Explain how atmospheric CO2 (gas) is forced into the oceans (liquid) ?

Well, we know that Henry's Law states that when a gas is in contact with water, some will dissolve into the water. The amount depends on the temperature of the water (ocean) and partial pressure of the gas (atmosphere). I think we can consider the pressure constant (the atmosphere on average) so the temperature of the ocean is definitely a factor.

So CO2 isn't "forced" into the ocean, it is dissolved. And it dissolves in cooler waters and out-gasses in warmer waters.

The chemical reaction for dissolving CO2 in water is: CO2 + H2O → H2CO3. The resulting compound is called carbonic acid.

CO2 dissolves in water so the mere fact that CO2 in the air is in contact with some water (the oceans) is sufficient cause. (So no acid rain required.)

Henry's Law gives you the relationship.

However, please note that not all the CO2 that gets into the oceans turns into carbonic acid. The ratio, according to wikipedia is about 600 to 1 dissolved CO2 to carbonic acid.

Also, the rivers and lakes usually have a lower pH value than the oceans meaning that the oceans are "less acidic" than the rivers.

(I put "less acidic" in quotes because the oceans are not acidic at all but alkaline - pH>7. Saying the oceans are becoming more acidic is like saying that water at 19°C is becoming more frozen when it cools to 18°C! Yes, it is cooler but it is not frozen.)

Time to recall high school physics... You'll need to look into partial pressure and Henry's Law. Partial pressure will dealing with CO2 in the atmosphere and Henry's Law will show the change in partial pressure changes the amount in the liquid, in this case the ocean.

Partial Pressure:

(V_x) x (p_tot) = (V_tot) x (p_x)

V_x is the partial volume of any individual gas component (X)

V_tot is the total volume in gas mixture

p_x is the partial pressure of gas X

p_tot is the total pressure of gas mixture

Henry's Law:

p_x = (k_H) x (c)

p_x is the partial pressure of of gas X

k_H is a temperature-dependent constant (will be unique for each gas and liquid) [ with water and CO2 it is 29.4 L · atm/mol at 298K {~25C}]

c is the concentration of gas dissolved in the liquid.

Combine the two to get -

((V_x) x (p_tot))/ (V_tot) = (k_H) x (c)

Solve for c

It gets carbonated like your favorite soda.

www.skepticalscience.com/How-Increasing-Carbon-Dioxide-Heats-The-Ocean.html