OCEAN ACIDIFICATIONThe 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.

http://www.pmel.noaa.gov/co2/people

AcidificationFundamental changes in seawater chemistry are occurring throughout

the world's oceans. Industrial revolution

Increased the amount of CO2 in the atmosphere. The ocean absorbs about a quarter of the CO2 we release into the

atmosphere every yearAtmospheric CO2 increase ocean CO2, 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 — The CO2 absorbed by the ocean is changing the chemistry of the seawater.

We call this ACIDIFICATION

AcidificationCO2 reacts with water

molecules (H2O) and forms the weak acid H2CO3 (carbonic acid). This leads to higher acidity mostly near the surface.

Most of this acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).

CO2 (aq) + H2O H2CO3 HCO3− + H+ CO3

2− + 2 H+

AcidificationThe 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.

AcidificationMarine crustaceans shells are

made of calcium carbonate (CaCO3) as are various species of corals. In order to form these shells calcium molecules must combine with carbon dioxide, or carbonate CO3

-2, (called calcification).Increase H+ levels causes a

reversal of the reaction to dissolve crustacean shells

H2CO3 + Ca+2 ↔ CaCO3 + 2H+ HCO3- + 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.

It takes about one year to equilibrate CO2 in the surface ocean with atmospheric CO2, so it is not unusual to observe large air-sea differences in CO2 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 CO2 modifies the climate which in turn impacts ocean circulation

and therefore ocean CO2 uptake. Changes in marine ecosystems resulting from rising CO2 and/or changing

climate can also result in changes in air-sea CO2 exchange.These feedbacks can change the role of the oceans in taking up

atmospheric CO2 making it very difficult to predict how the ocean carbon cycle will operate in the future.

CO2 Uptake vs. Carbon StorageCO2 uptake is not

the same as carbon storage. Anthropogenic CO2 absorbed in one region may be moved by ocean circulation and ultimately stored in a different region

CO2 Uptake vs. Carbon StorageA map of the anthropogenic CO2

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)

CO2 Uptake vs. Carbon StorageUptake 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.