Environmental Engineering Reference
In-Depth Information
i rst 1,500 years of the Common Era, from about 100 Mha at its beginning to about
220 Mha at the onset of the early modern era (HYDE 2011).
By 1800 that total had more than doubled, to just over 500 Mha, and then it
expanded 2.5 times during the nineteenth century (to nearly 1.3 Gha), mainly as a
result of the westward advance of permanently settled territories in the United
States, Canada, and Australia (for a gain of some 250 Mha of pastures) and a large
expansion of grazing throughout sub-Saharan Africa (a gain of some 200 Mha) and
in Central Asia. The expansion rate was even slightly higher during the twentieth
century (2.6-fold gain), when the greatest gains came in Latin America (where the
grazing area had quadrupled since 1900 to about 560 Mha), followed by sub-
Saharan Africa (more than doubling to about 900 Mha) and several regions of Asia.
As already noted, at the beginning of the twenty-i rst century permanent meadows
and pastures occupied about 34 million km
2
, or more than twice the area devoted
to the cultivation of annual and perennial crops (15.3 million km
2
)—and Europe is
now the only continent where less than 50% of all land used for food production
is in pastures (the rate is just short of 40%). In North America the share is just over
50%, in Asia it is over 60%, in Africa over 80%, and in Australia close to 90%
(Rodriguez 2008; FAO 2011f). Countries with the largest areas of grazing land are
China (400 Mha), Kazakhstan (185 Mha), and Mongolia (115 Mha) in Asia, the
United States (nearly 240 Mha) and Brazil (almost 200 Mha) in the Americas, and
Sudan (close to 120 Mha) in Africa.
Deforestation and Reforestation
While there is no major uncertainty about what constitutes annually cropped land
or permanent tree plantations, the dei nition of forest depends heavily on arbitrary
limits set on the spatial extent and density of growth. By the year 2000 more than
650 dei nitions of forests had been collected in the course of the FAO's Global Forest
Resources Assessment (FAO 2001). The most commonly cited national, continental,
and global totals regularly published by the FAO (and available in the organization's
electronic database) are now based on fairly relaxed criteria: they include all areas
larger than 0.5 ha (while the 1990 assessment required a minimum of 100 ha) with
trees higher than 5 m and a canopy cover of more than 10%, or trees able to reach
these thresholds in situ. It does not include land that is predominantly under agri-
cultural or urban use (FAO 2005).
The explanatory notes make even more liberal allowances: any areas under
reforestation that are expected to reach the above-listed criteria can be included,
