A crucial climate mystery is just under our feet

Written by: Nathanael Johnson

What Jonathan Sanderman really wanted was some old dirt. He called everyone he could think of who might know where he could get some. He emailed colleagues and read through old studies looking for clues, but he kept coming up empty.

Sanderman was looking for old dirt because it would let him test a plan to save the world. Soil scientists had been talking about this idea for decades: farmers could turn their fields into giant greenhouse gas sponges, potentially offsetting as much as 15 percent of global fossil fuel emissions a year, simply by coaxing crops to suck more CO2 out of the air.

There was one big problem with this idea: It could backfire. When plants absorb CO2 they either turn it into food or stash it in the ground. The risk is that if you treat farms as carbon banks, it could lead to smaller harvests, which would spur farmers to plow more land and pump more carbon into the air than before.


The question that Sanderman wanted to answer was laid out by the Canadian soil scientist Henry Janzen. In 2006, Janzen published a paper, “The soil carbon dilemma: Shall we hoard it or use it?” Janzen pointed out that since the dawn of agriculture, farmers have been breeding crops that suck carbon out of the air and put it on our plates, rather than leaving it behind in the soil.

“Grain is 45 percent carbon by weight,” Janzen told me. “So when you truck away a load of grain, you are exporting carbon which, in a natural system, would have mostly returned to the soil.”

Janzen has the rare ability to explain complicated things with such clarity that, when talking to him, you may catch yourself struck with wonder at an utterly new glimpse of how the world works. Plants, he explained, perform a kind of alchemy. They combine air, water, and the sun’s fire to make food. And this alchemical combination that we call food is, in fact, a battery — a molecular trap for the sun’s energy made of broken-down CO2 and H2O (you know, air and water).