Understanding Ocean pH Decrease Due to Global Warming

Global warming leads to an increase in atmospheric carbon dioxide ($CO_2$) levels. Oceans absorb a significant portion of this excess $CO_2$. This absorption process is the primary reason for the observed decrease in ocean water pH, a phenomenon known as ocean acidification.

Chemical Process of Ocean Acidification

When $CO_2$ dissolves in seawater, it reacts with water ($H_2O$) to form carbonic acid ($H_2CO_3$):

$ CO_2 + H_2O \rightleftharpoons H_2CO_3 $

Carbonic acid then dissociates, releasing hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$):

$ H_2CO_3 \rightleftharpoons H^+ + HCO_3^- $

The increase in hydrogen ion concentration ($[H^+]$) directly lowers the pH of the seawater (since $pH = -\log_{10}[H^+]$). This process explains why increased $CO_2$ uptake causes ocean water to become more acidic.

Evaluating the Options

• Option 1: larger uptake of $CO_2$ by ocean water - This is consistent with the chemical process described above. Increased atmospheric $CO_2$ leads to greater absorption by oceans, resulting in lower pH.
• Option 2: lesser uptake of $CO_2$ by ocean water - This contradicts the known effects of increased atmospheric $CO_2$ on oceans.
• Option 3 & 4: uptake of atmospheric nitrogen - While nitrogen plays a role in marine ecosystems, the direct cause of the pH decrease linked to global warming is the uptake of $CO_2$, not nitrogen.

Therefore, the ongoing decrease in ocean water pH is due to the larger uptake of $CO_2$ by ocean water.