APES 9.7 Ocean Acidification

Ocean Acidification Overview

  • Definition: Ocean acidification refers to the process whereby ocean water becomes more acidic due to the absorption of excess atmospheric CO2.

Causes of Ocean Acidification

  • Primary Cause: Increased levels of atmospheric CO2.

  • Mechanism: Carbon dioxide (CO2) from the atmosphere dissolves into ocean water, forming carbonic acid (H2CO3).

    • Equation: CO2 + H2O ⇌ H2CO3

  • Dissociation Process:

    • Carbonic acid dissociates to release hydrogen ions (H+), which increases acidity.

    • H2CO3 → H+ + HCO3^− (bicarbonate)

Role of Carbonate Ions

  • Importance: The carbonate ion (CO3^2−) is critical for marine organisms such as corals and mollusks for shell-building (calcium carbonate shells).

  • Impact of Acidification:

    • Increased H+ leads to reduced availability of carbonate ions, as they combine to form bicarbonate:

      • H+ + CO3^2− → HCO3^−

    • Result: Less carbonate is available for organisms to utilize in shell formation.

Effects on Marine Life

  • Calcification Process: Shell-building organisms require both carbonate ions and calcium to create their shells. The acidification reduces the availability of carbonate.

  • Projected pH Changes:

    • Historical pH around 8.1 (late 1800s) projected to decrease to 7.8 by 2100.

  • Consequences:

    • Less calcification leads to weaker shells, increase in deterioration over time, and reduced ability for organisms to repair their shells.

Observational Evidence

  • A shell placed in a solution at a pH of 7.8 deteriorates significantly within 45 days due to lack of carbonate ions.

Human Contributions to Ocean Acidification

  • Anthropogenic Sources:

    • Combustion of fossil fuels (vehicles, electricity generation).

    • Industrial processes contributing CO2 emissions.

  • Secondary Contributors: Nitric and sulfuric acid from NOx and SOx emissions, though primary driver is CO2.

Historical Data Correlation

  • Inverse Relationship:

    • As atmospheric CO2 levels rise, ocean pH decreases (more acidic).

  • Example: Data from 1985-2005 shows increasing atmospheric CO2 correlates with declining ocean pH.

Understanding pH Changes

  • Logarithmic Scale:

    • A movement from pH 8.2 to 8.1 represents a 30% increase in acidity, highlighting the significance of seemingly small changes.

  • Historical Context: The ocean is now 30% more acidic than it was 150 years ago.