5.9: Net primary production and global change
Probably the single most fundamental measure of "global change" of practical interest to humankind is change in terrestrial biological productivity. Biological productivity is the source of all the food, fiber, and fuel that humans survive on, so it defines most fundamentally the habitability of Earth. The spatial variability of net primary productivity (NPP) over the globe is enormous, from about 1000 grams of carbon per cubic meter per year for evergreen tropical rain forests to less than 30 grams of carbon per cubic meter per year for deserts. Primary productivity is largely balanced by ecosystem respiration (RE). The difference between GPP and RE is the terrestrial carbon balance, called net ecosystem productivity (NEP). With increased atmospheric abundance of carbon dioxide, NPP is expected to increase but ongoing global warming is likely to increase RE as well, so that the effect of predicted climate change on the terrestrial carbon uptake or release is largely a matter of ongoing research.
Understanding regional variability in carbon cycle processes requires a dramatically more spatially detailed analysis of global land surface processes than was available to climate scientists before 2000. In March 2000, the Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA's Terra and Aqua satellites, began producing a regular global estimate of near-weekly gross photosynthesis (GPP) and annual net primary production (NPP) of the entire terrestrial Earth surface at up to 1 km spatial resolution. However, ecosystem respiration, and therefore the full terrestrial carbon balance, cannot be estimated from satellite measurements, but only from models or ground-based measurements.
The GPP and NPP products are designed to provide an accurate, regular measure of the production activity or growth of terrestrial vegetation. (Note: only NPP is available in the NEO site.) These products have both theoretical and practical utility. The theoretical use is primarily for defining the seasonally dynamic terrestrial surface carbon dioxide uptake for global carbon cycle studies such as answering the "missing sink question" of carbon. The spatial and seasonal dynamics of carbon dioxide flux are also of high interest in global climate modeling because carbon dioxide is an important greenhouse gas. Both GPP and RE, and therefore the terrestrial carbon balance, can be modeled. Currently, global carbon cycle models are being integrated with climate models with the goal of creating integrated Earth Systems Models that will represent the dynamic interactions between the atmosphere, biosphere, and oceans. The weekly and monthly MODIS GPP and NPP products are useful to constrain models that are used to answer carbon dioxide flux questions.
The practical utility of these GPP/ NPP products includes routine measurements of crop yield, range forage, and forest production, and other economically and socially significant products of vegetation growth. The value of an unbiased, regular source of crop, range, and forest production estimates for global political and economic decision-making is immense. Moreover, these products are freely available for all users worldwide. This daily computed GPP more correctly defines terrestrial carbon dioxide fluxes than simple Normalized Difference Vegetation Index (NDVI) correlations currently done to increase understanding on how the seasonal fluxes of net photosynthesis are related to seasonal variations of driving factors such as atmospheric carbon dioxide, temperature, light, and precipitation.