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Soil Temperature & Global Warming

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Author: Randall Frost, Ph.D.

Soil temperature varies from one month to the next depending upon such factors as solar radiation, rainfall, seasonal swings in overhead air temperature, plant cover, soil type, and depth in the earth. Seasonal changes in soil temperatures deep in the ground are smaller than those at the surface, and lag behind seasonal changes in air temperature. In spring the soil warms more slowly than the air. By summer it is cooler than the air overhead.

The temperature of the soil at the earth’s surface is very sensitive to air temperature. In a semi-arid climate, the soil temperature 2 inches below the surface can fluctuate by 10 deg C over the course of 24 hours. Under the same conditions, the temperature a foot down might fluctuate by only 1 deg C. At depths ranging from 15 to 30 ft, the soil temperature does not fluctuate.

Global surface temperatures have increased by an average of 0.74 deg C (plus or minus 0.18 deg C) since the late 19th century. The trend over the past 50 years corresponds to an increase in temperature of 0.13 deg C (plus of minus 0.03 deg C) per decade. This is more than twice that for the past 100 years. Seven of the eight warmest years on record have occurred since 2001. The ten warmest years on record have all occurred since 1995.

Problem:

Is soil temperature a good indicator of global warming trends? How does soil temperature vary with depth from the surface?

Materials:

  • Auger hand post hole digger
  • Extension shafts
  • Soil thermometer
  • Tape measure

Procedure:

  1. Read about global surface temperature trends and global warming. Also read about soil layering.
  2. Formulate a hypothesis about whether soil temperature data could be used to reconstruct global warming trends.
  3. Select a location to obtain a soil temperature profile.
  4. Measure the temperature of the soil at ground level.
  5. Dig 6 inches into the ground, and measure the soil temperature there.
  6. Dig down another 6 inches and check the soil temperature again.
  7. Continue these measurements every 6 inches until there are no further variations in soil temperature with depth or until you can dig no further.
  8. Repeat these measurements at two more locations, at times spaced a month apart.
  9. Estimate the soil temperature at the depth where the temperature no longer depends on surface temperature (or use your measured value) for each of the three months in your study.
  10. Tabulate and plot this temperature versus time.
  11. Evaluate your hypothesis and revise it if necessary.

Depth (ft)

Soil Temperature (deg C)

 

Month 1

Month 2

Month 3

 

 

 

 

0

 

 

 

0.5

 

 

 

 

15

 

 

 

 

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