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Humans as a geological force
There are 149 million square kilometers of land, including all the desert, mountains, jungle and tundra. There are about 7 billion humans. This comes out to 2.12 hectares per person, or in snowflake units, 5.2 acres. An American football field with endzones is 5353 square meters, or 1.33 acres; a pro football/soccer field is 7140 square meters, or 1.76 acres. So, depending on where you live, everyone gets 3-4 football fields of their own.
I strongly suspect that with fire and metal tools, let alone power ones, even a wimp like me could totally deforest all that area. Probably not keep tree shoots and saplings from springing up -- that'd be what goats are for -- but chop down all the multi-year large trees and keep new ones from growing far. Also expect that with guns I could kill all the large animals I didn't want. So basically, humans evenly distributed could make all large animals and many trees go extinct, or undergo severe selective pressure.
As for terraforming in the most literal sense, you can imagine how much dirt you could schlep around your five acres. A deep grave is 2x1 meters x 2 deep, and seems doable in at least a day, if not a few hours. (2x2 x 1 deep would probably be easier.) 28 years to dig out your land at that rate. Non-trivial, but conversely the human race could turn over the Earth's whole surface two meters deep in half a lifetime. That's pretty geological. And that's people with shovels, not backhoes.
Also you get to imagine staying alive by farming 5 acres. Or, probably half of that or less, what with the mountains and deserts and tundra and such.
Earth's atmosphere is 5e18 kg. We breathe roughly 10 liters a minutes, or 15 kg/day. So the human race breathes 3.8e13 kg a year, or about 1/100,000 of the mass of the atmosphere. Okay, that doesn't seem huge.
A human's metabolic energy is about 100 Watts. An American uses 10,000 watts via various means, as do Canadians and Scandinavians. So an American is using the energy of 100 humans -- and the 300 million Americans are using enough energy for 30 billion people, and 4x the metabolic energy -- and respired air -- of the whole human race. Humans as a whole are using 1.7e13 watts, vs. American 3e12 watts, and world metabolic 7e11 watts, so with fossil fuels we're "breathing" 1/3,000th of the atmosphere per year.
There's about 3e15 kg of CO2 in the air, vs. the roughly 1e15 kg of air that we "breathe" industrially.
Average land rainfall is 72 cm/year, leading to about 14,500 m3 of water on your 5 acres. Americans use 1880 a year, about half of which is for power plant cooling. Collecting all the water that falls on your land may be a challenge, depending on terrain and climate. So, not using all the water, but definitely making a dent.
I strongly suspect that with fire and metal tools, let alone power ones, even a wimp like me could totally deforest all that area. Probably not keep tree shoots and saplings from springing up -- that'd be what goats are for -- but chop down all the multi-year large trees and keep new ones from growing far. Also expect that with guns I could kill all the large animals I didn't want. So basically, humans evenly distributed could make all large animals and many trees go extinct, or undergo severe selective pressure.
As for terraforming in the most literal sense, you can imagine how much dirt you could schlep around your five acres. A deep grave is 2x1 meters x 2 deep, and seems doable in at least a day, if not a few hours. (2x2 x 1 deep would probably be easier.) 28 years to dig out your land at that rate. Non-trivial, but conversely the human race could turn over the Earth's whole surface two meters deep in half a lifetime. That's pretty geological. And that's people with shovels, not backhoes.
Also you get to imagine staying alive by farming 5 acres. Or, probably half of that or less, what with the mountains and deserts and tundra and such.
Earth's atmosphere is 5e18 kg. We breathe roughly 10 liters a minutes, or 15 kg/day. So the human race breathes 3.8e13 kg a year, or about 1/100,000 of the mass of the atmosphere. Okay, that doesn't seem huge.
A human's metabolic energy is about 100 Watts. An American uses 10,000 watts via various means, as do Canadians and Scandinavians. So an American is using the energy of 100 humans -- and the 300 million Americans are using enough energy for 30 billion people, and 4x the metabolic energy -- and respired air -- of the whole human race. Humans as a whole are using 1.7e13 watts, vs. American 3e12 watts, and world metabolic 7e11 watts, so with fossil fuels we're "breathing" 1/3,000th of the atmosphere per year.
There's about 3e15 kg of CO2 in the air, vs. the roughly 1e15 kg of air that we "breathe" industrially.
Average land rainfall is 72 cm/year, leading to about 14,500 m3 of water on your 5 acres. Americans use 1880 a year, about half of which is for power plant cooling. Collecting all the water that falls on your land may be a challenge, depending on terrain and climate. So, not using all the water, but definitely making a dent.