IPCC Climate Change Data: GFDL99 B2a Model: 2020 Mean Temperature

The experiments with the GFDL model used here were performed
using the coupled ocean-atmosphere model described in Manabe et
al. (1991) and Stouffer et al., (1994) and references therein.
The model has interactive clouds and seasonally varying solar
insolation. The atmospheric component has nine finite difference
(sigma) levels in the vertical. This version of the model was
run at a rhomboidal resolution of 15 waves (R15) yielding an
equivalent resolution of about 4.5 degrees latitude by 7.5
degrees longitude. The model has global geography consistent
with its computational resolution and seasonal (but not diurnal)
variation of insolation. The ocean model is based on that of
Byan and Lewis (1979) with a spacing between gridpoints of 4.5
degrees latitude and 3.7 degrees longitude. It has 12 unevenly
spaced levels in the vertical dimension. To reduce model drift,
the fluxes of heat and water are adjusted by amounts which vary
seasonally and geographically, but do not change from one year
to another. The model also includes a dynamic sea-ice model
(Bryan, 1969) which allows the system additional degrees of
freedom. The 1000-year unforced simulation used here is
described in Manabe and Stouffer (1996). The drift in
global-mean temperature during this unforced simulation is very
small at about -0.023 degrees C per century. The two GFDL-R15
climate change experiments used here use the IS92a scenario of
estimated past and future greenhouse gas (GGa1) and combined
greenhouse gas and sulphate aerosol (GSa1) forcing for the
period 1765-2065 (Haywood et al., 1997). For the GGa1 experiment
only the 100-year segment from 1958-2057 are available through
the IPCC DDC. The radiative effects of all greenhouse gases is
represented in terms of an equivalent CO2 concentration, and the
direct radiative sulphate aerosol forcing is parameterised in
terms of specified spatially dependent surface albedo changes
(following Mitchell et al., 1995). Results from these climate
change experiments are discussed in Haywood et al. (1997). The
model's climate sensitivity is about 3.7 degrees C. Like B1, the B2 world is one of increased concern for
environmental and social sustainability, but the character of
this world differs substantially. Education and welfare
programs are widely pursued leading to reductions in mortality
and, to a lesser extent, fertility. The population reaches about
10 billion people by 2100, consistent with both the United
Nations and IIASA median projections. Income per capita grows at
an intermediary rate to reach about US$12,000 by 2050. By 2100
the global economy might expand to reach some US$250 trillion.
International income differences decrease, although not as
rapidly as in scenarios of higher global convergence (A1, B1).
Local inequity is reduced considerably through the development
of stronger community support networks. Generally high
educational levels promote both development and environmental
protection. Indeed, environmental protection is one of the few
remaining truly international priorities. However, strategies to
address global environmental challenges are less successful than
in B1, as governments have difficulty designing and implementing
agreements that combine environmental protection with mutual
economic benefits. The B2 storyline presents a particularly
favorable climate for community initiative and social
innovation, especially in view of high educational levels.
Technological frontiers are pushed less than in A1 and B1 and
innovations are also regionally more heterogeneous. Globally,
investment in R and D continues its current declining trend, and
mechanisms for international diffusion of technology and
know-how remain weaker than in scenarios A1 and B1 (but higher
than in scenario A2). Some regions with rapid economic
development and limited natural resources place particular
emphasis on technology development and bilateral co-operation.
Technical change is therefore uneven. The energy intensity of
GDP declines at about one percent per year, in line with the
average historical experience of the last two centuries.
Land-use management becomes better integrated at the local level
in the B2 world. Urban and transport infrastructure is a
particular focus of community innovation, contributing to a low
level of car dependence and less urban sprawl. An emphasis on
food self-reliance contributes to a shift in dietary patterns
towards local products, with reduced meat consumption in
countries with high population densities. Energy systems differ
from region to region, depending on the availability of natural
resources. The need to use energy and other resources more
efficiently spurs the development of less carbon-intensive
technology in some regions. Environment policy cooperation at
the regional level leads to success in the management of some
transboundary environmental problems, such as acidification due
to SO2, especially to sustain regional self-reliance in
agricultural production. Regional cooperation also results in
lower emissions of NOx and VOCs, reducing the incidence of
elevated tropospheric ozone levels. Although globally the energy
system remains predominantly hydrocarbon-based to 2100, there is
a gradual transition away from the current share of fossil
resources in world energy supply, with a corresponding reduction
in carbon intensity.