So, imagine we’re trying to reduce the amount of CO2 with some custom-made carbon-guzzling lifeform but it all goes horribly wrong and it eats up all the CO2 in the atmosphere.
What happens next? I guess all the plants die, which has the effect of starving most complex life on Earth, but what about the temperature? Would we get an iceball Earth till the Sun expands or what? What other effects would there be and what timeline are we talking about?
Earth I would expect freeze over, mostly or completely; the Snowball Earth scenario. Not exactly an iceball - the oceans stay liquid underneath the ice. If the CO2 eater dies as well, then eventually over tens of millions of years enough CO2 may accumulate from volcanoes to unfreeze the planet. Otherwise lacking greenhouse gases Earth would logically stay frozen until the Sun warms significantly.
The Sun is slowly warming over time, but I don’t know if it would warm enough to melt the ice before it goes red giant.
In fact, Earth has only about a billion years left, out of the 5 billion or so that the Sun has, before it becomes uninhabitable, since even with no CO2 (and the geological record shows that over the long term CO2 levels rather closely track inversely to solar irradiance, not a coincidence since the rate of weathering is affected by temperature) it will get so hot that the oceans boil off into space (actually, “boil off” isn’t the right term since the temperature isn’t expected to be that hot yet, but water will be lost from the upper atmosphere). Even before then (see mention of CO2 and irradiance), the level of CO2 will become too low to support plant life (current plants can survive with levels down to 10 ppm, depending on their photosynthetic pathway).
I was kind of wondering how far the temperature could drop? The change in albedo makes it tricky, but I’d assume the oxygen and nitrogen in the atmosphere would provide some kind of insulation (I guess we’d lose nearly all the water vapour as the CO2 went.)
Many mammals, at least humans, or possibly other vertebrates (I’m not sure of the extent) will die in their sleep. Your carotid process (tissue surrounding the carotid artery in your neck) constantly “tastes” your blood for pH caused by dissolved CO2. If there’s none there, it will tell your brain’s breathing center there is plenty of oxygen in your blood, and to go ahead and slow down or stop breathing. Awake people will feel dizzy and faint and force themselves to pant, but there must be the typical CO2 in the air when people sleep – our spacecraft engineers have to be very careful of this when they design the air-scrubbers. I know it sounds utterly bass-ackwards, but our bodies can’t test for “enough oxygen in the blood”, it can only test for, “most of the CO2 is gone.”
According to the study here, temperature would fall by about 35°C over several decades, and yes, it is largely because water vapor would condense out of the atmosphere (water vapor is actually the main greenhouse gas but because it can condense out it can’t act by itself, which is a good thing or it would cause a runaway greenhouse effect):
I’m skeptical of this, since CO2 would still be produced in your body and the concentration in exhaled air is much higher (several percent) than in inhaled air (0.04%), so reducing that to zero would have hardly any effect. The situation you describe sounds more like what happens in you breathe air that has no oxygen in it (you just pass out and die without realizing it).
The long term trend for CO2 levels is downwards, as weathering is slowly removing it from our atmosphere. Without interference from anthropogenic CO2, our planet would get colder and colder over the next few tens of milions of years, at least until the Sun warms up enough to melt the ice. By that time our planet wouldn’t have enough carbon dioxide to support the sort of plants that live on Earth today (although life might adapt to lower levels - plants that use C4 carbon fixation are already adapted to the lower CO2 levels that have existed since the death of the dinosaurs).
If humans or their descendants are around during this period of cooling they might decide to release extra CO2 to compensate for the long-term loss; that is one reason why we shouldn’t use up all the fossil fuels in the current era.
Ah, that’s an interesting read. 35 degrees is a fair amount in just 50 years! Lucky we’re not heating that quickly 
I hadn’t realised that plants would be happy down at 10ppm. I wonder what the lowest level we could live with would be? 50ppm or so?
Um, no. Weathering has always removed CO2 from the atmosphere, except possibly during Snowball periods. And animals and processes like volcanoes have in turn added more to replace it. It’s a dynamic balance, it’s not a matter of all CO2 being added to the atmosphere some time in prehistory and it slowly being removed by weathering.
Unfortunately it is not a complete balance. CO2 levels are decreasing over time, as the volcanoes are not replacing CO2 fast enough. We will freeze before we burn. Unless of course, anthropogenic global warming causes us to burn before we freeze before we burn.
Yeah, the plants may actually survive,but not thrive of course. Us mammals are deaders.
That seems highly implausible. We are in an Ice Age interglacial period right now; an unusually cool period of history, not a warm one. Without human interference it’s to be expected that the world would warm up, not cool down; just not so fast, and not right now.
The vast majority of CO2 in your blood is produced by your cells’ metabolism; the tiny amount of CO2 in the air shouldn’t make much difference.
The less CO[sub]2[/sub] in the atmosphere, the more comes out of your blood when you exhale. It may not be a lot on each breath, but after enough breaths, it becomes significant.
I don’t see why that would be the case, and it doesn’t seem to make sense to me physiologically, but if you have a cite I’d like to see it.
Currently the Earth is in an ‘icehouse state’, with a low level of greenhouse gases in the atmosphere.
We are in a warm phase during this icehouse state, an ‘interglacial period’. Calculations based on the Milankovic cycles suggest that this interglacial would be quite a long one, if there were no anthropogenic interference; see Berger and Loutre
http://www.climate.unibe.ch/~born/share/papers/eemian_and_lgi/berger_loutre02.sci.pdf
but eventually the glacial conditions will return, and carbon dioxide will continue to decrease in our atmosphere for as long as there is an active weathering cycle. Perhaps CO2 would build up if we went into a Snowball Earth condition, as our planet may have done in the distant past; but the gradually warming Sun will probably prevent that, so I suspect we will be in an icehouse condition for tens of millions of years, until the CO2 is nearly all gone.
It’s not physiology, it’s physical chemistry.
Consider putting out two cups of water, one in a low humidity area (say about 20% humidity) and the other in a high humidity area (about 80%). Which one will evaporate first? From experience, you probably know that it’s the low humidity one. But why does it happen?
Evaporation is basically water molecules leaving the cup of water. However, water molecules in the air will sometimes also re-enter the water in the cup. In the high humidity area, there are more water molecules in the air that can re-enter the cup. That means the net flow of water molecules out of the cup in the high humidity area is lower than the other area. So it evaporates slower.
Well the same thing applies to molecules dissolved in water, in this case CO[sub]2[/sub]. In an environment that lacks carbon dioxide, it’ll leave your blood faster than when there’s some already in the air.
Let’s tweak your analogy a bit. Instead of two cups of water in 20%RH and 80%RH areas, let us suppose we have two damp (not soaked) paper towels. One is in an environment with 0% relative humidity and the other is in an environment with 0.04% relative humidity. Neither towel has much water to give before it’s dry, and both are in extremely dry environs, so both are gonna dry out pretty damn fast; the difference between one and the other is very, very small.
I would be interested to hear from a doctor how much CO2 remains in the blood after it leaves the lungs. If it’s already very close to zero, then switching to a CO2-free atmosphere won’t lower blood CO2 levels much at all.
Even if it did lower blood CO2 levels a little bit more and we breathed a little more slowly…so what? there’s lots and lots of oxygen in a lungful of air; we won’t die if we breathe a little more slowly, and I’d be surprised if our blood were significantly less saturated with O2. If you’re healthy, you’ll find that you can actually breathe comfortably at a pace as slow as one breath per minute; I’ve done this for several minutes at a time before, just to see if I could do it. Inhale for 30 seconds, exhale for 30 seconds, repeat until bored.
One problem with this is that CO[sub]2[/sub] does not go directly from the blood to the air, since we don’t have gills. CO[sub]2[/sub] enters the pulmonary circulation at a partial pressure of about 46 mmHg. It freely diffuses from the capillaries across the alveolar membranes into the alveoli, the terminal portions of the lung’s respiratory tree. The alveoli have a CO[sub]2[/sub] partial pressure of 40 mmHg, so the CO[sub]2[/sub] in the blood quickly equilibrates to that level before leaving the pulmonary circulation.
The level of CO[sub]2[/sub] in the alveoli is based on an equilibrium between the amount of CO[sub]2[/sub] produced by the body and its ventilation into the outside air when you breath.
And yes, it is physiology, which can be seen as a study of the mechanisms used by living organisms to self-regulate and maintain homeostasis in changing conditions.
To follow up on my previous post: Let’s say that the tiny decrease in atmospheric CO[sub]2[/sub] from 400 ppm, or 0.04%, to 0 ppm means that the alveolar partial pressure of CO[sub]2[/sub] is now 39.5 mmHg instead of 40 mmHg, because some of that CO[sub]2[/sub] did come from inhaled atmospheric air. Therefore the CO[sub]2[/sub] level in blood leaving the lungs is also 39.5 mmHg. Your body will now decrease your respiratory rate very slightly, because the the chemoreceptors in your medulla and elsewhere are slightly less stimulated by CO[sub]2[/sub]. This will cause a slight decrease in ventilation of alveolar gas, which will cause the CO[sub]2[/sub] levels in the alveoli to rise slightly, which will cause the amount of CO[sub]2[/sub] detected by the chemoreceptors to increase.
In other words, there may be an insignificantly small decrease in respiratory rate, but an equilibrium will quickly be reached.