Scientists at UCSD's Scripps Institution of Oceanography have used satellite technology to develop a new approach for answering one of the key questions facing climate science: how much organic carbon is present in the upper layer of the world's oceans in the form of microscopic particles.
Carbon is a primary element that plays a central role in regulating the Earth's climate. A buildup of carbon in the atmosphere in the form of carbon dioxide can contribute to global warming because it acts as a blanket that traps heat against the Earth. The oceans help counteract this effect through a mechanism known as the "biological pump" that removes carbon dioxide from the atmosphere. Of the some 100 billion tons of carbon dioxide removed from the atmosphere each year through the process of photosynthesis, roughly half is thought to be taken up by the large numbers of phytoplankton in the surface layer of the world's oceans. These microscopic plants, the organisms that feed on them, and their debris comprise the oceanic reservoir of particulate organic carbon. Some of this carbon eventually sinks to the deep ocean, where it is sequestered for long periods of time and cannot interact with the overlying atmosphere.
To date, scientists have had little data about how much of this particulate organic carbon is actually present throughout the world's oceans because they have been forced to rely on sporadic sampling from ships. Using a new method they developed using satellite technology, however, Dariusz Stramski, a professor in the Scripps Marine Physical Laboratory, and colleagues report in the July 9 issue of Science that they were able to use satellite observations of ocean color to generate the first large-scale maps depicting the distribution of particulate organic carbon in the upper layer of a wide expanse of the Southern Ocean.
"In order to develop a better understanding of how the ocean regulates climate and how it responds to climate change, we need to know how much carbon is present in the upper ocean," Stramski said. "This is the first time we have been able to derive this information from satellite observations and the first time it has been shown on such a large spatial scale."
The new technique estimates the amount of particulate organic carbon contained in the upper tens of meters of the ocean. By combining this data with computer models, scientists can use it to estimate how much particulate organic carbon is being exported from the upper ocean to its deep interior.
"The sinking of particles containing carbon is essential to climate because it is actually removing carbon from the upper ocean and sequestering it in the ocean's interior where it can remain out of contact with the atmosphere for hundreds or even thousands of years," Stramski said.
The amount of carbon contained in microscopic particles in the ocean is small compared to that contained in terrestrial plants or dissolved carbon in the ocean. It plays a critical role, however, because it represents an important path by which carbon is transported from the upper ocean to the deeper ocean.
The rate at which carbon is exported to the deep ocean from surface waters is also important because the faster carbon is pumped to the oceans' depths, the more carbon can be absorbed by the surface layers from the atmosphere, Stramski said.
"We believe our approach will be essential to providing global-scale estimates of carbon export from the surface to the deep ocean with better accuracy than what is available now," Stramski said.
The new technique is based upon using a satellite sensor to measure the amount of light backscattered by particles within the ocean surface layer. Stramski's team developed an algorithm to calculate how much particulate organic carbon is present in the ocean's upper layer based on the backscatter data.
"For the past 20 years, scientists have used ocean color satellite data to determine the concentration of ocean phytoplankton -- that revolutionized our understanding of distributions of planktonic biomass and primary productivity," Stramski said. "But this is the first time we have had the capability to determine particulate organic carbon concentrations."
Stramski's team developed their algorithm based on data collected during two cruises of the U.S. Joint Global Ocean Flux Study (JGOFS) within the Antarctic Polar Zone from January through March of 1998 and one cruise within the Ross Sea in November and December of 1997. This algorithm was used with data from the SeaWiFS satellite sensor to produce maps of particulate organic carbon in the Southern Ocean.
This new capability to measure particulate organic carbon concentrations from space should be an important component of satellite programs for observing the ocean, Stramski said, and is timely with the launch of Terra, the first of many Earth Observing System satellites to be launched by NASA. Terra, which will be equipped with multiple sensors for observing the Earth, including those that measure ocean color, is currently scheduled for launch later this year.
Co-authors on the paper include Rick A. Reynolds, Mati Kahru, and B. Greg Mitchell, all of Scripps. The research was funded by NASA and the National Science Foundation.
Materials provided by Scripps Institution Of Oceanography, University Of California, San Diego. Note: Content may be edited for style and length.
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