Thus, the ocean has a major part in modifying the impact of fuel use on the
atmospheric CO 2 levels. It is believed that the positive estimate for land uptake, L, in
recent years can be accounted for by reforestation in northern hemisphere temperate
areas, including eastern North America and Scandinavia. As agriculture has become
more intensely focused in the most productive areas, sizeable places such as New
England have returned to forest since about 1950. The stock of biomass in these
forests is still increasing. Harvest rates have slowed in some temperate, lumber-
producing forests, as in the west of the US, allowing some reaccumulation of biomass.
Photosynthesis generating that biomass returns oxygen to the atmosphere, causing the
lowering of O 2 and O 2 /N 2 to be less than expected from the inventory of burned fuel.
Photosynthesis in the ocean does not have this effect since, as frequently stated in this
text, almost all marine photosynthesis is respired very soon after it is produced,
converting photosynthetic oxygen back to CO 2 . The budget can be worked in more
detail, keeping seasonal-scale track of CO 2 uptake and return (Fig. 16.8). Oxygen
released to the atmosphere in spring–summer is taken up again in fall–winter. This
must be partly physical and due to the effect of the temperature cycle on solubility.
Biology has a part, generating O 2 by photosynthesis and consuming it by metabolism.
Data on tropical forests suggest that their biomass continues on balance to be
transferred to the atmosphere, since we are cutting them down and burning most of