Geoscience Reference
In-Depth Information
in these areas between about AD 700 and 1700 as
populations slowly grew, but it did not take place
in North America until the westward movement
of settlement in the eighteenth and nineteenth
centuries. During the past half-century extensive
deforestation has occurred in the tropical
rainforests of Southeast Asia, Africa and South
America. Estimates of current tropical deforesta-
tion suggest losses of 105km
2
/year out of a total
tropical forest area of 9
E PROJECTIONS OF
TEMPERATURE CHANGE
THROUGH THE TWENTY-FIRST
CENTURY
1 Applications of General
Circulation Models
The most powerful tools for examining the
emerging signatures of climate change and
projecting changes through the twenty-first
century are General Circulation Models (GCMs),
the most sophisticated being those that are fully
coupled to the ocean, known as Atmosphere-
Ocean GCMs, or AOGCMs. As outlined in
Chapter 8, such global models are based on
detailed mathematical representations of the
structure and operation of the earth-atmosphere-
ocean system. The possible future (as well as past)
states of the system can be simulated by applying
assumed climate forcings, such as greenhouse gas
concentrations, solar irradiance and (in the
case of paleoclimate studies) ice sheet extent and
topography. GCMs are very powerful but involve
the need for a thorough understanding of the
variables, states, feedbacks, transfers and forcings
of the complex system, together with the laws of
physics of the atmosphere and oceans on which
they are based.
10
6
km
2
. This annual
figure is more than half the total land surface
currently under irrigation. Forest destruction
causes an increase in albedo of about 10 percent
locally, with consequences for surface energy and
moisture budgets.
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. Desertification also
contributes to an increase of wind-blown soil. The
'dust-bowl' years of the 1930s in the United States
and the African Sahel drought since 1972 illustrate
this, as well as dust transported from western
China across the Pacific to Hawaii, and from the
Sahara westward across the North Atlantic. The
process of vegetation change and associated soil
degradation is not solely attributable to human-
induced changes, as it can be triggered by natural
rainfall fluctuations leading to droughts.
Deforestation and associated biomass burning
has also contributed to rising carbon dioxide
concentrations. Forests store great amounts of
carbon, and left alone buffer the carbon dioxide
cycle in the atmosphere. The carbon retained in
the vegetation of the Amazon basin alone is
equivalent to at least 20 percent of the entire
atmospheric carbon dioxide load. Deforestation
and biomass burning in the Amazon and
elsewhere is estimated to account for about 25
percent of the increase in atmospheric carbon
dioxide since pre-industrial times.
×
2
The IPCC simulations
GCMs have been developed by modeling groups
all over the world. The Intergovernmental Panel
on Climate Change (IPCC) has served as a key
focal point for model development. As introduced
in Chapter 1, a goal of the IPCC is to assess the
impacts of projected increases in greenhouse gas
concentrations and other anthropogenic climate
forcings through the twenty-first century. The
IPCC has issued four comprehensive reports, (in
1990, 1995 and 2001 and 2007), each making use
of increasingly sophisticated models.
Models used in the First Assessment Report
(1990) were primitive by today's standards. Only
