[MaeveChondrally (OP)]

What happens to the pH or acidity of the ocean as the CO2 level rises?
Does the calcium carbonate or magnesium carbonate buffer saturate? when?
could it kill the phytoplankton , algae and seaweed in the ocean  if it changes discontinuously?
would it cut the planet off from a lot of oxygen at this point?  How much would we lose?
How much oxygen is being depleted by fossil fuels?
would this lack of oxygen be fatal for humans at that point?
Would it kill the base of the food chain in the ocean?
would this kill all the marine life?
at what CO2 level will this happen and when?
What can we do to stop the CO2 level from reaching this point?
i think there has been a misunderstanding the oxygen level will drop when the pH level changes discontinuously because it will kill the plants.
What is the maximum temperature increase we can allow to keep the CO3 level below this critical limit with a safety margin and how big should this safety margin be?

[evan_au]

Some algae use calcium carbonate as a skeleton. Presumably these algae would be impaired if they have to spend more energy continually replenishing calcium carbonate eaten away by a more acidic ocean.
- The same goes for corals
- Large Seaweed like kelp does not have a calcium carbonate backbone, so they should fare better

Humans would suffer badly if the level of oxygen were halved from 20% to 10%.
- People with genetic adaptions to high-altitude living (eg inhabitants of Tibet) would fare better than most.
- But to halve the amount of oxygen, CO₂ would need to increase by a factor of 20,000, from 0.04% to 10%.
- An increase of CO2 to this extent would really have us gasping for breath - and probably boost the surface temperatures considerably
- But provided we can leave enough of the greenery alive in land and sea, I doubt it will get that bad anytime this century due to human actions alone.

There are indications that many species survived similar conditions in the past couple of million years.
- But the main problem at present is the speed of change, and the triple attack on pH, temperature and habitat destruction and fragmentation.
- That makes it really hard for species to move poleward, and to have enough genetic diversity to adapt to changed conditions.

[alancalverd]

3% CO2 is uncomfortable, continuous 5% is lethal.

[walnutclose]

How much oxygen is being depleted by fossil fuels?
would this lack of oxygen be fatal for humans at that point?

The changes to oxygen concentration in the atmosphere due to human activity are insignificant to life processes on earth.  At the current rate of oxygen consumption due to fossil fuel burning, it would take 50 years to reduce O₂ concentration by .1%.   It's just not enough to matter.   

[Petrochemicals]

Carbon dioxide is higher around cities and oxygen is lower.  I am not sure what factors cause this, but I should think fossil fuels are a significant part.

3% CO2 is uncomfortable, continuous 5% is lethal.

Is that at the 20 percent atmospheric level?

https://en.m.wikipedia.org/wiki/Carbogen

[alancalverd]

I don't recall 5 = 20 in conventional mathematics.

Carbogen was only ever administered in one or two breaths and consisted of 75% oxygen - quite different from continuous inhalation of 5% CO2 and 20% O2 diluted with 75% N2.

CO2 rapidly stimulates the breathing response. It is recycled in anesthetic gas systems to regulate respiration and the excess CO2 in exhaled breath is valuable in mouth-to-mouth resuscitation - better than fresh air! But substantial excess promotes hyperventilation, panic, and eventually death.   

10% of all anthropogenic CO2 is generated by humans breathing, hence the high concentration in cities and paticularly indoors and inside trains and buses. As it is a dense gas, it tends to concentrate at street level where there is little wind to disperse it.

[Bored chemist]

I don't recall 5 = 20 in conventional mathematics.

Do you remember ellipsis in English?

Is that at the 20 percent atmospheric level?

Didn't you realise that's a reference to O2, even though it's unspecified?

As it is a dense gas, it tends to concentrate at street level

Would you like to calculate the extent of that concentration?

[alancalverd]

No. It's unlikely to exceed 5000 ppm in any occupied space, but at floor level in the Grotta del Cane (Naples) it has been measured at 80%, whilst the air at head height was adequately breathable.

[Bored chemist]

but at floor level in the Grotta del Cane (Naples) it has been measured at 80%, whilst the air at head height was adequately breathable.

And you think that has something to do with this?

10% of all anthropogenic CO2 is generated by humans breathing, hence the high concentration in cities and paticularly indoors and inside trains and buses. As it is a dense gas, it tends to concentrate at street level

Do you think that 80% was due to people exhaling it (via their feet) and magically removing the nitrogen?

[alancalverd]

Don't be silly.

[Petrochemicals]

I don't recall 5 = 20 in conventional mathematics.

That would be in atmospheric concentrations of oxygen, 5 percent co2, 20 percent or there abouts oxygen.

From wikipedia

Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour"

again this is not exactly stipulating oxygen content.

[MaeveChondrally (OP)]

if you google 'how much oxygen does the ocean produce?'  it says 50 to 85% of the worlds oxygen has been produced by plankton in the ocean,  not the trees, plants or bacteria
plankton population is 40% of what it was in 1955 and is dying and continuing to die because of temperature, pollution and pH changes. 
when the calcium carbonate buffer saturates at 493 +/-5 ppm in 2051 or sooner,  the pH will suddenly decrease by .5 pH units and may kill off the rest of the plankton and kill the base of the food chain in the ocean!
I believe their estimates of the pH at 2051 are wrong.  they are saying it will be 7.8 at 2100 and i think it is more like 7.3 then due to the saturation of the calcium carbonate buffer at 2051,  and we must make sure we do not reach this point.

check out videos....
youtube.com
introduction to ocean chemistry part 1 by andrew G. Dickson
and introduction to ocean chemistry
and
ocean acidity,  why should we care!