Environmental Engineering Reference
In-Depth Information
10
Changing Land Cover and Land Use
The most obvious, and conceptually the simplest, indicator of the human impact on
the biosphere's productivity and phytomass storage is the total area of the natural
ecosystems that have been transformed by human action. To put these changes into
an evolutionary context, I will begin with a brief review of phytomass storage during
the past 20 millennia, since the last glacial maximum (LGM), when North America
north of 50°N and much of Europe beginning at only a slightly higher latitude were
covered by massive continental glaciers. Primary productivity and carbon strongly
rebounded during the next 15 millennia, reaching maxima by the mid-Holocene,
some 5,000-6,000 years ago at the time of the i rst complex civilizations. This was
followed by millennia of locally severe and regionally substantial transformations
whose global impact remained still relatively minor.
The most common kind of conversion was to create new i elds by converting
forests, grasslands, and wetlands (and in some regions also deserts) to new croplands
and pastures. Next in overall importance have been the claims made on forest
phytomass to remove timber and i rewood and, starting in the nineteenth century,
wood for making paper. These human transformations of ecosystems began to
accelerate during the early modern era (after 1600) and reached an unprecedented
pace and extent thanks to the post-1850 combination of rapid population increases
and economic growth, marked by extensive urbanization, industrialization, and the
construction of transportation networks.
Tracing these changes can be revealing even when most of the conclusions must
be based on estimates and approximations, and a closer look is rewarded by a more
nuanced understanding. At one extreme are those areas whose plant cover has been
entirely lost by conversions to constructed impervious surfaces (such as pavement)
in urban and industrial areas and transportation corridors: the primary production
of these areas has been completely eliminated. At the other extreme are natural
