Dr. Bill Chameides, dean of Duke University's Nicholas School of the Environment and a member of the National Academy of Sciences, blogs regularly at theGreenGrok.com.
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).
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.
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:
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.
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.
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.
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 the missing 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.
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:
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.