we caught while working on this: even if you've designed your colony with no soil, we still show grass when you enter it.  My friend (and High Frontier advisory board member) Al Globus advocates for omitting soil from early colonies, as he feels it's not worth the expense.  So, in his honor, we'll also be making it so that if you have less than 0.25 m of soil in your colony, it's no grass for you!
Good evening. Quick question that I was wondering. Do the tree's you place in High Frontier absorb CO2 and emit O2?
Yes, they do! Both trees and grass are fully simulated with respect to the carbon cycle.