I always heard that the rain forests were "lungs of the Earth" as they use sunlight to convert Carbon Dioxide in to Oxygen which we need to breath. The idea is that they create the Oxygen we all need to live.
And then, more recently, I heard that they are pretty much neutral. Indeed this article points out that the Carbon they extract ends up decomposing and releasing that Carbon Dioxide back in to the atmosphere (using Oxygen in the process). This makes perfect sense and made me wonder why I did not question the "lungs of the Earth" idea in the first place. There are other articles that point out that they will vary absorbing or releasing Carbon Dioxide over periods of years, so a sort of "capacitor" for Carbon Dioxide on the planet, but not the power plant for Oxygen.
So I pondered where does the Oxygen I need to live come from. Well, there is an incredibly simple answer!
The Oxygen I need is used to burn (oxidise) the Carbon from my food. That Carbon in my food ultimately comes from plants (directly, or via other animals). Those plants get the Carbon from Carbon Dioxide in the atmosphere, and using sunlight as an energy source they release Oxygen when they absorb that Carbon. Indeed, the sunlight's energy used to split the Carbon Dioxide, is the energy released when Carbon is burned in my body.
So there is a perfect balance. The exact amount of Oxygen I need in order to burn the Carbon in the food I eat is exactly the amount that was released by the plants that absorbed that Carbon from the atmosphere in order to end up in my food!
Of course, I also end up with some Carbon in my body as part of my body, or leaving my body by other means. That Carbon gets released as Carbon Dioxide when my poop decomposes, and finally when my body decomposes after I die - that decomposition needs exactly the amount of Oxygen that was released by plants that absorbed that Carbon.
So, there should always be exactly enough Oxygen for us to burn the food we eat simply because that food only exists because exactly that much Oxygen was released by photosynthesis in the first place.
Nice isn't it :-)
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Up to a point, Lord Copper (that point is to a first approximation that *all* biological processes are in equilibrium, with equal amounts of atoms flowing in both directions: all enzymes do is change the equilibrium concentration on both sides). The majority contributor to photosynthetic oxygen in the atmosphere is probably the tiny blue-green alga Prochlorococcus marinus, the smallest photosynthetic organism known and perhaps the most numerous organism on the face of the Earth (with a population numbering in the octillions). The figures are unclear (P. marinus is so small it passes straight through most filters), but the best guess right now is that something between 30% and 70% of atmospheric oxygen replenishment is down to this one extremely diverse genus of cyanobacteria.
ReplyDeleteNote that atmospheric oxygen *replenishment* is different from *abundance*: any given atom of oxygen remains in the atmosphere for a very long time because there is just so much of it. Consider where the oxygen in the atmosphere came from in the first place. Our best guess is that it was bound up as CO2 in a Venusian atmosphere at perhaps a hundred times present-day pressure. So where did all that CO2 go? It was turned into carbonate rock, mostly by living things: the remnant oxygen mostly went to oxidize exposed sediment and the oceans (there are banded iron sediment beds *miles thick*), a process taking maybe a billion years all told: the oxygen remaining after that (maybe 10% of the total before mass oxidation of rocks began) just... built up in the atmosphere. (It is easy to imagine that alternative earths with a bit less atmosphere could have ended up with no oxygen at all after this process was over.)
The magnitude of this process was stunning. On the order of 20,000 times as much carbon is stored in carbonate rocks as is present in the entire atmosphere-and-living-organisms part of the carbon cycle. Now and then you hear the dimmer sort of environmentalist (the sort who ignores science in favour of saying that fracking will puncture the Earth and flood it, I wish this was a joke) saying that burning fossil fuels will use up all the oxygen in the air. The figure above shows just how far from true that is, unless the fossil fuel companies find a way to use teratons of limestone as fuel (yeah right, I don't think so).
They're a bit more than a capacitor, in that they permanently hold the topsoil and associated carbon in place. Slash and burn the forests and you release all the carbon tied up in the soil below and often turn that land from forest -> desert as the top soil gets blown into the Atlantic. We're not going to suffocate from this but it's not a smart thing to do.
ReplyDeleteNot quite the full picture.
ReplyDeleteRuminant animals - especially cows - create a lot of methane, which is 23-27 times more potent as a greenhouse gas (CO2e) than the air & food they consume.
Then there's the shit->ammonia->ozone+particulates processes, combined with transport and processing emissions.
Chickens are much better, but also not a permanent solution. People simply need to eat significantly less meat - roughly once per week or month. I try to avoid meat now, and cycle to work rather than drive. No flying either.
The amount of available oxygen doesn't really matter if half the planet turns to arid desert at 4C warming. Russia & Canada would produce & control almost the entire global food supply.
We might not be that far off from a runaway warming scenario - maybe 100 to 200 years, maybe even less. Scientists are scared shitless.