Land degradation caused by agriculture takes many forms and has many causes. Some of the most important types of land degradation (and those that we will focus on) include:
(1) Degradation related to overgrazing by livestock
(2) Degradation related to soil erosion , here related to inappropriate cultivation practices)
3) Degradation attributable to soil salinization (a build up of salts in soil that results from irrigation in certain situations)
(4) Degradation attributable to waterlogging (another problem related to irrigation)
Another major cause of land degradation is conversion of tropical forests to agriculture (crop or pasture land), however we won't discuss this until later when we discuss forest issues and threats to biological diversity explicitly.
We won't spend as much time on these subjects as they deserve, but read "Feeding Nine Billion," "Should Cows Chew Cheatgrass on the Common Lands, " and "Can Cows and Conservation Mix?" in your assigned readings for more information. There are also additional readings on these subjects on the supplementary reading list.
We'll use degradation (or "desertification") in the broad sense to mean impoverishment of the land by human activities and by natural causes (such as climate). We will focus on human activities as causal agents. Land is considered "degraded" when its productivity is diminished.
This is one of those places where it is hard to get good numbers! For example, you can read credible estimates ranging from 196 million km2 to 20 million km2 affected by human-induced soil degradation! (FEE 2008 6(3)). (One km2 = 100 hectares, or nearly 250 acres.) Estimates depend greatly on the methods used to obtain data, on definitions of what constitutes "significantly degraded," what land use types are included in estimates (i.e., are grazing lands included, or just cropped lands?). The information that we do have on the amount of Earth's surface affected by human-caused degradation comes from a combination of remote sensing and ground truthing.
The World Resources Institute estimated (1998/99) that each year, irreversible degradation claims ~ 5 - 6 million ha of crop land (across all kinds of crops) -- an area about equal to the size of Belgium. This translates into a loss of almost 1/2 of 1 % of global crop land per year -- sounds small, perhaps, but in the face of continued population growth, it isn't small at all!
A 1990 survey by the United Nations resulted in the following estimates:
About one fourth of the world's total crop land is affected by degradation severe enough to restrict its productivity. (Some estimates run as high as 38%...)
About 15.6% of this degraded agricultural land is "strongly" degraded land whose "original biotic functions" (e.g., nutrient cycling) are largely destroyed.
Another 51.7% of the world's degraded agricultural land is moderately degraded that is, exhibiting greatly reduced agricultural productivity.
Applying estimates of productivity loss due to degradation to the categories given in the UN study, production averaged 17% less on degraded lands than it would have been without the degradation.
Degradation is unevenly distributed around the world, varying by nation and by climatic region. For example, according to UNEP (the UN Environmental Programme) 60% of the agricultural land in nonhumid areas of the world is already affected by desertification (i.e. experiencing some loss of productivity). In the US, about 40% of the potentially productive dry land is desertified to some degree (this would include the wheat growing areas of our continent's interior).
This land degradation is for the most part rooted in human population pressures -- slowing the rate of human population growth is at the heart of the solution to this problem.
It is critically important to remember that a world that is growing at ~ 80 million persons per year can't afford to be losing the quality of its food base!
Desertification is not a new problem. It has been a problem in some areas since about 7000 BC. In fact, it is believed that land degradation contributed to, if not caused, the decline of some of the world's great civilizations, including those in Mesopotamia, Egypt, Greece, N Africa, and China.
In the following sections, we'll discuss each of the primary human causes of degradation. To read about overgrazing, click >> at the bottom of this page; for general reminders on how to move about within and among these pages, click "Navigate ."