June 07, 2006

Global Warming & The Soil

Mary & I were just on the phone talking about An Inconvenient Truth, which we've both now seen and thought was fabulous. She reminded me about Gore's point about how one of the ways global warming affects us is by increasing the rate at which water evaporates from the soil. This makes sense of course; everybody knows that heat causes water to evaporate faster. This is basic kitchen chemistry.

There's another reason more water is being lost from the soil, however, which is that a soil component that holds a lot of water is being lost. When the temperature goes up, decomposition happens faster because microbial metabolism speeds up and they break down more soil organic matter (SOM).

SOM is the waste or remains of plants and sometimes animals in the upper layers of soil, with the extremely decay resistant fraction known more specifically as humus. SOM improves the soil's ability to hold nutrients, promote plant growth, resist erosion and compaction, as well as its ability to retain water. The more SOM that's lost, the less water the soil can hold at any temperature, particularly in sandy soils.

According to The Nature and Properties of Soils: Thirteenth Edition by Brady & Weil, 2002, p. 500. Emphasis mine:

[G]lobally, the release of carbon from soils into the atmosphere is about 62 Pg/yr [Pg or Petagram: 1015 grams], while only about 60 Pg/yr enter the soils from the atmosphere via plant residues. This imbalance of about 2 Pg/yr, along with about 5 Pg/yr of carbon released by the burning of fossil fuels (in which carbon was sequestered from the atmosphere millions of years ago) is only partially offset by increased absorption of atmospheric carbon dioxide by the ocean. Fossil fuel burning and degrading land-use practices have increased the concentration of carbon dioxide in the atmosphere at an accelerating rate since the beginning of the Industrial Revolution, some 400 years ago. The levels have increased from 290 to 370 ppm [parts per million] during the past century alone.

When you take land out of native forest or prarie cover and leave it bare to the air and sun, the organic matter contained in the soil degrades faster. When all the plant material grown on a patch of soil is carted off somewhere else every year and little to none remains to replace fully decomposed SOM, the level of carbon dioxide sequestered in that soil in solid form goes down.

Our use and treatment of the land itself is every bit as much of a problem as our burning of fossil fuels. As we lose SOM, not only do we lose carbon dioxide directly to atmosphere, but the soil loses its ability to support the plant life that takes carbon dioxide out of the atmosphere and adds it to the food chain in the form of sugars. As the air above the soil warms, plants expend more of the carbon dioxide they take in through respiration, sending it back out as carbon dioxide instead of keeping it as sugar, suppressing plant growth in a way that is worsened by water stress.

We aren't just warming the planet, we're decreasing the total amount of food available to every living thing on it. Literally, we're on course to starve ourselves out. If we wait until this starts affecting temperate zone crop production, we're all going to be in for a rude, rude shock about how dependent we are on the ecosystems we live in.

Posted by natasha at June 7, 2006 05:00 PM | Environment | Technorati links |

check this out....

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