Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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notice the problem. Like a disease that remains undetected until its last
stages, by then it has already become a crisis.
Just as lifestyle influences a person’s life expectancy within the constraints
of the human life span, the way societies treat their soil influences
their longevity. Whether, and the degree to which, soil erosion exceeds soil
production depends on technology, farming methods, climate, and population
density. In the broadest sense, the life span of a civilization is limited
by the time needed for agricultural production to occupy the available
arable land and then erode through the topsoil. How long it takes to regenerate
the soil in a particular climate and geologic setting defines the time
required to reestablish an agricultural civilization—providing of course
that the soil is allowed to rebuild.
This view implies that the life expectancy for a civilization depends on
the ratio of the initial soil thickness to the net rate at which it loses soil.
Studies that compare recent erosion rates to long-term geologic rates find
increases of at least twofold and as much as a hundred times or more.
Human activities have increased erosion rates severalfold even in areas with
little apparent acceleration of erosion, while areas with acknowledged
problems erode a hundred to even a thousand times faster than what is
geologically normal. On average, people appear to have increased soil erosion
at least tenfold across the planet.
Several years ago, University of Michigan geologist Bruce Wilkinson
used the distribution and volume of sedimentary rocks to estimate rates of
erosion over geologic time. He estimated that the average erosion rate over
the last 500 million years was about an inch every 1,000 years, but that
today it takes erosion less than 40 years, on average, to strip an inch of soil
off agricultural fields—more than twenty times the geologic rate. Such
dramatic acceleration of erosion rates makes soil erosion a global ecological
crisis that, although less dramatic than an Ice Age or a comet impact,
can prove equally catastrophic—in time.
With soil production rates of inches per millennia and soil erosion rates
under conventional, plow-based agriculture of inches per decade to inches
per century, it would take several hundred to a couple thousand years to
erode through the one- to three-foot-thick soil profile typical of undisturbed
areas of temperate and tropical latitudes. This simple estimate of
the life span of civilizations predicts remarkably well the historical pattern
for major civilizations around the world.
Except for the fertile river valleys along which agriculture began, civilizations
generally lasted eight hundred to two thousand years, roughly
life span of civilizations