gas carbon dioxide. As deforestation proceeds, it is estimated to be responsible for
over a fifth of the world’s carbon dioxide emissions. The burning of trees signifi-
cantly decreases atmospheric visibility and local energy balances. Land cover
changes provide other feedbacks into the climate system. The loss of trees usually
decreases the amount of oxygen produced by photosynthesis. Because the main
source of atmospheric oxygen is photosynthesis, the loss of forest by deforestation
interferes with the planet’s oxygen cycle. The loss of trees lessens the amount of
energy associated with water vapor (latent heat), resulting in a local rise of temper-
ature. Finally, deforestation changing forest canopy to bare soil and incomplete crop
canopies changes local energy balances and tends to warm the atmosphere.
Demographic Transition Model
A model that defines four progressive stages of demographic development, in an
attempt to elucidate and analyze the various conditions and factors that influence
population growth. Some sources use the term “demographic transformation” to
refer to this model. The key relationship at the center of the model is the dynamic
between death rates and birth rates, and how these are altered by changing socio-
economic conditions over time.In particular, the model relates increasing rates
of economic advancement and higher levels of urbanization to a general, and at
least in theory, long-term, reduction in population growth. The model is based on
the experience of economically advanced countries that underwent industrializa-
tion in the late 19th and early 20th centuries.
The first stage of the model, typically called the “preindustrial” stage or “high
stable” stage, is marked by both high birth rates and high death rates. The total fer-
tility rate and infant mortality rate arealso both quite high, compared to later
stages of the model. Life expectancy for both genders is also typically reduced in
comparison to subsequent stages, due to the influence of food shortages, disease,
and frequent violent conflict.Populationgrowth in this stage is generally low,
because while the birth rate may exceed the death rate for a short interval, it is also
just as likely for the death rate to be higher, and neither is marked by a sustained
trend upward. The age structure of the population at this stage is typically youth-
ful, with large numbers of individuals in the lower-age cohorts, and few in the
older cohorts, reflecting the high rates of birth, fertility, and mortality, as few peo-
ple survive into “old age.” Represented by a population pyramid, a country or
region in this stage would show a pyramid with a broad base (high birth rate)
tapering steeply to a sharp pinnacle (high death rate). Few countries are considered
to be currently at this stage of the model.
Demographic Transition Model 93