FIGURE C.1 Time trend of surface water pCO₂ offshore Hawaii, showing the direct tracking with atmospheric CO₂ forcing and the resultant change in ocean pH. The change in pH results from reaction with dissolved carbonate ion and causes a decline in the buffer capacity of seawater. The penetration to depth can also be seen in the changing subsurface data. SOURCE: Dore et al. (2009). Copyright 2009 National Academy of Sciences, U.S.A.

surements, greatly improved knowledge of the functioning of natural cycles, and an enormous increase in the anthropogenic CO₂ signal.

The first demonstrated recovery of the anthropogenic CO₂ signal from direct ocean measurements was by Brewer (1978), who corrected for the subsurface changes in dissolved CO₂ due to respiration and carbonate dissolution and showed that the residual pCO₂ signal closely resembles the atmospheric CO₂ history of the water mass. An additional term to correct for local air-sea disequilibrium at the water mass source was applied by Gruber et al. (1996), and techniques such as these are widely used today. In addition, comparison of datasets from different cruise years now allows simple tracking of the changing ocean anthropogenic CO₂ burden. An example of the ability to record the increasing storage of anthropogenic CO₂ in the ocean is shown in Figure C.2.

The chemistry of ocean methane (CH₄) is complex (see the review by Reeburgh, 2007); determining the extent to which the atmosphere is affected and detecting and understanding regional changes (e.g., ocean basin scale, or preferably less) are considerable challenges. First, the global methane budget contains significant oceanic terms (Table C.2). The net ocean emissions to the atmosphere are only about 2 percent of the total, mostly because large amounts of methane originating in