Environmental Engineering Reference
In-Depth Information
production databases have helped narrow the most likely range to between 55 and
60 Gt C/yr.
The i rst modern attempt to estimate marine NPP (inadvisably extrapolating from
a small number of questionable values) was Noddack's (1937) total of 25.4 Gt
C/year for the open ocean and 3.2 Gt C/year for coastal seas. This was followed by
Riley's (1944) wide range of 44-208 Gt C/year based on just seven western Atlantic
sites. Koblents-Mishke et al. (1968) used data from 7,000 ocean sites to come up
with a range very similar to Noddack's (27-32 Gt C/year, excluding all benthic
production). De Vooys (1979) put the annual marine NPP at 46 Gt C/year. Estimates
of marine NPP had also benei ted from satellite monitoring.
Data from the Coastal Zone Color Scanner (CZCS) made it possible to monitor
global patterns of phytoplankton growth, most notably the seasonally high produc-
tivities in major upwelling regions off the western coast of Africa and South America.
Two satellite-based models of marine NPP came up with very similar results: 37-46
Gt C/year by Antoine et al. (1996) and 47.5 Gt C/year by Behrenfeld and Falkowski
(1997). There is a strong seasonal variation, with the monthly average ranging from
3.8 to 4.6 Gt C/year and longer-term anomalies driven by El Niño/La Niña l uctua-
tions. The Pacii c Ocean accounts for slightly more than 40% of the total, and the
marine NPP of the two hemispheres is roughly equal because the smaller expanse
of the northern ocean has about 60% higher productivity per unit area.
At the century's end, the Potsdam NPP Model Intercomparison meeting provided
a comprehensive survey of global phytomass productivity estimates based on 16
models that were compared using standardized input variables (Cramer et al. 1999).
After two extreme values were excluded, the results ranged from 44.3 to 66.3 Gt
C/year with the mean of 54.1 Gt C/year. This translates to roughly 89-132 Gt (mean,
108 Gt) of dry phytomass, or, assuming an average energy density close to 20 GJ/t,
to about 2 ZJ. Two conceptually similar models of terrestrial and marine NPP
resulted in a combined global total of 104.9 Gt C/year, with 56.4 Gt C/year on land
and 48.5 Gt C/year in the ocean (Field et al. 1998; Geider et al. 2001). The most
likely planetary total of satellite-based modeling of NPP was thus between 100 and
110 Gt C/year or 200-220 Gt/year of dry phytomass and at least 3.5 ZJ of energy
(an annual l ux of about 110 TW).
A superior tool for monitoring global NPP became available with the launching
of the
Terra
satellite in December 1999 (
Aqua
, devoted to ocean studies, was
launched in May 2002). The satellite carried a Moderate Resolution Imaging Spec-
troradiometer (MODIS, resolution of 0.25-1 km) designed to track rel ected radia-
tion in 36 discrete spectral bands. The combination of measurements of canopy
