Photosynthesis, hypothetical question

[Starkers]

Hi, I don’t tend to post here very often, but I had a query relating to a story I’m writing and since I can’t seem to find the answer I’m looking for via Google I thought I’d try people smarter, or at least more scientifically minded, than me

My hypothetical query relates to what would happen if photosynthesis ceased to work (for the sake of argument lets say aliens fire a beam at the earth that accomplishes this!).

Well, actually not so much a ‘what would happen’, as a ‘how long it would take’. I know we’d be screwed, plants would die meaning starvation, and worse they wouldn’t be pulling carbon dioxide out of the atmosphere and replacing it with oxygen so we’d all choke to death…my question is though, how long before the air became unbreathable?

Obviously the atmosphere still has a lot of oxygen, but how long before it dropped to levels where life could no longer function?

As I say the internet’s been a bit rubbish in answering this, I’ve seen estimates ranging from a few days or weeks (which seems a ridiculously short time) to several thousand years (which seems way too long).

Obviously I realise there won’t be a definitive answer (because a lot may depend on how we react. If we cull 90% of the population and stop pumping co2 into the air, it’ll last the survivors a lot longer than if we just carry on like nothing’s happened) I’m just looking for something that’s more than vaguely scientifically plausible.

Thanks in advance

 

[Robert Maxwell]

As always, this comes down to math.

According to this link, an average person consumes about 550 liters of oxygen per day.

According to this link, there are approximately 1 quintillion kilograms of oxygen in our atmosphere.

Based on oxygen's molecular weight, it takes 700 liters of oxygen to sum up to 1 kilogram. That means there are 700 quintillion liters of oxygen in our atmosphere. (Remember, this is all quite approximate.)

So, accounting only for humans, let's take 6.5 billion into 700 quintillion. That works out to about 107 billion days' worth of oxygen. Divide by 365 days a year, and we're talking about 295 million years to run out of oxygen.

Of course, we don't have to run out for it to be fatal. Current atmospheric concentration of oxygen is about 21%. I've seen various numbers but it looks like anything below 17% is getting into dangerous, possibly deadly, territory. That means a mere 20% in oxygen reduction in the atmosphere would doom us.

So, 20% of 295 million years is still... 59 million years. Once you start accounting for other oxygen-breathing species and other ways humans consume oxygen (combustion, etc.) you may yet shave quite a bit more off, but it looks to me like, if the carbon chain collapsed tomorrow, it'd be a very long time before lack of oxygen did us in. We'd all starve to death first, because without the carbon chain, there's no food chain.

 

[Starkers]

Thanks for the help, It does look like being able to breathe would be the least of our worries. I wonder if the oxygen in the oceans would deplete faster or slower than that in the atmosphere? I wonder what the potential would be for a purely carnivorous food chain to emerge, it'd be incredibly brutal if all carnivores were basically only able to hunt each other.

 

[Robert Maxwell]

In order to kill the carbon chain, you'd have to kill all the plankton, which would destroy the oceanic food chain pretty quickly. Plankton are the primary produces of the ocean, much as plants are the primary producers on land. Kill the primary producers, it's not long before everything else dies. I'd put it at a handful of years, somewhere between 2 and 5, with the carrion-eaters being the last to go.

You can't have a solely carnivorous food chain thanks to conservation of energy. The energy input into our food system comes from the sun, whose energy is captured by plants and plankton. Without that input, you have a closed system which will rapidly dwindle as it consumes all its energy. Large predators, for instance, have massive caloric requirements which would quickly deplete (now-foodless) prey and starve out the species as everything they eat would either starve to death or be eaten.

 

[Starkers]

An ever decreasing carnivorous circle as it were...interesting.

 

[iguana_tonante]

I'm too tired to look into the math right now, but atmospherically I would say that carbon dioxide accumulation would be quicker and more harmful than actual oxygen depletion.

But I agree that lack of food would be quicker than both.

(On the other hand, there are bacteria that don't need photosynthesis to survive. So it will not mean a total extinction of life. With time, more complex forms could evolve, resulting in a completely different evolutionary tree.)

 

[Starkers]

Yes I think life, of some kind, would find a way. whether something that wouldn’t need photosynthesis could evolve, or more likely be engineered, in time to save even a remnant of humanity is another matter of course.

If it was a case that there just wasn’t sunlight, i.e. nuclear winter, then at least there would be the option to use artificial lighting although how effective that would be would be up for debate.

 

[Robert Maxwell]

I agree that some sort of life would survive, but the end of the carbon cycle and the vast majority of terrestrial and aquatic food chains would ensure pretty much only the least complex life forms would endure. Humans could buy time with synthetic measures, but I'm not confident we could do that long-term. It might just be a long, slow decline.

In a nuclear winter situation, lack of oxygen (or abundance of carbon dioxide) wouldn't be a huge problem since we're "only" talking several years at most. A lot of plants would die, mind you. Artificial light is effective for raising some kinds of plants, but it would necessitate a lot of energy production to power those lights. Solar power is (obviously) out of the question, so that leaves fossil fuels, and maybe wind and hydro power. How much food we could reasonably produce in an artificial environment is anyone's guess. Not enough to support billions of people, though.

 

[rhubarbodendron]

It might actually go a good deal faster. Firstly, humans make only a tiny fraction of all oxigen breathing species on this planet. I'd be surprised if we'd reach 1% of the total oxigen consume.
Secondly, if plant's can't do photosynthesis they'll die and rot. This rotting will again use up a lot of oxigen. The same will happen to herbivorous species and their predators. The whole planet would become a huge compost heap, so to speak. The oxigen used by bacteria in the rotting process and by simple oxidation of substances would very likely be a good deal higher than the oxigen all organisms require for breathing.

However, since the ecosystem is so complex, we have an equation with billions of variables. It's impossible to even make an educated guess.

The survivors of such a catastrophy would be certain bacteria and a lot of the archaea