Soil Carbon: An Unappreciated Threat Grows

COPENHAGEN—Christopher Field of the Carnegie Institution had a sobering update in a session yesterday on the steadily worsening ability of the oceans and land to suck up carbon. Field says that the 2007 IPCC report "underemphasized" the growing challenge posed by changes in the terrestrial and ocean carbon cycle. Ocean and soils take up about 5 billion tons of carbon dioxide per year—for perspective, deforestation and energy-related emissions add 10 billion—but they may be losing that ability as the globe warms. They may even start leaking their own carbon. The science is rife with uncertainties, but since the 2007 report, a number of findings have solidified—and the news is wholly bad. The carbon cycle studies on which IPCC authors based their report didn't emphasize the vulnerabilities of the carbon stored in natural sinks, says Field, but those potential sources are "really important."

Scientists have also been desperately hoping that the increased temperatures and CO2 levels in a world marked by global warming would help plants grow better, increasing the amount of CO2 they absorb and slowing further climate changes. Field says studies are not finding that to be the case, calling estimates of this phenomenon overly "generous."

The permafrost—found mostly in frozen soil and bogs in the Northern Hemisphere—is the "scariest lever on the climate system," says Field. Scientists don’t know how or when this ticking bomb might go off, but they’ve recently quantified its size with more certainty. The old estimate was that 850 billion tons of CO2 was locked in this mysterious source, and the models quoted in the last IPCC report barely mentioned permafrost as a potential—and damaging—contributor to future carbon emissions. Now, after scientists examined the amount of carbon locked deeper in permafrost soils, the new estimate is that these soils worldwide hold up to 1.7 trillion tons of carbon. About a third of that is in a type of ancient permafrost called Yedoma, whose carbon would unfortunately be easily released as the soil melts. Much of it would also be emitted as methane, a potent greenhouse gas. The rest of the permafrost shouldn't release its carbon as easily, but it's more widespread than Yedoma and thus more likely to be affected in a warming world.

The biggest fear is that as a warming atmosphere or ocean causes carbon to be emitted— from soils, permafrosts, or the top layer of the sea—that extra carbon will cause additional warming and make the effort to decarbonize the world economy that much harder. For example, if the world was aiming to stabilize the concentration of CO2 in the atmosphere at 450 parts per million—a commonly discussed goal—it might have to reduce industrial carbon emissions an additional whopping 100 billion tons or more to compensate for carbon stocks released by soils and waters. "It’s like a big tank [of carbon], and if you knock the valve off you’ll spill a massive amount," says Field. "What we don’t know is whether it will be a little leak or a big gushing."

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