I'm not sure of the reason why, but the degree of insight on the marine side of the carbon cycle was considerably less than that on the terrestrial or atmospheric side in the TAR. Almost un-noticed, for the TAR, the IPCC moved from the use of ocean carbon cycle models as the primary method of gauging the ocean sink, to the use of atmospheric O2/N2 measurements. They did not notice (or at any rate did not highlight) the significance of the large discrepancy between these two techniques when applied to the period of the 1990s, which was a clear indication of something amiss in the assumptions underlying the O2/N2 method. One result is that their preferred estimates of the size of the land and ocean sinks were out of date before they were published, and quite substantially wrong. This has not enhanced the IPCC's reputation in this area of science. Getting the ocean CO2 fluxes right is important because we are much closer to being able to specify the ocean sink over wide areas from primary measurement and understanding, than is the case for terrestrial sinks and sources. Thus the main constraints on the natural CO2 sinks come from a combination of atmospheric measurements, and ocean studies. Being fully up to speed with what is happening in marine CO2 studies is therefore critical to the IPCC WG1.
9 minutes ago