We do not need to measure millions of years ago, only 200 years ago. (About 200 years ago is when we see the first major changes of ocean acidification)

And the number of hydrogens ion in a solution is the same 200 years ago as today for a relevant PH number. And we have very accurate equipment to measure ph in real time (aka not using things like rocks) since 1924 so we have real time data for about 70 years now. And we have scientific papers pointing out ocean acidification for over 30 years now using real time data.

But no we do not use rock measuring to determine ocean ph but instead we use air bubbles trapped in glaciers as the main method of measuring ph of 200,000 and earlier.

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And it does not matter what the ocean ph of millions of years ago, like the dinosaurs. What matters is can current life absorb a shock for current life is adapted to current conditions. Of course overall life will find a way (to borrow another dinosaur question) but system shocks can have consequences and economic effects that we do not predict. It can also lead to lower biodiversity, now some people say Fuck biodiversity and my answer is why do you say that, biodiversity is not the only thing on the planet, but biodiversity has many benefits and many costs so if we are going to say I do not care about biodiversity as a principle you still have to address the cost and benefits of biodiversity even if you do not care about a specific number of biodiversity.

And biodiversity is not just about the number of species, but instead another part of biodiversity should be seen as the amount of biomass the environment generates from inorganic sources and is able to then translate into organic sources that are active in the food web. Well when you lower biodiversity you lower the amount of life ability to actualize the planet resource as something to farm and harness as raw resources.

I am not being sensationalist via talking about cosmic time scales human beings can't understand. Instead I am talking about the here and now. The oceans are 28% more acidic than they used to be since pre industrial levels.

To put this in comparison a chain of 0.1 in chemistry from 7.35 to 7.45 is the difference from your body being in Acidosis and Alkalosis aka your blood is too acidic or too basic and both extremes are bad, very bad things. Luckily your body has various ways to balance this all out with a ph including using actual pumps to try to restore balance, but the ocean does not have this type of way to quickly change ph in a microsecond basis, and most of the life (not animal life, but life) on this planet do not have all these fancy tools we have in order to stay in the desired ph range. A change of 0.1 ph is the same type of change we are talking about with ocean acidification.