Geoscience Reference
In-Depth Information
albedo changes for 1750 to the present is estimated to
be -0.2 W m-
2
.
It should be noted that deforestation is difficult to
define and monitor; it can refer to loss of forest cover
with complete clearance and conversion to a different
land use, or species' impoverishment without major
changes in physical structure. The term desertification,
applied in semi-arid regions, creates similar difficulties.
The process of vegetation change and associated soil
degradation is not attributable solely to human-induced
changes but is triggered by natural rainfall fluctuations
leading to droughts.
Deforestation affects world climate in two main
ways - first, by altering the atmospheric composition
and, second, by affecting the hydrological cycle and
local soil conditions:
Models designed to simulate the operation of
Amazonian forests (with a 27°C air temperature and
a mean monthly rainfall of 220 mm) can predict the
impacts of their degradation to savanna conditions.
This would lead to a decrease of evapotranspiration
by up to 40 per cent, an increase of runoff from 14
per cent of rainfall to 43 per cent, and an average
increase of soil temperature from 27 to 32°C.
E MODEL STRATEGIES FOR THE
PREDICTION OF CLIMATE CHANGE
Concern regarding the longer term results of possible
anthropogenic effects has prompted much recent
research directed towards global climatic change during
the twenty-first century. For this purpose, global math-
ematical models (see Chapter 8) have been employed,
assuming radiative forcing by changes in atmospheric
composition and based on assumptions of feedback
within the atmosphere-earth-ocean-ice system (see
B.2, this chapter).
Three general modelling strategies may be identified,
as described below.
1
Forests store great amounts of carbon dioxide, so
buffering the carbon dioxide cycle in the atmosphere.
The carbon retained in the vegetation of the Amazon
basin is equivalent to at least 20 per cent of the
entire atmospheric CO
2
. Destruction of the vegeta-
tion would release about four-fifths of this to the
atmosphere, about half of which would dissolve
in the oceans, but the other half would be added to
the 16 per cent increase of atmospheric CO
2
already
observed this century. The effect of this would be to
accelerate global warming. A further effect of trop-
ical forest destruction would be to reduce the natural
production of nitrous oxide. Tropical forests and
their soils produce up to half of the world's nitrous
oxide, which helps to destroy stratospheric ozone.
Any increase in ozone would warm the stratosphere,
but lower global surface temperatures.
1
Black box modelling
involves the statistical
extrapolation of an historical time series (e.g. of
past temperatures) into the future, without real
concern for the mechanisms involved. Figure 13.15,
for example, shows a crude attempt to describe the
variation of mean annual temperature in the northern
hemisphere between 1880 and 1980 in terms of a
linear increase on which is superimposed a sinu-
soidal change. This has been projected to the end of
the twenty-second century to predict a temperature
increase of 0.6°C in 120 years. This prediction
disregards the poor fit of the simple mathematical
relationships to the scatter of points representing
temperature. It also makes the very questionable
assumption that during the next 120 years the mix
and weighting of the factors controlling temperature
in the northern hemisphere will resemble those that
operated during the twentieth century. Clearly, this
prediction is too simplistic
2
Grey box modelling
is based on the assumption
that the effects of the most important controlling
variables can be identified, measured and super-
imposed to produce a satisfactory simulation of a
past record, and that the resultant mathematical
2
Tropical rainforests have a great effect on the hydro-
logical cycle through their high evapotranspiration
and their reduction of surface runoff (about one-third
of the rain never reaches the ground, being inter-
cepted and evaporating off the leaves). Forest
destruction decreases evapotranspiration, atmos-
pheric humidity, local rainfall amounts, interception,
effective soil depth, and the height of the water-table
and surface roughness (and thereby atmospheric
turbulence and heat transfer). Conversely, defor-
estation increases the seasonality of rainfall, surface
runoff, soil erosion, soil temperatures and surface
albedo (and therefore near-surface air temperatures).
All these tendencies operate to degrade existing
primary and secondary tropical forests into savanna.
