IPCC Climate Change Data: GFDL99 A2a Model: 2020 Precipitation

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. For the A2 emissions scenario the main emphasis is on a
strengthening of regional and local culture, with a "return
to family values" in many regions. The A2 world
"consolidates" into a series of roughly continental
economic regions, emphasizing local cultural roots. In some
regions, increased religious participation leads many to reject
a materialist path and to focus attention on contributing to the
local community. Elsewhere, the trend is towards increased
investment in education and science and growth in economic
productivity. Social and political structures diversify, with
some regions moving towards stronger welfare systems and reduced
income inequality, while others move towards "lean"
government. Environmental concerns are relatively weak, although
some attention is paid to bringing local pollution under control
and maintaining local environmental amenities. The A2 world
sees more international tensions and less cooperation than in A1
or B1. People, ideas and capital are less mobile so that
technology diffuses slowly. International disparities in
productivity, and hence income per capita, are maintained or
increased. With the emphasis on family and community life,
fertility rates decline only slowly, although they vary among
regions. Hence, this scenario family has high population growth
(to 15 billion by 2100) with comparatively low incomes per
capita relative to the A1 and B1 worlds, at US$7,200 in 2050 and
US$16,000 in 2100. Technological change is rapid in some
regions and slow in others as industry adjusts to local resource
endowments, culture, and education levels. Regions with abundant
energy and mineral resources evolve more resource intensive
economies, while those poor in resources place very high
priority on minimizing import dependence through technological
innovation to improve resource efficiency and make use of
substitute inputs. The fuel mix in different regions is
determined primarily by resource availability. And divisions
among regions persist in terms of their mix of technologies,
with high-income but resource-poor regions shifting toward
advanced post fossil technologies (renewables in regions of
large land availability, nuclear in densely populated, resource
poor regions) and low-income resource-rich regions generally
relying on older fossil technologies. With substantial food
requirements, agricultural productivity is one of the main focus
areas for innovation and RD efforts in this future. Initially
high levels of soil erosion and water pollution are eventually
eased through the local development of more sustainable
high-yield agriculture. Although attention is given to
potential local and regional environmental damage, it is not
uniform across regions. For example, sulfur and particulate
emissions are reduced in Asia due to impacts on human health and
agricultural production but increase in Africa as a result of
the intensified exploitation of coal and other mineral
resources. The A2 world sees high energy and carbon intensity,
and correspondingly high GHG emissions. Its CO2 emissions are
the highest of all four scenario families.