Are We Dining on a Carbon Sink?

by Bill Chameides | March 2nd, 2009
posted by Erica Rowell (Editor)

Permalink | 4 comments

Some studies indicate that no-till farming, where seeds are planted directly into the soil, can help build carbon in some soils. Keeping carbon in the biosphere and out of the atmosphere can slow global warming. (USDA)

Worldwide, a whopping 27 percent of the globe’s ice-free land is under cultivation (source [pdf]). That’s 13 million square miles. Have you ever thought what might happen if we were able to use less of that land?

In a globally warming world, we would like as much carbon as possible to reside in the biosphere. Why? Because when it leaves the biosphere, carbon takes the form of carbon dioxide (CO2), that pesky greenhouse gas. Fortunately, the biosphere – the part of the earth that supports life – already contains a whole bunch of carbon: about 2 trillion tons, compared to a mere 750 billion tons in the atmosphere as CO2 (see graphic).

Most of the Earth’s Carbon Is in Soils

By far, most of the biospheric carbon – about 1.6 trillion tons – is found in soils. So, to keep the carbon here in the biosphere (and prevent its escape into the atmosphere), our best place to focus on is soils.

(Source: IPCC)

Which brings us to agriculture. Modern agricultural practices, including tilling, planting, and harvesting, depletes carbon from the soil – lots of it. Some reports suggest stored carbon has been reduced by as much as 50 percent, thanks to decades of continuous cultivation (source [pdf]). That’s a whole bunch of carbon escaping to the atmosphere, considering that:

  • there’s more than twice as much carbon in soils than in the atmosphere and
  • more than one-quarter of all ice-free land is under cultivation.

(This is why William Ruddiman has argued that people began interfering with the climate way back in 8000 AD with the advent of agriculture).

Slow Global Warming by Farming Different?

If agriculture has moved carbon from soils to the atmosphere, why can’t we slow global warming by coaxing some of that carbon back into the soils? The simple answer: we can.

No-Till Farming

One way to do this is to change agricultural practices. For instance, some studies indicate that no-till farming, where seeds are planted directly into the soil, can help build carbon in some soils (sources here and here). In fact, it has been proposed that farmers could find a new source of income in a carbon cap-and-trade system by adopting no-till and selling the extra carbon stored in the soil as a carbon offset.

Perennial crops

An even more promising approach is to develop deep-rooted perennial crops to replace the annual crops we currently cultivate. Perennial crops, which wouldn’t have to be uprooted and replanted each year, would allow more carbon to remain in the soil.

This idea of sustainable agriculture involving perennial crops probably has its modern roots in the so-called “permaculture” works by Bill Mollison and David Holmgren.

Today it’s a high-tech pursuit, with the hope of developing new forms of perennial instead of annual grain crops. (Sources: Encyclopedia of Soil Science and the Land Institute)

Au Natural

Another approach is very low-tech – simply allow cultivated land to return to its natural state. In that regard, the world carried out an interesting experiment in the 1990s. With the fall of the Soviet Union, some 66,000 square miles of cropland went fallow.

In a recent paper published in Global Biogeochemical Cycles Nicholas Vuichard of the University of Tuscia in Viterbo, Italy, and colleagues estimated that between 1991 and 2000 some 64 million tons of carbon were added to the fallow soils.

This is not enough in and of itself to have a major impact on the overall global carbon balance – for instance themissing sink of carbon is considerably larger (about 2.5 billion tons of carbon per year). But the rate per square mile was quite large – about 1.5 times the rate of the carbon absorption by Russian forests.

Shrinking Croplands While Still Meeting Global Food Demand

It would appear that planting fewer acres of land could put more carbon in the soil and less in the atmosphere. But in a world of growing populations and widespread hunger, how can we shrink croplands? Think about these facts:

  • Something like 30 percent of the Earth’s land surface is used for grazing by or growing grain for livestock (source);
  • There are an estimated 1.3 billion cattle on the Earth today (source) – that’s about one animal for every five human beings (source); and
  • Growing meat for protein is less efficient than growing grains for protein (source [pdf]). To be more precise, about
    • 4 times more protein from grains is needed to produce an equivalent amount of protein from chicken,
    • 10 times more protein from grains for an equivalent amount of protein from pork, and
    • 30 times more protein from grains for an equivalent amount of protein from beef.

Eating a little less meat could reduce the need for so many acres of land for cultivation and allow more carbon to stay in the soils instead of the atmosphere.

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  1. Hydra
    Mar 3, 2009

    No till farming involves intensive use of herbicides. However other countries already recognize the environmentql services provided by farmers – and pay for them. The best way to use agricutural products to sequester carbon is to harvest them and turn them into something valuable. Trees will sequester carbon far longer as timber used in construction than they will as trees left to rot. When you come up with a perrenial potato, let me know. Each of these ideas has it promises and tis limitations: providing one without the other is misleading. The au naturel and shrinking farmland approach are exactly wrong: with the right kind of cultivation and th right uses we can sequester far more carbon than natural processes can. Some people have even suggested growing perennials and converting them to charcoal which can be buried. Changing to a vegetarian diest is probably the single largest contribution that (Westerners and advanced economies) can make to GHG reduction. Not only as a matter of soil cultivation but animal flatulence. A vegetarian diet is also unlikely. The best set of solutions will be the ones that can be adopted at a profit.

    • Bill Chameides
      Mar 3, 2009

      Hydra: Interesting points. Some thoughts of my own: (1) The court is still out on the benefits of no-till; let’s wait and see. (2) Most carbon sequestration in a forest occurs in the soils. To the extent that cutting trees for timber causes carbon to be lost from the soil, it will lead to less sequestration. Which is not to say I am against harvesting trees for timber – it’s a forest-management decision involving a variety of issues, one of them being carbon. (3) Scientists are working on perennial crops and when they get there, I imagine you’ll hear about it on your own. (4) Not sure why you thought I was advocating “au naturel” farming. (5) I doubt if growing corn the way we do today will sequester more carbon than a fallow field – please send me the data on that one. (6) Agree obviously about the vegetarianism, and while it is unlikely that most Westerners would adopt a vegetarian diet, many would be willing to eat less meat, don’t you think? (7) Agree on the profit thing, but that just means we need to realign markets.

  2. ErikVance
    Mar 3, 2009

    Interesting point. It makes me wonder though – between fertilizer runoff, pesticide use, more biodiverse crops, fewer GMOs, paying living wages, and now CO2 awareness – are we asking a little too much of our farmers? Can they be good stewards on all those fronts? also, I think the IPCC image needs a caption. Not sure what the red arrows mean.

    • Bill Chameides
      Mar 3, 2009

      Erik – It is a tall order, but if we can align markets so that farmers can enhance their revenues or lower their expenses by addressing these problems, it will be a lot easier.

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