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Warehousing CO2: What Lies Beneath

Seagrass meadows are some of the most productive ecosystems on the planet. New research shows that they’re enormous carbon sinks, too.

The trouble with carbon dioxide is that it’s invisible and doesn’t reek to high heaven. As dejected environmentalists have long pointed out, climate change would rank a lot higher on Americans’ worry lists if the gas were, say, purple, or smelled like rotting fish. As it is, we’re at war with an unseen enemy, and the war is not going well.

The latest round of climate talks concluded in Bonn, Germany, last month utterly gridlocked. Kyoto is set to expire at the end of the year with no successor in sight. And according to the International Energy Agency, CO2 emissions from fossil fuels reached an all-time high in 2011—after falling off a bit during the global recession—with coal and China leading the way.

And that's the good news, that 10 percent increase in Chinese emissions. It could have been a lot worse. “What China has done over such a short period of time to improve energy efficiency and deploy clean energy is already paying major dividends to the global environment,” said IAE chief economist Fatih Birol.

Amid the dreariness, climate scientists got a bit of cheer from a new study, published in Nature Geoscience, which assessed, for the first time, the ability of seagrass meadows to sequester organic carbon.

Researchers have long documented how terrestrial forests—boreal, tropical, and otherwise—lock up carbon in their trunks and roots. So-called “carbon sinks” are taken into account when annual “greenhouse gas inventories” are prepared. In 2010, according to the U.S. Environmental Protection Agency, the U.S. emitted 5.7 billion tons of CO2 into the atmosphere, nearly all of it from burning fossil fuels; at the same time, American forests and farm fields sequestered 1.1 billion tons of CO2, or 18 percent of the total produced.

The ocean, too, is a carbon sink, sequestering a full quarter of atmospheric carbon. (Because warm water traps less CO2 than cold water, scientists fear that as sea temperatures continue to rise, the release of oceanic carbon will contribute to a dangerous positive-feedback loop.) While seagrass covers less than one percent of the ocean floor, oceanographers had previously estimated that it was responsible for sequestering 10 percent of the organic carbon trapped in the ocean each year.

Hoping to check this figure, a team of researchers led by James Fourqurean of Florida International University built a database with samples from 946 sites around the world, from the Gulf Coast to the Mediterranean to Southeast Asia. Off the coast of Spain, his team discovered meadows rooted in “mattes” of soil nine feet thick and 1,200 years old. Very little carbon was sequestered in the plants’ “biomass” itself—leaves, stem, and roots; most was held in the uppermost meter of soil beneath the plants, which, Fourqurean writes, can hold twice as much carbon as terrestrial soil. The database was far from perfect, as meadows vary widely in their size and health, but it allowed the researchers to extrapolate global figures.

Per acre, Fourqurean found, seagrass meadows are capable of storing as much organic carbon as some forests. And where carbon sequestered in trees and cover crops is released back into the atmosphere when the wood is burned and fields tilled—accounting for 10 to 20 percent of annual global emissions—carbon held in the ocean’s cool, oxygen-poor soil can be preserved for millennia. As the authors note without a trace of obvious humor, “the lack of fires underwater” contributes to such longevity.

All told, the authors conclude, seagrass meadows sequester far more than the 10 percent of the oceanic carbon they were once thought to do. In fact, they probably bury twice as much.

Which makes it all the more damning a problem that they’re disappearing. Due to dredging and worsening water quality, seagrass meadows are one of the planet’s most threatened ecosystems. Nearly a third of all seagrass known to exist 100 years ago has disappeared, the authors note, replaced by unvegetated mud. Just as deforestation puts terrestrial carbon back into the atmosphere, so, too, do meadow die-offs. Worse, the sink is lost forever. More carbon in the atmosphere, fewer places to store it.

To say that the cycle will only speed up is not alarmist. It’s not alarmist enough.