Ocean’s carbon dioxide uptake varies year to year

Data taken hourly by cargo ships show that how much of the greenhouse gas North Atlantic waters absorb varies more than thought.

Data gathered by cargo ships plying the North Atlantic between 2002 and 2007 show that the ability of surface waters there to sop up CO2 varies considerably but somewhat predictably from year to year. The finding may help scientists better estimate the future rate at which the planet-warming gas will build up in the atmosphere.

CARGO OF DATA Data gathered by cargo ships plying the North Atlantic from 2002 through 2007 show that carbon dioxide uptake in those waters can vary greatly from year to year. That variation can affect how the greenhouse gas accumulates in the atmosphere. Atlantic Container Lines Inc.

GOOD COVERAGE The cargo ships gathered data throughout the North Atlantic (routes shown). Watson et al./Science

Ups and downs in the ocean’s CO2 uptake are important to atmospheric levels because globally, the portion of the gas that isn’t absorbed by oceans or taken up by land plants accumulates in the atmosphere.

The newly released, six-year set of oceanographic data is the largest of its kind, says Arne Körtzinger, a chemical oceanographer at the Leibniz Institute for Marine Sciences in Kiel, Germany. While scientific research vessels are limited to occasional cruises, cargo ships constantly cross the oceans, he notes.

Vessels traveling along seven North Atlantic deep-water shipping routes pulled water samples from depths of between 3 and 8 meters at least twice each hour and measured how much dissolved carbon dioxide those waters held. Combined with the relatively small amount of data gathered by research vessels, the data for 2005 alone include more than 125,000 measurements, Körtzinger and his colleagues report in the December 4 Science.

The most consistently traveled routes were those linking the English Channel with ports in the eastern Caribbean. The team’s data suggest that in 2005, each square kilometer of ocean along that swath soaked up about 41 million metric tons of carbon dioxide over the course of the year. But in 2002 and 2007, each square kilometer of those waters absorbed less than 18 million metric tons of the greenhouse gas each year.

“This variation is much larger than anticipated,” says Körtzinger.

Those year-to-year fluctuations in carbon dioxide uptake are not totally unpredictable, however, and can be linked to two factors. One is the sea surface temperature: Warm waters can’t hold as much dissolved gas as cold seas can, so when temperatures are high, surface waters don’t absorb carbon dioxide from the air as readily. The other factor is the thickness of the surface waters that had been thoroughly mixed by winds, waves and turbulence during the previous winter. The deeper that mixed surface layer is, the more nutrients are available for surface-dwelling, CO2-gulping phytoplankton during spring and summer months.

The team’s findings show that similar data-gathering programs in other regions thick with shipping routes, such as the North Pacific, could track year-to-year variability in carbon dioxide uptake in a meaningful way, says Körtzinger.

“I like their methods, making use of existing cargo ships makes data gathering more efficient,” says Pieter Tans, an atmospheric chemist with the National Oceanic and Atmospheric Administration in Boulder, Colo. Although the quality control on data may not be quite as rigorous as it would be on a scientific research vessel, “they seem to be doing an excellent job,” he notes.

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