Trees require CO2 to breathe, and release oxygen afterwards (iirc). So when will we run out of CO2?
The carbon cycle is the general term for the flow of carbon with an emphasis on carbon dioxide.
Plants and other biota convert CO2 into more complex forms of carbon. CO2 mostly gets released back into the atmosphere when the living organisms die and are degraded by other organisms. But some isn’t. Critters that build carbonate shells for instance. These materials can get further converted into minerals when the biota dies and the residue gets involved in geological processes. Limestone for instance. So you get carbonates incorporated into geology and removed from the cycle. Long term tectonic processes drive this back into circulation via volcanic activity. This is a very very long cycle.
Eventually even tectonic processes will stop. If life becomes extinguished and the plates stop moving CO2 may vanish. A billion years or so.
CO2 is the default - O2 is the anomaly. Both Mars and Venus have atmospheres largely consisting of CO2, and if life were extinguished on Earth, we’d probably revert to an atmosphere just like theirs.
6H20 + 6CO2 <=> C6H12O6 + 6O2
Going left to right, that’s photosynthesis.
Going right to left, that’s respiration.
Plants don’t need CO2 to breathe, they require it to produce carbohydrates.
Plants also respire, in the same chemical pathway to burn carbohydrate and produce energy that the non-photosynthetic life forms do. Ergo, trees breathe O2, like we do.
Now that we have factual answers, I’ll just add that there’s a trend, these past few years, to paint CO2 as really valuable. You’ll see posts on social media (and here) saying that plant cover is increasing thanks to increasing CO2 levels, isn’t it wonderful, we can grow plants to feed everyone, etc. It’s meant to influence public opinion on policies aimed at reducing CO2 emissions to fight climate change. Presumably this is sponsored (directly or indirectly) by the oil industry.
CO2 is heavy enough so that, barring some planetary catastrophe, it isn’t going to be lost to outer space. Also, the CO2 cycle, meaning that it alternates between a gas and being part of solid plant life, keeps us in balance.
CO2 and nitrogen, yes. That was what the prebiotic atmosphere on Earth was like. Earth has lots of carbon and oxygen, oxygen tends to combine with carbon, CO2 is therefore common in the atmosphere.
We’ll likely “run out of it” when the Sun grows large and hot enough to boil away Earth’s atmosphere entirely near the end of its life.
In addition to what others have said, plants only consume CO2 and release oxygen while they’re growing. A stable forest that’s already at full size is, on average, producing the same amount of CO2 as it’s consuming.
The easiest way to see this is that atoms are conserved. The total amount of carbon on the Earth can’t change. All that can change is what form it’s in: Mostly, either in the compounds that living things are made of, or simpler compounds that are left over millions of years after those living things die, or in the form of carbon dioxide gas. When a tree or any other plant is growing, it’s making more of the compounds that living things are made of, and it’s using carbon from carbon dioxide in the atmosphere to do so. When a plant gets eaten by an animal, or when it rots away (which is also getting eaten, just by smaller organisms), the carbon is converted back into carbon dioxide.
Just looking at the biology side of it, with plants growing and other things eating them, things tend to stay more or less balanced. The problem comes in with that third way that carbon is stored in the environment. Very rarely occasionally, when something dies, it gets buried enough that the carbon can’t return to the atmosphere. Over hundreds of millions of years, that carbon turns into fossil fuels like coal and petroleum. But now, we’re taking what took all those aeons to build up, and we’re burning it (and releasing the carbon back into the atmosphere) a million times faster than it took to accumulate. This is resulting in carbon dioxide levels in the atmosphere far higher than they’ve been for hundreds of millions of years, and rising very quickly. This is already causing horrible disasters, and will just get even more horrible if we don’t stop what we’re doing.
We should put plastic bags over those people’s heads and then ask them how much they like CO2.
okay thanks
in several hundred million years the earth will run out of CO2, killing pretty much all life on earth
Earth has about 420ppm of CO2 in the atmosphere. Pre-industrial times had 280ppm. C3 photosynthesis starts to fail when CO2 levels fall below 100-150ppm. The vast majority of plants use C3 photosynthesis, so in 600 million years or so, the vast majority of plants will starve to death due to lack of CO2.
C4 photosynthesis fails when CO2 levels fall below 10-20ppm. Corn, sugarcane and sorghum use C4. So there will be a period where CO2 levels fall enough that all the C3 plants are extinct, and all that is left are C4 plants. But those will eventually go extinct too.
Every billion years, the sun gets about 10% brighter. As the sun gets brighter it’ll cause more CO2 to be pulled from the atmosphere and locked into the soil as rocks. Over time the brighter sun will cause more CO2 to dissolve in the rain, and get locked into the soil as rock formations.
So roughly 600 million years from now, the sun will be bright enough that C3 photosynthesis stops due to lower Co2. Then in about a billion years, enough CO2 has been locked away in rocks that C4 photosynthesis stops. This will cause starvation and death of pretty much all animal life. All that will be left is microbes that use geothermal energy as fuel.
Also with no plants, the O2 in the atmosphere drops to near zero also eventually.
Right. I always thought that life on Earth might persist until the Sun reached the red giant phase. Turns out that life on Earth will likely go extinct (with the possible exception of extremophile bacteria) long before that.
It’s a sobering thought to realize that life on Earth is past the mid-point of its existence (i.e. has less time in front of in the future than the past).
Kind of like me personally on a vastly different time scale. 
On the same timeframe, it’s expected that the Sun will be so hot it evaporates the oceans. So take your pick of the solar disasters a half a billion years from now.
On shorter timeframes, besides releasing lots of CO2 from fossil fuels, we are also releasing a significant amount of CO2 from rocks when we make lime for cement. So CO2 loss to weathering of rocks or incorporation into shells/limestone is not an immediate problem.
Yeah there is a narrow window for life on land. Life first moved onto land about 400-500 million years ago (plants and animals). And then in another 600 million years or so from now, most of the plant life will go extinct.
However I’m assuming the C4 crops will just continue to evolve and change since they can survive in environments as low as 10ppm CO2. Animals will use them for food and they will hopefully produce enough O2 to keep life alive. But in a billion years all those plants and animals will be dead too.
Yeah. While the suns increased luminosity will remove CO2 from the atmosphere, it’ll remove CO2 at a rate of about 1ppm per million years. So over the course of 500-800 million years or so, CO2 will go from 400ppm down to 0ppm or so.
Human civilization adds 4ppm every year, so the numbers aren’t even close.
It would be extremely easy for an intelligent civilization to keep life alive through all this. They’d just need to add CO2 back into the atmosphere, but also reduce the amount of sunlight energy that hits earth. One method of doing this is using the gravity of meteors to slowly pull the earth away from the sun to match the sun’s increased size and luminosity. The sun gets bigger and brighter, so we just slowly pull the earth away from the sun to decrease the amount of energy that hits the earth.
Do you mean asteroids and not meteors? There was a proposal a few years back about setting up an asteroid to transfer orbital energy from Jupiter to the Earth by periodically swinging by one planet and then the other. Do it the right way and the Earth would gradually move away from the Sun while Jupiter moves very slightly towards the Sun. The swing-bys would only happen about once every 6,000 years. The transfer asteroid would be in an orbit that would be roughly perpendicular to that of the planets.