5.7: Global estimates of net primary production and biomass
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.
Below are a series of maps showing global net primary production and biomass. Some are linked to interactive maps on other sites.
Terrestrial Net Primary Production
The World Atlas of Desertification has an interactive map of net primary production a snapshot of which is shown in Figure \(\PageIndex{1}\) and is based on data from the NASA Earth Observatory .
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.
Marine Net Primary Production
At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. Like plants on land, phytoplankton use chlorophyll and other light-harvesting pigments to carry out photosynthesis, absorbing atmospheric carbon dioxide to produce sugars for fuel. Chlorophyll in the water changes the way it reflects and absorbs sunlight, allowing scientists to map the amount and location of phytoplankton. These measurements give scientists valuable insights into the health of the ocean environment, and help scientists study the ocean carbon cycle.
These chlorophyll maps show milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are dark green. The observations come from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite. Land is dark gray, and places where MODIS could not collect data because of sea ice, polar darkness, or clouds are light gray.
The highest chlorophyll concentrations, where tiny surface-dwelling ocean plants are thriving, are in cold polar waters or in places where ocean currents bring cold water to the surface, such as around the equator and along the shores of continents. It is not the cold water itself that stimulates the phytoplankton. Instead, the cool temperatures are often a sign that the water has welled up to the surface from deeper in the ocean, carrying nutrients that have built up over time. In polar waters, nutrients accumulate in surface waters during the dark winter months when plants can't grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.
A band of cool, plant-rich waters circles the globe at the Equator, with the strongest signal in the Atlantic Ocean and the open waters of the Pacific Ocean. This zone of enhanced phytoplankton growth comes from the frequent upwelling of cooler, deeper water as a result of the dominant easterly trade winds blowing across the ocean surface. In many coastal areas, the rising slope of the sea floor pushes cold water from the lowest layers of the ocean to the surface. The rising, or upwelling water carries iron and other nutrients from the ocean floor. Cold coastal upwelling and subsequent phytoplankton growth are most evident along the west coasts of North and South America and southern Africa.
Biomass Maps
NASA's Oak Ridge National Laboratory Distributed Active Archive Center has a map of above ground and below ground biomass. Details are provided in the open article at Nature, Scientific Data by Spawn, Sullivan, and Lark.
The Global Climate Observing System has a map of above ground biomass in global forests
References
Spawn, S.A., Sullivan, C.C., Lark, T.J. et al. Harmonized global maps of above and below ground biomass carbon density in the year 2010. Sci Data 7 , 112 (2020). https://doi.org/10.1038/s41597-020-0444-4
Chlorophyll, NASA Earth Observatory Team, using data provided by the MODIS Ocean Science Team. Public Domain Accessed December 2023 from https://earthobservatory.nasa.gov/global-maps/MY1DMM_CHLORA
Net Primary Production, NASA images compiled by Reto Stockli, NASA's Earth Observatory Team, using data provided by the MODIS Land Science Team. Public Domain. Accessed December 2023 from https://neo.gsfc.nasa.gov/view.php?datasetId=MOD17A3H_Y_NPP