The Other Carbon Dioxide Problem Ocean acidification is the term given to the chemical changes in the ocean as a result of carbon dioxide emissions

To understand the changing chemistry of the oceans and the impacts of ocean acidification on marine ecosystems. Our observations of key physical, chemical, and biological parameters support NOAA's overall efforts to predict how marine ecosystems will respond and to develop management strategies for adapting to the consequences of ocean acidification.

Acidification Fundamental changes in seawater chemistry are occurring throughout the world's oceans. Industrial revolution Increased the amount of CO 2 in the atmosphere. The ocean absorbs about a quarter of the CO 2 we release into the atmosphere every year Atmospheric CO 2 increase  ocean CO 2, increase. Initially, many scientists focused on the benefits of the ocean removing this greenhouse gas from the atmosphere. Observations now show that there is also a downside — ACIDIFICATION The CO 2 absorbed by the ocean is changing the chemistry of the seawater. We call this ACIDIFICATION

Acidification CO 2 reacts with water molecules (H 2 O) and forms the weak acid H 2 CO 3 (carbonic acid). This leads to higher acidity mostly near the surface. Most of this acid dissociates into hydrogen ions (H + ) and bicarbonate ions (HCO 3- ). CO 2 (aq) + H 2 O  H 2 CO 3  HCO 3 − + H +  CO 3 2− + 2 H +

Acidification The increase in H + ions reduces pH and the oceans acidify, that is they become more acidic. Over the past 300 million years, ocean pH has been slightly basic, averaging about 8.2. Today, it is around 8.1, a drop of 0.1 pH units, representing a 25- percent increase in acidity over the past two centuries.

Acidification Marine crustaceans shells are made of calcium carbonate (CaCO 3 ) as are various species of corals. In order to form these shells calcium molecules must combine with carbon dioxide, or carbonate CO 3 -2, (called calcification). Increase H + levels causes a reversal of the reaction to dissolve crustacean shells H 2 CO 3 + Ca +2 ↔ CaCO 3 + 2H +   HCO Ca +2

Ocean Carbon Uptake Air-sea gas exchange is primarily controlled by the air-sea difference in gas concentrations and the exchange coefficient, which determines how quickly a molecule of gas can move across the ocean-atmosphere boundary. about one year It takes about one year to equilibrate CO 2 in the surface ocean with atmospheric CO 2, so it is not unusual to observe large air-sea differences in CO 2 concentrations. Most of the differences are caused by variability in the oceans due to biology and ocean circulation.

Dynamics of Gas Exchange Whenever the partial pressure of a gas is increased in the atmosphere over a body of water, the gas will diffuse into that water until the partial pressures across the air-water interface are equilibrated. However, because the global carbon cycle is intimately embedded in the physical climate system there exist several feedback loops between the two systems. Increasing CO 2 modifies the climate which in turn impacts ocean circulation and therefore ocean CO 2 uptake. Changes in marine ecosystems resulting from rising CO 2 and/or changing climate can also result in changes in air-sea CO 2 exchange. These feedbacks can change the role of the oceans in taking up atmospheric CO 2 making it very difficult to predict how the ocean carbon cycle will operate in the future.

CO 2 Uptake vs. Carbon Storage CO 2 uptake is not the same as carbon storage. Anthropogenic CO 2 absorbed in one region may be moved by ocean circulation and ultimately stored in a different region

CO 2 Uptake vs. Carbon Storage A map of the anthropogenic CO 2 ocean column inventory shows that the carbon is not evenly distributed in space. More than 23% of the inventory can be found in the North Atlantic, a region covering approximately 15% of the global ocean. By contrast, the region south of 50°S represents approximately the same ocean area but only has ~9% of the global inventory. Map of the column inventory of anthropogenic CO2 in 1994 (Sabine et al., 2004)

CO 2 Uptake vs. Carbon Storage Uptake at the surface does not fully explain the spatial differences in storage because of the slow mixing time in the ocean interior and the fact that waters only move into the deep ocean in a few locations. The highest inventories are found in the locations where the water is intermediate (cold and low salinity). Global map of the average annual exchange CO2 flux (mol-C m-2 a-1) across the sea surface.