Soil Part II: An Optimistic Global Warming Column
I was inspired to write this two-part series on soil by reading Kristin Ohlson’s recent book, The Soil Will Save Us. Part I focused on the management of minerals in the soil and suggested that increased use of seaweed-based fertilizers could improve our agricultural productivity as well as the quality of our diets. In Part II, I’ll explore Ms. Ohlson’s suggestion that improved soil management and farming practices could slow, stop, or even reverse global warming.
Before discussing how soil management could impact global warming, we need to start with an explanation of the overall carbon balance for the biosphere. By limiting the discussion to the biosphere, I am omitting carbon which exists deep underground and deep in the ocean, which effectively keeps it out of the carbon cycle. The approximate distribution of carbon in the biosphere in gigatons (billions of tons) is shown below.
Gigatons of Carbon
|Fossil Fuel Reserves||
When the Industrial Revolution began in the mid-1800s, the atmosphere contained 570 gigatons of carbon. Since then, through the combined effects of fossil fuel use, deforestation and (as we will explore further) soil degradation, we have added 270 gigatons, to reach the current level of 840. At present, humanity is releasing approximately 9 gigatons of carbon per year into the atmosphere, of which 4 gigatons are absorbed into the oceans, soil, and vegetation, resulting in a net increase of 5 gigatons per year in the atmosphere.
In order to maintain a climate similar to that in which human civilization evolved, we need to find a way to reduce the amount of carbon in the atmosphere to between 570 and 735 gigatons. So let’s take a look at the numbers in the table above and consider scenarios which might result in the removal of 150 to 200 gigatons of carbon from the atmosphere.
Fossil Fuel Reserves
Without the intervention of human beings, fossil fuel reserves are effectively excluded from the biosphere. When we burn fossil fuels, we add carbon to the biosphere which must then find a home in one of the other reservoirs listed above. If we have any hope of reducing the amount of carbon in the atmosphere, we need to transition to low and no carbon sources of energy and leave most of these reserves in the ground.
While it is certainly possible to increase the amount of vegetation on the earth to some degree, unless one assumes a significant drop in human population, it’s unlikely that this reservoir will grow by very much. Adding a significant amount of vegetation to the earth would require massive conversion of farmland to forest, which would dramatically reduce food supply.
The capacity of the upper ocean to absorb carbon from the atmosphere is already becoming limited as the concentration of dissolved carbon dioxide approaches saturation. Therefore, even if we wanted to induce significantly more carbon to dissolve in the oceans, we would not be able to. We really would not want this to happen, as it causes acidification of the oceans which is harmful to aquatic life.
Thus we are left with soil. The appeal of soil as a carbon reservoir stems from two factors. First, since it is already a rather large reservoir, an increase in carbon content of just 10% would absorb 150 gigatons of carbon from the air. Secondly, since we know that our land-use practices have significantly depleted the amount of carbon in our soils, increasing the amount by 10% seems within the realm of possibility.
As I reviewed in Part I, a healthy soil contains approximately 10% organic matter. (Organic matter is comprised of carbon containing molecules.) Prior to reading Ms. Ohlson’s book, my concept of the addition of organic matter to the soil was limited to either adding compost or plowing under cover crops. It turns out that I was missing a key mechanism, one that just might change the world.
During photosynthesis, plants absorb carbon dioxide from the air and water from the soil to produce glucose, a simple sugar. Plants use the sugar both as an energy source and as a raw material to make other more complex organic molecules to form stems, leaves, flowers, and fruits. In addition, plants “intentionally” allow up to 40% of the glucose they make to leach out of their roots and into the soil. By introducing sugar to nearby soil, the plants “recruit” a host of bacteria, fungi, insects, and earthworms whose assistance they need for health and growth. The glucose added to the soil by the plants provides the foundation for an entire underground food chain, the end result of which is a dark organic product typically referred to as humus. If left undisturbed, humus will remain in the ground for long periods of time. Unfortunately, at least on our farms, we don’t leave the ground undisturbed. We till it and till it and till it and till it. Each time we do so, we break up the humus and bring it to the surface where it is both eroded away and oxidized into to carbon dioxide and released into the atmosphere. The loss of carbon from the soil over the years has been a significant contributor to the increased level of carbon dioxide in our atmosphere.
I often write about problems for which the solutions are either complex or unknown. In this case, the answer is obvious: stop tilling the soil. Other than out of habit, the primary reason that farmers till a field prior to planting crops is to prevent other plants from shading seedlings. In a no-till approach, the farmer maintains cover crops, such as clover, in the fields and mows them prior to planting. This does not kill the cover crop but still allows sunlight to reach the seedlings while they become established. The no-till approach has been shown to raise yields, reduce erosion, and help the soil to maintain water content.
At present, 10% of farms in the U.S. and 7% globally are using a no-till approach, and these percentages are growing every year. While the United States Department of Agriculture has several programs in place to encourage it, conversion of farms to a no-till approach is hampered by the costs involved in replacing expensive equipment which they use for their current traditional approach, and by the reluctance of farmers to make such a significant change in their operations.
Given the potential value of soil as a carbon sink, we should try everything possible, including financial incentives, to convert farms all over the world to no-till practices. We can also improve our management of other lands as well. For example, grass lawns are much less efficient in adding organic content to soils compared to meadows. You can chip in to this process by converting part of your yard to a no-till natural area.
Who knows, maybe the soil really will save us.
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