The absorption of human-generated CO₂ by the oceans __________.

• A. increases the oceanic concentration of carbonic acid
• B. increases the oceans' acidity and pH
• C. reduces the carbonate ion concentration in the oceans and threatens calcifying organisms in marine ecosystems
• D. increases the hydrogen ion concentration in the oceans but decreases the carbonate ion concentration and threatens the livability of the oceans for organisms that produce calcium carbonate shells

Answer: (D)

Verified step by step guidance

1. Understand that when CO2 is absorbed by the ocean, it reacts with water to form carbonic acid (H2CO3). This is a chemical equilibrium process.
2. Recognize that carbonic acid dissociates into bicarbonate (HCO3-) and hydrogen ions (H+), which increases the acidity of the ocean. This can be represented by the equation: ${H2\mathrm{CO}3}_{}\to {H\mathrm{CO}3}_{}+H_{}+$
3. Note that the increase in hydrogen ion concentration (H+) leads to a decrease in pH, making the ocean more acidic.
4. Understand that the increased acidity reduces the concentration of carbonate ions (CO3^2-), which are crucial for marine organisms that build calcium carbonate (CaCO3) shells and skeletons.
5. Conclude that the reduction in carbonate ions threatens the survival of calcifying organisms, such as corals and some shellfish, impacting marine ecosystems.