Oceans regulate the amount of CO2 in the air through absorption equilibrium

Equilibrium is rapid and total as indicated by many sources of evidence. One, the graph for CO2 in the air is an extremely precise line. If nothing were regulating, it would vary wildly. Two, if equilibrium were not established, oceans would be absorbing or losing CO2 at a high rate, yet no detectable change occurs beyond the stab in the dark guess that it might have dropped 0.15 pH units over the past century.
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Oceans do absorb atmospheric CO2. And under normal conditions the oceans and the biosphere will generally keep the amount of CO2 in the atmosphere constant. However, human activities are producing CO2 at a much higher rate than the planet's oceans and biosphere are able to absorb it. This imbalance is responsible for virtually the whole of the 20th century rise in CO2.CO2. 
Also, estimating pH changes need not involve any stabbing in the dark thanks to fossil records and certain species being restricted by pH (e.g. shelled molluscs) due to their physiological requirements. And 0.15 pH units may not sound like much but pH is measured along a logarithmic scale.