Natural sinks still sopping up carbon

Ecosystems haven’t maxed out ability to absorb fossil fuel emissions

BOULDER, Colo. — Earth’s ecosystems keep soaking up more carbon as greenhouse gases accumulate in the atmosphere, new measurements find.

The research contradicts several recent studies suggesting that “carbon sinks” have reached or passed their capacity. By looking at global measurements of atmospheric carbon dioxide, the new work calculates instead that total sinks have increased roughly in line with rising emissions.

“The sinks have been more than able to keep up with emissions,” said Pieter Tans, an atmospheric scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo. Tans presented the findings May 15 at an annual conference on global monitoring hosted by the lab.

Careful measurements of atmospheric carbon dioxide taken in the rarefied air atop Mauna Loa, Hawaii, and elsewhere have established that levels of the gas are rising steadily, from 316 parts per million in 1959 to 392 parts per million today. The question is how Earth’s great ecosystems respond to that increase. Forests can suck down carbon dioxide through photosynthesis, whereas oceans take it up proportionally as levels rise in the air.

Previous work has relied on carbon inventories that gather data from multiple sources to try to estimate how much is being put into the atmosphere and how much is being taken out every year. For the new study, Tans and his colleagues went back to basics, choosing 42 marine sites where carbon dioxide levels have been measured for decades.

The researchers then analyzed how much carbon dioxide was in the atmosphere above each of these sites over time. “Less carbon dioxide has remained in the atmosphere, relative to the amount of fossil fuel emissions, today compared to 50 years ago,” Tans said. Even including the effects of land use change, which may alter carbon sinks, produced no measurable trend, he added.

Exactly where the sinks are isn’t clear. One possibility is that forests are regrowing in parts of the world more than scientists had thought, sucking up carbon in the process. Or the oceans may be taking up significantly more carbon than researchers had estimated.

Ralph Keeling, an atmospheric scientist at the Scripps Institution of Oceanography in San Diego, agrees that both land and the oceans aren’t yet done absorbing all the carbon they can. “The land is responding in a big way” to increasing fossil fuel emissions, he says.

Both Keeling and Tans warn that society shouldn’t get complacent just because carbon is still being absorbed. Rising levels of atmospheric greenhouse gases are triggering other planet-wide changes, such as alterations to the oceans’ chemistry. “The situation is bad enough,” Keeling says, “even with the sinks hanging in there.”

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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