5.4.5 Biological Changes Relevant to Ocean Biogeochemistry

Changes in biological activity are an important part of the carbon cycle but are difficult to quantify at the global scale. Marine export production (the fraction of primary production that is not respired at the ocean surface and thus sinks to depth) is the biological process that has the largest influence on element cycles. There are no global observations on changes in export production or respiration. However, estimates of changes in primary production provide partial information. A reduction in global oceanic primary production by about 6% between the early 1980s and the late 1990s was estimated based on the comparison of chlorophyll data from two satellites (Gregg et al., 2003). The errors in this estimate are potentially large because it is based on the comparison of data from two different sensors. Nevertheless, a change in biological fluxes of this order of magnitude is plausible considering that biological production is controlled primarily by nutrient input from intermediate waters, and that a decrease in intermediate water renewal has been observed during that period as indicated by the decrease in O₂. Shifts and trends in plankton biomass have been observed for instance in the North Atlantic (Beaugrand and Reid, 2003), the North Pacific (Karl, 1999; Chavez et al., 2003) and in the Southern Indian Ocean (Hirawake et al., 2005), but the spatial and temporal coverage is limited. The potential impacts of changes in marine ecosystems or dissolved organic matter on climate are discussed in Section 7.3.4, and the impact of climate on marine ecosystems in Chapter 4 of the Working Group II contribution to the IPCC Fourth Assessment Report.

5.4.6 Consistency with Physical Changes

It is clearly established that climate variability affects the oceanic content of natural and anthropogenic DIC and the air-sea flux of CO₂, although the amplitude and physical processes responsible for the changes are less well known. Variability in the marine carbon cycle has been observed in response to physical changes associated with the dominant modes of climate variability such as El Niño events and the PDO (Feely et al., 1999; Takahashi et al., 2006), and the NAO (Bates et al., 2002; Johnson and Gruber, 2007). The regional patterns of anthropogenic CO₂ storage are consistent with those of CFCs and with changes in heat content. The observed trends in CO₂, DIC, pH and carbonate species can be primarily explained by the response of the ocean to the increase in atmospheric CO₂.

Large-scale changes in the O₂ content of the thermocline have been observed between the 1970s and the late 1990s. These changes are everywhere consistent with the local changes in ocean ventilation as identified either by changes in density gradients or by changes in apparent CFC ages. Nevertheless, an influence of changes in marine biology cannot be ruled out. The available data are insufficient to say if the changes in O₂ are caused by natural variability or are trends that are likely to persist in the future, but they do indicate that large-scale changes in ocean physics influence natural biogeochemical cycles, and thus the cycles of O₂ and CO₂ are likely to undergo changes if ocean circulation changes persist in the future.