PMEL Programs and Plans
Accomplishments in FY 99 and Plans for FY 00
Carbon Dioxide Project
Accomplishments in FY 99
Carbon dioxide is one of the most important gases in the atmosphere affecting the radiative balance of the earth. Atmospheric CO2
concentrations in the past 400,000 years have oscillated from around 200 to 280 ppm. Current atmospheric concentrations are
now around 367 ppm as a result of industrial and agricultural activities. In the past few decades, only half of the CO2 released by
human activity has remained in the atmosphere; on average, about 30% of the CO2 is taken up by the ocean and about 20% by the
terrestrial biosphere. Because carbon reservoirs in the ocean, atmosphere and terrestrial biosphere are irrevocably linked, the CCSP
calls for an integrated approach to studying the carbon cycle. The ocean plays a critical role in the global carbon cycle since it has a vast
reservoir of CO2 containing approximately 50 times more CO2 than the atmosphere, and therefore, exerts a controlling influence on
atmospheric levels.
The primary objective of NOAA's Global Carbon Cycle (GCC) Program is to quantitatively assess the fate of CO2 in the atmosphere
and oceans. In order to accomplish this goal the natural sources and sinks of carbon dioxide must be determined. During FY 99, the
PMEL CO2 group determined the distribution of pCO2 in the equatorial Pacific during the transition between the 1997-98 El Niño and
the 1998-99 La Niña event. Data from the eastern equatorial Pacific during and after the 1997-98 El Niño shows that, during the mature
phase of the1997-98 El Niño, surface-water pCO2 was generally below atmospheric levels (Figure 1). By June of 1998 recovery from
the El Niño had begun, and high surface-water pCO2 was measured at 0E, 155EW. The recovery at the mooring at 2ES, 170EW occurred
later; waters low in pCO2 were found at this site in June 1998. Levels increased rapidly in July, followed by large excursions associated
with the passage of tropical instability waves.
Plans for FY 00
In the 1990s, the cooperative multi-agency efforts of the World Ocean Circulation Experiment (WOCE), Joint Global Ocean Flux Study
(JGOFS), and Ocean-Atmosphere Carbon Exchange Study (OACES) together with parallel international programs greatly improved the
global ocean carbon database. Total carbon dioxide (TCO2) data sets accurate to 2-3 µmol/kg, equivalent to approximately 2 to 3 years'
uptake of anthropogenic CO2 in near-surface waters, are now available for hydrographic transects representing most of the world's oceans.
These data, together with recently improved interpretive techniques, will allow a comprehensive global estimate of anthropogenic CO2 in the
oceans that will serve as a bench mark for future observational programs. The spatial distribution of the data-based inventory estimates also
provide robust constraints for the evaluation of prognostic ocean carbon models. Figure 1. Zonal mean anthropogenic CO2 column inventories.
Atlantic results from Gruber (1998), Indian results from Sabine et al. (1999), Pacific results based on preliminary estimates along P16 (155ºW) only.
The preliminary results are shown in Figure 2 as zonal means of the anthropogenic CO2 column inventories. This figure clearly shows the
large North Atlantic sink for CO2 as well as substantial sinks in the Southern Hemisphere associated with the Subtropical Convergence
in both the Atlantic and Indian Oceans. These calculations imply that relatively little anthropogenic CO2 is stored in the high latitude
Southern Ocean. Analysis of the Pacific data is currently underway. A first look at the meridional trends in the Pacific can be estimated from
the preliminary column inventory estimates along WOCE line P16 (~155°W). The location of the largest inventories in all three basins are
generally consistent with the location of the largest net surface ocean CO2 sink regions as compiled by Takahashi et al. (1999). During
FY 2000, the Global Carbon Cycle Program will continue the analysis of the field data from the Atlantic, Pacific and Indian Oceans.
In particular, the group will compare data sets obtained on other WOCE-WHP cruises and will provide internally consistent data sets
encompassing roughly 16 cruises in the Pacific Ocean, 15 cruises in the Indian Ocean, and 10 cruises in the Atlantic. These data
will be used by the modeling community for setting boundary conditions for general ocean circulation models, to determine the DIC
inventory in each basin using several independent methods, and to estimate anthropogenic CO2 increases in the ocean. To facilitate
comparisons of models and observations, the data will be gridded into similar box sizes as currently used in the models.
In addition to this activity, we will continue our pCO2 instrument development activities with the group at MBARI, directed by Francisco
Chavez, to provide a suite of chemical and biological sensors deployed on the 155EW and 170EW TAO morring array in the equatorial
Pacific. The work leverages on developmental efforts carried out by MBARI (with support from NOAA, NASA, and PMEL) over the
past several years. The primary objectives of this project are: (1) to determine the relationships between physical forcing, primary
production and the exchange of carbon dioxide between ocean and atmosphere; (2) to determine the biological and chemical responses
to climatic and ocean variability in the equatorial Pacific; (3) to determine the spatial, seasonal and interannual variability in primary
production, carbon dioxide, and nutrient distributions; and (4) to determine the spatial, seasonal and interannual variability of sea surface
pigment distributions to groundtruth sattelite measurements of ocean color.
|
