At a steady rate they consume twice the CO2 humans put into the air; but they also die, thus releasing all of that carbon back as CO2. Short answer, there is NO simple solution like trees or algae etc, largely due to the above.
The math says that each year humans ncollectively emit 40 billion tons (scientific notation, 8E13 US pounds of CO2 due to all the coal, methane, and liquid carbon-based fuels we consume.
We jam 8E13 pounds of CO2 into the air each year, so pulling all of it back out would leave us at a steady state THAT ISN’T REALLY STEADY. Why isn’t it steady? Because the effect of each gigaton of C02 is cumulative. The planet continues to swelter under each annual new layer or blanket until the heat leaking back out into outer space (there is no place left for it to go inside the atmospheric envelope) finally catches up with that increment. It’s cumulative!
So let’s be simple and say we’ll harvest 8E13 pounds of CO2 per year; the climate is still going to get hotter year over year, for some while. Thus we need to harvest more than that, say twice–1.6E14 pounds of CO2 annually for multiple decades to put the genie back into the bottle.
Since around 1700 we’ve taken the climate’s steady state machine and thrown sand in the gears by nearly doubling the amount of CO2 in the air, and if we’re not quite there yet, we will be soon.
What does this mean? We need a way to permanently sequester carbon; so far no one has patented an ecomonically sound answer to that wee small riddle.
Second, doing that requires staggering amounts of energy. When C2 meets two O2 and make two CO2, they release a lot of energy, such as fire; see what I mean? Prying them back apart by brute force means finding all of that energy (atomic fusion) or doing something a little more clever like making calcium carbonate (CaCO3 – an O2 plus a couple of Ca’s plus two CO2’s would be the raw ingredients. Shellfish do this all the time, and corals likewise.
So, how much calcium carbonate would we need to remove two annual loads of CO2 from the atmosphere? Skip the math part:
((Numerically, one CO2 has a mass of 44 while CaCO3 has a mass of 100. So to clear 8.8E13 pounds of CO2 we need, in abstract units, 12 of C, 48 of O, 40 of Ca. CO2 = 12 + 32 = 44, and CaCO3 40 + 12 + 489 = 100.))
((Or in other words we need 40 of Ca to add to 44 of CO2 plus 16 of O (the air is loaded with that) to produce 100 CaCO3 per 44 of CO2. SO: 1.6E14 of CO2 will make 160/44 E14 of CaCO3, or 3 5/8 E14 pounds of calcium carbonate.))
((How big is that? Its density is 2.71 gm/cc; converting pounds into grams we get (1.34 * 453.6)E14 cubic centimeters of CaCO3, or roughly 6.1E(16–9) or in round numbers))
6E7 cubic meters of marble. For instance, 1000 meters by 1000 meters by 60 meters. Or in cubic kilometers, 60.
Anybody ready to re-face all the buildings in the world, annually, with man-made marble?