Thanks to Audubon magazine for the interview in their current issue with one of the go-to explainers we most frequently seek out on food sustainability issues:
Celebrated author Michael Pollan talks climate change, and how farming can help stop it.
BY RENE EBERSOLE Published: November-December 2014
Q: Should we be looking more closely at how the food we eat affects the climate?
A: I think there’s a growing recognition that you can’t really address climate without looking at the food system. Yet exactly how you do that, what that means from a policy point of view, is a lot more complicated than regulating coal-fired power plants.
Agriculture is a large source of global warming emissions. Yet you propose that it can help reverse climate change?
We certainly need to mitigate the amount of carbon and methane and nitrous oxide that agriculture adds to the atmosphere, but we can do much more than mitigate, because we can use agriculture to sequester large amounts of carbon. In fact, a third of the carbon in the atmosphere today was originally in the soil. Not in the form of fossil fuels but in the form of soil carbon.
So you’re saying farming can recapture carbon in the soil?
The body of science telling us exactly how to do it is still fairly undeveloped. There’s lots of experimentation going on, but farmers can show you that a patch of soil over there used to be exposed rock, and now there’s six inches of soil. Much of that is carbon.
We usually talk about sequestering carbon in plants, and we know how trees do it. When plants die, many of their parts break down and enter the soil as carbon. But what most people don’t realize is that the roots do it, too, and may do it in a more permanent way. It all has to do with the plant’s production of sugar, much of which leaks into the soil through the roots. What happens is when grasses grow they feed sugar to beneficial microbes that live around the rhizosphere, or root zone. In exchange the microbes supply other nutrients, minerals in particular, to the plant. This is how carbon enters the microbial ecosystem and goes through the food chain. Depending on the kind of microbes you have, they can store carbon in a very stable form, where it remains in the soil for an extremely long time.
The other way carbon sequestration happens is when plants are grazed by ruminants. Plants shed, or kill off, a certain amount of root mass to match the mass they’ve lost to the ruminant. They’re trying to keep their roots and shoots in balance. The roots they shed are broken down by worms, nematodes, microbes, and fungi and turned into soil organic matter. That’s how organic matter originally was formed.
We began losing this bank of soil carbon 10,000 years ago, when we started agriculture. And even more dramatically after we started using a plow. Because as soon as you break the skin of perennials that cover the soil, you stimulate microbial activity that releases a lot of carbon into the air. So the challenge is whether we can reverse that. The evidence is pretty good that, at least to some extent, we can.
What about the mitigation part of the equation?
We have to look at a few things. The food system is responsible for somewhere between 20 percent and 33 percent of all greenhouse gas emissions. People argue about that figure, and a lot of it has to do with how you count the carbon emitted by the meat system. It also depends on if you include deforestation, which is part of agriculture, too. The part that people know the least about is what the fertilizer contributes to the problem. When you use synthetic nitrogen fertilizers (ammonium nitrate is the main one), what the plants fail to take up can turn into nitrous oxide when exposed to water, and that goes into the atmosphere. Nitrous oxide is like methane–it’s a much more effective gas at trapping heat than carbon dioxide. Plus, the way the fertilizer is manufactured involves a very high-heat, high-energy process.
Cutting down on nitrogen fertilizer would be one of the best things we could do to mitigate agriculture’s contribution to climate change. It would also have the additional effect of improving soil health, because when you spread a lot of nitrogen fertilizer on the soil you basically make it salty, which kills many of the microbes.
To give you an idea of how serious this fertilizer problem is, when Wal-Mart decided they wanted to measure their carbon footprint as a corporation, they looked at everything–transportation, factories, the building of the stores, heating and air-conditioning, packaging–and discovered that their single biggest contributor to climate change was the fertilizer being used to grow the corn that was getting turned into the whole range of products that they sell, from Coca-Cola to meat and pet food.
Are there particular types of agriculture that sequester more carbon than others?
We know that organic agriculture sequesters more carbon than conventional; that perennial agriculture sequesters a lot more carbon than annual agriculture; that tilling releases carbon into the air, so it needs to be reduced if we’re going to sequester more carbon; and that raising animals properly on grasses, which is to say, rotationally–moving them frequently and giving the grass time to recover–sequesters very large amounts of carbon. I think there’s a terrific reservoir of hope here that we’re not paying nearly enough attention to.
So organic farms can do this, but what about Big Ag?
You don’t have to go organic to reduce agriculture’s environmental footprint. History tells us what’s possible. Beginning in the Depression, we created incentives in our farm programs to curb soil erosion. We offered farmers incentives not to plant hillsides, and leave them in perennials. We invented soil management strategies. We required our farmers to take advantage of a certain number of USDA programs to protect soil–the big environmental problem of that time.
Today the big environmental problem relating to agriculture is climate change. So why can’t we have a set of programs that give farmers incentives to, for example, not till? Most farmers will tell you that they use about twice as much synthetic nitrogen fertilizer as recommended because they see it as a form of crop insurance. We should put a tax on the excessive use of fertilizer–above a certain amount per acre you have to pay more for it. We could pay farmers for cover cropping–keeping plants on their fields through the winter. (The more you leave land black over the course of the year, the more carbon you’re losing.)
Even in conventional farms, if we organized our agricultural policies with the aim of dealing with climate change, there are a whole lot of things we could come up with to encourage farmers to build soil carbon. Plus, we could invest significant amounts of agricultural research in addressing the problem.
Is anyone doing this?
In Australia today they have something called pasture cropping, where they actually seed annual crops directly into perennial pastures. They time things very carefully so that the annual you’re putting into the pasture gets a head start on the perennial grasses around it. To put barley or corn right into a pasture–you get your grain and at the same time you’re leaving the land covered the entire time, and in the process you’re sequestering lots of carbon. There are tens of thousands of acres in Australia that are now being managed this way…
Read the whole interview here.