Ocean Acidification source -

Simplified Carbon Cycle

Keeling Curve

Credit: Richard A. Feely, Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, USA, with atmospheric data from Pieter Tans and seawater data from David Karl. Adapted from Feely (2008) in Levinson and Lawrimore (eds), Bull. Am. Meteorol. Soc, 89(7): S58.

How acidic are the oceans? The oceans are not, in fact, acidic, but slightly basic. Acidity is measured using the pH scale, where 7.0 is defined as neutral, with higher levels called "basic" and lower levels called "acidic". Historical global mean seawater values are approximately 8.16 on this scale, making them slightly basic. To put this in perspective, pure water has a pH of 7.0 (neutral), whereas household bleach has a pH of 12 (highly basic) and battery acid has a pH of zero (highly acidic). However, even a small change in pH may lead to large changes in ocean chemistry and ecosystem functioning. Over the past 300 million years, global mean ocean pH values have probably never been more than 0.6 units lower than today (6). Ocean ecosystems have thus evolved over time in a very stable pH environment, and it is unknown if they can adapt to such large and rapid changes. Figure reproduced from the Pew Charitable Trust Policy Brief "Carbon Dioxide and Our Ocean Legacy", by Feely, Sabine, and Fa

Rising CO2 is changing the chemistry of the oceans

What can we expect in the future ?

Coral Bleaching Unbleached coralBleached coral CaCO 3 → CaO + CO 2

Foraminifera Laboratory experiments show that calcification rates of many marine calcifyers, like certain corals, foraminifera, coccolithophores and shellfish, reduce under more acidic (high CO2) conditions. Currently, the average pH of the world’s oceans is about 8.1, which is already about 0.1 units lower compared to pre- industrial times and corresponds to an increase in acidity of 30%. Model calculations show that by the end of the 21st century pH levels will have dropped another 0.4 units under a business as usual scenario and possibly even 0.7 units lower than pre- industrial values by the year Due to the logarithmic nature of the pH scale this corresponds to three (0.5 units) and five (0.7 units) fold increase of H+ ions compared to the pre- industrial situation. These changes are probably larger than any pH change in the past 300 million years because of the extreme speed at which it takes place. The ability of species to adapt to their changing environment might therefore be limited.

Ocean Acidification Lab Day 1 Step 1: Wait at seat for teacher to bring samples and lab worksheet Step 2: Trace shells in area provided on worksheet Step 3: Measure each shell and record length and width Step 4: Take samples to balance and weigh each sample Step 5: Go to microscope and sketch details of shell on your paper Step 6: Bring sample to microscope camera and take photograph of shells (with assistance from your teacher)

How is atmospheric CO2 responsible for ocean acidification? When CO2 dissolves in seawater, carbonic acid is produced via the reaction: This carbonic acid dissociates in the water, releasing hydrogen ions and bicarbonate : The increase in the hydrogen ion concentration causes an increase in acidity, since acidity is defined by the pH scale, where pH = -log [H+] (so as hydrogen increases, the pH decreases). This log scale means that for every unit decrease on the pH scale, the hydrogen ion concentration has increased 10-fold. One result of the release of hydrogen ions is that they combine with any carbonate ions in the water to form bicarbonate: This removes carbonate ions from the water, making it more difficult for organisms to form the CaCO3 they need for their shells.

Ocean Acidification Lab Day 2 Step 1: Label three beakers with the following information. Step 2: Photograph your shells with microscope camera. Step 3: Prepare solution for each treatment. Step 4: Measure pH and record values at the top of your table. Step 5: Place shells in appropriate container. Step 6: On page 1 of lab, record observations about each treatment. BasicNeutralAcidic Group name Group name Group name