Fundamental changes in seawater chemistry are occurring throughout Earth’s oceans. Since the beginning of the industrial revolution, the release of carbon dioxide (CO2) from industrial and agricultural activities has increased the amount of CO2 in the atmosphere. Ocean acidification is the term given to the chemical changes in the ocean as a result of carbon dioxide emissions.
Oceanographic measurements worldwide indicate that the pH of seawater is decreasing—that is, the ocean is becoming more acidic. This is due to the fact that seawater absorbs CO2 from the atmosphere, and the concentration of CO2 in the atmosphere has been steadily rising owing primarily to the burning of coal, oil and gas for transportation, heating, electricity generation, and other industrial activities. The amount of CO2 produced by human activities is small compared to that released naturally through biological and geological processess, but it is large enough that forests, grasslands, and aquatic plant communities can’t absorb it all, so every year the amount in the atmosphere rises. The rate of atmospheric CO2 accumulation has steadily increased in recent decades, reaching a record high of 31.6 gt in 2011 (IEA 2012).
In addition to the CO2 from natural sources that it absorbs, the ocean absorbs approximately 30% of the CO2 emitted into the atmosphere by humans (Sabine et al. 2004). Consequently, seawater worldwide is becoming more acidic.
The average pH of ocean surface water has decreased from a calculated value of 8.2 in 1750 to a measured value of approximately 8.1 today. Although it seems small, since the pH scale is logarithmic, this decline actually represents 30% greater acidity overall. It is important to note that the ocean is not ‘acid’, its pH is greater than 7 and will in all likelihood remain so. However, it is becoming more acidic, and this acidification will have profound biological effects.
Ocean Health Index, August 2012. Web page (text and infographics).
