Model Estimates of the Land and Ocean Contributions to Biospheric Carbon and Water Fluxes Using MODIS Satellite Data

Land and ocean are often treated separately in modeling studies despite their close links through the carbon, water, and energy cycles. However, biospheric models, particularly when used in conjunction with recent satellite datasets, provide a new, fully coupled, global perspective. The current investigation uses a new version of the Grid Enabled Integrated Earth system (GENIE-SF) to compare both the magnitude and the seasonal and zonal variation in water flux [evaporation E and precipitation (PPT)] and carbon flux [net primary productivity (NPP)] above land and ocean. GENIE-SF contains state-of-the-art representations of photosynthesis and is driven by the phenological cycles of leaf area index (LAI) and marine chlorophyll concentration, both recorded with the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite sensors. The current study reveals the striking uniformity of the ocean–atmosphere carbon and water flux exchange, both temporally and spatially, compared to the corresponding land–atmosphere exchange. Although biospheric annual NPP (108 ± 27 GtC yr−1) is split almost equally between land (52% ± 9%) and ocean (48% ± 9%), the oceanic contribution to biospheric annual E exceeds that of the land by a factor of 6.7 ± 1.7. Simulations conducted over a 50-yr period (1951–2000) suggest that a 16% increase in land NPP, owing mainly to CO2 fertilization, may be partially offset by a decline in marine productivity.